 |
| Watt "Imperium" (tm) Fuel Cell Propane Consumption |
September 10, 2021. Because Roadtrek took down the Watt video, I added a link to the website.
October 10, 2018. Added Roadtrek video on the Watt propane generator
Originally Posted March 20, 2018
Class B RV battery systems continue to evolve. Recently Winnebago announced a new version of the Travato with high voltage lithium-ion (LiB) coach batteries. This sparked some conjecture and interest on social media. WGO is joining Roadtrek and Advanced RV in providing Li-ion battery systems in Class B's. Coachmen has an announced Li-ion system for their Galleria. Not all Class B manufacturer's are on board and some of the new systems may not achieve long term expectations. There is reason for buyers to be cautious with these higher power, somewhat expensive and technically sophisticated systems. Nevertheless, there is a trend to larger battery powered systems in Class B coaches.
This post builds upon my recent ones about solar, AGM batteries, Li-ion batteries (LiBs) and charging systems. See the "Important Notes" at the end of this post.
The Only Constant is "Change"
Lithium-ion batteries (LiBs) are a "work in progress" and there are a number of changes coming which may further improve these systems. LiBs will be getting more powerful, and soon.
This post focuses on some of the things that are going on, will be here soon, some of the approaches, some issues and provides some comparisons. Expanded and larger battery systems may be a necessity if RV manufacturers replace propane fueled devices including refrigerators, hot water heaters, furnaces and range tops with DC electrics. The total available portable energy in these coaches may not be changing, but is shifting to electricity as the primary power source.
Because these are expensive systems, I suspect some buyers will overspend, getting features (power ratings) they don't need or seldom use. Components such as inverters and converters may only have 3-year warranties, or less. It will be up to the RV manufacturers to decide how much warranty they are willing to provide for these systems and owners will have to decide how much financial risk they are willing to take. See the "System Comparison Caveats" at the end of this post for important information about ratings, etc.
However, the core information is very positive for these systems. It seems there is continuing divergence as different technologies appear and each has inherent advantages and disadvantages. For my personal use I will be comparing this to alternatives, including standard AGMs and carbon foam battery technology. A lot of that data has been put into earlier posts. "This really isn't rocket science" although these are systems of varying complexities. Manufacturers and tekkies may use jargon and other terms which can confuse the casual reader. I'm sure I'm guilty of that, too. There are also "apples to oranges" comparisons about systems and ratings. These can confuse, particularly when statements rely upon presenting the "best" case.
I assume you are a casual, but interested, reader and this may be helpful to you. I've spent a lot of time in advanced technology systems for industrial applications and after years "on the leading edge" and some experience "on the bleeding edge" I am cautious and I have no interest in being an early adopter, and I will avoid certain types of experiments with my Class B. However, there are others who are far more adventurous than I am.
Meanwhile, for an alternative reality using Propane Fuel Cells
Hymer Group (Roadtrek) announced a propane fuel cell. The Roadtek video has since been taken down. I am replacing it with a link to the manufacturer's website (Sept. 10, 2021). "WATT Imperium’s™ quiet, always-working operation is about as loud as a ceiling fan or normal background conversation (45 dB at 3ft.), and won’t cause interruptions or distractions. You can now replace cumbersome and noisy conventional gas generators that emit toxic exhaust fumes with Imperium’s™ state-of-the-art compact, quiet and clean power generator":
Here is the original press announcement by Hymer:
"CAMBRIDGE, Ontario, March 6, 2018 /PRNewswire/ -- Erwin Hymer Group North America, Inc. has signed an exclusive supply and engineering cooperative agreement with WATT Fuel Cell Corporation located in Mount Pleasant, Pennsylvania, to introduce the WATT Fuel Cell technology into the RV market........The integration of the WATT Imperium™ Fuel Cell allows the use of propane to create clean electricity and heat, and allows Erwin Hymer Group North America, Inc. to affordably put in place a method of creating clean electricity."
October 10, 2018:
"One of the leading manufacturers of class B motorhomes in North America, EHGNA placed their first order with WATT after a successful pilot of the Imperium on board their E-Trek autonomous recreational vehicle earlier this year. The Imperium will provide clean power on demand, allowing EHGNA customers to automatically create, access and manage power for all their on-board appliances and devices wherever their adventures take them.........Manufactured at WATT’s facility in Southwestern Pennsylvania, the Imperium is a hybrid SOFC power management system that creates small-scale power, 500W to 1.5kW, from readily available and easily accessible fuels and manages renewable energy sources. The Imperium SOFC system delivered to EHGNA will utilize the hybrid power manager to integrate the fuel cell with solar generation while optimizing on-board energy storage. It will create power efficiently and quietly from propane and solar energy with little to no engine noise or harmful exhaust."
https://www.wattfuelcell.com/news/imperium-shipments-erwin-hymer-group/
Improved Li-ion batteries (LiBs) are Coming.
Tesla has begun using improved Lithium batteries in the Model S electric car. These batteries reportedly extend range by 6%. How was this accomplished? By changing the battery construction to include silicon anodes. That change, which sounds simple but in fact is not, allows more lithium in the batteries and that increases power capacity. Graphite (crystalline carbon) does not meet the high energy demands of electric vehicles.
It has been predicted that within two or three years manufacturer's will be shifting to this more powerful silicon anode battery construction. Experts suggest that we'll be seeing improvement in the range of 10 to 15%. This approach may soon provide some batteries capable of 10 to 30% more power. These improved LiBs will be used in applications ranging from smart phones to laptops to electric vehicles. It remains to be seen if these technologies will make it to Class B RVs at an affordable price.
A few years more distant there are even better batteries. These will have primarily silicon anodes which will have the possibility of improving energy storage by up to 40% over todays Lithium-Ion batteries.
However, it might be best to keep in mind the actual numbers which Tesla Motors has achieved with silicon in the anodes. There is a huge difference between a 40% potential improvement and a realized 6% improvement.
There are serious hurdles to be overcome to get above the 15% improvement, so don't hold your breath. A few companies involved in this include Angstron Materials, Enovix, Enevate, MilliporeSigma, Panasonic and Sila Nanotechnologies.
Larger Batteries
RV manufacturers are now providing larger capacity battery systems, even in smaller RVs, such as Class Bs. These use existing LiB battery technology. Not long ago Class B's came with 2.4kW (220Ah AGM batteries) or less, and this can be compared to the LiB technology offerings, which generally provide two times the battery power, and upwards. Here are a few of the recent battery packs based upon LiB technology in Class Bs. Note that specifications are published by RV manufacturers and are subject to change, as is the information they provide:
Those larger batteries need a lot of power to recharge them. This has led to larger underhood generators and larger shore power chargers. Using solar to recharge those batteries? Consider that in a typical day we might only get about 8 hours of bright sunlight at an angle to the solar panels sufficient to provide maximum charging. Solar is limited because of daylight hours and rooftop space. 400W of rooftop solar provides only about 33A at peak charging amperes. A 200W solar array provides half that. Consider that a 100W solar array provides 0.1kW of power. That's useful for augmenting or reducing dependence upon batteries, but with 4.8kW and larger battery systems, that 0.1kW solar is going to be a trickle battery charger.
The problem can be exemplified by the Tesla Model S electric automobile, which is normally charged on a high ampere 240V AC circuit. However it can be charged on a 120V circuit. Such a circuit limits charging amperes to 15-20 (about 2.4kw max charging power drawn from "shore power").
How far can a Model S go after an all night charge at such reduced charging?
Here are a few typical choices available as coach battery power recharge sources:
Here's a comparison of the power ratings of several underhood generators. These alternators require a minimum RPMs to provide usable power. That means that the vehicle engine must run at a "fast idle" and higher to provide sufficient power. Alternators generally put out low amperes at normal idle (actual output is determined by the speed of the alternator, and increases dramatically once we get above fast idle). Very low quoted charging times may require highway speeds to get the underhood generator to peak power:
Projected Lifespan, Warranties and Opportunity Costs
There are a variety of figures published about the lifespan of these battery systems. Some say 10 years, with the ability to provide 80% of the published power over that period of time. Ultimately, the only thing one can count upon is the RV builders warranty. Equipment manufacturers may only provide 1-3 years. This makes the RV builder's warranty very important.
Justification of these high power battery systems requires long lifespans. For example, the "opportunity cost" of a $10,000 system which operates reliably and provides at least 80% of published power will be:
So what do we gain with higher DC voltages? To use those higher power alternators (underhood generators) requires either larger conductors or increasing the voltage. The significance is this:
"CAMBRIDGE, Ontario, March 6, 2018 /PRNewswire/ -- Erwin Hymer Group North America, Inc. has signed an exclusive supply and engineering cooperative agreement with WATT Fuel Cell Corporation located in Mount Pleasant, Pennsylvania, to introduce the WATT Fuel Cell technology into the RV market........The integration of the WATT Imperium™ Fuel Cell allows the use of propane to create clean electricity and heat, and allows Erwin Hymer Group North America, Inc. to affordably put in place a method of creating clean electricity."
October 10, 2018:
"One of the leading manufacturers of class B motorhomes in North America, EHGNA placed their first order with WATT after a successful pilot of the Imperium on board their E-Trek autonomous recreational vehicle earlier this year. The Imperium will provide clean power on demand, allowing EHGNA customers to automatically create, access and manage power for all their on-board appliances and devices wherever their adventures take them.........Manufactured at WATT’s facility in Southwestern Pennsylvania, the Imperium is a hybrid SOFC power management system that creates small-scale power, 500W to 1.5kW, from readily available and easily accessible fuels and manages renewable energy sources. The Imperium SOFC system delivered to EHGNA will utilize the hybrid power manager to integrate the fuel cell with solar generation while optimizing on-board energy storage. It will create power efficiently and quietly from propane and solar energy with little to no engine noise or harmful exhaust."
https://www.wattfuelcell.com/news/imperium-shipments-erwin-hymer-group/
Improved Li-ion batteries (LiBs) are Coming.
Tesla has begun using improved Lithium batteries in the Model S electric car. These batteries reportedly extend range by 6%. How was this accomplished? By changing the battery construction to include silicon anodes. That change, which sounds simple but in fact is not, allows more lithium in the batteries and that increases power capacity. Graphite (crystalline carbon) does not meet the high energy demands of electric vehicles.
It has been predicted that within two or three years manufacturer's will be shifting to this more powerful silicon anode battery construction. Experts suggest that we'll be seeing improvement in the range of 10 to 15%. This approach may soon provide some batteries capable of 10 to 30% more power. These improved LiBs will be used in applications ranging from smart phones to laptops to electric vehicles. It remains to be seen if these technologies will make it to Class B RVs at an affordable price.
A few years more distant there are even better batteries. These will have primarily silicon anodes which will have the possibility of improving energy storage by up to 40% over todays Lithium-Ion batteries.
However, it might be best to keep in mind the actual numbers which Tesla Motors has achieved with silicon in the anodes. There is a huge difference between a 40% potential improvement and a realized 6% improvement.
There are serious hurdles to be overcome to get above the 15% improvement, so don't hold your breath. A few companies involved in this include Angstron Materials, Enovix, Enevate, MilliporeSigma, Panasonic and Sila Nanotechnologies.
Larger Batteries
RV manufacturers are now providing larger capacity battery systems, even in smaller RVs, such as Class Bs. These use existing LiB battery technology. Not long ago Class B's came with 2.4kW (220Ah AGM batteries) or less, and this can be compared to the LiB technology offerings, which generally provide two times the battery power, and upwards. Here are a few of the recent battery packs based upon LiB technology in Class Bs. Note that specifications are published by RV manufacturers and are subject to change, as is the information they provide:
- Roadtrek: Ecotrek 400, 400Ah or about 4.8kW (and upwards beyond 800Ah).
- Winnebago Travato, 725Ah or about 8.7kW at about 50V.
- Coachmen Li3, 600Ah or about 7.2kW
- Advanced RV, 400 to 800 Amp hours at 12V (4.8kW to 9.6kW).
Those larger batteries need a lot of power to recharge them. This has led to larger underhood generators and larger shore power chargers. Using solar to recharge those batteries? Consider that in a typical day we might only get about 8 hours of bright sunlight at an angle to the solar panels sufficient to provide maximum charging. Solar is limited because of daylight hours and rooftop space. 400W of rooftop solar provides only about 33A at peak charging amperes. A 200W solar array provides half that. Consider that a 100W solar array provides 0.1kW of power. That's useful for augmenting or reducing dependence upon batteries, but with 4.8kW and larger battery systems, that 0.1kW solar is going to be a trickle battery charger.
The problem can be exemplified by the Tesla Model S electric automobile, which is normally charged on a high ampere 240V AC circuit. However it can be charged on a 120V circuit. Such a circuit limits charging amperes to 15-20 (about 2.4kw max charging power drawn from "shore power").
How far can a Model S go after an all night charge at such reduced charging?
- Answer: about 30 miles after an overnight charge at 120V! Yet, the Model S has a normal battery range of 249-315 miles. (According to Tesla the 60-kwh battery provides a range of up to 232 miles (the EPA pegs it at 208 miles), and the 85-kwh battery (a $10,000 option) provides up to 300 miles (the EPA puts it at 265 miles).
- It would seem that the best the Tesla can do on a 120V circuit overnight is to get about 6-8kW of charge.
Here are a few typical choices available as coach battery power recharge sources:
- 120VAC "shore power" circuit with standard 12V charger: 45A = 540W (e.g. Tripp-lite).
- 120VAC "shore power" circuit with high capacity charger: Up to 30A at 120VAC = 3600W (Volta).
- Onan gasoline/propane 120V generator: powers "shore power" charger, above.
- Underhood 12V generator (e.g. Roadtrek GU): 300A max. at fast idle = 3600W.
- Solar: e.g. 200W panel about 17A = 200W.
Note: There are conditions for each of the above. Here are a few:
- For the underhood generator the engine must be at a fast idle to provide sufficient power from the alternator. That will use about 0.6gph of fuel according to some sources. That's more than a gasoline generator, but what vehicle engine RPMs are required to provide those quoted "peak" alternator charging rates? However, underhood generators can provide more DC power at peak alternator speeds..
- The Onan gasoline generator (2.8kW) in my 210P can provide a maximum 23.6A at 120V. If charging batteries at maximum using the Tripp-Lite, the other AC consumption in the coach must be limited to 13.6A or less (about 1.6kW). Generator power output decreases above 1,500 ft elevation and temperatures above 85F. "Typical" gasoline consumption at full load: 0.43gph; at half load: 0.3 gph.
- Solar charging is determined by the size of the panels in watts as well as the type of charge controller. MPPT type will outperform PWM type. Temperature and amount of sunlight also determines how much energy can be "harvested" from the sunlight striking the panel(s). 200W is probably the maximum roof real estate that can be provided for solar panels on a Class B, but there will always be exceptions. Rooftop solar has one large limitation for those parking and charging; the RV must be in full sun to get full benefit. That sometimes turns the metal can into an oven. Using portable solar panels may overcome this, but they might have a tendency to "walk."
- A Tripp-Lite "Powerverter" inverter-charger includes "load sharing" settings, to limit the charging current when running off of an AC circuit. If not used, AC input of 10A or 1200W may occur for charging batteries at 45A. This is the device in my Roadtrek 210P.
- LiBs can charge faster than AGM batteries. However, charge times are determined not only by battery chemistry but also by charging power available.
The Problem in a Nutshell
From the above, the problem facing RV manufacturers for their large, LiB systems can be summarized simply:
Inverters are required to change battery voltage to 120 VAC power. These have peak efficiencies of about 90%, but as AC loads decrease so does the efficiency of the inverter, which can be as low as 50% at light loads. In other words, 250W in and only 125W output power This can create a dilemma. To get maximum use of the batteries requires running inverters at higher efficiencies. To do so means using more battery power, and using more power requires larger batteries. Eventually we reach a point where the battery systems are approaching the ability to replace that 30A shore power, at least for a few hours a day.
Manufacturers are moving away from propane/electric coaches. This may be for economic reasons and it may also be a result of those larger coach batteries. An example is replacing absorptive refrigerators with compressor refrigerators. Some compressor refrigerators operate on 12VDC (or 24VDC) and require a 15A dedicated circuit (180W maximum).
If 12V DC refrigerators are used on 12V battery systems, then there are no converter losses. If 12V DC refrigerators are used on 48VDC battery systems there will be converter losses. If 120V compressor refrigerators are used, then there are inverter losses.
"There is no free ride." LiB battery prices can be $2,000 per 2.4kW. As RV manufacturers embrace "all electric" approaches, the battery requirements increase.
My personal trekking experience a few years ago in an "100% electric, solar powered" Class B RV gave me some insights and I do have some concerns about recent trends. There is a movement away from multi-fuel systems such as propane/electric to all electric coaches. This places larger demands on the electrical power systems of the coach. However, most battery power systems are not up to full replacement of shore power on a continuous basis. That would require battery systems capable of providing up to 3.6kW continuous electrical energy, ignoring inefficiencies of any inverters or converters.
There are methods in use on some larger RVs to control peak power consumption. These are "load shedding" systems which monitor the coach power demands and automatically turn off features based upon priorities so as not to trip circuit breakers, overload inverters, converters, etc.
For example, in an all electric coach there may be a resistance heater stove top, a microwave/convection oven, a coffee pot, a compressor refrigerator, an electric hot water heater and environmental heat and air conditioning. Vehicle engine heat may be used to augment heating/hot water.
Only the larger battery systems can power all of these, which may even overpower the 30A shore power capabilities of a Class B if an attempt is made to use hot water, air conditioning and do cooking at the same time.
It is relatively easy to automatically turn off, or "shed" some of these loads on a priority basis. For example when using the electric range, other appliances may be prevented from powering up. However, I don't see Class B manufacturers doing this. It is also easy to manually turn off certain features, if the coach is so equipped. Using circuit breakers to manually shed loads is not a good approach. Circuit breakers are not intended to be used as "off-on" switches.
Higher Voltage DC Systems:
From the above, the problem facing RV manufacturers for their large, LiB systems can be summarized simply:
- It takes a lot of power to quickly recharge those large battery packs!
- Solar, small underhood generators, and other charging systems may be inadequate to fully charge the batteries in the time available.
- Going to higher battery voltages requires DC/DC converters and to get fast "shore power" charging will require larger "shore powered" chargers.
- As power requirements of RVers increase, so does the need for larger inverters to change that DC power into 120VAC.
- All of the above equipment adds weight. So there are trade-offs.
Inverters are required to change battery voltage to 120 VAC power. These have peak efficiencies of about 90%, but as AC loads decrease so does the efficiency of the inverter, which can be as low as 50% at light loads. In other words, 250W in and only 125W output power This can create a dilemma. To get maximum use of the batteries requires running inverters at higher efficiencies. To do so means using more battery power, and using more power requires larger batteries. Eventually we reach a point where the battery systems are approaching the ability to replace that 30A shore power, at least for a few hours a day.
Manufacturers are moving away from propane/electric coaches. This may be for economic reasons and it may also be a result of those larger coach batteries. An example is replacing absorptive refrigerators with compressor refrigerators. Some compressor refrigerators operate on 12VDC (or 24VDC) and require a 15A dedicated circuit (180W maximum).
If 12V DC refrigerators are used on 12V battery systems, then there are no converter losses. If 12V DC refrigerators are used on 48VDC battery systems there will be converter losses. If 120V compressor refrigerators are used, then there are inverter losses.
"There is no free ride." LiB battery prices can be $2,000 per 2.4kW. As RV manufacturers embrace "all electric" approaches, the battery requirements increase.
My personal trekking experience a few years ago in an "100% electric, solar powered" Class B RV gave me some insights and I do have some concerns about recent trends. There is a movement away from multi-fuel systems such as propane/electric to all electric coaches. This places larger demands on the electrical power systems of the coach. However, most battery power systems are not up to full replacement of shore power on a continuous basis. That would require battery systems capable of providing up to 3.6kW continuous electrical energy, ignoring inefficiencies of any inverters or converters.
There are methods in use on some larger RVs to control peak power consumption. These are "load shedding" systems which monitor the coach power demands and automatically turn off features based upon priorities so as not to trip circuit breakers, overload inverters, converters, etc.
For example, in an all electric coach there may be a resistance heater stove top, a microwave/convection oven, a coffee pot, a compressor refrigerator, an electric hot water heater and environmental heat and air conditioning. Vehicle engine heat may be used to augment heating/hot water.
Only the larger battery systems can power all of these, which may even overpower the 30A shore power capabilities of a Class B if an attempt is made to use hot water, air conditioning and do cooking at the same time.
It is relatively easy to automatically turn off, or "shed" some of these loads on a priority basis. For example when using the electric range, other appliances may be prevented from powering up. However, I don't see Class B manufacturers doing this. It is also easy to manually turn off certain features, if the coach is so equipped. Using circuit breakers to manually shed loads is not a good approach. Circuit breakers are not intended to be used as "off-on" switches.
Higher Voltage DC Systems:
There is some interest in higher voltage DC battery systems. The moving force for these has been electric vehicles and marine applications. Some Class B RVs offer batteries and underhood generators at higher voltages. Typically 24 or 48VDC (peak 58VDC). An example is the Volta System, which is the basis for the Winnebago Travato Li-Ion battery system.
The higher voltage systems may use a variety of chemistries and LiB construction. Lithium Iron Phosphate(LiFePO4) is most common in 12VDC LiB systems. The higher voltage systems depart from this.
Higher voltage battery packs in electric vehicles may use Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2) which is also called "NMC" or they may use Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2) also called "NCA".
LiFePO4 is generally used in 12-24V systems. "NMC" seems to be more preferred in the higher voltage systems such as 48VDC. There are differences between each of these battery systems which yield different power per unit weight. In other words, some battery packs are lighter than others, even with the same power ratings. The weight and power density varies with the technology used.
Are Higher DC Voltages Better?
I have seen some apparent confusion about the lower versus higher voltages. The bottom line is straightforward with comparison of the copper conductors and the alternators. As we move into alternative battery chemistries, the density (power per unit volume and the weight) also changes. Keep in mind that higher voltage DC systems do require a "converter" to get 12VDC from the 48V alternator and batteries. That 12VDC is required for standard RV DC power systems.
Be aware that every time we change voltages, there are losses, or "inefficiencies". These inefficiencies are inherent in the power conversion, and they usually show up as waste heat. Wonderful in the winter, but unwelcome in the summer. Those converters probably lose upwards of about 10% of the battery power put into them.
When considering efficiencies, we need to consider the weight of batteries and the weight of alternators and wiring, but also the weight of inverter/chargers and converters (e.g. 48V to 12V DC). Stationary weight is not a consideration, but increasing accessory weight does displace other things we can carry within the chassis weight limits.
Another term sometimes used for this is "overhead." Whatever term we choose to use, not all of that battery power may get to the appliance, and there are weight differences, which lower the vehicle MPG and may limit the useful carrying weight of the vehicle.
How much inefficiency is there? It is reasonable to use 10% as the minimum inefficiency of a converter. In practical terms, that means that a 7.0kW high voltage battery system will only provide 6.3kW of useable 12VDC power, or less. Of course inverters which change DC battery power to 120VAC power also have inefficiencies.
One advantage of a 48V alternator is size and weight. Such an "underhood generator" will be smaller and lighter than a 12V alternator providing the identical power. The 48V generator will also require smaller conductors (wires) to convey its power to the batteries. However, the trade-off is the weight of the converter required to change that higher voltage to 12VDC. Volta's Z5104200-0121250 converter weighs 42 pounds and provides 1500 continuous watts of 12VDC.
Of course, under hood generators are less weight than a gasoline generator. An Onan "microlite" 2.8kW generator weighs 113 pounds.
The 12VDC demands may decrease as RV manufacturers juggle and size the voltages of appliances and power requirements. This may allow smaller, lighter and less powerful converters. Transferring requirements from 12VDC to 120VAC will increase the inverter size and weight. Volta's weighs 68 lbs and can provide 3000 watts of continuous 120VAC power. It can functionally replace the 30A "shore power" connection for as long as there is sufficient battery power available.
Higher voltage battery packs in electric vehicles may use Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2) which is also called "NMC" or they may use Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2) also called "NCA".
LiFePO4 is generally used in 12-24V systems. "NMC" seems to be more preferred in the higher voltage systems such as 48VDC. There are differences between each of these battery systems which yield different power per unit weight. In other words, some battery packs are lighter than others, even with the same power ratings. The weight and power density varies with the technology used.
Are Higher DC Voltages Better?
I have seen some apparent confusion about the lower versus higher voltages. The bottom line is straightforward with comparison of the copper conductors and the alternators. As we move into alternative battery chemistries, the density (power per unit volume and the weight) also changes. Keep in mind that higher voltage DC systems do require a "converter" to get 12VDC from the 48V alternator and batteries. That 12VDC is required for standard RV DC power systems.
Be aware that every time we change voltages, there are losses, or "inefficiencies". These inefficiencies are inherent in the power conversion, and they usually show up as waste heat. Wonderful in the winter, but unwelcome in the summer. Those converters probably lose upwards of about 10% of the battery power put into them.
When considering efficiencies, we need to consider the weight of batteries and the weight of alternators and wiring, but also the weight of inverter/chargers and converters (e.g. 48V to 12V DC). Stationary weight is not a consideration, but increasing accessory weight does displace other things we can carry within the chassis weight limits.
Another term sometimes used for this is "overhead." Whatever term we choose to use, not all of that battery power may get to the appliance, and there are weight differences, which lower the vehicle MPG and may limit the useful carrying weight of the vehicle.
How much inefficiency is there? It is reasonable to use 10% as the minimum inefficiency of a converter. In practical terms, that means that a 7.0kW high voltage battery system will only provide 6.3kW of useable 12VDC power, or less. Of course inverters which change DC battery power to 120VAC power also have inefficiencies.
One advantage of a 48V alternator is size and weight. Such an "underhood generator" will be smaller and lighter than a 12V alternator providing the identical power. The 48V generator will also require smaller conductors (wires) to convey its power to the batteries. However, the trade-off is the weight of the converter required to change that higher voltage to 12VDC. Volta's Z5104200-0121250 converter weighs 42 pounds and provides 1500 continuous watts of 12VDC.
Of course, under hood generators are less weight than a gasoline generator. An Onan "microlite" 2.8kW generator weighs 113 pounds.
The 12VDC demands may decrease as RV manufacturers juggle and size the voltages of appliances and power requirements. This may allow smaller, lighter and less powerful converters. Transferring requirements from 12VDC to 120VAC will increase the inverter size and weight. Volta's weighs 68 lbs and can provide 3000 watts of continuous 120VAC power. It can functionally replace the 30A "shore power" connection for as long as there is sufficient battery power available.
Here's a comparison of the power ratings of several underhood generators. These alternators require a minimum RPMs to provide usable power. That means that the vehicle engine must run at a "fast idle" and higher to provide sufficient power. Alternators generally put out low amperes at normal idle (actual output is determined by the speed of the alternator, and increases dramatically once we get above fast idle). Very low quoted charging times may require highway speeds to get the underhood generator to peak power:
- Roadtrek GU: 300A at 12VDC, rated 3,600W (3.6kW) at ?? RPM.
- Volta 120FTAN-58: 160A at 48V, rated 6,000 W (6.0kW) at 4,000 alternator RPM.
- Volta 160GM92V-58: 120A at 48V, rated 8,000 W (8.0kW) at 7,500 alternator RPM.
Projected Lifespan, Warranties and Opportunity Costs
There are a variety of figures published about the lifespan of these battery systems. Some say 10 years, with the ability to provide 80% of the published power over that period of time. Ultimately, the only thing one can count upon is the RV builders warranty. Equipment manufacturers may only provide 1-3 years. This makes the RV builder's warranty very important.
Justification of these high power battery systems requires long lifespans. For example, the "opportunity cost" of a $10,000 system which operates reliably and provides at least 80% of published power will be:
- $1,000 per year if the system provides 10 years of service.
- $2,000 per year if the system provides 5 years of service.
System Comparison Caveats
The weight and capabilities of these systems varies considerably. In general, here are some things to keep in mind:
- LiB batteries weigh substantially less than AGM equivalents. Comparing 12VDC battery packs, for example: 200A Roadtrek "Ecotrek" modules weigh about 80 lbs. 200A AGM batteries weigh about 126 lbs. This gives LiBs a weight advantage if we consider only the batteries.
- To compare these different systems, we should consider the useable kW. However, "useable" makes some assumptions, and manufacturer's use this in their published data. For maximum lifespan, LiBs should probably not be discharged below 80%, but this does vary with battery chemistry and there are some differences of opinion. For maximum lifespan, AGMs should not be discharged below 50%. However, alternative AGM technology reputedly allows discharge to 80%. Comparing discharge cycles a 2.4kW LiB battery can provide about 1.9kW while an 2.4 kW AGM can provide about 1.2kW. Alternative Carbon AGM technology batteries rated 2.4kW can reputedly provide 1.9kW.
- Losses and inefficiencies are important, too. A 12VDC battery system powering a 12VDC appliance has zero "conversion" losses. A high voltage battery system using a converter will have about 10% conversion loss. In other words, the available 2.4kW of such a system will decrease to 2.16kW useable for appliances.
- Inverters which change battery DC to 120VAC also have inefficiencies. We never get 100% of that DC power to the AC appliances. At lighter loads, inverter efficiency is less and as the load on the inverter increases, the efficiency improves. Generally, inverter manufacturers will state "maximum" efficiency, which occurs at higher AC loads. So, for example, a 2.4kw battery with a highly loaded, 90% efficient inverter can only provide about 2.16kW of AC power; of course at those loads we can quickly draw down batteries. But at lighter loads the actual efficiency may fall to 50%, In other words, half of the battery power may be lost when changing DC to AC with lighter loads. So at lighter loads, we get longer battery availability, but the actual kW available will be less than that at the higher efficiencies. Confusing, isn't it?
- Efficiencies can be difficult to figure out in the "real world" using batteries to power DC appliances and 120VAC appliances, with varying loads and if augmented by solar, with varying sunlight. Which is why there are statements made all over the map. We do make assumptions when specifications are stated (even engine rpm changes the battery charge time using that underhood generator). So it is not surprising to me that it is difficult to come to some idea of what we can actually achieve with these different systems.
- When comparing systems, it is necessary to compare battery costs. By comparing the cost of equivalent kWs of batteries, LiBs win. Comparing usable kW costs and disregarding battery lifespans, the differences change significantly. Here's a post which provides a comparison: A comparison of AGM batteries to Lithium-ion
- Manufacturers may be inclined to stretch the projected life spans, abilities and so on, as well as to make other statements to justify the costs to buyers.
- To determine the weight of a system requires adding the weight of all of the following:
- Generator
- Batteries
- Inverter/Charger
- Converter
- Solar Panel
- Solar Controller
- Wiring
- Mounting Hardware
Volts, Amperes and Wiring (Conductors)
In higher voltage battery systems the conductors required to carry power will also be smaller than they would be in a 12V system providing the identical power (kW).. The actual size of the conductors will be determined by the current (amperes) they are required to carry as well as the allowable voltage drop.
All wires have resistance. That creates "voltage drop" which is wasted power and it is dissipated as heat. Voltage drop reduces the DC voltage available to the appliances, etc. on the circuit. Here's an example in which we want the voltage drop not to exceed 3%. In a 12VDC circuit, a 3% drop means that with a battery voltage of 12.0 we'll see 11.64V at the end of the wire when carrying the rated amperes :
Note: The smaller the AWG number the larger in diameter and current carrying capacity is the conductor. AWG = American Wire Gauge.In higher voltage battery systems the conductors required to carry power will also be smaller than they would be in a 12V system providing the identical power (kW).. The actual size of the conductors will be determined by the current (amperes) they are required to carry as well as the allowable voltage drop.
All wires have resistance. That creates "voltage drop" which is wasted power and it is dissipated as heat. Voltage drop reduces the DC voltage available to the appliances, etc. on the circuit. Here's an example in which we want the voltage drop not to exceed 3%. In a 12VDC circuit, a 3% drop means that with a battery voltage of 12.0 we'll see 11.64V at the end of the wire when carrying the rated amperes :
- For a 30A circuit, we would use a #10AWGwire for a circuit up to 10 ft. in length.
- For a 30A circuit with lengths of 15 ft. we would use a #8AWG conductor.
- At 20 ft. we'd go to #6AWG.
- etc.
So what do we gain with higher DC voltages? To use those higher power alternators (underhood generators) requires either larger conductors or increasing the voltage. The significance is this:
- At 12V a 30A circuit carries 360 watts.
- At 48V the same 30A circuit carries 1,440 watts (four times more "power" than the 12V circuit).
- DC power (watts) = volts x amperes.
- All trademarks, etc. belong to the respective equipment and RV manufacturers.
- Information presented here is from the equipment manufacturer's and RV manufacturer's published sources. No effort has been made to certify the validity or accuracy of that information.
- All information is "subject to change" by the respective manufacturers.
- This post is Copyright (c) 2018 Norman Retzke "All Rights Reserved"
- I don't work for any RV or battery manufacturer, nor am I compensated in any way by them. These posts are provided with the intention to provide factual and useful information. Nevertheless, they are my personal opinion.
- I have no inherent preference for any of the LiB systems in this post. At the time of this writing I am trekking in a Class B which is equipped with an Onan gasoline generator, 220AH AGM batteries and an inverter/charger. I added a supplemental solar panel to assist in charging the coach batteries when I am parked off the grid.
- It has been reported that global lithium-ion battery revenue is expected to expand to $53.7 billion in 2020, up from $11.8 billion in 2010.
- Tech Note: Silicon (Si) has attracted substantial attention as an improved LiB battery material because of "its specific capacity of 4,200 mAhg-1, volume capacity of 9,786 mAh cm-3, relatively low working potential (0.5 V vs. Li/Li+), the abundance of the element Si and the environmental benignity of Si."
