Transitioning to LiFePO batteries

LiFePO4 Battery on shelf, during installation

 

After doing my most recent research, I decided to replace the faltering AGM coach batteries with a LiFePO4 battery.  The decision was the easy part. Next came selecting a battery, installing it and integrating it into the existing Roadtrek 12VDC system.  I also have a solar system and de-sulfating controller.  Such a controller is not recommended with the LiFePO4.

The battery can be mounted in any position, and the space available provided two options.  I decided to go with terminals up. I did elevate the battery slightly by resting it on plywood.  It has a metal case and I don't want it sitting in water.  I've never had an issue with water in the battery pan but I don't ford streams or drive through flooded underpasses. There is always a first time.

The new battery will use the existing shunt and power wiring in the compartment.  The integration of other components did require some additional work on my part. I didn't want to make alterations to the existing wiring of the coach: no shortening or removal.  I wanted the wiring to be recognizable to a technician familiar with the Roadtrek or a future owner.  I also wanted to provide the ability to transition back to AGM batteries at some future time, although I don't expect to do that.

Maintenance Free LiFePO4 batteries?

There are limitations with any coach battery and LiFePO4 batteries have their own.  I decided to address these in my installation. 

It is true the LiFePO4 batteries are "maintenance free".  One significant advantage over other battery types is the internal, electronic battery management system (BMS) which is there to protect the battery.  (See Note 1).

However, to achieve optimal battery life (4000+ cycles) it must be operated within the design parameters.  That may require some external hardware.  I concluded that in my Roadtrek, replacing AGM batteries with LiFePO4 is not quite "plug and play" or "drop in and forget". 

Integrating the various 12VDC Components - a List

The LiFePO4 batteries aren't simply a "drop in" proposition. Here are the things I considered:

1. Tripp-Lite charger/inverter is to remain in place.
2. Retain Tripp-Lite 750W inverter function (~59A at 12.8V).
3. Make the provision for a future LiFePO4 charger. 
4. Solar charging system - update to compatible LiFePO4 controller.
5. Upgrade the battery manual disconnect.
6. Retain existing volt-ammeter and shunt.
7. Add a low voltage automatic disconnect to enhance the BMS cutoff (to preserve the battery).
8. Add battery compartment supplemental AC/DC heat to extend charging time.

Low Temperature Battery Considerations

One of the issues with the LiFePO4 batteries is their intolerance to cold.  The BMS will not allow charging if it determines that the battery internal temperature is at 32F or below.  Power output will vary with ambient temperature, but not as much as with the AGMs I'm replacing.   

High ambient temperature may accelerate the aging of the battery while low temperature may reduce output power capability. In general LiFePO4 batteries perform better at low temperatures than do the AGMs I'm replacing. 

I decided to install the battery in the unheated compartment where the AGM batteries were installed.  Minimal 12VDC power wiring changes would be required.  In that location the battery will be exposed to freezing temperatures at any time the outside ambient is below freezing.  Of course, I do have the option of relocating it to the interior of the coach at some time in the future.

Keep in mind that if stored outdoors all Class B's will have their interior temperature decrease to below 32F in fall and winter if the vehicle is north the freeze line. In cold weather the coach interior will be at cold until it is warmed by running the engine.  So too will any batteries stored within.  If the coach is parked in the sun the interior may be warmer than outside. The Roadtrek coach battery can be discharged when cold, but the LiFePO4 battery can't be charged until the battery internal temperature rises above 32F.

I concluded that to store the Roadtrek in winter, if that is necessary below freezing conditions that I'll simply remove the batteries.  If I use it at below freezing temperatures some supplemental heat to warm the batteries would be desireable. That will extend the charging of the LiFePO4 battery as weather cools.  

I do realize that at some point the ambient temperature will be so low as to nullify the supplemental heat and the BMS will prevent charging the battery. I don't expect to encounter that situation. LOL.

Some batteries are available with internal 12V heaters but that draws down the battery in cold weather if the vehicle is not running and not connected to shore power.  Furthermore, if this is an internal battery function and can't be controlled by the user it simply runs the battery down faster if off the grid.  In my opinion that's undesirable in a RV.

I'll be installing both 120VAC and 12VDC supplemental heaters, which I can control.  I'll be monitoring the compartment temperature to get some data about the effectiveness. 

Avoid Over Charging and Over-discharging

Another issue is the possibility the internal battery cells can be damaged if they're discharged below a certain threshold. That low point is approximately 5 percent of total capacity. If the cells are discharged below this threshold their capacity can be permanently reduced.  The BMS will protect the battery, but at too low a threshold to preserve optimum life. 

The solution is an external, automatic cutoff or a low voltage alarm (or both).  

I have installed a low-voltage automatic cutoff. 

Three methods of Charging

I have three methods for charging the batteries in my Roadtrek. Ideally, each method would provide the appropriate charging voltage and current for LiFePO4 batteries, but they don't:

1. Tripp-Lite charger inverter using 120VAC power (3-stage voltage and current control).
2. Solar using solar panels and a controller (voltage and current control).   
3. Engine alternator (simple voltage regulation, no control).

The first two methods can be adjusted to adhere to the battery charging specifications.  The alternator has no such adjustments.  To avoid over-charging the coach batteries I can use the battery separator to disconnect from the alternator and rely upon solar charging.  Alternately, a DC-to-DC charging system could be installed.  There are practical limits to how much I'm willing to spend on this.

External low voltage disconnect - Details

The battery manufacturer recommends that an external low voltage disconnect be used.  The manufacturer suggests 11.2V as the disconnect point.  

• Battery low voltage disconnect < 11.2V

Such a voltage represents about 5% battery capacity remaining.

There are a couple of methods to achieve this:

1. Manual switch.
2. Automatic switch or relay.

It is true that the battery management system will protect the battery from complete discharge. However, 10.4V is about 2% battery capacity.

• BMS Low-Voltage disconnect <10.4V

In general, a battery constructed of Grade A cells can probably achieve the specified cycles if the battery is operated within the manufacturer's parameters.  Discharge to very low voltage is to be avoided. 

A battery low voltage alarm is a possibility, too.  For example:

• Battery low voltage alarm < 11.8V

Such an alarm would occur when the battery capacity has decreased to about 8%. 

Adjusting to a new reality.  As I use the LiFePO4 battery I'll have to adjust.  For one thing, the output voltage versus State of Charge is different than the AGMs.  The new battery is more stable than the AGMs.  After charging, and intermittent use for a week, the battery voltage has decreased by 0.06V.  This battery seems to be much "stiffer" than the AGMs.  This may change as the battery ages and capacity decreases to about 80%. 

State of Charge - SoC - Details

I provide a typical chart later in this post.  State of Charge (SoC) is a very useful battery measurement. It states the present, actual capacity of the battery compared to its total capacity. 

State of Charge is a percentage: 

• 100% SoC means the battery is fully charged, new and undamaged.
•  0% means fully discharged. 

 SoC is calculated this way: 

• State of charge (%) = Remaining Capacity (Ah) / Total Capacity (Ah).

LiFePO4 batteries are chemical devices and so they operate similarly to the AGM lead acid batteries we are familiar with. There are several SoC values to keep in mind.  These are the absolute minimum SoC, the preferred SoC and the actual SoC.  

Absolute minimum SoC is the most discharged state with the lowest possible terminal voltage that doesn't destroy the battery.  For my battery this is 10.4V.  However, discharging to such a low voltage will diminish the capacity and useful cycles of the battery. 

For maximum battery life, only discharge it down to the Preferred level, but no lower. Hmmm, that seems reminiscent of the recommendations for AGM lead-acid batteries, doesn't it?  Here are the three values:

• Alarm and recharge at <11.8V (the Preferred minimum state of charge, 8-10%). To maintain reasonable battery longevity and performance do not discharge below this value.
• External battery cutoff <11.2V (Actual minimum state of charge, 5%). This is the realistic minimum.  The battery can be discharged to this state, but some battery degradation and performance will occur is the battery is operated this way.  This discharge level is a trade-off between available power and battery life.
• Battery BMS low voltage cutoff <10.4V (Absolute minimum state of charge, 0%). This is approaching the point of battery destruction.  The BMS will stop battery discharge when this state is reached. 

LiFePO4 Terminal Voltage and SoC

A typical 12V LiFePO4 battery is constructed of four cells called a 4S battery pack. The battery output voltage decreases as the batteries discharge and remaining capacity decreases.  A typical 12.8V battery output will vary from 13.8V to as low as 10.0V (completely discharged).  

The nominal output voltage is 12.8V.  There are differences among manufacturers, but here are typical voltages:

• Absorption Voltage (charging) 14.4-14.6V  (3.6-3.65V per cell).
• Float (fully charged) 13.6-13.8V (3.4-3.45V per cell).
• Full discharge 10.0V (2.50V per cell).
• Nominal voltage 12.8V (3V per cell)

The terminal voltage will decrease as the battery discharges and the capacity diminishes.  To get an accurate reading of SoC the battery must be rested.   Here's a typical chart. You will note that the battery output voltage is reasonably flat throughout the useable discharge cycle, from about 100% down to 10%:

• 13.6 V = 100% SoC (Fully charged)
• 12.1V  = 10% SoC (Preferred minimum)
• 1.5 V output decrease from Fully charged to minimum SoC.

LiFePO4 Capacity versus Battery Voltage 

Note:

1. There are different battery constructions out there.  Some have as many as 8 internal cells. This is an evolving and improving technology. Low temperature automatic protection by the BMS and internal 12V heaters are relatively new.  Some batteries are well constructed internally, and some are not. Some have steel cases, while most have plastic. Prices range from about $325 to $900 for a 12V, 100Ah battery.

(c) N Retzke 2022

At The FMCA Rally - Batteries

 

Electric Vehicle

While at the FMCA Rally at the Pima County Fairgrounds I noticed this electric vehicle on display.  The batteries had been removed from the chassis and are sitting on the bed.  Of course, there are seminars about RV batteries and also vendors.  I found this old vehicle to be interesting.

Wooden crated lead-acid batteries

Close-up view of the batteries

Gould was the battery manufacturer.  At one time they produced electrical switchgear, automation products and all sorts of electrical distribution hardware.

(c) N. Retzke 2022

Lithium Batteries and Solar Power, Revisited

Winter approaches and we are near the end of our first two years with a camper van. It is time to winterize and to review my "to do" list. One item has been improving the coach battery system. We did store the camper for two winters with the batteries on a solar panel.

This worked fairly well. But, I wasn't happy with the amount of power provided by these 220Ah batteries, and began exploring alternatives in late 2013, immediately on purchase. I'd done the numbers and knew we would be on the edge based upon our intended use of the vehicle. Why do I say "on the edge?" It is because of the available capacity of these batteries, which is not 220Ah if one wants them to have a reasonable life expectancy of 5 or more years.

A Short Review
What is the true available 12VDC power? 220Ah of AGM batteries can only provide 110Ah while keeping them above a "safe" discharge level of 50%.  What does that mean? If one wants to run some appliances and an inverter, the batteries can provide the following power:

• 5 amperes (60 watts @ 12 volts) for 22 hours, or 
• 6.5 amperes (78 watts @ 12 volts) for 16 hours, or 
• 8.5 amperes (102 watts @ 12 volts) for 12 hours, or 
• 12 amperes (144 watts @ 12 volts) for 8 hours. 

The real issue is having sufficient battery capacity to run necessities through the night while off shore power and without a generator. At a minimum this would be the refrigerator and a vent fan or the furnace plus some lighting. The loads of your rig might be different than mine. To run any 120V appliances including a PC would require the inverter which has losses as part of the process of converting 12VDC to 120VAC. It would be best to use 12VDC appliances and a PC which can run from 12VDC. We've got a propane range top and so we can use that for cooking. If the weather is mild, it is possible to use the propane BBQ or if allowed, a wood burning campfire. These things reduce the electrical requirements for the batteries. Of course, we can simply fire up the generator. However it is my desire to make it through a typical night in mild (40F to 85F) weather without shore power or running the generator. That is not necessarily a daily requirement. In fact, based on our actual experience. we only need overnight battery capacity on an occasional basis. That translates into the cost-benefit analysis of the battery and solar system.

The existing AGM batteries, if in excellent condition could provide about 6.5 amperes for 16 hours. I do need to emphasize that I am assuming the batteries are in good condition and fully charged. If not, then less power would be available. Why 16 hours? That's maximum for winter with darkness and operating on batteries from 5pm to 9am. I'm assuming cool temperatures in which there would be no need for supplemental coach heat beyond use of the furnace. However, I've done some analysis of our electric blanket and that is a viable alternative.

If my refrigerator operates on propane I do need some 12VDC for the controls. The furnace electronics and fan also require 12VDC. Lighting loads vary. I've got fluorescent and LEDs. In a pinch we could use hockey puck LED lights which run of AA batteries. But we don't want our "tiny home on wheels" to morph into "our tiny cave on wheels."

How Much DC Power is Used?

• Suburban Furnace = 2.8A (intermittent)
• Max-Air Fan @ Medium Speed = 1.5A
• Refrigerator 12V electronics = estimated 1A
• Propane/CO Alarm = 0.1A
• Smoke Alarm = 0A (9V battery)

These  items consume 5.4 amperes. That is approaching the maximum 16 hour capacity of the batteries, and based upon experience that's a realistic maximum for daily hours with less than perfect batteries. It would seem that Roadtrek did a good job sizing this system. Remember that the furnace runs intermittently. That's why I didn't include the lighting load. However, if one uses the inverter, then the requirements increase by about 1.5 amperes or more. That's because of the inefficiency and losses in the inverter.

Do I Need Lithium Batteries?
Based upon our actual experience since December 2013, I would say that we do not. It is true that our current battery system is marginal. However, based upon actual needs, we don't need to do an upgrade at this time.

That said, it is possible I will replace the AGMs with LiFePO4 batteries when that time comes. Here is my reality: It is all about cost-benefit analysis. I do have a design and it will be easy to adjust the design as time goes on. With each design modification I'll get current prices and current technology.  Trigger events to upgrade would include battery failure, inverter/charger failure, a desire for more solar than I currently have, and so on.

It is also possible that some day I may take this on as a "hobby" project. But I am under no pressure to make the modifications at this time.

Here are some links to earlier posts on this subject:

http://roadtrek210.blogspot.com/2015/02/agm-battery-alternatives.html

http://roadtrek210.blogspot.com/2015/02/are-lithium-coach-batteries-expensive.html

Here is a handy calculator to help you determine how long your coach batteries can handle a specific load in amperes:

http://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html

Note: Edited amps used to add CO/Propane detector, misc.

AGM Battery Alternatives

It's again that time of year when attention goes toward the coach batteries in the RV. When the RV is in frequent use, this is never an issue. Running the engine will charge those batteries, as will plugging into shore power.

However, in winter, some of us put our RV into storage. Those AGM coach batteries then self-discharge. Sulfation may occur and if the batteries are sufficiently discharged they may freeze and undergo permanent damage. In winter, our attention turns to the coach batteries. When spring arrives some of us may be unhappy to find that our batteries have only 75-85% of rated capacity, or less.

Is There a Better Way?
In my case, I installed a 50 watt solar panel and charging system to help those AGM batteries. I also installed a smaller solar panel to offset parasitic drain for the engine battery. Both of these have seemed to help. I went to the storage facility for my RT and the coach battery monitor indicated my coach batteries were in "fair" condition. That has been typical; I suspect my RV which was purchased one year after manufacture had damaged AGM batteries. My current interest is in getting the maximum benefit from whatever coach battery system I have.

On my most recent RV inspection the engine started fine and I charged the coach batteries for a half-hour. Not a lot, but intended to augment the solar charging system.

But is there really a better way? I think lithium batteries are the way to go. I've begun exploring this.

1. Smaller lithium battery systems have been proven in sailboats.
2. Faster recharge times. 
3. Lithium batteries have half the weight of the AGM batteries on a weight versus output basis; that's another 70 lbs. of gear, or improved fuel economy, in my case. Or more battery capacity at the same weight!
4. Lithium batteries, while more costly initially do have a higher number of charge-discharge cycles and can tolerate deeper discharges. In simple terms, they will last far longer than the AGM batteries. 
5. There is some evidence that lithium batteries cost less over the life of the battery than do AGM batteries. For anyone who intends to use a RV for 10 years or so, this is significant. 
6. Lithium batteries don't freeze at low temperatures and have the winter problems of AGM lead-acid/water batteries. In other words, fewer winter maintenance issues. Roadtrek states in their 2014 210P manual "AGM Battery Warranty....... is voided if AGM batteries are tampered with, topped off with distilled water or allowed to sulfate or freeze due to lack of charge."
7. Lithium batteries have lower self-discharge rates. In other words, they can be stored for extended periods at full charge and don't self deplete. 
8. Lithium batteries can tolerate deeper discharge than can AGM batteries.
9. AGM batteries have high ambient temperature restrictions.  AGM batteries are designed for an average annual temperature of 77F (25C). If the average annual temperature is 95F (35C) then the battery life will be reduced by about 50 percent. 

Improving Solar Response
I've also been researching improved solar panels and I've decided it would be pointless to put them on an AGM battery system.  Improved solar panels would benefit with an improved electrical storage system. In other words, the system is limited by the weakest link in the chain.

I'll continue my research and will post the upgrades as I make them.

Why Do Manufacturers Use AGM Coach Batteries?
That's a question you might ask. The reasons are straightforward.

1. AGM technology is well established and existing system designs are in place.
2. Alternative battery technologies, such as lithium, are at a higher initial cost. This increases the purchase price of the RV.
3. New designs will require engineering manhours which is an additional cost to the RV manufacturer.
4. Other technologies are new and are not well understood. In other words, while technically superior, some technologies have not yet been widely offered because the sales, marketing, management and engineering departments at RV manufacturers have not yet come to grips with the benefits.
5. It's a competitive world. Most users (RV buyers) compare total cost to overall performance. It's only after purchase that the limitations become apparent.
6. Leadership entails risks. However, most western companies are risk-averse. My company recognized this. We were leaders in our field and as president I would remind our employees that "There is the leading edge, and then there is the bleeding edge." I can say that we did find ourselves from time to time on the "bleeding edge." Good engineering, attention to detail and serious prototyping kept us and our clients from going over the edge. I would say that lithium batteries, which are well proven in smaller systems such as sail boats are not at all like what I was facing with hardware and software which didn't perform as expected. This isn't 2008, nor was the microprocessor invented only four years ago. This is 2015 and there are a lot of Tesla motor vehicle on the road, and lithium batteries are not uncommon on sailing vessels. Lithium batteries are entering the mainstream.