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| 100 Ah LiFePO4 Battery |
During the FMCA Rally, I had an opportunity to attend a battery seminar and also discuss features and current prices with several battery vendors. In February 2015 I posted here about Lithium-Ion LiFePO4 batteries. In that post I said: "a similarly rated lithium battery system will probably deliver about 45% more power than a similarly rated AGM battery system, and it will probably do so for at least 6-7 years."
Here's a link to the earlier post:
Are Lithium RV Coach Batteries Expensive?
In August 2017 I posted this comparison:
Comparing AGM and Lithium RV Battery Systems
In March 2018 I posted this:
Lithium Battery and RV DC Power System Developments
In October 2019 I posted an update:
Coach Batteries - AGM versus Lithium-Ion Update
The battery seminar was cancelled. I'd been hoping to get current information and insights. I discussed with FMCA and advised them that I could provide a 1-hour presentation. For one thing, I have nothing to sell and I'm very familiar with what's available, the costs, as well as the pros and cons of each battery type. However, there were several battery vendors at the rally, and I had the opportunity to talk to Battle-Born, RVConnection, etc.
One of the Roadtrekkers brought up coach batteries in a conversation and told me he was interested in building a LiFePO4 battery. That piqued my interest. After the rally I did some additional research.
My AGMs were installed in January 2017. They are 5-years old. I do test the capacity of my batteries from time to time and I had determined it was about time to replace, as the capacity has diminished, which is normal. BTW, using the Roadtrek LED indicator isn't very useful for this. I measure the battery voltage under load and monitor the decrease in voltage over time under load. By discharging the battery at a constant current, I can observe the decrease in battery voltage. That's how I know the battery condition. I admit it is a rough estimate, but adequate for my needs.
Some Roadtrekkers use the LED L-F-G-C volt indicators. I don't. The "G" indicator is illuminated at about 11.9V and above. My AGM batteries are at 25% SOC at 11.9V OC (Open circuit), which is below the minimum 50% state of charge necessary to achieve rated life.
Is it time to change to Lithium?
I had once again been thinking about switching from AGM coach batteries to Lithium-Ion. I too was interested in building my own batteries. I decided to check prices, which have changed significantly in the past couple of years.
What I discovered was I can purchase a complete battery for about the same price as building one. In one of those oddities, the price of the components is now nearly equal to the price of some of the assembled batteries; I guess the marketing people in the factories in China figured it out. A year or so ago the parts were less costly than the batteries, but no longer. This is not surprising because a 12V LiFePo4 battery requires these components:
- (4) 3.2V LiFePO4 Cells.
- (1) Battery Management System (BMS).
- Screws, buss-bars, wire and terminators for the BMS.
- (1) Case, which is sometimes called a Rack, to hold the cells.
Add up the cost of all of the above and compare to the cost of some of the batteries available. At present there isn't much of a difference. I won't put the component numbers here. However, I decided that while assembling my own battery would me a fun exercise, there was no financial incentive for me to do so.
Note: There are some very low-cost Lithium LiFePO4 batteries available. I've seen prices as low as $300 for a 12V 100Ah battery. However, the price range is from $300 to $900. I decided to do research into testimonials and specific battery reviews as part of the selection process. I also decided to make a simple specification so I could do a reasonable comparison.
Be aware that there are different grades of 3.2V LiFePO4 cells; A, B, C, etc. Those cells are assembled in a case with a battery management system (BMS) to make a 12V, 24V or 48V battery. Grade A cells are more expensive than Grade B cells. Grade B cells are characterized as "unqualified" at manufacture. Maximum benefit is achieved from a LiFePO4 battery constructed from Class A cells. But different manufacturers use different standards. So, a Class A cell from one manufacturer may be the equivalent of a Class B from another. Confusing, isn't it? That's why it is a good idea to purchase a battery from a known manufacturer (or assembler) who provides a proper warranty.
Battery Prices and available capacity:
One thing that is unchanged is the price differential of AGM batteries versus LiFePO4. True, the lowest priced lithium batteries are competitive with AGMs, but may not offer features or warranties. Determining battery quality isn't easy. I decided to avoid the lowest cost lithium because there are different quality levels in the cells.
Bottom line is, the AGMs are less costly, out-of-pocket. However, if one compares the capacity of the batteries and charge-discharge cycles, the LiFePO4 offer superior performance.
Furthermore, because we seldom boondock, I don't need a lot of Ampere-hours from the batteries.
Here's an "out of pocket" price comparison. I priced an inexpensive 12V AGM battery as follows:
- AGM (lead acid) = $219 + shipping + tax
- LiFePO4 = $569 + shipping + tax
There is a capacity difference and that should be considered. Considering how I use my Roadtrek, the fact that I do have solar available, and I have an Onan generator, I really don't need two 100 Ah Lithium batteries. However, a single 110Ah AGM could only provide 55Ah which is not sufficient. As a consequence, I need to compare 220Ah AGM to 100Ah lithium. This would be adequate based on available capacity:
- (1) 220Ah AGM = 110 Ah available (50% maximum discharge)
- (1) 100 Ah LiFePO4 = 80 Ah available (80% maximum discharge)
Battery price per Watt-hour (Wh):
The following is a simple cost comparison and doesn't consider lifespan, number of cycles or battery performance degradation. In fact, it would be more accurate to use 50% capacity for the AGM and 80% for the LiFePO4. Discharging below those levels can ruin the AGM and may reduce the lifespan of the lithium battery.
For example, one 12V, 100 Amper-hour (Ah) assembled LiFePO4 battery which has acceptable specifications has a price of $569 plus shipping plus tax. That's $0.44 per Watt-hour (Wh), as follows:
$0.44 = $569 / (12.8V x 100 Ah)
A 12V, 100 Ampere-hour (Ah) AGM battery has a minimum price of $219. That's $0.18 per Watt-hour (Wh):
1 battery $0.18 = $219 / (12.5V x 100 Ah)
2 batteries = $0.36 per watt-hour
If I were to purchase a single 206 Ah LiFePO4 battery, the cost per Wh would be less, while the price of two AGMs would be twice the capacity for twice the price:
$0.39 = $1,029 / (12.8V x 206 Ah)
Prices above are "raw" which is to say, a simple cost per Wh. However, if we consider the life of the batteries, the numbers change. An AGM battery can provide 500 charge-discharge cycles. A Lithium-Ion battery can provide 4,000 charge discharge cycles. The LiFePO4 battery can be repeatedly discharged 80% and the AGM 50% to provide this cycle lifespan.
Realistic battery price per Watt-hour (Wh):
In fact, the AGM battery I would be inclined to purchase would be the Deka 8AGC2. This is a 220Ah 6 V battery which closely matches the original furnished with the Roadtrek. Two would be required to get 12V, but at 50% useable capacity they would provide 110Ah at 12.5V. The cost each is $300 + shipping + tax. This is the realistic AGM battery cost per Watt-hour:
AGM batteries: $0.44 = $600 / (12.5V x 110 Ah)
One 12V, 100 Amper-hour (Ah) assembled LiFePO4 battery which has acceptable specifications has a price of $569 plus shipping plus tax. It has a 80% useable capacity, 80Ah. That's $0.56 per Watt-hour (Wh), as follows:
100Ah Lithium: $0.56 = $569 / (12.8V x 80 Ah)
The AGMs when new can provide energy at a lower cost than the Lithium. However, the Lithium batteries will provide more power over their lifetime because they can tolerate greater number of charge-discharge cycles.
Comparing Charge-Discharge Cycles
If we consider charge-discharge cycles, then these batteries can provide these Wh over their lifespan:
Lithium: 3,840 KWh = 4000 cycles x 1200 Wh x 0.8
AGM: 300 KWh = 500 cycles x 1200 Wh x 0.5
As can be seen above, the lithium can provide 12.8 times greater power (3840 / 300) over its lifespan. Considering battery lifespan, the lithium batteries are substantially less costly.
Cost over the life of the batteries
If I take the initial cost and divide by the number of useful years, I can arrive at an estimated cost to own per year. I am using my experience with my AGM batteries, which provided 5-years of useful life in my Roadtrek. For comparison I'm using the warranty period of the LiFePO4 batteries. Note that the warranty is 7 years but can be extended to 10 years. To make this comparison, I'm using two AGM's (100 Ah useful) and one LiFePO4 (80Ah useful). I'm ignoring shipping and tax, and I am assuming that the existing Tripp-lite charger/inverter will work with both batteries. That's the configuration I'm considering.
Lowest cost AGM: $88 per year = (2 x $219) / 5 years
Deka AGM: $120 per year = (2 x $300) / 5 years
LiFePO4: = $82 per year = $569 / 7 years
If one can get a 10-year warranty for the LiFePO4 batteries, the cost per year is further reduced, and the Lithium battery is less costly than the AGM:
LiFePO4: = $57 per year = $569/10 years
Advantages and disadvantages
Lithium LiFePO4 batteries have these characteristics:
- Can be repeatedly discharged to 80% with no reduction in battery life. In other words, a 100 Ah battery (1200 Wh) can provide about 960 Wh safely without decreasing battery life. A similar AGM battery should not be discharged more than about 50% repeatedly. The AGM can provide about 600 Wh, which is 63% of the power provided by the LiFePo4 battery.
- The Lithium can be charged-discharged about 4,000 to 8,000 times, or cycles. The AGM battery, if discharged repeatedly to 50% can be charged-discharged about 500 times, or cycles. The lithium can be charged-discharged at least 8 times more cycles than the AGM.
- The Lithium battery provides power at 13.2VDC. This may gradually decrease to 13.1V when discharged to 40% capacity. At 20% remaining the battery will be about 12.9V. The AGM begins at about 12.9V and decreases to about 12.3V at 50% state of charge. At 20% remaining the battery voltage will be about 11.7V. Actual output may vary by manufacturer.
- The Lithium cannot be charged at temperatures below 32F (0C). To do so the battery will be ruined. Using Lithium batteries in below-freezing conditions takes some forethought. However, they can be discharged at any temperature. Adding a battery heater can solve this problem.
- The LiFePO4 battery may be mounted in any position (but check with specific manufacturers for their limitations and recommendations).
My Specifications & Tripp-Lite Charger-Inverter Settings
To make an apples-to-apples comparison of LiFePO4 batteries from various suppliers I had a simple specification:
- Minimum 4,000 cycle life.
- Internal Battery Management Systems (BMS).
- Low temperature cut-off to protect the batteries.
- Over charge & over discharge protection
- Over current & short circuit protection.
- High temperature disconnect.
- Storage as low as -22F. Discharge temperature range -22°F to 140°F.
- Charging temperature range 32°F to 140°F.
- Charging Current range 0 to 20A (maximum 50A).
- Maximum output 100A.
- Be compatible with my Tripp-lite charger (14.4V Charging Voltage and 13.5V float, maximum 45A charging current). - Note 1, 2, 3 below.
- A minimum 7-year warranty.
- Prefer a removeable cover and replaceable BMS, but not mandatory.
- Blue-tooth (r) communications optional.
Note 1: The Tripp-lite has two charging settings for two different types of batteries:
- "Wet Cell (vented)" (DIP Switch A1 in the DOWN position) and
- "Gel Cell (sealed)" or AGM (DIP Switch A1 in the UP position).
Note 2: The Tripp-lite has three stages for charging: BULK, ABSORPTION and FLOAT. The "Absorption" and "Float" stage voltages are adjusted by selecting the battery type:
- Wet Cell: 14.4 VDC "Absorption", 13.5 V "Float".
- AGM: 14.1 VDC "Absorption", 13.6 V "Float".
Tripp-lite BULK Stage: In
this
stage,
the
battery
is
brought
up
to
about
80%
capacity
using
a
constant
charge
current;
the
charge
voltage
can
vary
in
this
stage.
Note 3: The Tripp-lite has two charging current rates:
- 11 Amperes (DIP Switch B4 in the UP position).
- 45 Amperes (DIP Switch B4 in the DOWN position).
Battery Selection & Sizing Considerations To choose LiFePO4 batteries it would require these decisions:
- Choose (1) or (2) 100Ah batteries (total 100Ah or 200Ah capacity; 80Ah or 160Ah useable).
- Or choose (1) 200Ah battery (160Ah useable).
- Determine mounting location: (1) or (2) 100Ah mounted outside or (1) large 200Ah battery mounted inside the coach (the 200Ah battery is too large for the Roadtrek battery tray). Inside mounting would add cost, as additional 6AWG wiring would be required.
- A single 100Ah battery could provide 20A for 300 minutes (about 240W for 5 hours).
- A 200Ah battery could provide 20A for 600 minutes (about 240W for 10 hours).
- The Tripp-lite inverter is rated 750W continuous. Ignoring losses, the inverter input for 750W AC output would be about 60A at 12.8 VDC. The battery should be capable of providing that current output. The inverter can provide 150% output for a short period of time, so a battery capable of 100A output would support that. Of course, higher battery current decreases available battery time; 60A output could deplete the 100 Ah battery within 100 minutes.
To protect the battery, a low voltage cut-off device at 11.2V is recommended. The BMS of the battery I am considering will cut-off at 10.4VDC. However, this very low cut-off voltage may reduce battery life. An in-line fuse is also recommended. I do have a digital voltmeter installed, so I can rely upon that at the lowest cost approach. Keep in mind that battery voltage is an indicator of the battery capacity, but the battery must be at rest for this method do be accurate. "At rest" means nothing drawing a load. In my Roadtrek I can use the battery voltage as an indicator if 1) The chassis/coach battery separator is in the "off" state, 2) the battery disconnect is "off" and 3) the inverter is "off". About 15 minutes after disconnecting the LiFePO4 batteries I'll consider the voltmeter reading to be indicative of the battery capacity.
Because the LiFePO4 can't be charged below 32F, mounting inside the coach has an advantage. The interior coach temperatures can be maintained above 32F while the coach is in use. However, if the battery were in the outside compartment a heating pad could be attached to the battery to provide some supplemental heat in the winter, thereby keeping the battery or batteries above freezing. It would be my preference to mount outside to conserve interior space. The heater will add cost.
Notes:
- Certain RV organizations including FMCA and Escapees may offer price discounts to members for specific batteries.
- Certain manufacturers offer batteries which have internal heaters and have blue-tooth (r) communications. However, this is at greater cost.
- The Roadtrek battery tray is outside of the coach. Care must be taken to assure that the LiFePO4 batteries mounted within stay dry.
- The charging requirements of the battery selected must be compared to the capability of the Tripp-lite charger/inverter to assure compatibility. Otherwise, a new charger and inverter would be required.
- I decided to purchase a single 100AH SOK battery pictured at the beginning of this post. If purchased directly from the U.S. supplier, the warranty is extended from 7 years to 10 years. My cost for (1) battery was $624.93. This included tax and shipping.
- If I decide to add a thermostat and 12V heater to keep the battery warm, my out-of-pocket additional expense would be about $40 (fuse, pad, thermostat). A second 120VAC 84W heating pad with internal 45F thermostat is about $39. Why two? Well, if I decide to winter camp on 120VAC, I can heat the battery using shore power or the generator. Alternately, I can use the chassis alternator to heat the battery using 12VDC while the Roadtrek is in motion or I can use coach 12VDC to maintain the LiFePO4 above freezing. I have not purchased any parts, but I do anticipate installing at least the 12V heater.
- My Solar system will work with the LiFePO4 battery.
- Low voltage disconnect at 11.2V is recommended for maximum battery life. If I want to add an automatic cut-off that would be at additional cost. I currently have a DC display with alarm. Here's the post link: New Voltmeter-Ammeter-Wattmeter for AGM batteries
(c) N. Retzke 2022
