Introduction: Lithium Iron Phosphate (LifePO4) Solar Storage Battery Bank
LifePO4 batteries are fairly new for solar use. My home originally used a 24volt bank of lead acid (see pic on the right). These failed only after a year of use. This was a costly mistake. My replacement bank is LifePO4 25.6volt batteries as in the left photo. These I bought off Amazon for 241usd each.
Each battery has built in BMS for under/over voltage protection, cell balancing and temperature safe guard.
LifePO4 (and any lithium secondary cell) requires 2 stage (constant current followed by constant voltage) charging. In my arrangement, a float voltage was allowed since a load is always drawn by an inverter.
LifePO4 does not explode or catch fire like lithium cobalt (found in cell phones and laptops etc). It does not have toxic chemicals and offers four times the power density at a third of the volume compared to lead acid. For these reasons I felt safe to have such batteries in my Home.
Also the lifePO4 chemistry prefers partial charge and partial discharge. You will have no danger of undercharging these batteries compared to lead acid which will sulphate if undercharged.
If you are serious about solar powering your home, definitely go with lifePO4. Also make your lighting led and use inverter type air conditioners. Be energy efficient and your cost will be much lower.
Step 1: Connections to Each Battery
Since the internal BMS provides short circuit protection, I used a simple single pole single throw toggle switch to provide isolation for maintenance. The current rating of this switch is 20amp and is quite low cost.
For very reduced cost and less materials I soldered the head of a bolt onto one terminal of the switch. This provides a mechanically strong and electrically low resistance connection. To secure the switch to the positive battery pole, just screw in and tighten the nut.
Similarly for the negative battery pole I tightened a nut and bolt. The head of this bolt I soldered a wire directly.
The size wiring is used for each battery is 12awg obtained at a local car audio store. You can of course use a larger cable but the maximum voltage drop when my home draws peak current is 1.2 percent.
Step 2: Metering Per Battery Set.
In order to measure the amp hour capacity of each battery set (I have my batteries in groups of parallel sets), I used a low cost power meter from Amazon. The maximum current through each of these meters in my setup is 10amp as a worst case design load. In practice the current is less than 3amps per meter.
I have parallel sets batteries per meter to allow a low cost metering solution versus a meter per battery. I have saved on cable costs and ancillary hardware (nuts, washers, bolts, lugs).
Generally if the amp hour capacity of a battery bank drops below 60% of the original value then it's time to replace. The energy meters per battery sets will provide such monitoring.
Step 3: Provide a Plug N Play Means to Deliver Power.
My Old lead acid bank was hardwired with large Guage wiring complete with circuit breaker per 24volt string. For my new setup, I wanted an easier way to add/remove batteries from the solar power system.
I used two standard nema-15r outlets each with 6 outlets. Please note that this is not a standard use for such outlets however I put a sign up to warn of the purpose of these outlets. The advantages of this arrangement is that I can safely plug in and out any battery group without affecting the rest of the power system.
The connector on the battery side is a two prong polarized nema-15 plug. I chose the bigger blade as my positive. There is no possibility of the wrong polarity being inserted into the receptacles. Safe, high current, readily available and low cost.
The main cable from the receptacles to the DC loads/solar mppt controller is 2awg that came with an old inverter.
Note that the DC current rating of cables and electrical connectors (not switches or relays) is higher than the AC current rating of the same cable. This is due to no skin effect with DC.
Step 4: Thermal Imaging of Connections.
I took thermal scans of the batteries during night time maximum load. No problems with any of the connections were observed. I also mechanically verified the connections by attempting to rock/rotate each.
Step 5: Having Correct Voltages for Charging.
All lithium technology prefers two stage (constant current followed by constant voltage) charging. My solar charge controller is an outback flexmax 80amp mppt unit. This offers three stage (last stage being float) for lead acids. Below are my settings.
CC: 0.5c per battery or 5amps: total 80amps.
Absorption (CV) timer: 120 minutes
Due to the fact my Inverter is always connected to the Lifepo4 bank, the float voltage would not negatively affect my batteries. If you use Lifepo4 batteries in only standby use then float charging them may become an issue: you will have to use 2-stage (CC followed by CV) charging.
Currently none of the major solar charge controller manufacturers offer profiles for Lifepo4. I have emailed outback power systems and they are not yet willing to make this move. Once you have a programmable charge controller, using the settings I provided will allow you to use the Lifepo4 but please note that there must always be a load attached to the batteries. For example using a Lifepo4 on a desktop ups I will not currently recommend since that float charge in standby service may degrade the Lifepo4.