Lithium Iron Phosphate (LifePO4) Solar Storage Battery Bank

Introduction: Lithium Iron Phosphate (LifePO4) Solar Storage Battery Bank

About: Update 12 September 2017: A very special thanks to Sam Elder, a manager here at Instructables, who tracked down the cause of my lost publications and fixed the issue. Take a bow Sam!

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.
CV: 29.0v
Absorption (CV) timer: 120 minutes
Float: 27.5v

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.

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52 Discussions

Hi, I have read your solar thingy and am impressed. Am wondering if you are still active so that I may ask a question?

Thanks.

Mary

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LenP8

8 months ago

Just looking at your photos, the one with the plugins (step 3) you have the positive on the negative proper code is the smaller size plug is the load + and the larger plug is the return -

2 replies

This is a custom connection using DC. The code does not apply here. All voltage and amperage ratings are satisfied however.

Hi there! Great looking set up you have!

1)I was just wondering how your lithium batteries are doing? 2)Can you kindly tell me the make and model?

I have a 1000Ah battery bank (off-grid Vancouver BC) that has 1 weak battery and I am considering replacing the whole bank..if it doesn't break the bank!

Do you have a generator to charge your batteries other than solar? or are you on the grid?

Please and thank you in advance!

To infinity and beyond
Denis
please reply to accounting@fcpg.ca (my work email..my hotmail has too much junk mail)

Hi thank you for this review.

Common lead acid batteries are not meant to be fully discharged or even halfway discharged.

My question is... Are LiPO4 batteries ok to be discharged more than 50% and still be reliable for the long run?

4 replies

Technically you can discharge to zero with lithium. BUT, with no more than 20 percent discharge you get 3000 charge cycles, and with no more than 50 percent discharge you get 8000 charge cycles. So yes, lithium does tolerate greater depth of discharge, but you literally get eight times the life of lead acid batteries if you mostly don't (Which is like a decades use). At worst, you should try to keep from going past 20, as a rule of thumb. Because lithium also tolerates higher discharge rates, and has less resting discharge, and uses less space, AND is safer, the lifetime alone makes it worthwhile - it's probably worth getting as much power input from solar, wind, water etc as possible, so that you generally stay highly charged.

The other thing is, the capacity drops off slower. At that 3000 cycles or 8000, your lithium is supposed to be at 80 percent (ie you don't really need to replace it yet), whereas with lead acid, the capacity drops like a stone once it reaches its 1000-1500 cycles.

Greetings:

I am running a totally off-grid 12VDC system. The ONLY lead acid batteries you should consider are the "so-called" "Deep-Cycle" or "Marine" batteries. I've been using my bat bank for over 2 years now (Careful care and maintenance!)

Good luck. It's worth it!

Mr. B

Take a look at this site, ElectroDacus. It's a solar controller for lithium batteries, I got one and am really happy with it. There are several youtube videos on the use and capabilities.

Hello, I'm very interesting in using a lifepo4 batteries and the most hidden information is reliable of the battery. How long in terms of days will it hold its capacity over 80% of nominal. I have never saw such information in datasheet from seller of the lifepo4, only lifetime in terms of cycling (about 1500 to 3000 cycles 80 DOD). Have you ever test your batteries for capacity and internal resistance?
Great job! Good site!

1 reply

The capacity is actually greater than the amp hour rating on its label per battery. Any battery with internal bms, you cannot test the internal resistance. In terms of the self discharge rate, these beauties can hold their charge for at least a year.

Thanks for the thumbs up!

WRT.

Step 3: Provide a plug n play means to deliver power.

Good idea on the disconnect for the batteries.

BUT

It is dangerous, very dangerous, yes I did read your warnings. But you only need a John Doe to walk in and connect a battery to an 110v extension cord, and BOOOOM!

Also: The pins of the plugs are uninsulated, and if dropped onto the battery that are connected to, or onto a metal surface, guess what, BOOOM!

Sorry to be negative, great idea, simple to implement, and cheap. I like cheap!!! I am going to implement a similar scheme using RCA connectors, so who am I to criticize?

BUT

I have a suggestion, for $2.00 per connection you could use Anderson Power Poles. I use these connectors for bi-directional 12 volt connections for my Ham Radio equipment. They are genderless (I think that's the word) simple, and fully insulated, and finally, not reversible (within limits). Link below.

http://www.powerwerx.com/anderson-powerpoles/power...

1 reply

Great suggestions. I just wanted the plug and play feature and I'm the only person who is anywhere near these batteries.

At 60 Hz in copper, the skin depth is about 8.5 mm, so skin effect is not an issue at the low frequencies found in house wiring. DC has greater arc quenching requirements, so switches and breakers rated for ac have to be derated for DC.

1 reply

I will debate the skin effect with u but the breakers you are correct.

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FahdA3

2 years ago

Hi there! Your guide is really inspiring for people like me who are new to the world of solar home systems.

Quick question: I have read on some sites that LiFePO4 batteries are not ideal for solar home systems because they are not deep cycle like conventional batteries. Your article clearly shows this not to be the case. Could there be different types of LiFePO4 batteries (maybe some designed for vehicles, others for backup power etc)?

1 reply

LiFePO4 chemistry is perfectly suited for deep cycle and engine start applications. To date all my LiFePO4 batteries are performing admirably. My wish is that the prices will drop in the near future. There is only one type of LiFePO4 unlike lead acid that has a few variants (Gel, AGM, Flooded, various plate alloys etc). I hope my response was helpful to you.

Nice setup, the way battery prices are going the lead battery will to costly to buy. Nice to know LifePO4 can be used.