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Robaroni 11-29-19 08:55 AM

A 10Kw LiFeP04 battery bank
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About 2 years ago, before LIFEP04 batteries became the backup standard, I scored 640 new Tenergy 32700 LiFeP04 batteries for about $900 USD including shipping. These were purchased to replace my 13 year old L/A bank.

A little background:
I have two systems here, in 2006 I installed my first PV array with backup. I went to backup, as opposed to off-grid, because I have ready access to grid power and running batteries everyday is an expensive battery solution. I knew as I built my system I would install an intertie down the line and selling my excess to the grid was a better option. This way the grid keeps my batteries charged for when I need them and I still sell my excess after all my needs are met.
In 2011 I installed my intertie system which just sells back to the grid. So I have two systems, the intertie sells back to the grid and the intertie/off grid sells back after my needs are met.

First and foremost was safety - I wanted the bank to be bullet proof safe
and I wanted at least 2 to 2-1/2 days of power in complete darkness - which should never occur.


The build:

Because these batteries are stationary I built them in 4 groups of 4S36P (4 series and 36 parallel).
The batteries are rated at 3.2V, 5.5A, this meant that 16S (4x4S) would give me 51.2V for my 48V system.

I used one piece of finished plywood (~$25 at Home Depot) cut into 4 equal pieces (see pictures)
I machined jigs to cut and form each battery holder in the system. I wanted access to each battery and I wanted to be able to remove each battery. I also wanted each battery fused - just in case one went south. I did test a battery with a dead short to see how it would react. No fire no flames, just got warm and died. LiFeP04's are ideal for backup.

For fusing I used bare 24AWG wire on each battery and for the bus bars I used 1/2" copper pipe (the heavier pipe, L?) I connected the four 12V modules using AWG 2/0 battery cable.

The clips were made from surplus 316 SS 3/4" x .020" 100 foot rolls of banding and the battery wooden rails from hard maple. Because you can't solder directly to 316SS I spot welded nickel tabs to the SS when running fuse wires.


As it stands now I have a little over 10Kw of PV. My energy supplier owes me several hundred dollars and I have driven my EV for the last 2-1/2 years at a total cost of less than $75 for maintenance, inspections and charging. If the grid fails I expect to run easily for several weeks, even at my 42 latitude in the winter, without issues. I tested the system this winter accidentally leaving a space heater on during the test. It ran about 24 hours and the system recharged the next day which was partially overcast.

Daox 11-30-19 03:25 PM

Wow, that setup looks great!

What kind of inverter are you running?

Robaroni 11-30-19 04:41 PM

Thanks Daox!
I haven't posted in awhile, been busy with projects but I thought you guys would like to see what I've been up too.

I'm using two Outback GFX3648 inverters for a total of 7.2Kw with a Flex80 and a Flex60 charge controller.

The project took about 2 mos. to complete but I saved thousands of dollars so I think it was worth it.
Next I hope (finally!) to get to wind and microhydro but I needed to put them aside as I had no backup during power failures, my L/A batteries were shot.

philb 11-30-19 05:23 PM

Really impressive! Nice setup.

Your only using Flex 60's and 80's. No expensive BMS?

I have 24 GC2 6volt batteries on one system that die when the sun goes down. Most of them are 8 years old. $3800 for a traction demo battery. Maybe you could point me to a LIFEP04 retailer?

This post came at just the right time!
Thanks Rob!

Robaroni 11-30-19 05:52 PM


Originally Posted by philb (Post 61678)
Really impressive! Nice setup.

Your only using Flex 60's and 80's. No expensive BMS?

I have 24 GC2 6volt batteries on one system that die when the sun goes down. Most of them are 8 years old. $3800 for a traction demo battery. Maybe you could point me to a LIFEP04 retailer?

This post came at just the right time!
Thanks Rob!

I initially set up the system with a BMS system but after exhaustive testing
I found the best solution was to charge the bank to 3.4V (each cell 54.4V for the bank) max which, according to the data sheets on my batteries brings them up to almost 100% charge. I also spoke to the engineers at Tenergy who corroborated this.
I did carefully bring all the batteries into balance and I carefully checked all 576 batteries to make sure they were within 50mV of each other before putting them into the system.

Cheap Chinese BMS boards can't control batteries above a few hundred mV so it's best not to use them at all. The wires they use are too thin and the voltage drop across the wires makes it hard to regulate batteries well.

There's a YT guy named Will Prowse who's doing a lot of battery testing. My problem is that I simply don't have access to all the batteries so I can't give you hands on information.
From what I've seen the best LiFeP04's around is the Battleborn. I think it may have an internal BMS. Several people are using them with good results. If I had to go with a commercial battery this would be it.

Daox 11-30-19 08:31 PM

I would suggest Battery Hookup. I ordered some 18650 cells from them not too long ago. Great prices!

randen 12-01-19 12:13 PM


Nice looking system and the price also excellent. 10 kWh for $900.00 unheard-of !! To have an autonomous system is very nice in-deed. 2 days reserve power amazing !!

Could you give a idea to what you power in your home. I think that impresses the value of a system. You had mentioned about charging your car and driving on sunshine now thats something. Do you remember buying gasoline to get to work??


where2 12-01-19 08:41 PM

Having a 4.4kW solar array, and having a neighbor who regularly asks me when I'm going to come up with a "backup plan" for when the grid is down, Rob wins the prize for having created his own solution to this common question. Now I just need to incorporate this concept into my upcoming 5.6kW PV array. Having a grid tied array is easy, having a backup plan to recharge my e-Golf is a better plan...

arnesr 12-09-19 01:22 PM

Fantastic work, thanks for sharing! I am truly envious, that seems like a great DIY backup system.

wlbryce 12-20-19 02:47 PM

Great Job, But, you still should use a BMS. The BMS is not just for balancing, it is the safety for the battery bank. Inverters and especially charge controllers are not foolproof in regards to the charging and DOD control.

The BMS, will insure that you do not overcharge or over discharge the battery.
It not just good practice but its adds a level of safety for the battery bank.
Overcharging LA battery just ruins the battery but over charging Lithium even LiFE chemistry might end with a diffrent result.

instagram. com/wlbryce

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