A 10Kw LiFeP04 battery bank
 

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.

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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.

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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.

Wow, that setup looks great!

What kind of inverter are you running?

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.
Rob

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.

https://battlebornbatteries.com/?gcl...SAAEgKTHfD_BwE

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

Robaroni

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??

randen

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...

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

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.

Logan
instagram. com/wlbryce

thanks.
Two Outback GVX4836 ; 7.2Kw total
Flex 80 Charge controller
Flex 60 Charge controller

Haven't bought gas in 23k miles.

All my critical loads are backed up. Well pump, fridge, freezer, phones, lighting, computers.

Thank you!

Logan thanks for the input.
I originally designed the system with BMS in mind but the Outbacks are so good that I don't need it. I cycled through a few charges running all my critical loads. with no problems.

What I did was to set max voltage to the bank at ~3.400v per cell. The data sheet shows the batteries to be very stable at under 3.600v so I even took an extra measure of safety. Also I cut the batteries off at ~3.00V cell which is well above critical. Internal battery resistance is very stable at these voltages. In fact the differential is under 40mV for the 16 legs.
LiFePo4's are the best batteries around for off grid and backup, really bullet proof.

I agree on both your points, the Outbacks are awesome and so is the chemistry of the cells.

Lets think of an insurance policy for your system. Do a Contactor with a voltage controlled switch. That way IF the controller hangs because of a hardware or a software issue you will be protected. Cheap insurance.

On my DIY system I protect both the High side and the DOD. It is cheap insurance. I have seen many controllers from several major brands do crazy things especially if networked into a "System".

One day Ill be able to post my DIY battery on here.

Regardless Great battery bank!

Cheers!

instagram .com/wlbryce

Might be time to jump on here and make a thread about my solar with battery back up.... but to answer the battery question asked above: I like simple phi lithium iron phosphate battery’s. Three of them make 10.5kw. At 10,000 cycles and built in BMS they work great for load shifting and back up. I paired it with a Schnider xw+ so I could ac couple to my grid tied inverter. If I did it again I would look long and hard at Sol-ark for its many more features and modern design.

Weather Spotter; As far as new installs we have installed over 500 Kwh of Fortress batteries. The price point is extremely competitive and come in large formats.
I also like the Sol-ark. We have installed several. The onboard dual channel MPPT makes for a easy and clean install.
One important issue that needs to be identified. If using Lithium you must insure that the battery is NEVER removed from the Sol-Ark when the chargers are charging hard. If you do, it will blow up the chargers. Sol-Ark now has a warning to this problem (also is a issue for other 600V charge controllers not just Sol-Ark, ask how I know! LOL). So, if the BMS trips the battery offline for some problem when the charger is charging it can kill the onboard charger.

On my system I built a 40f super cap bank that will hold the charger power long enough for the Fets to shut off the charging.

Just a Lithium FYI, or a LA issue if you trip the breaker somehow.

Logan

instagram .com/wlbryce

I have not used the sol-arks yet, last year at this time when I installed my system they did not support ac coupling. Mid January they did and I kicked myself :(. I wonder if the issue of lithium battery’s you are talking about would apply to my xw+? Tech support was saying that they have built in capacitors on the dc side.... which was why the xw+ needed the battery monitor add on to properly read battery voltage.... the caps make a voltage lag or something....

What are fortress battery’s?

In AC capacitance circuits current lags voltage.