LiFePO4 Battery Build

One Lesson learned. Calibrate your Cheap ASS China PS (power supply), before you balance. 3.6V on the PS display gets me 3.65 on my DMM. Once you hit 3.375, you need to be checking a lot more often. It goes really quick!!

Here's the SoC vs Voltage curve. Once you hit 3.375 your at about 99%.

LiFePO4 SoC vs Voltage.PNG


Top Balance Complete!!
 
dtfeld said:
... Here's the SoC vs Voltage curve. Once you hit 3.375 your at about 99%.
Click to expand...

Does that last turn up ~3.63-3.75 bother you? I see that as dead charge, but only from my experience with RC LiPO's. I don't have much experience with these, very curious.
 
3.375 to 3.65 is only for the initial top balance. I intend to limit it between 90% and 20% SoC so 3.375 down to about 3.15 in actual practice. There isn't much energy at the far end of the curve.
 
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dtfeld said:
3.375 to 3.65 is only for the initial top balance. I intend to limit it between 20% and 90% SoC so 3.375 down to about 3.15 in actual practice. There isn't much energy at the far end of the curve.
Click to expand...

Agree, that makes sense. Good plan as well. What do you see as your max discharge rate?
 
There is a ton of energy on a very flat curve between to two points.
 
A couple more tasks competed.

I pulled the specifications for the cells I have (EVE 304Ah)

1. Loaded the specific values for these cells into the BMS. Cost to have the "Pro" version of the app was $7. Items like Max Cell Voltage (3.65V), min Cell Voltage (2.5V) etc.
2. Calibrated the cell voltage and current in the BMS.

C264924F-07A7-4989-BB65-D4486C1A2D5E.png


3. I blew up the State of Charge (SoC) vs voltage chart EVE supplied and came up with custom values for SoC. BMS allows input at 80%,60%,40%, and 20%. Supposed to increase its accuracy in the ipad app. Mind you its all just a guestimate, but beats nothing. Inverter already has a pretty accurate shunt/SoC monitor on it.

20211105_153615.jpg
 
Also performed an initial capacity check utilizing both batteries in parallel, and charged up to 14.0V (about 98% Full). I used a 750W inverter powering about 700W of lights as a load and an inexpensive battery monitor to keep track of the total Ah out.

Unfortunately, I Programmed the battery monitor to count down from 608Ah and it hit 0 before the batteries or inverter hit a low voltage point (once it hit 0 Ah remaining, it quit counting, it wont go negative.). But I know generally, these cells have more than 304 Ah each. :)

I did keep track of the amperage out and the time it ran, so I can also make an estimate that way. Roughly, each battery was supplying about 30.5A and it ran 10 1/2 hours (10:33) until the inverted dropped off due to low voltage.

So. 30.5A* 10.5 hours = 320 Ah /each battery.

This is what others were getting, so I think I received the cells I paid for and they seem to live up to the initial expectations. These are supposed to drop down to 304Ah after a number of cycles. In practice, I'll limit the charge and discharge voltages in the inverter to operate in the middle 80% of that range to increase life.

In the process of charging them back up, but its gonna take a while with a 30A charger.

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Still gathering materials for the build.
 

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Ran a second capacity test using a cheap battery monitor. Per the instructions, set the battery capacity artificially high (800Ah) and ran the same test using 700W lights on a 750W inverter till the inverter shut down due to low voltage. Final reading showed 167.7 Ah "left", so the difference 800-167.7= 632.3 total Ah from the 2 batteries, or about 316 Ah from each battery. Thats about 3x from a good AGM or FLA.

Before.jpg
After.jpg
 
Hey Dave, I've reached out to Jenny for a quote for mine. I'm going to put as many cells in as will fit in the space I have, so it's either a 2x or 3x bank. She didn't answer my question about cell sizes, so I'm wondering if you could grab a quick measurement of each cell for me.

Thanks,
Kevin
 
GnrlPatton said:
Cool, thanks David. It looks like the space I currently have my 4x 6V golf cart batteries in will easily fit 12 of these, so I'll probably go with that. Almost 1000aH of storage sounds nice.
Click to expand...

These are roughly the size of a group 31. I have a CAD drawing, I’ll post when I get home.
 
GnrlPatton said:
Cool, thanks David. It looks like the space I currently have my 4x 6V golf cart batteries in will easily fit 12 of these, so I'll probably go with that. Almost 1000aH of storage sounds nice.
Click to expand...

If you have 12 of the 304 (might as well as about same price and size as the 280 Ah), you’ll have about 912 Ah nominal with 80% usable of 730 Ah.

The other thing you’ll have to consider is how to charge such a large battery bank. It could take 6-12 hours (assuming you have a 100A or so chager on your inverter) if fully depleted. A lot longer if all you have is something like the standard Pronautic that came with most Sea Rays.

Tom might have some input on charging these puppies up, but it is quite a change to go to high output alternators which would be my first choice.
 
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