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timbo avatar image
timbo asked

Evaluate battery capacity, again

There were some discussions on battery capacity evaluations but they did not really answer my question/concerns.

I have 16 CALB Lifepo4 prismatic cells 400Ah connected in series to get 48v. So, nominal capacity is about 20kWh. Battery is 7 years old (time flies!). Batteries promised 2000 cycles for 80% DD. This is a good manufacturer and their batteries were tested before to deliver this promise. 2000 cycles is about 7 years. But I never went that far down. My typical DD 25-49%, not more. So, I should expect some life in the battery

I want to expand my total storage capacity. I try to evaluate remaining capacity to understand: is it worth to add a similar battery pack in parallel or I need to build a new system with new batteries.

All methods with "charge to 100% and then discharge to 50% or 20%" don't work for me. They are impractical. I live off-grid from this battery charging from solar. I need energy for the house, and charging is not up to me - it depends on sun. It is not so easy to discharge 10-16kWh in one go, and if I do it then there is a chance I might not be able to charge it back as there is no sun.

Hence, the only way for me is a natural experiment and observations.

I assume that Multi derives SOC based on the voltage, and Multi knows that I use Lifepo4 battery. So, if I collect SOC and consumption numbers from VRM portal, I might be able to get some idea of the remaining capacity.

For example, SOC after sunset s1 = 98.5% and consumption so far is c1 = 4.9kWh. Then I check the same parameters next day before sunrise, s2 = 66% and c2 = (7.61+3.48) = 11.09kWh as there are two days.

Estimated capacity should be (c2 - c1) / (s2 - s1) = (11.09 - 4.9) / (0.985 - 0.66) = 19.0kWh

I tried this approach several times for different SOCs and time periods and I got numbers around 19kWh. If SOC is low or time interval is short, then estimation might be lower, e.g. 18kWh. Sometimes it is higher, e.g. 20kWh. But most time it is very close to 19kWh.

Could you please comment on this approach and reliability of my estimation?

19kWh looks good, it might make sense just to add another 16 cells in parallel and use them for the next 7 years.

Lithium Batterybattery capacity
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Alexandra avatar image Alexandra ♦ commented ·
@timbo

Are you using ess and keep batteries charged before beginning the test cycle?

With the multi you might see it charge up (using its internal battery monitor) even when at 100% on optimized with battery life.

And you would have to do the charge and discharge test without solar attached. But really without a shunt it is clever guesswork.

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timbo avatar image timbo Alexandra ♦ commented ·

No, I don't use ESS. I use a BMS assistant together with the PV inverter assistant (AC-coupled system).

The battery might be charged to 100% or it might not. It depends on the weather. I take numbers only after the sunset to eliminate sun charge effect. Then I have only consumption till next morning.

I take numbers from VRN portal, which gets them from Victron Color Control, which gets them directly from Multi. When all of them show 100% SOC, charging stops. There might be a bit plus or minus as this is a live system, there are always consumers of the energy and sun-inverter is connected. But Multi does a good job in general to balance generation and consumption. Charging stops at 100% SOC.

My main concern is how precise estimations of SOC and consumption I get from Multi. Obviously, there are rounding errors. For example, SOC is measured with 0.5% precision. It has its effect, especially when time difference and SOC difference (s2-s1) are small.

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snoobler avatar image snoobler timbo commented ·

MP does not use voltage except to synchronize to 100%. It counts current in and out. It's only as accurate as its programming, i.e., you've programmed it for 400Ah, so it's X% of 400Ah. It in no way knows if your battery is deteriorated.


Because of this, your experimental method will yield very nearly 20kWh all the time.


The only way to reliably determine your battery capacity is to test it with a discharge down to 2.5V/cell following a charge.





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timbo avatar image timbo snoobler commented ·
Thank you. This is very important.

Discharge-charge is impractical or even dangerous in my case. I guess, I should try to calculate my own SOC based on the voltage and discharge curve.

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nickdb avatar image nickdb ♦♦ timbo commented ·
It is impractical to use voltage as the measure of charge in lithium batteries.
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Nev avatar image Nev timbo commented ·

I doubt there is a better way. Without a full discharge capacity test you may never know.

If you ever manage to achieve that better way you may just revolutionise LiFePO4 SOC calculation.

Our 4 cell 300Ah Sinopoly battery has survived 8 years of full time travelling in our RV.

I perform an annual capacity test each November. Constant 30A discharge and stop when any cell reaches 2.8V.

Capacity for the last test was 4008Wh - c305Ah. They are holding up well.

The battery both starts the 3.9TD diesel truck and powers our house behind.

I look forward to any easier capacity test method one day.

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