Reliable way to measure battery capacity

No very easy or 100% reliable way. The only accurate way is to charge then discharge completely and measure the output.

What I stated above is basically true for any battery type. Remeber that 100% SOC and 0% SOC is arbitrary for many battery types, such as LiIon. 100% SOC is defined as a safe voltage to charge to. You can charge further probably, but at one point the battery will explode and before that it will take damage from high voltage. Similarly with 0% SOC - it's the lowest voltage you allow the battery to discharge to. If you let a LiIon battery discharge to 0 Volts it is ruined for ever. So that's why I'm saying charge to the predefined 100% SOC then discharge to the defined 0% SOC and whatever total ammount of energy you got out during that discharge is the usable capacity of the battery (was the useable capacity for that cycle). If you discharge at 0.5C, 1C or higher C shouldn't matter in theory but in reality except the total capacity you get out of the battery may vary due to thermal issues etc. depending on discharge rate.

Are you asking from a practical or theoretical standpoint? What type of battery are you referring to?

ertobald said:

Thanks for the funded answer! I'm asking from a practical point of view, because I'm trying to do some cycle life tests on LiFePO4 batteries manufactured by BYD (200Ah) and CALB(100Ah and 180Ah). And I wanted to think of a way to test the capacity without discharging it completely, I thought maybe it is possible to discharge it for example only to a defined 50% SOC and then calculating the theoretical capacity without testing the last 50%.
Because which indicator could be used to measure capacity in a real time situation, for example if the battery is used in a vehicle or as an energy storage system at home or even in a power plant?
I know that there are a lot of models ( data-driven or electrochemical-based ) to estimate the total capacity or capacity fading over time, but how does the BMS actually measure the capacity?
Is it also just an estimation based on a battery model?

Click to expand...

From my understanding the way BMS calculates battery capacity is a lot of "black magic" and there are many real-world examples (for example from EVs) of how this is often not very accurate... Voltage of course is the most important variable to to equate SOC with voltage is a super oversimplification. There was a great discussion here on the forum, with some very expert posts on methods for estimating SOC and capacity, but I can't find that thread right now... I'll have a look later.

so the expected battery capacity can be calculated based on the molecular weight of the active material in the electrode, the number of electrons transferred and Faradays constant. For example the commercial cathode for Li-ion is LiCoO2. It undergoes one electron reduction of Co(IV) to Co(III) and uptake of 1Li+ to maintain charge neutrality on discharge and oxidation of Co(III) back to Co(IV) with release of the 1Li+ upon charge. Its molecular weight (MW) is 97.87 g/mol, it undergoes 1 electron (e-) transfer and Faraday's constant (Fc) is 96485.34 C/mol.

The math is as follows:

1/MW x Fc x #e- x (1/3600) x 1000 = 274 mAh/g

However, the crystal structure of LiCoO2 undergoes irreversible structural changes if discharged more than 50% so the commercial Li-ion cathode only utilizes 50% of its capacity to retain its reversability, or ~140 mAh/g capacity.

Experimentally, the amount of current the battery can supply per unit of time (in other words energy - Ah, etc) can be recorded by any battery cycles or potentiostat.