question

Personally, I would do a 50% load test then take a terminal voltage reading under load and then turn off load and wait an hour for your terminal voltage to settle. Then compare both of your readings this with the values from the battery/cell manufacturer datasheets. There should be a depth of discharge terminal voltage table for various C discharge rates and also the standing voltage. If not, use a generic table(s) for your battery type gel/AGM etc to work out the estimated state of charge at 50%. See if it matches and adjust higher/ lower capacity as required. It's never going to be 100% accurate because your capacity will change depending on your discharge rate/ age of batteries/ temperature etc.

To load them up you can build a simple load bank from some high wattage resistors in parallel and maybe an MCB. They will get pretty hot, sort of a DC space heater. You can use simple R/V calculation. Or, easier if you have an inverter unit, plug in a 1KW heater or similar which is a purely resistive load.

Only problem is you'll need a starting point to work out when you think you're down to 50%. I'd use the capacity on the side of the battery/ cell as a starting point. Then calculate how long you'd need to keep the heater on. So for example, if you had a 100Ah bank and were using 24V battery. Then a 1KW load would need 4A AC or 8A AC (230V/115V). Convert this to the DC current which would be 40A. So after an hour, you'd have used 40Ah which is around 40% depth of discharge. Obviously this does not take into account any losses. For 50% you'd need to go another 10% so 1 hour and 20 minutes roughly. Then switch off and see where you are. It wouldn't be as accurate as full load test but maybe safer if you're not sure on the capacity. No reason why you couldn't use the same method for an 80% discharge though, leave heater on for 1 hour and 40 minutes (adjust figures accordingly depending on your capacity/ voltage/ load power rating etc)

I use this method to test out batteries in the field with the DC resistor pack.

I'm struggling to find a graph or table showing terminal voltage / float voltage vs SOC even for generic batteries - how you perhaps got one?