the voltage drop under load is the result of internal resistance in the battery, and so will be proportional to the current
you are actually measuring voltage at the battery terminals then you should be able to measure the internal resistance of your batteries. You might also get it from the manufacturer, but that depends.
With the batteries rested, put a load with known current
on the system for a short period (~30 seconds or less?). Check the voltage under load.
E=IR > R= E/I, so:
Internal Resistance = (Resting Voltage - Working Voltage)/Load Current
In other words, divide the measured voltage drop by the load current you applied and you will get the internal resistance.
For lead-acid I would expect something under 0.02 Ohm. With that you can build a little table for voltage drop vs. current. If I use 0.01 Ohm as an example then:
- 10A = 0.1V
- 20A = 0.2V
- 30A = 0.3V
Internal resistance in your battery will change with age and with state of charge, so do this little test when you think you are near 50% DoD and it should give you a general idea of the voltage drop to expect. IEEE 450 actually requires (slightly more sophisticated) testing of IR on a regular basis, and uses the changes in measured IR to determine when batteries should be replaced.
Most of this goes out the window if your volt meter is not connected pretty much directly at the battery posts, as you will then have to account for additional drop factors in the wiring
, etc. so will not be measuring the IR of the battery, but rather that of the system.