So I am creating a battery voltage measure, and it takes a reading both when the battery is in open circuit, and when it is loaded with a 100mA constant current. I have set it up and the reading is being taken right after the load is triggered and I've been getting some rather peculiar readings, at least, peculiar in my eyes.
I'd calculated the loaded voltage to be around 2V less that the open circuit reading (quite an old and used battery) but when I get the readings from the ADC it shows a drop of only around 1.2V.
I have found that if I leave the load on a then measure it with a DVM the output is significantly lower than the ADC reads and closer to what I am actually expecting, so I was wondering if there is some sort of delay in the battery 'adjusting' to the new load, it's all being triggered by a micro-processor so the difference in time between the load being triggered and the reading being taken is next to none as they're one line after each other, I was wondering whether it was the ADC that could have been the problem but after doing some tests I think it is more likely that there is a delay, I just wanted some confirmation of this and also some reasons as to why!
Is is a characteristic of all batteries or just old and withering ones?
Thanks in advance.
Some information on the circuit:
Battery being tested: CR123A
There is a constant current of 0.5mA always over the battery during test, this is what I am classing as 'open circuit'.
The load that is on when I am trying to measure the loaded voltage is a constant current drain, draining 100mA from the battery each time it is triggered.
For arguments sake lets say the triggering timings are as simple as: it is triggered every 25ms -> triggered on -> measurement taken -> 25ms delay -> trigger load off -> repeat.
Edit: This is part of an accelerated life test for a battery so the time before the reading is taken can't really be increased to negate this problem.