I've designed and built a prototype PIC based Sealed Lead Acid Battery Tester, but I'm having confusing and conflicting results and would like some input on whether the strategy I'm using is sensible ...
It's based around a constant-current load, controllable by the PIC chip. The current can be set over a range of C/40 C/20 C/10 C/5 C/2 and C (where C is the nominal Ah rating of the battery). The unit can monitor the voltage of the battery. All the voltage measuring/current measuring and current setting is appropriately calibrated, so any problems are down to approach or duff assumptions on my part :(
The basic idea is a "brute force" test of the capacity of the battery: Load it up at C/n and time how long the battery takes to reach a discharged state. In theory, it will take "n" hours for a 100% battery (aside from the obvious that discharging faster will reduce apparent capacity etc.)
From my research so far a fully charged SLA cell should read 2.16v at rest, and 1.75v when discharged. So for a 12v battery, the useful capacity is found between terminal voltages of 10.5v and 12.96v. Right?
The capacity of the battery can be estimated using this voltage range, which I'm assuming to be a linear function (it nearly is...). So the first feature on the tester is a battery capacity estimator, based on offload voltage.
Then loading the battery at (some constant current) begins the discharge process, and the voltage starts to fall. I time how long it takes for the voltage to reach 10.5v, and then stop the timer, and compute the capacity actually achieved. It should be that simple, however ...
I have a dilemma: When things say "don't discharge the battery below 10.5v" do they mean on-load or off-load? There is a difference!
I noticed that loading the battery, especially at higher currents, causes the terminal voltage to fall, due to the internal resistance of the battery.
So I added a feature to try and measure this resistance at the start of the discharge process. I did this by taking the off load voltage, the on-load voltage at C/n, and working out the voltage being dropped across that resistance. This gives me a figure of = capacity 1. This seems to give reasonable figures, of capacities that are