hi there, I have a project which uses two 7 mAh batteries for mains failure. These are being constantly charged. What I would like to do is some sort of battery test on them on a push of a button. I thought the best way to test them is a mAh test? But I have seen others that have done this but they take a fully charged battery and discharge it all the way down but I don't have the time to do that or take the risk of having a flat battery at the end of it. Is there another way I can do this?
Lead acid batteries are sluggish to respond. So if you simply remove the float charge, the voltage takes a while to settle down to the resting voltage. Unfortunately that time may be on the order of hours. Unfortunate, because you can tell fairly accurately what the charge state is by voltage, if the battery has been left without charging or discharging for a fair chunk of time, as long as you compensate for temperature.
What is the manufacturer and model number, or manufacturer and voltage/Ah?
Presume you mean 7000mAH per battery.
Forget the theory and all the fancy gizmos. If you want to know how long your batteries will last under mains failure conditions then the simplest (and most realistic) test is to turn off the mains and monitor the actual discharge voltage as the batteries supply current to your system. When voltage has dropped to your predetermined minimum (say 11v) then the time it took to get there is the true application capacity. OK that takes time, but do you want a truthful answer or an answer that makes you feel good.
Discharging is the only way to test the energy capacity of the battery. You don't have to discharge it all the way down to zero. I think a battery is considered discharged when the voltage gets down to 80% of it's nominal value.
But, you should be able to get a general idea of the battery's health by measuring the voltage with no-load or some known load. You'll probably have to experiment and "guess" what the voltage would be when the battery would be considered no-good.
which uses two 7 mAh batteries for mains failure.
I'll bet that should say 7Ah. 7 mAh would be a really small battery.
Yes I did mean 7am
So the general concessive is to switch off the wains to it check the voltage and then put it under a load and see how far the voltage drops?
On the other hand you can buy a battery testers which can test the Am within about 20s or so, so how do these things work?
Or are they not particularly accurate? But hope not as some of them are over £200
Joes:
hi there, I have a project which uses two 7 mAh batteries for mains failure. These are being constantly charged. What I would like to do is some sort of battery test on them on a push of a button. I thought the best way to test them is a mAh test? But I have seen others that have done this but they take a fully charged battery and discharge it all the way down but I don't have the time to do that or take the risk of having a flat battery at the end of it. Is there another way I can do this?
any help will be appreciated
thanks Joe
You need to measure the internal resistance.
This involves loading and unloading the battery (say by PWM at a low frequency)
and measuring the amplitude of the variation in terminal voltage. divide the voltage
variation by the load current variation and you get the internal resistance of the battery.
Something like a 10A dummy load might be about right for a 7Ah battery, and the
voltage variation is still going to be small with a healthy battery, so it might need
amplification (DC-blocking capacitor can be used of course)
You wouldn't run this test for more than a few dozen cycles, just enough to get a
reasonable average. The resistance should vary somewhat with stage-of-charge -
if the battery is nearly discharged the internal resistance will start to rise markedly
since it will struggle on the high current pulses.
7 mAh would be a really small battery.