How to measure battery capacity with a programmable load.

Hi guys!

I’ve just made a fairly basic programmable load and I want to try to measure a battery capacity.

Can I just discharge a battery at 0.5 amp and when voltage will drop to near zero then time * 0.5 will be battery capacity in Amp/hours?

Or do I have to discharge it in constant watt mode? (I have not made a constant watt mode yet…)

Thank you!

P.s. This is what my programmable load is like:
programable load.jpg

Battery capacity is expressed in Amp Hours, so current and time.
Some batteries are made for fast discharge, some for slow discharge.
Cutoff point varies per chemistry.
You can't e.g. test a LiPo by discharging it to 0volt without ruining it.

Sure, lithium batteries require a lot of TLC (should be kept dry and never fed after midnight!!) :smiley:

I'm thinking about testing AA alkaline batteries first - cheap and safe.

Thanks Leo!

Me thinks a discharge current of 1/10 of the expected capacity, to a 0.9volt cutoff.

Unless you want to know how long that battery will last in a device with a known current draw.

Unfortunately the 'BatteryUniversity' website is not very useful as the hi-tech methods of capacity testing by "multi-model electrochemical impedance spectroscopy" will be hard to reproduce at home..

But thank you for confirming my initial suspicion - battery capacity is current over time.
Thanks Leo!

If you care about a battery capacity for your specific application, discharge to 0 v. won't be very applicable. What is the lowest voltage your application can run at? What is it's typical current draw? Set up that rate of drain, and measure the time that the battery operates above or at your minimum accepted voltage. Since the capacity rating on a battery is a marketing tool, I've found that often they don't apply in the real world.

Battery capacity is a somewhat ephemeral concept. The manufacturers have no shame.

It is one thing to know how much energy you can harvest from a battery in a single discharge event. It is another thing entirely to figure out how much energy you can harvest in repeated charge and discharge cycles over a 12 month period. Discharging batteries to 0v is not good for longevity.