# how to work out a capacity of a battery

hi, just brought a few battery's (18650's Li-ion) for a project brought them from china im pretty sure they are not counterfeit as they seem to be heavy, another one i brought seemed very light i guess that one has a much lower capacity.

anyway just wondering how to work out the capacity of it, im wondering if i could work out the capacity of it by the charging rate? im charging it at 500ma, so if it takes 6 hours to charge can i assume its roughly 3000mah battery? or can it only be worked out on the known current discharge rate? thanks

For all you know there could be a lump of lead inside your battery (for feel-good weight) and a low capacity battery to give an output. One way to determine capacity is to fully charge it then conduct a controlled discharge, measuring current and voltage at specific periods of time. From that data you draw a curve of watts versus time then totalise your readings to get watt-hours, divide the results by average voltage to establish a rough idea of AH capacity. It would be even better if you could discharge at constant current but that is a little more complicated to do.

Charge of a battery is measured during discharge under specified conditions (often the 10-hour rate) and after a specified full-charge cycle. The amount of charge it takes to recharge is usually 20+% more than the capacity.

Actual practical capacities are less because you don't always start using a battery immediately after a full charge cycle, and don't discharge uniformly at the recommended temperature.

Measuring a discharge curve requires either a specialist battery discharge monitor or a suitable load-resistor and regular measurements by hand - divide the voltage values by R and integrate to get the charge.

Most battery types do not tolerate over-discharge (lithium batteries actually become dangerous), so never discharge a battery into a dummy load unsupervised, or you'll just trash the battery. Observe the minimum voltage and stop the discharge, immediately recharge.

jonwhite: hi, just brought a few battery's (18650's Li-ion) for a project brought them from china im pretty sure they are not counterfeit as they seem to be heavy, another one i brought seemed very light i guess that one has a much lower capacity.

I've seen videos on youtube where there's a chunk of metal in a "battery" and a couple of tiny coin cells in the top to make you think it's working when you buy it.

I'm guessing they feel heavy.

jonwhite: im charging it at 500ma, so if it takes 6 hours to charge can i assume its roughly 3000mah battery?

I could put in a resistor to shunt 495ma of that around the teeny battery I put inside.

jonwhite: or can it only be worked out on the known current discharge rate? thanks

Only the discharge counts.

thanks for the info, going to try to test the battery im going to aim for a 250ma load using suitable r (high wattage r), 5w 20 ohm.

i stumbled across this guys site if anybodys has any interest in batterys capacitys, this guy must have tested every battery available amazing

http://lygte-info.dk/info/Batteries2012Info%20UK.html

(im never ever going to get in an plane comming out of china im sure the holds are full to the brim with these batterys id rather walk)

thank you