# How to measure the amount of power/energy a lithium battery holds

Discussion in 'General Electronics Chat' started by Keo_young, Feb 8, 2015.

1. ### Keo_young Thread Starter New Member

Feb 8, 2015
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Sorry for the most basic question..but how do you measure the battery(lithium) capacity ( I am not sure that's the right word ).. how much power does a battery holds or stored.

Go easy on me.. for I a metal retard with electronic stuff but ready to learn for I am only 57 yrs old.

2. ### ErnieM AAC Fanatic!

Apr 24, 2011
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The best you can do is drain all the useful energy output of a battery and record what it once held.

Not very useful.

Far better though less exact is see the manufacturer's data sheet and read what they state is should hold.

The units are in amp-hours (AH), or how many amps times how many hours. Do not this is not one single value, meaning if you draw less current you can draw that for more hours.

That's kind of confusing, meaning if the rating is 20 AH you can draw 1 amp for 20 hours, but of you draw .5 amps you may get 45 hours of life.

3. ### Denesius Member

Feb 5, 2014
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Not to confuse the issue, but there's also the consideration of ambient temperature (batteries are fundamentally nothing but chemical reactions, and temperature dependent) and cooling effect of the environment as the battery is discharged.

4. ### Keo_young Thread Starter New Member

Feb 8, 2015
2
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I needed to measure the amount of "power" inside a battery (lithium)... a battery may says 10Ah, how do I measure whether the fully charged battery is indeed 10Ah? many thanks

5. ### ErnieM AAC Fanatic!

Apr 24, 2011
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As I said the only way to measure this is to drain the battery and see what came out.

To actually accomplish something useful with that energy you have to follow the manufacturer's specfication of AH and use that as a guide.

6. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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And, to be technically accurate, batteries do not store "power". They store energy.

7. ### takao21203 Distinguished Member

Apr 28, 2012
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They are chemical charge pumps. The electric charge is distributed to components and then converted into some kind of electrical work. That can be measured as electric power: Voltage * Current

8. ### Denesius Member

Feb 5, 2014
89
14
I think what you're asking is how to test a battery to see if the actual capacity is what the manufacturer claims (to test either the integrity of the manufacturer, or the battery)?
ErnieM gave the correct answer. A practical method would be:
1-discharge the battery (drain, then short it to ensure complete depletion- people are going to squawk that this is bad for the battery, and that's true, but we're doing fundamental testing here!)
2-fully charge (either a smart charger, or based on constant current for given time per battery capacity. If you want details, ask!)
3-top off (if you want better accuracy in your results- this accounts for about 5% of additional power storage)
4-put a constant current load on the battery & time to depletion. For example, if I was testing a 12v battery, I'd use a LED driver and hook it to a power LED or incandescent bulb (significant percentage of the battery's capacity), setup a camera to record the light, turn everything on and go on vacation. Upon return recharge the camera (!), download the video file and fast forward to where the LED kicked off. It will be near the battery's depletion point, as most constant current power supplies have significant boost capability. It won't be lab grade accurate, but it'll give you a pretty good idea of how the battery performs.

Of course by now the forum squawk will be (esp if it's a Li battery) you've pretty much killed it (draining it to zero twice), but at least you'll know what it WAS capable of doing. You can do the test without draining to null, by partial charge/discharge cycles & extrapolation, but I'll leave that answer to the experts on this site.

9. ### BobTPH Active Member

Jun 5, 2013
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The capacity you get when you measure will likely be different than what the manufacturer claims. That is because the capacity is actually dependent on the rate of drain.

Bob