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Author Topic: measuring battery capacity  (Read 1762 times)

guitar man

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measuring battery capacity
« on: October 25, 2014, 03:33:05 pm »

Hi all
couple of questions. This battery stuff is so over my head?
1.
 Is there a way of measuring how much capacity is left in a nimh battery, I have a charger that will tell me what it put in but does not I think take into account what is already left in there
2.
Could I discharge a pack of nimhs using say a light bulb so I could recharge from a low level/flat pack so I would then know how much charge my charger has put in but I guess I could discharge to much and kill the battery??
3.
My charger does have a discharge feature but I am not understanding it to well at present.

Any help in simple terms is much appreciated 

Many thanks in advance

Tony
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Calimero

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Re: measuring battery capacity
« Reply #1 on: October 25, 2014, 04:03:30 pm »

Quote from: guitar
3.
My charger does have a discharge feature but I am not understanding it to well at present.


That's probably what you want to look into. Set the target voltage and let the charger discharge your battery.


But keep in mind that actual capacity will greatly decrease with higher current draw.


ie: a 4000mah battery might be able to deliver 4000mah if you draw around 500mA of current. If current draw is 10 or 15 amps it might be able to only deliver 2500mah. Very few chargers can provide such discharge current. Most can only pull around 5W to 10W.
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inertia

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Re: measuring battery capacity
« Reply #2 on: October 25, 2014, 04:04:46 pm »

Tony
1) Not easily, unless you know that it was fully-charged and you have closely monitored the current drawn and the time elapsed since.
2) Yes. Discharge the pack until the voltage across it reads 1 volt for each cell in the pack. Don't let it discharge any further. Connect a voltmeter across the bulb terminals to monitor it and be ready to disconnect the pack quickly - the voltage drop can quickly gallop away once it gets to this level. You'll need to measure the current while you recharge the pack. A digital multimeter connected in series (in line) will do the job.
3) The charger will automatically discharge the pack for you down to a preset limit, then usually recharge it without you needing to do anything.
Hope this explains things.
Dave M
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grendel

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Re: measuring battery capacity
« Reply #3 on: October 25, 2014, 04:21:26 pm »

I have a charger with the discharge facility, mine will discharge then recharge, thus letting me know the capacity, mine also has a setting that discharges and recharges 3 times to recondition the battery.
Grendel
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roycv

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Re: measuring battery capacity
« Reply #4 on: October 25, 2014, 04:59:52 pm »

Hi all, Nimh and Nicad are rated at the 5 hour rate.  The example quoted gives you a good idea of what to expect.  4000mAh.  take the 4000 and divide by 5 (hours) = 800 (m.amps)  so the battery will give you 5 hours at 0.8 amps (800 mamps)
.
If you want more current then the battery is less efficient and will run for less time than say 4 amps for 1 hour.  The extra power going into warming the battery, a good clue as to taking too much current at once.  The whole process of re-charging needs 40% extra current to give a full charge.

Lead acid cells are rated at the 20 hour rate so do not appear to have the same power.
Spiral wound cylindrical lead acid cells are rated at the 5 hour rate.

A word of warning if you are buying from China, you cannot rely on the battery label as to capacity.  If suspicious after purchase weigh them in compaison with a known quality battery, they are much lighter, i.e less content.

After getting caught a year or so back with labels saying 3800mAh which when measured turned out to be 1450mAh, I rely on our well known British suppliers.  You pay more because you get more.

regards Roy

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