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There is no way you can measure battery capacity with a multimeter. There are devices though "micro controller based" that would charge the battery to 100% and then discharge it under specific load and calculate the capacity based on the load value and discharge time.
Something like this:
http://www.ebay.com/itm/150W-Consta...743421?hash=item281f4ba6bd:g:8SwAAOSwHMJYELBp
but i am not sure if they work good or not.
Something like this:
http://www.ebay.com/itm/150W-Consta...743421?hash=item281f4ba6bd:g:8SwAAOSwHMJYELBp
but i am not sure if they work good or not.
What about something like this? I've seen these but are they the same thing as what you showed me?
http://www.ebay.com.au/itm/USB-DEVI...666513?hash=item2821acd451:g:ICoAAOSwLnBX85lw
http://www.ebay.com.au/itm/USB-DEVI...666513?hash=item2821acd451:g:ICoAAOSwLnBX85lw
If you're only testing USB output power banks, a meter like the one shown in your ebay link can work but as Hellas mentioned, the results are only meaningful if the load is known or controlled.
This is the best one I've found and used.
http://www.droking.com/drok-new-ver...charger-detector-a-must-have-tool-for-a-geek/
This is the best one I've found and used.
http://www.droking.com/drok-new-ver...charger-detector-a-must-have-tool-for-a-geek/
Not really. The one in your post can only monitor current output.
Set the battery up under a known load and see how long it lasts, You will find you get a different mA-Hr result at different currents. As a general rule for most common batteries figure an 8 hour life. Divide mA-Hr rating by 8 to calculate load resistance.
Well, divide it by 8 to get the 8 hour discharge current in mA.
e.g. 1600mAh equates to 200mA discharge current for a projected 8 hour discharge.
Then to calculate the load resistance, get the battery voltage and divide by the current in AMPS to get the load resistance in ohms.
Let's say it's a nominally 7.2V battery, the load resistance will be 7.2/0.2 = 36 ohms.
If you're testing a battery, the voltage will fall during discharge. This presents 2 problems.
The first is that as the voltage falls, the load draws less current. In addition, the battery may well start at a voltage higher than its nominal rating. Thus the load current is not constant and this introduces errors into your calculation. The answer to this is to use a constant current load, but that's a little more complex.
The second problem is where to stop? The best measure is to find what the manufacturer recommends as the discharge voltage, however this may be too low for a typical device to operate at, and thus you may choose a higher voltage.
Preferably you have some means of detecting the stop point, and this will stop the clock and remove the load.
The two problems mentioned above don't really happen if you're testing one of those USB power sources because they have a regulated 5V output and will automatically shut down when the battery is flat (or at least they should). SO your load will always be "correct" and all you need do is wait until the output voltage suddenly drops to zero.
Good observation. I use Excel and note batter voltage and current, calculating amphours on a per hour basis, then totally them as it progressed.
When to stop? consult the battery manufacturer.
How precise do you imagine the battery rating is? Is 10% an acceptable error? 20%?
If you test four batteries would you expect them all to be within 1% of each other and the rated value?
What does the manufacturer suggest?
Just to let you know why I want to do this, I also want to test some smartphone batteries to see if they are actually the 1800mAh rating they are claimed to be.
I have bought a china made grey import phone and would like to determine if the battery has the mAh rating it's claimed to have. They seem to run out a little fast so I'm dubious.
Will also test some battery packs as previously mentioned.
I have bought a china made grey import phone and would like to determine if the battery has the mAh rating it's claimed to have. They seem to run out a little fast so I'm dubious.
Will also test some battery packs as previously mentioned.
I don't have an easy answer for you, but I have some resources for you 
Depending on what you are testing, you may be able to find test results already.
There has already been a comment about the current draw... the mAh rating will vary when you compare a a heavy to a light load on the battery. The resources I've attached will show this as well.
Take a peek and see if you have more questions.
http://denishennessy.com/2012/04/08/measuring-battery-capacity-with-an-arduino/
http://www.batteryshowdown.com/
http://www.lygte-info.dk/info/indexBatteriesAndChargers UK.html
Depending on what you are testing, you may be able to find test results already.
There has already been a comment about the current draw... the mAh rating will vary when you compare a a heavy to a light load on the battery. The resources I've attached will show this as well.
Take a peek and see if you have more questions.
http://denishennessy.com/2012/04/08/measuring-battery-capacity-with-an-arduino/
http://www.batteryshowdown.com/
http://www.lygte-info.dk/info/indexBatteriesAndChargers UK.html
Attached is a pdf version of an Excel file similar to what I used the last time did a similar test. It is not a real representation. Battery voltage is measured every hour. Current is calculated. Amp-hours are calculated and summed up as you go along.
Stop whenever you feel like it. I was comparing rechargeable AA batteries. My concern was relative, not absolute as yours is. As long as I stopped at the same time in each comparison my test was acceptable. Amp-hours we pretty close to advertised.
I use one of these for testing 18650 cells:
http://www.ebay.co.uk/itm/7-in-1-OL...337036?hash=item43eeed494c:g:EI8AAOSwQaJXQtrQ
I use a standard tail-light lamp (12V 21W) as a dummy load, passing the current through the meter. I fully charge the 18650 (to 4.2V) then discharge it through the lamp until the voltage across the cell is 3.0V, and read off the mAh figure from the meter.
http://www.ebay.co.uk/itm/7-in-1-OL...337036?hash=item43eeed494c:g:EI8AAOSwQaJXQtrQ
I use a standard tail-light lamp (12V 21W) as a dummy load, passing the current through the meter. I fully charge the 18650 (to 4.2V) then discharge it through the lamp until the voltage across the cell is 3.0V, and read off the mAh figure from the meter.
I use these:
http://modelshopleeds.co.uk/catalog/product_info.php?products_id=19601
Also a linear regulator into a resistor is a constant current load, saves messing around with a spreadsheet, with the advantage of not overdischarging at the bottom end if you set it up right.
Bill
www.fire-tics.co.uk
http://modelshopleeds.co.uk/catalog/product_info.php?products_id=19601
Also a linear regulator into a resistor is a constant current load, saves messing around with a spreadsheet, with the advantage of not overdischarging at the bottom end if you set it up right.
Bill
www.fire-tics.co.uk
@mww:
Don't need to mess around with spreadsheets using the meter I indicated, since it has a mAh measurement on the readout that is zeroed when the meter is powered up and when it is reset with the reset button.
Regarding overdischarging at the bottom end... yeah, done that... got distracted and forgot about it. Cell went to zero for maybe an hour, but it showed me that these 18650 Li-Ion cells are more resilient than many people would have you believe... after a full charge and a new capacity test it showed no damage, but I now make sure it doesn't happen again by using the following device (which is much cheaper if you want more than one and buy from China and don't mind a long delivery time). As well as looking after the charging process, the device switches off the discharging current when the cell falls below 2.5V and re-connects when the cell recovers to some overdischarge release threshold (can't remember what that is but around 2.9V or 3.0V, and notably a feature that would make bottom balancing simple if you're inclined that way):
http://www.ebay.co.uk/itm/TP4056-WI...213092?hash=item3ad0902e24:g:QXsAAOSwU-pXrhEe
I've also pulled the protection circuit out of a dead protected LiPo cell and successfully used that for the same purpose.
Just a few more ideas to add to your suggestions.
Edit: see this vid:
Don't need to mess around with spreadsheets using the meter I indicated, since it has a mAh measurement on the readout that is zeroed when the meter is powered up and when it is reset with the reset button.
Regarding overdischarging at the bottom end... yeah, done that... got distracted and forgot about it. Cell went to zero for maybe an hour, but it showed me that these 18650 Li-Ion cells are more resilient than many people would have you believe... after a full charge and a new capacity test it showed no damage, but I now make sure it doesn't happen again by using the following device (which is much cheaper if you want more than one and buy from China and don't mind a long delivery time). As well as looking after the charging process, the device switches off the discharging current when the cell falls below 2.5V and re-connects when the cell recovers to some overdischarge release threshold (can't remember what that is but around 2.9V or 3.0V, and notably a feature that would make bottom balancing simple if you're inclined that way):
http://www.ebay.co.uk/itm/TP4056-WI...213092?hash=item3ad0902e24:g:QXsAAOSwU-pXrhEe
I've also pulled the protection circuit out of a dead protected LiPo cell and successfully used that for the same purpose.
Just a few more ideas to add to your suggestions.
Edit: see this vid:
davenn
Moderator
This is a VERY old thread, please try and avoid responding to ones like this
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