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is there a way to measure exact mAh or Wh of a battery?

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akikhia

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i want to check true mAh of a battery just cant think of any devices that could do that. only thing i can think of is to video record charging or discharging and do a by hand integral of the whole thing? haha but thats painful there has to be a simpler way
 
David Jones did some testing of that sort on some of his EEVBlog episodes. Consider also that the capacity in mAh will be higher at lower discharge rates due to reduced loss in heating the battery's internal resistance.

https://www.eevblog.com/
 
i want to check true mAh of a battery just cant think of any devices that could do that. only thing i can think of is to video record charging or discharging and do a by hand integral of the whole thing? haha but thats painful there has to be a simpler way


Hi,

I made my own 4 channel volt meter that interfaces with the computer so i could take many readings over time. For one battery though we only need one channel, so you could use any meter made to interface with the computer.
What you do is set it up with a known resistive load that is approximately what the manufacturer uses and then you log the voltage readings. Using that data, you can then calculate the current and use the time logs as the time reference and so compute the ampere hours over the discharge.
Back when i did it by hand i took readings every five or ten minutes. It's much easier to use a computer meter, and i even built in an ampere hour measurement feature right in with the software. You could do this too if you know how to write software.
 
that should be offered in multimeters. i dont have anything i can hook up to my computer and for my purposes its not worth it.
 
for my purposes its not worth it.
What is your purpose? If you measure a non-rechargeable cell by the above method then the cell is dead at the end of the measurement; so that's a waste. The next cell you pick up won't necessarily have the same mAh as the first, even if it's the same brand and nominal capacity.
 
What is your purpose? If you measure a non-rechargeable cell by the above method then the cell is dead at the end of the measurement; so that's a waste. The next cell you pick up won't necessarily have the same mAh as the first, even if it's the same brand and nominal capacity.

haha well i am referring specifically to rechargeable batteries lithium ion
 
Hi again,


Well another technique is to use a constant current (active) load, where the current drain is always the same regardless of the battery voltage. This way you only have to make a few measurements over time and do a little math to figure out the rating. The active load would be made using an op amp and transistor rather than just a resistor. Not to hard to do though if you are interested.

For example, say we set the constant current load to 100ma. That means no matter when we look at it it will always be drawing 100ma. And lets say that when we turn it on, the voltage is 4.10 volts. We come back two hours later and the voltage is 4.05 volts, so we know we have gotten 0.1 times 2 which is 0.2 ampere hours out of it already. We come back 8 more hours later, and it is at 4.00 volts. As long as it is not under 3 volts we dont have to do anything really. We come back 5 hours later (a total of 15 hours now) and it is at 3.2 volts. We know we're getting closer to 3.0 volts now so we come back more often. We come back another 2 hours later and it's at 3.01 volts. We decide that is low enough, so we disconnect the load.
Now since it took 17 hours to complete this experiment with a 0.1 ampere load, that means the battery had exhibited a capacity of 17 times 0.1 which is 1.7 ampere hours. That's the capacity when we discharge at 100ma down to 3.0 volts.
So you can see how much simpler this is now. We only have to catch it when it reaches 3.0 volts (or whatever target voltage we choose that wont hurt the battery).
 
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Hi again,


Well another technique is to use a constant current (active) load, where the current drain is always the same regardless of the battery voltage. This way you only have to make a few measurements over time and do a little math to figure out the rating. The active load would be made using an op amp and transistor rather than just a resistor. Not to hard to do though if you are interested.

For example, say we set the constant current load to 100ma. That means no matter when we look at it it will always be drawing 100ma. And lets say that when we turn it on, the voltage is 4.10 volts. We come back two hours later and the voltage is 4.05 volts, so we know we have gotten 0.1 times 2 which is 0.2 ampere hours out of it already. We come back 8 more hours later, and it is at 4.00 volts. As long as it is not under 3 volts we dont have to do anything really. We come back 5 hours later (a total of 15 hours now) and it is at 3.2 volts. We know we're getting closer to 3.0 volts now so we come back more often. We come back another 2 hours later and it's at 3.01 volts. We decide that is low enough, so we disconnect the load.
Now since it took 17 hours to complete this experiment with a 0.1 ampere load, that means the battery had exhibited a capacity of 17 times 0.1 which is 1.7 ampere hours. That's the capacity when we discharge at 100ma down to 3.0 volts.
So you can see how much simpler this is now. We only have to catch it when it reaches 3.0 volts (or whatever target voltage we choose that wont hurt the battery).

of course that would be grea but with a variable voltage how can we ensure a constant current draw throughout??
 
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of course that would be grea but with a variable voltage how can we ensure a constant current draw throughout??

Hi again,


There are a number of ways, KeepIt's post mentioned an LT device that looks good. You have to be careful with the LM317 if you have to discharge down to 3v or lower it may not function properly.

Another simple way is shown in the attachment. This uses one general purpose op amp and a transistor like 2N2222 for currents up to about 300ma. A separate source V1 is shown for the supply voltage, and V2 can be a pot that goes from V1 to ground using the wiper arm terminal for the input to the op amp. R3 is part of the battery, it's internal resistance, and V3 is the battery internal voltage.
R1 is shown as 5 ohms and that is sized to keep some of the power out of the transistor while still allowing some current draw from the battery. With R1 being 5 ohms, the output current is then equal to:
I=V2/R1=V2/5
so when V2 is 1 volt, the output current is:
I=V2/5=1/5=200ma
The power in R1 is:
PR1=I^2*R1=0.2 watts, so a 1/2 watt resistor would be good.
The power in the transistor is:
PQ1=(V3-I*R1)*I
so for I=200ma we have:
PQ1=0.4 watts when the battery voltage is 3 volts, and we have PQ1=0.6 watts when the battery voltage is 4 volts.
If the transistor gets too hot then you can increase R1 up to about 10 ohms for a 200ma current sink, but you'll also have to adjust V2 for 2 volts (or close to that).

If we use a rail to rail op amp and a voltage reference diode for V2 we can use the battery under test to power the circuit too.
 
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Here's a very simple 100mA discharger. Current is constant (+- 1.5%) from ~10V down to 3V. Tweak R1 to select/trim the current.
 
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