# Measuring Li-ion battery manage to get a number in mAh

Joined Aug 30, 2017
84
How can a circuit functioning to measure Li-ion or else type battery manage to get its mAh capacity, how is this circuit's method ?

Found it out from a reviewer fed into an e-commerce site I forget, likely China's
Here, though a bit hard, we can still read 542 mAh which I think it's either battery capacity the tester device refers to and the consumer then pled or complained the seller's fraud

#### Papabravo

Joined Feb 24, 2006
21,030
My guess would be that it measures the terminal voltage and consults a database of Li-Ion battery discharge characteristics stored in a memory device in the battery.

Joined Aug 30, 2017
84
How do we solve the uncertainty and our confusion on an as light as feather China's 18650 battery being claimed 8800 mAh capacity (moreover yet far cheaper!)?

#### Papabravo

Joined Feb 24, 2006
21,030
Short of gathering the information yourself, I do not see a solution.

#### MrAl

Joined Jun 17, 2014
11,281
How can a circuit functioning to measure Li-ion or else type battery manage to get its mAh capacity, how is this circuit's method ?

Found it out from a reviewer fed into an e-commerce site I forget, likely China's
Here, though a bit hard, we can still read 542 mAh which I think it's either battery capacity the tester device refers to and the consumer then pled or complained the seller's fraud
What you can do is look around the web and find out some info on the best energy density for an Li-ion battery and then compare it to the one advertised that you might want to purchase.

For example going by rating comparison, if you see that the best Li-ion is rated for 2200mAh and then you see one for sale that is 22000mAh, you know right away that it is and exaggeration of the real rating so you probably dont want to buy that one. Instead look for one rated 2200mAh. If you see one rated even 3000mAh when the best you can find is 2200mAh again, you know something is up there too. Only when you see one that is 2200mAh or less you can assume they are not just making up the rating. Note i am using 2200mAh as a comparison here but there may be better than that out there today.
Going strictly by energy density, you can look up the mass energy density of Li-ion chemistry of late and compare it to the weight of the item you might purchase. If the latest energy density is say 2000mAh per ounce and the product you want to buy weighs 1 ounce, then the maximum rating (within some reasonable tolerance) should be 2000mAh and if they say 3000mAh then they are making it up.

As far as measuring it yourself, the best way is to fully charge the battery and then run it down with a known load and measure the current over time. The accumulated current over time gives you the ampere hour rating as measured. If it is too low compared to the advertised level then they lied about the rating. Of course you allow for some reasonable tolerance like 20 percent. This is the best test.

As Papabravo said, sometimes they use just the voltage measurement to calculate an approximate charge state. This is not an absolute measurement it is a relative measurement meaning that the result tells you the state of charge relative to the maximum for that battery. So say the max is 1000mAh and you read 3 volts and that correlates to 50 percent charge, then the charge at that time in the battery is 500mAh. With this kind of measurement a typical value for 100 percent is 4.2 volts but some now go up to 4.35 volts (for a single cell that is).
Since this kind of measurement works decently it works well to calculate the result in percent. IF the lowest value of the voltage after discharge is 3 volts and the highest 4.2 volts then each percentage point is:
pp=(4.2-3)/100=0.012 volts per each percentage point (0 to 100).
So if you read 3.012 volts the battery is at just 1 percent of full charge. If it reads 3.024 volts then it is at just 2 percent of full charge, and so on and so forth.
If the lowest is 2.5v then:
pp=(4.2-2.5)/100=0.017 volts per each percentage point.
Now reading 2.517 volts means the battery has just 1 percent of full charge left in it.

There are combination battery chargers and testers for sale that do these kinds of tests, notably the discharge test. You stick the battery in the tester then start the discharge test and when it is done some time later it displays the total charge that was in the battery. This gives you a good idea how well the battery is doing. I used to make a chip myself that did this kind of test and displayed the results on the PC computer, but the ones for sale are already built completely so you dont have to make any PC board or assemble anything. For example there is one made by "Opus" that i have found to work quite well for these tests. They probably make even more models by now, but there are several other companies that also make these things and they are somewhat reasonably priced. That's the simplest way to test batteries. I even test alkaline batteries when i first buy a batch. I have to sacrifice one battery to get an idea if i got what i paid for I think it is worth it though. Testing NiMH cells goes the same way.

Let us know how you make out if you can get back here later.

The attachment is a chart of cell voltage vs state of charge using a linear estimate. It is good enough for your basic comparison, but since the cell discharges faster near the top end you could modify it a little to make it more accurate over the full range.
There is one example calculation for a cell that measures 3.77 volts.

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