I just got a charger that tells me how much mAh I put into my 18650 batteries in a particular charging cycle (XTAR VC2). What I actually got the charger for is to test if my batteries are good or not. In other words check the capacity (I know, it’s just one indicator, but an important one). I know that I can check that, if I discharge the battery fully (meaning just below 3V) and then recharge. I rarely ever discharge to that point though. Is there a way to calculate the true capacity from let’s say starting voltage, voltage when full and mAh put in? As an example, Yesterday I put a 18650 in with 3.5V. Fully charged it was obviously 4.2V and I put a total of 1440 mAh into it. Can I find out what the capacity is from those numbers?
Calculating Capacity
- Thread starter cnickl
- Start date
wesnewell
100 GW
Select the Xtar battery (or any other) you have here and look at the test data.
http://lygte-info.dk/review/batteries2012/Common18650comparator.php
http://lygte-info.dk/review/batteries2012/Xtar%2018650%202100mAh%20%28Black%29%20UK.html
http://lygte-info.dk/review/batteries2012/Common18650comparator.php
http://lygte-info.dk/review/batteries2012/Xtar%2018650%202100mAh%20%28Black%29%20UK.html
dnmun
1 PW
to test capacity you have to charge each cell in the battery to full charge of 4.20V/cell and then discharge the battery until you have the first cell reach 2.70V. then stop and charge up that low cell enuff to continue the discharge until you reach the point where the next cell reaches 2.70V and record the capacity for it and then recharge it and then continue until you have recorded the capacity of each cell.
discharge through a watt meter into a dummy load like a heater or lights. record the Ah for each cell. write it on the can or pouch.
discharge through a watt meter into a dummy load like a heater or lights. record the Ah for each cell. write it on the can or pouch.
wesnewell said:
The battery I have is not in the list.
dnmun said:
I think there is a misunderstanding here. I'm working with single 18650 cells at this stage. The charger is designed to charge two at a time. I just want to know if I can take the initial voltage and calculate the total capacity from the mAh put it to "top off" the cell.
dnmun
1 PW
no. the only way to know the capacity is to measure it. to measure it you have to fully charge it and then fully discharge it. the voltage does not mean a thing.
wesnewell
100 GW
Not with much accuracy.. Also capacity varies quite a bit depending on the current draw. At 0.2A you should get the full rated capacity of Xtar batteries. But at 5A, you probably won't get much more than 80%. And there's tons of voltage sag at higher amp draws. All 18650 cells are best used at no more than a 1C draw, and you'll still get considerable sag and capacity loss at that.
dnmun said:
I believe you. I don’t want to start an argument here. Just for clarification. When I tried the following calculation:
Taking the example from my OP. A 18650 cell has 1.5V of “usable” voltage range (4.2V-2.7V)
I charged from 3.5V to 4.2V = 0.7V, putting 1440 mAh into the cell.
x/1.5V = 1440mAh/0.7V => solving for x => 3086 mAh total capacity
The capacity I calculated, 3086mAh, matches what’s on the battery label, 3000 mAh. Give or take a few mAh that could be off because the initial voltage might have been 3.47V or something like that. Is that just a coincidence?
icecube57
10 MW
Sounds like you dont want to actually check the batteries. Voltage isnt a good indicator at all on how much capacity is left in a cell or what the over all capacity. No you can not use your starting voltage and how much capacity you put in to the cell calculate the capacity. Capacity can only be calculated by doing a full discharge from 4.2 to 3.0 ... or whatever the cut out voltage is for your sale. Do not take short cuts on checking your battery it can surely bite you real good in the end. One bad seed can spoil the whole bunch and much more.
icecube57
10 MW
cnickl said:
Your formula sounds logical and you can know all the specs of the cell and everything and in theory your formula seems solid but from years of experience this isn't the case. Ive checked plenty of batteries when I recieve them and the voltage looks good and in theory its supposed to have X capacity and even though all the voltages matched on the pack the cells had very different capacities when I actually did a discharge test.
OK, so calculating is a no go. The next question I have is: What is more accurate or reliable. Measuring the mAh during discharge (full charge to cut off) vs. what my charger is doing. Measuring the mAh during charging (Cut off to full charge). It seems to me from the answers I got so far that the discharge is the better way to go.
icecube57
10 MW
Your charger should be able to discharge and charge the cell and give you MAH for both.
Depends:
Do you want to know how much it takes to charge them up?
Or do you want to know how much you will get out of them during discharge under the condtions you will actually be using them at?
Normally only the latter is important, and that's what I'd test, were I doing this experiment.
Do you want to know how much it takes to charge them up?
Or do you want to know how much you will get out of them during discharge under the condtions you will actually be using them at?
Normally only the latter is important, and that's what I'd test, were I doing this experiment.
amberwolf said:Depends:
Do you want to know how much it takes to charge them up?
Or do you want to know how much you will get out of them during discharge under the condtions you will actually be using them at?
Normally only the latter is important, and that's what I'd test, were I doing this experiment.
Well... I use 18650 for two things. I recently build my first, and so far only, pack for my e-bike. I bought cheap batteries from a source that shall not be named because I didn't want to be out too much $ in case I mess it up. What I wanted to know is by how much the "manufacturer", whose name shall not be named, is lying about the capacity that's printed on the cell.
The second application is in my e-cigarette mod. Obviously I'm using high quality cells in there and what I want to know in this case is if the cell is still good and save to use after a myriad of charge/discharge cycles. So in essence the same thing: Does the capacity of the cell match whats it's supposed to have. I assume what ever is printed on the label is discharge mAh done under defined, standardized conditions.
Hillhater
100 TW
Have you read any of the previous threads on capacity testing 18650s ?
If not...start here...
https://endless-sphere.com/forums/viewtopic.php?f=14&t=53017
If not...start here...
https://endless-sphere.com/forums/viewtopic.php?f=14&t=53017
dnmun
1 PW
he does not do capacity tests. he has his own charging theory.
there is only one way to do a capacity test and i explained it.
there is only one way to do a capacity test and i explained it.
dnmun said:
"He" has read your comment and "he" has acknowledged it by saying "he" believes you. Not sure what your whiny way of suggesting that I have "My own charging theory" is all about. I was asking a question, asked for clarification on something that seemed odd to me, and I have learned something as a result. Do I suggest somewhere that I disagree with you? If so, that was not my intention.
Hillhater said:
I started reading that thread, but my question was not answered. so I asked.
dnmun
1 PW
i was talking about the dark angle guys' ideas of how to determine capacity. he does not determine capacity by doing discharge tests. he looks for self discharge rates.
Hillhater
100 TW
dnmun said:
DA s analysis is a capacity map or profile.
What it does do is show very clearly where the majority of the cell capacity is relative to voltage..
for example, on this Sanyo 18650, it shows that its pointless looking for any more capacity below 3.65 v , or above 4.05v..
I think its a fair guess, that unless you have a very unusual cell, you wont find much more useful capacity below 3.5volts :wink:
