So since I do not have a battery balance thing yet I have been working, starting yesterday, on some li-ion cells because I need to find capacity of the cells. And I have a few questions to make sure I am doing my math right.


I am using 4 computer fans to discharge the cell. Each fan pulls .18 amps. But there 12v fans and this is a 4v battery. So is the fan still pulling .18 amps a hour? And there is 4 in parallel to pull .72 amps a hour. So how much are they pulling because I am simply not sure. And is there a formula on figuring that out? What would it be? Along with the pull I have a multi-meter on the battery to watch the voltage. There at 4v and discharging to 3v. But have nothing to charge them bringing me to my next part.


The only thing I have is a 5v 1 amp a hour wall charger for a phone. I stripped the wires. All I was thinking of doing was having that directly on the battery and again using a multi-meter to watch the voltage and then stop at 4v. But how long would I expect this to take? Say the battery could hold 2 amps. It would then take 2 hours to charge but I worry that would charge to 5v. So how do I calculate how long it would take to charge to 4v?

The whole purpose is to find the capacity of the battery. Also note these are about 5 years old. Some are good some are not good. Brand new they where able to hold about 2A

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    \$\begingroup\$ Go read battery university about how to test and charge batteries properly avoiding <3.1 and > 4.1V while charging for longer life batteryuniversity.com/learn/article/… \$\endgroup\$ Commented Dec 30, 2016 at 23:34
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    \$\begingroup\$ Especially learn the difference between amps, amp hours and amps per hour. The last of those is nonsense. \$\endgroup\$
    – user16324
    Commented Dec 30, 2016 at 23:49
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    \$\begingroup\$ A fan spec'd at 12 volts, 0.18 Amps is highly unlikely to draw 0.18 Amps at 4 volts - the only way to know what current it draws at that voltage is to measure it. It would be much better to used a suitable resistor as a load on your batteries. \$\endgroup\$ Commented Dec 31, 2016 at 2:39

1 Answer 1


Lithium-Ion batteries are very dangerous and need to be handled with care. They are sensitive to over-charging, over-discharging, current (while charging or discharging) above what they are rated for, temperature, mechanical force (eg. puncture), trickle charging... just to name a few. Their immediate failure mode is to catch fire.

Even if they have built-in protection circuitry, it is always best to have a solid understanding of the risks and safety precautions involved.

Also, with respect to charging, there are specific profiles that must be followed. Parameters will be given by the battery manufacturer for constant current and constant voltage levels required. A compatible charge circuit is absolutely required. Unless you have in-depth experience with this, just buy a reputable one, pre-made. Absolutely do not simply apply 5V and monitor your battery!

For discharging, if you are trying to characterize these batteries, use a known resistance and measure the voltage drop. Be sure that the resistor used is of a sufficient power rating. You can then calculate the current drawn. It will decrease as the battery voltage decreases. Plot a graph of Battery Voltage as a function of Time, with constant resistance.

Better yet, you could apply a constant-current load to your battery, and again plot the Battery Voltage as a function of Time. Both methods work, but I prefer the constant-current method.

To re-iterate, you should really do some research on Lithium-Ion battery safety before you continue with any of this experimentation!

Try looking online. Places like Battery University are great resources.

Be careful, don't burn down your house.

  • \$\begingroup\$ Ya I figured applying 5v was not good idea. What if I added a resister to take it down to 4v, but what resistance would you need? \$\endgroup\$
    – Ljk2000
    Commented Dec 31, 2016 at 0:16
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    \$\begingroup\$ That still isn't a good idea. Take a look at a Charge Curve for a Li-Ion battery. The voltage will start well below 4 Volts in the Constant-Current section of charging. There is a good EEVblog video going over the basic concepts of charging. Also check out Battery University, linked above. \$\endgroup\$
    – Daniel
    Commented Dec 31, 2016 at 0:29
  • \$\begingroup\$ Well I was looking at my power supplies and I found a 12v at 1 amp per hr. So I could do three in series and charge them all at once, unless you think that is even a bad idea. I measured and it came to 12.5v. so that is just over 4v per battery. \$\endgroup\$
    – Ljk2000
    Commented Dec 31, 2016 at 1:02
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    \$\begingroup\$ Also, as @BrianDrummond said above, it is not 'amps per <unit of time>'. Just amps. \$\endgroup\$
    – Daniel
    Commented Dec 31, 2016 at 1:07
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    \$\begingroup\$ @Ljk2000 And in the same manner it isn't a 4.2 V 2 A battery, it's a 2 Ah battery. This difference in physical quality is crucial to your understanding of how to calculate charge and discharge times for batteries. \$\endgroup\$
    – winny
    Commented Dec 31, 2016 at 8:38

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