Planning to build a 72V 18650 battery pack with used laptop batteries. I have a few questions to what happens when you have different batteries.

  1. What happens to batteries in parallel with different voltages? Say, five batteries are rated at 4V and one is rated at 3V. What will be the overall voltage of the group?

  2. What happens if batteries are in parallel with different capacities? Say, five batteries have 4000 mAh and one has 3000 mAh?

  3. What happens to batteries in series with different capacities? If one group has 10 Ah and one has 20?

Overall, if I have batteries with different internal resistances, capacities and voltages, which are the most important? Should I group based on internal resistance, capacity or votltage?


OP here 2 years later haha. I ended up making this battery. I got a bunch of used laptop batteries for free and connected the ones with similar capacities, voltages and resistances into a large 48 V pack. It worked perfectly for 1.5 years and then it failed (didn't catch fire). Overall a risky but fun project!

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    \$\begingroup\$ There are many questions on this site discussing practically the same, start with the list in the column on the right. Before you start doing anything, educate yourself on how to use Li-Ion cells as they can and will smoke/become hot/catch fire/explode when treated the wrong way. Your question 1) is an example of making a cell overheat and explode. \$\endgroup\$ Commented Nov 21, 2017 at 7:49
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    \$\begingroup\$ Thanks, I know they're dangerous and that's what I've been doing for these last couple days. I'll check other places first \$\endgroup\$
    – teepers
    Commented Nov 21, 2017 at 8:23
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    \$\begingroup\$ Just a brief comment to question 1. What happens when you put a fully charged (4.2V) cell in parallel with an equal but discharged (3.6V) cell? This depends on the internal resistance of the cells, but since they are usually high current cells, well, the current will be quite large (in this video 3S cells were used but the current was more than 10A). A lot of heat and the cells will suffer from it. This is why before parallelizing you should equalize the voltages. If you want to use DIFFERENT chemistries in parallel, well, don't do it \$\endgroup\$
    – frarugi87
    Commented Nov 21, 2017 at 10:27
  • \$\begingroup\$ @frarugi87 thanks for your comment. I exaggerated the numbers. More likely the cells will be around 4.3V and 4.2V. \$\endgroup\$
    – teepers
    Commented Nov 22, 2017 at 20:22
  • \$\begingroup\$ @BrianTipold My advice is never mix the chemistries; be sure what you are attaching and, if you have doubts, better avoid to have problems in the future (LiPo, LiIon, LiFePO, ...). If they have the same chemistry before putting them in parallel connect them through a resistor in order to slowly balance them. For instance, connect the grounds together and then the positives with a 10 ohm 3W resistor; this way the voltages will be the same; then put them together and treat them as one. It is best if they have the same wearing (don't mix new and old batteries, you can lower the life of them) \$\endgroup\$
    – frarugi87
    Commented Nov 23, 2017 at 12:43

1 Answer 1


Short answer: do not play matchmaker with Li-ion batteries.

1) As explained in the comments, parallel batteries with different voltages and low resistances will probably catch fire and certainly destroy one another if connected for any length of time. Do not do this. You will cause a fire.

2) Batteries at the same voltage with different capacities will have to remain at the same voltage all the time because they are in parallel. In a perfect world, they would drain according to their capacity, remaining at equal voltages the entire time. This is not a perfect world. Because of things like internal resistance and general errors (both age and manufacturing defects can mess with the batteries specs), there is a high chance of overheating. I would strongly advise against trying this. If you were really desperate, I would use diodes on the terminals of each battery to make sure that the current is always flowing in the correct direction (away from the batteries). The voltage across the diodes will increase if the batteries voltages diverge, but the diode will (hopefully) be able to handle it. I would choose diodes rated to at least half the voltage of the largest battery just to be safe.

3) Batteries in series are, actually, largely fine. The voltages will add. In terms of which ones would drain first, it would be a bit complicated, but you should stop using them after the first battery in the chain is drained. That means that if you have a 100 ah and a 200 ah in series, you would only get 100 ah total. Once the first battery is drained, the internal resistance will rise and it will start heating up. This can result in cell breaches and the like. I would strongly advise against using them after this point.

If you have any problems with any of the batteries, dispose of them safely. They are very dangerous and not something to be toyed with lightly. For god's sake don't touch the terminals together either or you will wreck them permanently.

  • \$\begingroup\$ Thank you for your concerns and advice. I'm not 100% ignorant and I exaggerated the numbers to make it easier to describe the effects. I'm planning on getting over 500 cells so that I can pick and choose which ones I put in the groups. In reality the difference between voltages would be something like 4.2-4.3 V, and capacities differing from 2000-2500 mAh. I'm testing all the batteries internal resistance, capacity, voltage and if they get warm I'm tossing them. \$\endgroup\$
    – teepers
    Commented Nov 21, 2017 at 17:13
  • \$\begingroup\$ I always write answers assuming the reader is 100% ignorant in case someone else stumbles upon this and is completely ignorant. By the way, the voltage difference won't matter. If there is a different number of cells (the only way to change the nominal voltage), the batteries will discharge and heat up until they eventually fail. \$\endgroup\$
    – Sam Spade
    Commented Nov 21, 2017 at 17:15
  • \$\begingroup\$ Thank you, it's a good thing to do. I'm not sure if I'm going to go through with this yet, but assuming I group properly, add short circuit protection, monitor temperatures and carry a fire extinguisher, do you forsee major problems? \$\endgroup\$
    – teepers
    Commented Nov 21, 2017 at 17:19
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    \$\begingroup\$ Thanks, i'm going to be charging this while awake, with a fire extinguisher in my house and strapped to my bike. \$\endgroup\$
    – teepers
    Commented Dec 17, 2017 at 19:24
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    \$\begingroup\$ From what I have learned a fire extinguisher won't stop this kind of fire. As soon as oxygen gets back it will fire back up. \$\endgroup\$
    – Lonnie
    Commented Jun 10, 2021 at 13:14

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