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Having two 3.7V "3000mAh" Rechargeable Li-ion batteries, I tested connecting them in series and parallel.

Using a multimeter, I measured the amperage of single batteries and it's about 6A. When connecting them in series, the voltage is doubled (it becomes 7.4V).

When connecting them in parallel, I expected that the amperage will be doubled, but it's not. It remains 6A.

Why does this happen? How can I increase the amperage using more batteries?

From what I know, when connecting batteries in series makes the voltage to be increased and when they are connected in parallel, the amperage is increased–but in my tests this last expectation didn't happen. Why?

This is how my circuit looks like:

My two batteries, in parallel, connected to ampermeter.

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    \$\begingroup\$ Did you know that it is VERY DANGEROUS to short circuit Li-Ion batteries ? And that you should not connect them in parallel without balancing them first ? And what is "Intensity ?" I think you mean "short circuit current". Perhaps you are lucky and the batteries have a short circuit protection circuit which limits the current to 6A. \$\endgroup\$ – Bimpelrekkie Sep 7 '15 at 9:22
  • \$\begingroup\$ Oh, good to know. :-) By Intesity I meant Amperage. \$\endgroup\$ – Ionică Bizău Sep 7 '15 at 9:39
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    \$\begingroup\$ As mentioned by others the cells need to be balanced, why this matters is that the cell with the higher voltage will carry the bulk of the load and the other will sit idle until the common terminal voltage falls below that ones voltage. The over current protection may have tripped already. \$\endgroup\$ – KalleMP Sep 7 '15 at 11:52
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    \$\begingroup\$ The short circuit current may be determined by the resistance of your meter's current measurement circuit, rather than by the current capability of the batteries. Measuring short-circuit current is rarely useful, as it gives no indication of the actual useful current capability at the desired voltage (and may damage the battery or other power source, or your meter.) \$\endgroup\$ – Peter Bennett Sep 7 '15 at 16:37
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    \$\begingroup\$ The balancing of lithium cells is done by smart chargers during charge. The charger monitors each cells voltage and charges the pack as the rate the slowest to charge cell dictates. If you parallel cells you may have to balance the cells at discharge time either with a small resistance on each or some kind of current mirror circuit (more complex) so they discharge evenly. \$\endgroup\$ – KalleMP Sep 9 '15 at 16:30
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As the comment mentions, paralleling and short circuiting lithium batteries is potentially very dangerous if you don't know exactly what you are doing. Most Li-Ion batteries have a protection circuit which includes overvoltage and overcurrent protection, but it's still not a good idea at all. Much better would be to test this using a couple of small alkaline batteries (e.g. AAA) and still do it briefly.

If you absolutely must test the current on the Li-ions, then read the datasheet, see what the maximum current rating is and for how long, then use external current limiting on each battery.

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  • \$\begingroup\$ Thanks. However, I see exactly the same behavior in case of connecting in parallel two small solar panels. Isn't it supposed to increase the amperage? Also, how to increase the amperage using Li-ions batteries? \$\endgroup\$ – Ionică Bizău Sep 7 '15 at 9:41
  • \$\begingroup\$ Yes the short circuit current should increase if the cells are paralleled. Could you post exactly how you are connecting everything? (including multimeter) \$\endgroup\$ – Oli Glaser Sep 7 '15 at 9:44
  • \$\begingroup\$ I drew a diagram how my batteries are connected. I tried the same with my solar cells. Shouldn't the amperage be increased in case of Li-ions batteries too? Also, what means dangerous in this case? :-) \$\endgroup\$ – Ionică Bizău Sep 7 '15 at 9:54
  • \$\begingroup\$ Yes batteries should also increase current when paralleling (in theory - your circuit resistance (contacts, wires) may be limiting current, you need less than ~0.3 Ohms for 12A at 3.7V) . Dangerous is short circuiting Li-ion batteries at all, I would strongly advise against this. Usually the max discharge current is around 1-2C (C being the capacity of the battery, e.g if the battery is 800mA capacity, then between 800-1600mA max. Some batteries like those designed for RC vehicles are built for faster discharge) I would test with smaller, higher internal resistance batteries as mentioned. \$\endgroup\$ – Oli Glaser Sep 7 '15 at 10:08
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Look, let's get one thing straight: V=IR. If you take two "ideal" 3.5V batteries and put them in parallel, what you read across the parallel pair is still 3.5V. If you put that voltage across a 1000 Ohm resistor, you'll get 3.5mA - the same as you'd get with one cell alone! That would be true no matter how many 3.5 volt batteries you put across the parallel bundle. If you try to "burst" the current by shorting + and - terminals, the current you get depends on a whole bunch of stuff: internal resistance, battery chemistry, the rearrangement of charges in the battery electrolyte, temperature change, etc. And, as was pointed out, it's never a good idea to just short the terminals out, particularly in a lithium battery.

What does change when you put the cells in parallel is the amount of energy stored in the whole system. This is frequently expressed in Watt.Hours (or, if you know the open circuit voltage of the cell, Amp.Hours.) So in an ideal case, doubling the cell volume (putting two cells in parallel) could double the Amp.Hours, NOT Amps!

That is, there'd be twice the energy stored in total system. You'd get the same current for a longer time. But even this may not be quite true. Nothing is ideal! If the open circuit voltages and internal resistances of the cells you put in parallel were different, you may stress out one battery over another (as pointed out elsewhere) and that battery may not supply the same amount of energy as the one next to it. Quite simply, the "stronger" battery might pump current into the adjacent battery and rob the load of useful energy in the long run. I know a lot of other people responding were driving at this. I thought I'd just put it in a more physics/chemistry setting.

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  • \$\begingroup\$ I think this answer answers OP's question. Capacity versus current. \$\endgroup\$ – Oskar Skog Mar 9 '17 at 14:19
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Many very helpful pointers that this is a bad idea, but I haven't seen anyone actually answer your question.

Putting two voltage sources in parallel does not increase amperage in the circuit. Ohm's law tells us V=IR, so the only way to increase current is to increase voltage, or decrease resistance. Putting two voltage sources in parallel doesn't do either of those things.

If you want to double the current by adding batteries, you put them in series. This will double the voltage (approximately) and leave the resistance (nearly) unchanged, so the current will double. However, in your case, you're SHORTING the batteries, so doubling the current is an even worse idea, because it's just way too much current for those batteries.

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    \$\begingroup\$ He's changing the "R" in this case, by halving the internal resistance of the battery by putting two in parallel. It absolutely should increase current available, assuming the short circuit is a lower resistance than the battery's internal voltage. \$\endgroup\$ – Bryan Boettcher Dec 2 '15 at 22:00
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    \$\begingroup\$ uhm, nothing is an ideal voltage source. each of those batteries have to some degree, a Thevinin or Norton equivalent, with a characteristic internal resistance. putting two identical Thevinin equivalent voltage sources in parallel should theoretically add their currents if the load resistance is much lower than the internal resistance of the sources. \$\endgroup\$ – robert bristow-johnson Dec 3 '15 at 1:00
  • \$\begingroup\$ Both of your points are well taken. I didn't mention internal resistance because it's really only relevant to the question if the internal resistance is significant compared to the load. Obviously in this case of a short circuit it is, but I'm assuming in most realistic cases it is not. I was assuming the OP was asking a general question about connecting batteries in parallel to multiply the current in a sizable load, analogous to multiplying the voltage by putting them in series, which I wanted to clarify. But I may have been overgeneralizing the question. \$\endgroup\$ – brianmearns Dec 3 '15 at 1:21
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Your ampmeter could not possibly read 6amp on a 3.7 3amp battery. "Having two 3.7V "3000mAh" "

So if you parallel 2 3amp will result in 6amp.

The best way to check for sure is to discharge the 2 paralleled batteries and measure their discharged amp capacity from 3.7v to their cut off minimal voltage, normally 3v.

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    \$\begingroup\$ You are confusing two different ratings. Amp hours are not amps. The peak current capability of cell is distinct from its product of current and time rating. Some types of cells do better under high discharge than others, but to know specifically you have see a rating which has not been given. Also realize that at least half of the issue in the question is the severe misuse of an ammeter by the poster. \$\endgroup\$ – Chris Stratton Oct 3 '16 at 0:24

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