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I recently bought some 12V lithium ion batteries off of Aliexpress. I probably should've paid more attention to the item description as I realised that the pack has internal circuitry which limits the discharge current to 4.8A and also protects the battery from short circuits. I need it to be capable for "short" (10 second) pulses of 30A. Can I just remove the circuitry in the pack and just wire the cells in series? If this isn't possible, are there 12V lithium ion batteries available which have no or very large discharge current limits?

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    \$\begingroup\$ "battery size: 18mm * 58mm * 65mm" "rated capacity: 9900mAh" - yeah right. \$\endgroup\$ Commented Feb 16, 2023 at 9:19
  • \$\begingroup\$ So choose a different chemistry - lead acid batteries are used in trucks where the starter motor needs 1000A or more. If you can't use a different chemistry build a power pack of your chosen 12V Li batteries and put as many needed in parallel so your target current can be achieved. Do use a suitable BMS for it though. \$\endgroup\$
    – Solar Mike
    Commented Feb 16, 2023 at 9:19
  • \$\begingroup\$ Yeah I normally use SLA but I was going for something more compact so I thought lithium ion would be the best option. Won't be able to make a power pack from the ones I've got since I'd need quite a few in parallel to reach my 30A target. Are there singular packs available which can supply high current and if so are they any smaller physically than SLA? \$\endgroup\$ Commented Feb 16, 2023 at 9:36
  • \$\begingroup\$ There's absolutely no way in the world those thin wires can sustain 30A for 10 seconds without the insulation melting right off. Same probably applies to the internals of the battery. Also what @BruceAbbott is hinting at is that the posted ratings are so obviously fake that there's little chance the cells themselves are up to the task. (They probably summed 3300 mAh times the three individual cells, even though connecting them in series doesn't let you do that. Even if the cells were really 3300 mAh, you would want to load them with nearly 10C discharge, which is ambitious at best.) \$\endgroup\$
    – TooTea
    Commented Feb 16, 2023 at 10:00
  • \$\begingroup\$ "Can I just remove the circuitry in the pack and just wire the cells in series?" - Absolutely not. Lithium-ion cells are dangerous without protection circuitry. \$\endgroup\$
    – marcelm
    Commented Feb 16, 2023 at 10:28

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I recently bought some 12V lithium ion batteries

The pack is made of three 18650 cells in series and claims 9900mAh capacity. There are no 9900mAh 18650 cells on the market (from memory the maximum should be around 3600mAh), so the capacity specification is either:

  • Sum of capacity of the three cells, which would be correct if they were in parallel, but they're not
  • Or just fake.

In any case, why buy this stuff?...

I need it to be capable for "short" (10 sec) pulses of 30A

"Lithium-ion" batteries come in many varieties and each is a compromise between safety, cost, weight, capacity, power...

"Li-Ion" batteries used in laptops, cellphones etc use chemistries optimized for capacity instead of high current (LiCoO2) so they won't work for your application. If you remove the BMS and exceed the maximum discharge current they will explode. The pack you bought probably uses this type of cells, most likely second hand or from the famous "Shenzen dumpster" source. Probably not safe either.

Some Lithium-Polymer (LiPo) cells can do what you want. These are common in RC toys, quadcopters, etc because LiPo is optimized for high current. Maximum discharge rate is usually expressed in "C" which is the capacity of the pack in Ah, so a for a 2Ah pack, 20C would mean 40A. However... while all lithium based chemistries have pyromaniac tendencies, some are more civilized than others, and LiPo really takes the crown of self-combustion. Not recommended indoors.

IMR/INR 18650 cells, often used in E-cigarettes and power tool batteries, can usually do 20A ; some will do 30A (per cell). They're more expensive and heavier than LiPolymer, but much safer. An e-cigarette is basically a circuit that dumps high power from a battery into a low value resistor, and that's pretty close to what you want to do.

Well, "safe" is a relative term with Lithium, if you put an IMR battery in your pocket with some keys or coins to short it you're going to have a very bad day. "Less unsafe" would be a better term.

If you have a 12V power tool with 5Ah or more lithium-ion batteries, that will also probably work, and you get the charger and protection circuits. I'm mentioning 5Ah because that usually means the packs are 3S2P, which means they'll do 30A. Lower capacity packs only have 3S1P so I doubt they'll do 30A.

The safest option would be a small lead acid battery, for example a model for scooters/motorcycles. A car battery would also work, but it's much heavier and more expensive for only 20 Amps. The only way to make an explosion with a lead acid battery is to overcharge it so much it electrolyzes the electrolyte into H2+O2, in a non-ventilated room, and then make a spark, boom. However one must be careful because they can definitely deliver enough current to melt wires and cause fires and burns.

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  • \$\begingroup\$ Right ok. Yeah I thought that advertised capacity sound too good to be true given that inside the pack is just standard 3.8V cells in series. From what you've written it sounds like SLA batteries may be better suited for the job and safer to work with (I think?). Thanks for the advice! \$\endgroup\$ Commented Feb 16, 2023 at 10:32
  • \$\begingroup\$ SLA should work absolutely fine, but big and heavy. Motorcycle battery should be less big and heavy, and if it can crank the starter motor, no worries about 30 Amps... \$\endgroup\$
    – bobflux
    Commented Feb 16, 2023 at 10:38

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