18650 cells in series safely?

I got a hold of some pretty high quality genuine Sony Murata 18650 cells for a project. I know each cell has a 30A max output capability but will the cells be damaged if I run them in series of 3 for just over 12V charged and then pull 70ish amps through all of them simultaneously? Or will the resistance of each cell cause overheating?

I'm hoping to run an inverter on them and I figured 4.2V x 3 = 12.6V with 90A so 1134W then I thought no, that's stupid, it's probably 4.2V per cell @ 30A = 126 watts per cell times 3 = 378 watts max. So let me know if that second one is more accurate and if it's safe to run that many amps through each cell if they're in series of three. I have enough cells to make 3-4 parallel lines of 3 in series for a 400W inverter if that's safer.

series increases voltage not amps. 3s1p is only 30amps not 90amps

^^^ This.

goatman said:

series increases voltage not amps. 3s1p is only 30amps not 90amps

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Except if you're talking about batteries. Each battery cannot output more than 30 amps. So if I put 3 in a row, I can draw 90 amps from each battery in the series. So then I'd be drawing 30 amps from the first battery, 30 amps from the 2nd battery, and 30 amps from the third battery so the last battery in the chain would have the 60 extra amps running through it. I just want to make sure that wouldn't destroy the batteries.

I bet you if I put 12 batteries in series and hooked up a 48V inverter then turned on a 1500 watt space heater on it, the last battery in the series would burst into flames by having the 360 amps from the other batteries running through it. Or would it? Do they have such low resistance and high effective "wire gauge" in the housing that it'd pass over just fine?

If you put three 30A cells in series, you will get 12V at 30A. If you put three 30A cells in parallel, you will get 4V at 90A.

If you put 9 cells in a 3S / 3P configuration, you will get 12V at 90A

lvl99MemeWizard said:

Except if you're talking about batteries. Each battery cannot output more than 30 amps. So if I put 3 in a row, I can draw 90 amps from each battery in the series. So then I'd be drawing 30 amps from the first battery, 30 amps from the 2nd battery, and 30 amps from the third battery so the last battery in the chain would have the 60 extra amps running through it. I just want to make sure that wouldn't destroy the batteries.

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What did you think he was talking about? Of course batteries.

I'd listen to what the other two have told you, or you're in for a bad time.

I just realized the change in the voltage represents the increase in energy and there's no way to add a battery without doubling the voltage which negates what would be an amp increase because the device cares about voltage so that does make sense. Grrrr so now i need 3 parallel circuits because the overhead on my inverter is so crap

lvl99MemeWizard said:

If you have a 30A capable battery at 4.2V and you're running a circuit to a device that, according to its resistance and overall design, should draw 100 amps at that voltage like a power tool, it won't. It would draw 30 because the battery can only do 30.

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A battery (cell in this case) that's rated for 30A doesn't limit current consumption to 30A. That's just the rated maximum current you can draw from it, but it's resistance will be low enough that you can actually draw more than 30A.

lvl99MemeWizard said:

The limiting factor is the battery. So if you add a battery in series, both of which can do 30 amps, and the device you're powering still wants to operate at 100 amps, it should draw 60 because in reality it wants 100 anyway and is being limited by the batteries. How would each battery now somehow magically only do 15 amps each?

So yeah boring, basic ohm's law circuit design says series != more amps but if you're at the limit of amps and you add a battery, tada, there's more amps assuming the load wanted to draw more in the first place, right?

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The same current flows through all devices in series circuit. Adding more cells in series doesn't make each single cell capable of putting out more current. What does change is that the battery voltage is higher now, and if the same load resistance was connected to it, the current draw would be higher. You'd still have to find some way to limit the current draw to 30A if that's what your cells are rated for.

Ohhhhh yeah doubling the voltage and keeping the amps the same IS double the "electricity" and the extra wattage from the 2nd battery goes. Now I get it. Electronics 1 in high school was a looong time ago lol.