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Connect 18650 to car battery in series?


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Hey guys,

This is kinda random but my car wasn't starting earlier and I didn't have another car to jumpstart from so I decided to try connect a 12V gel battery that has been lying around for a few months, but the car still would not start.

Figured the only thing to do is to just leave the jumper cables connected and wait for the batteries to slowly equalize over the next several hours.

That got me wondering though, would it be possible to take a (or a bunch of) 18650 batteries and connect that in series with the gel battery to get the voltage up to +/- 15V in order to vastly accelerate the charging/equalizing process?

Obviously I did not try that out of fear that it would lead to a lithium bomb type scenario...

This should theoretically be possible though right?

Then the question is how many paralleled 18650s would be needed to be safe?

And finally, do you think it would be possible to connect the 18650s to the gel battery in that fashion and immediately start the car before the 18650s drain into the car battery?

Edited by tetrasection
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16 hours ago, tetrasection said:

I decided to try connect a 12V gel battery that has been lying around for a few months, but the car still would not start.

Unless your car is an electric vehicle, the starting current is of the order of 200 A. A smallish gel battery isn't going to be able to provide that sort of current. It might have enough energy (if nearly full) to put a surface charge on the car's 12 V battery. But if the car's battery is damaged or very low in charge, that won't work.

I really don't like the idea of the gel battery and/or 18650 cells; just get yourself a proper car battery charger. Just a ≈6A one should be fine, though again it might take some time (say half an hour) to put enough charge into the car battery that it will be able to start.

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What @Coulomb said. With the currents involved here, you could easily reverse the polarity of the 18650 cells in series with another battery, which would lead to a very dangerous situation. It is best to jump your car from another car or car battery that is charged.

On a related note - jumpers are usually designed to have a relatively high impedance. That is to say you won't start a car without a battery with jumpers bought at a store. That is intentional - it is a safety feature because a high percentage of people get the polarity wrong, and also you actually only want to boost the car battery. If your battery is sufficiently flat, you won't be able to jump it, unless you have a set of jumpers made from 16mm² or better cables.

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Taking highly flammable batteries (Li-Ion) without protection and then connecting them to anything is reckless.

That said, there is channel where a guy brilliantly tests a bunch of jump starter packs.  What I've learned from that is, Lithium batteries far out perform lead acid in starting applications.  The small little jump starter packs were easily able to provide far more amps than the equivalent lead acid intended for the application.


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