As the title suggests, I would like to repurpose an old ebike battery (36v) to be a supplementary car starter, which is a 12v system, but I would like the voltage at the car battery terminals to be 14v.

 

Is this doable? I would also like to have the battery as a backup ebike battery, so prefer not to make any permanent changes internally.

 

Any ideas about this are appreciated!

The type of battery intended for car starting have extremely high immediate current capacity .. they call it cranking power in the order of 300 to 500 amps. The liquid in lead acid batteries can absorb the heat gain over the few seconds. Lithium ion would not be as forgiving. Even were you to reconfigure the wiring to give you 3 X 4.2 or 4x4.2 , it would not be playing to the strengths of the battery technology. The excess heat,at start will progressively damage the cells.

AFAIK the newer purpose-made car jump-starters use Lithium Phosphate cells to deliver the high current burst required... 4 cells at 3.3v nominal in series to give 13.2v. The cells used are typically only 25-75Wh... not big at all, but just enough to provide that quick burst of energy. These jump-starters be bought from as little as £30 on ebay/Amazon. A Li-Ion celled e-bike battery - even if you could make it work - just wouldn't be suitable (IMHO) as Danidl has pointed out.

It,s simple. Your ebike battery can't deliver enough power (current) to operate your starter motor.

Thanks for the replies. So an ebike battery cannot safely produce enough current for this purpose, but what it might be able to do is charge the car's battery enough to enable it the start the car. Agree/disagree?

Just get a lithium jump starter for a few quid

 

http://www.autoexpress.co.uk/accessories-tyres/92728/best-mini-jump-starter-packs-2018

Thanks for the replies. So an ebike battery cannot safely produce enough current for this purpose, but what it might be able to do is charge the car's battery enough to enable it the start the car. Agree/disagree?

Yes of course. They are lighter and more energy conservation, so if the current is kept below 20 amps, they won't heat up much. . but it is now starting to look that you have a solution looking for a problem

Very dodgy to try and charge a car battery from a lithium one as you have no control over the current. It's clear that you don't understand how these things work and it''s fraught with risks and serious consequences. I'd forget the idea and get a proper jump-starter or a new battery.

E-bike lithium batteries have at times been described as "household bombs". They can violently explode and produce fierce fires if mistreated, so one should never take any risks with them.

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