How do portable car-battery boosters work?

They all lie about the 1000+ amp capacity, but they can supply into the hundreds of amps, some of them slowly charge a capacitor from the battery, then dump that over a short period.

No magic; nothing new under the sun. You're basically exchanging a long time charging at a low current for a short time discharging at a high current. Capacitors can do this as well. I have one of the most powerful jump-start packs just to be on the safe side. It can crank over big American V8 engines faster than most lead acid batteries can, but whereas the lead acid battery can keep chugging for several minutes if rested at intervals, my jump pack will only manage about half a dozen spin-ups before it needs a recharge. And giving it a rest between starts won't extend that.

It's not a few Amps but hundreds of Amps for a very short period.

Bigclive shorts out a battery pack and then takes it apart afterwards and finds out why it can supply 100s of Amps

It cannot. a typical lithium 20 AH cell will easily deliver 400A

Just not for long...

The current divided by the Ah rating is known as the 'C' value . When I first started using cells 10C was pretty good but the cells lasted longer at 5C...today 20 years on there are up to 140C batteries ...

Few hundred A , yes. But limited to only around 11V typically

See above. Up to half a minute typically. Most alternators can deliver well over 50A charge current. And will, to a flat battery

If from a reputable manufacturer, these things are FA. (f****ng amazing)

This is probably worth risking money on

No. they do not. The peak discharge rate of Lithium batteries has taken leaps and bounds. That is why they can so easily catch fire if shorted.

A ten ampere hour battery should these days be easily capable of 1000A.

I'm sure some batteries can, but none of the jump starters tested met their claimed current, best was 700A of a claimed 3000A

At what voltage? The best do not claim 7000A . But 1000A is very achievable at 10V or so

The flip side of the jump pack being able to deliver a lot of current quickly, is that lead acid batteries can't accept current quickly.

If the lead acid was flat and you connect another battery across it, if the lead acid could accept current at the same rate as the lithium cell, suddenly there would be a big drain on the lithium pack and it would be flat before you got in the car to turn the key.

But what happens when you apply let's say 14V across a battery at 11V is it only charges at a few amps, which means that most of the current of the lithium pack remains available to crank the engine. Once the engine is started the alternator provides enough current to run the car's electrical loads and to slow-charge the 12V lead acid. The same happens with a lead acid jump start.

If cars had lithium batteries in them instead of lead acids, jumping them would be more difficult because you'd suck a lot more current out of the jump battery, potentially making the jumping car unable to run.

(some EVs do have lithium 12V batteries, and I believe there's special arrangements for jumping those)

Theo

Go and study some electrical theory.

Lead acid batteries can accept hundreds of amps. If they can deliver it, they can accept it.

Lithium cells are typically 12.4V absolute max whereas to fully charge a lead acid it takes around 14.4V. Ergo the lead acid cannot drain the lithium

The highest voltage I remember hearing was 16V, but didn't sustain that for the freezing cold test.

That would be 4 lithium cells in series...Not a good source of charge for a car battery

Thank you for that explanation - makes sense to me.

it was a sodium one, which I think are slightly lower per-cell than lithium?

Pity it is mostly bollocks though

Ah. All bets are off. no practical experience of those

A 12V LiFePO4 would be almost flat at 12.4V

In message <10oh8hh$1gu9r$ snipped-for-privacy@dont-email.me, at 13:17:37 on Sat, 7 Mar

I have a jump-start battery, made for Stanley, and in reassuringly bright yellow. It's worked fine the few times I've needed it. Spec is:

Lithium Polymer, 7,200mAh/26.64Wh. 700A peak, 350A cranking.

It charges using a 2A USB-Micro-B, and has secondary outputs of

Fair point (I got the voltages wrong above) - but the idea stands that the chemistry differences means the lithium pack is not immediately sucked dry by charging the lead acid. That means it's able to hold up while running the starter motor (even if it drops to eg 3.6V per cell = 10.8V for the 3S pack) which the flat lead acid won't do. So you can put it across a flat battery and start the car without having to disconnect the lead acid from the system.

Theo