12V charging question

[MogLite]

I've got an el-cheapo jumpstart pack at home. Its a useful piece of kit, but it isn't up to starting a big diesel, and I've seen them with inverters in them, which is a brilliant idea.

So I put some thought into building one, which is easy enough apart from the charging aspect.

My bought one, I can connect to a cigarette lighter to charge from 12V, which is a nice feature.

But assuming the vehicle battery is fully charged, and the jumpstart pack is pretty discharged - what stops 100's of amps flowing from the vehicle battery to the jumpstart pack ?

Is there some kind of 12V -> 12V trickle charger module I can build in ?

Thanks

[Scotian]

Unless there is some electrickery involved the flow of power will keep going untill both batteries are at the same level.

[FridgeFreezer]

I suppose an inline resistor of choice would do it, would need to be a chunky power one if you want a decent charge current. Assuming the battery would never be much flatter than 10v a 1 Ohm power resistor would limit the charge current to ~4A when the alt was kicking out 14v. 0.5 Ohm would give 8A which would be about the max a cig lighter socket would ever want to be drawing. There's probably a more elegant way of doing it, I'm sure an adult will be along soon.

[TheRecklessEngineer]

I expect you could come up with a circuit using a MOSFET, Op-amp of some sort and a couple of discretes...

In fact, how about gutting one of these optimate jobs?

[sotal]

I took mine apart so that I could charge the battery from my optimate and I wanted to bypass any charging circuitry.

They do have a little circuit, the one in mine was about an inch square circuit board, I seem to remember a diode and not much else.

I did realise once it was apart that the jump leads themselves were attached directly to the battery so to charge using an optimate or similar you can just connect the charger to the jump leads and turn the jump start on!

[MogLite]

I took mine apart so that I could charge the battery from my optimate and I wanted to bypass any charging circuitry.

They do have a little circuit, the one in mine was about an inch square circuit board, I seem to remember a diode and not much else.

I did realise once it was apart that the jump leads themselves were attached directly to the battery so to charge using an optimate or similar you can just connect the charger to the jump leads and turn the jump start on!

I think thats where I'm at - I think I'll rape my current jumpstart pack for the charging circuit and voltmeter etc.

I'm certainly not into/upto building MOSFET circuits

Andy

[sotal]

Something like this should do you:

http://ludens.cl/Electron/12Vcharger/12Vcharger.html

Nothing especially hard there

[sotal]

How about finding a cheap one of these:

http://www.vat19.com/dvds/easy-quick-jumpe...tart-cables.cfm

And using that as a quick charger?

[BogMonster]

I think I'll rape my current jumpstart pack

That conjures up an unpleasant image of somebody with their hair standing on end doing lots of screaming

[mike4444244]

might be why its called an inverter