Charging 12v battery from 12v cigar lighter socket?

they have different offload voltages?

you could with a dc-dc convertor, but I'm not seeing any point in doing so.

you need to check out ebay & iffy chinese sites. A few quid should get you a low power switching regulator. But I still don't see what it would gain.

alternators have charge controllers built in - they're for wet cells of cou rse, so would want voltage dropping some. I presume that could be done with a diode or 2.

Charge it when the car runs, not when it isn't. The latter gains you nothin g.

NT

If they're both the same technology (i.e. lead acid, liquid electrolyte - or - lead acid, gel - or - whatever) then I don't really see any problem with simply parallelling them to get the 'extra' one charged. It would probably be worth disconnecting it if leaving long term.

Pretty low demand pushing up the price, I'd say.

You need well above the nominal battery voltage to charge it fully.

Er, yes, there is presumably something charging the 'first' battery so it will charge the added parallel one as well.

Yes, I am intending to charge it when the car is running; I don't want to flatten the car battery.

IME, cigar lighter sockets are hopeless for long duration current rating (anything over a couple of amps). The contact resistance is too high and they overheat, because they don't have contacts with large enough surface contact and high enough pressure.

Quite agree, it was a very silly idea to try and use them as power connectors. I wasn't suggesting that above, just that parallelling the two batteries would probably work fine (but best not to use the cigar lighter socket!).

In article , David writes

Then jut connect up a voltage sensitive relay and off you go. That's all the caravan battery has. The regulation system on the alternator will prevent overcharging. When you switch off the engine the relay will ensure the two batteries are not connected in parallel.

I'd have thought this could be done simply by just having a switch to charge one or the other of them. Don't like the idea of having a charging battery in the passenger compartment withou ventilation.

The lighter is not going to sink the sort of current you need to charge it really, and yes you could current limit this by a simple circuit (Look in many of them old Bibani Books!)

I'm sure little modules exist for this nowadays.

Brian

on 09/05/2017, Brian Gaff supposed :

It may not charge at full speed, but charge it would.

No need, the resistance of the wiring and the small voltage difference between vehicle battery and car battery will provide an effective limit to the charge current.

I would no want a battery on charge in the passenger compartment.

There was some form of electronic controller you fitted inside the boot that would charge each battery independently and then when you wanted 12V in say a tent, you unplugged it, lifted it out and flicked a big rocker switch from 'Charge' to '12V'.

If you needed a charge you just put it back in the car when you were out and about, or using a 12V mains charger of course.

Before campsites offered mains hookups, most places would allow you to charge such things in their garage / store for a few pence. Not sure 'the rules' would allow them to do such these days?

Cheers, T i m

I am ancient enough to have installed and used a similar system myself.

The caravan was quite advanced, and came with its battery in a box with an external socket, to match that on the van. You could then fit a mating connector in the car boot to top up when you went for a run.

You used to be able to fit a simple split charge relay, which is amongst the methods described here:

But cars and life seem to have become more complicated these days.

Chris

Our Touareg has the battery under the passenger front seat.

Our Hymer motor home had the leisure battery under the driver's seat.

Our caravan has the battery under a seat in the lounge area.

All of them are charged whilst people are around.

I fail to see the problem, frankly.

Cheers

Dave R

Charging regulation failed in one of my history pieces. The first I knew of it was when the interior suddenly filled with choking sulphuric acid fumes . In the space of under a second I couldn't breathe at all. I clamped ancho rs and leapt out. It had no effective bulkhead. Maybe if it had had fusing (it had none anywhere) that could have prevented it. If it had had an ammet er I might have noticed something wrong.

Modern vehicles are different of course. Interior batteries will either be boxed & vented to the outside, or monitored in various ways to detect any m alfunction & shut down - I presume.

NT

David used his keyboard to write :

Batteries under charge vent a mix of H and O, the two mixed are very, very explosive pair of gases. I would hope that any vents from those internally installed batteries are safely piped out side. My caravan has its battery under a seat in the lounge, but it is safely contained in a cabinet sealed from the living space and only accessible plus vented to the outside.

My old Beetle had its battery under the rear seat. No venting and only minimal protection for the terminals.

Caravans used to have the battery in the drawbar locker, along with the gas cylinders. Times have changed.

Chris

Vented

Vented?

Old Land rovers had the fuel tank under the driver's seat.