On Monday, 8 May 2017 14:48:47 UTC+1, David WE Roberts (Google) wrote:
I am getting to the point where it seems a cunning plan isn't.

We have a caravan with a leisure battery.

We also have a spare leisure battery which gives us the option to carry it
for extra capacity and possibly use it with an inverter to power some low
demand 240v devices.

It is a few years old and doesn't match the new one in the caravan so just
linking the two together may not be the best option.

they have different offload voltages?

I thought I might be able to trickle charge it from a 12v socket in the
car for the times when we will not have an Electrical Hook Up (EHU)
available over night.

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

However, although there are DC-DC chargers these seem to start around £70
which is not far off the price of just buying another leisure battery and
fitting it to the caravan.

Does anyone know of a cheap but effective DC-DC charger?

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.

To make it easy to use it needs to work off a cigar lighter socket; I know
I would probably get far superior charging by fitting a charge controller
to the output of the car alternator to feed the battery but this looks to
be in the £200+ price range.

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

Just looking for a cheap and simple way to use the spare battery without
any major electrical surgery to the car or caravan.

Charge it when the car runs, not when it isn't. The latter gains you nothing.


NT