Charging 12v battery from 12v cigar lighter socket?

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.
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.
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?
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.
Just looking for a cheap and simple way to use the spare battery without any major electrical surgery to the car or caravan.
Cheers
Dave R
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On Monday, 8 May 2017 14:48:47 UTC+1, David WE Roberts (Google) wrote:

t

t

they have different offload voltages?

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

?70

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.

w

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
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<ssnip>

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

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On Mon, 08 May 2017 22:11:05 +0100, bert wrote:

Does this work directly off the alternator, or via a 12v cigar lighter socket?
The requirement is for something cheap and simple which does not involve any extra wiring in the car.
I realise the output from a 12v socket will not have the same characteristics as a direct feed from the alternator via a charge controller.
Cheers
Dave R
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David wrote:

The only difference will be due to wiring and fuse resistance in the feed to to the 12v socket.
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David brought next idea :

It would provide a more rapid and consistent charge for a second battery. If you tow the caravan and have the 12S socket or the fully wired 15(?) pin socket, then you should have a voltage controlled relay already system wired up in boot.
I have and tapped off that with a none reversable plug and socket, to allow a battery to be charged in the boot in emergencies.
The problem with using lighter sockets is they are current limited, there will be lots of voltage dropped in the wiring and the connections are just not reliable at the plug and socket.
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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.
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You need well above the nominal battery voltage to charge it fully.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Er, yes, there is presumably something charging the 'first' battery so it will charge the added parallel one as well.
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Chris Green
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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.
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Andrew Gabriel
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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!).
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Pretty low demand pushing up the price, I'd say.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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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
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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.
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On Tue, 09 May 2017 09:53:30 +0100, Harry Bloomfield

*Years ago*, Dad bought a portable battery system that consisted of 2 x 6V lead acid batteries in a fibreglass carry case that strapped into the boot of the car and took power from the car 12V system when the engine was running.
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
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T i m wrote:

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:
http://www.12voltplanet.co.uk/split-charging.html
But cars and life seem to have become more complicated these days.
Chris
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wrote:

There aren't many of us left. ;-)

I think there were several such solutions as these things evolved.

Car and motorbikes ... and why I keep my kitcar based on a 1978 Ford Escort and whilst I don't think any of my bikes are quite old enough to have points, none of them have CAN busses and the like. ;-)
It's funny, we had the 2L GL Sierra Estate for 23 years in the end and in all that time the only things that failed were a window winder wheel runner (broke in the icy weather), a brake caliper seized slightly and the cambelt broke (safe engine and I had replaced in within the hour).
Oh, the clutch cable snapped but I carried a spare (like you did in those days) and I changed it at the side of the road using no more than my Leatherman PST II. ;-)
Not quite so easy to change a concentric hydraulic slave cylinder and unlike the Rover 218SD or the Meriva, on the Sierra I never had any issues with the aircon, central locking, immobiliser, the ECU or electric windows because it didn't have them (and somehow we used it everywhere for everything) and to have a new key cut cost £5. ;-)
Cheers, T i m
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But then there were the new plugs, plug leads, points, rotor arm, dizzy cap etc at regular intervals - and checking the ignition timing too. Then antifreeze each winter and replacing a few hoses too. To say nothing about 5000 mile oil changes. And then the new engine at perhaps 50,000 miles...
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On Thursday, 11 May 2017 15:32:04 UTC+1, Dave Plowman (News) wrote:

t
and wondering why the timing light showed the sparks scattered all over abo ut 1/4 of the flywheel.
NT
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