Charging leisure battery from 12v socket

[daldroflas]

First post here, so first of all, Hello!

I have a 1.6D Desire Multispace 06 and need to top up the charge on a 12v 74Ah leisure battery whilst camping.
The battery will be fully charged before the trip begins and should not be fully discharged each day (hopefully).

I want to be able to connect the battery to the 12v internal socket and charge it back up as we drive around during the day.
It has a proper battery box with a lid.

The plan is to place it in the rear seat foot well and connect it to the 12v socket via a male to male 12v socket connector and the crocodile clips/female 12v socket on the battery. This would be done once the Berlingo has been started and disconnected when the engine is not running.

Is this likely to blow any fuses or cause damage?

Can you buy a 12v powered leisure battery charger?

I have Googled a lot and have found a lot of different opinions of charging leisure batteries on the move.

Some suggest using a split charge relay to prevent flattening batteries but I cannot find any for sale that use a 12v 'cigarette lighter' type socket. Does anyone know where these can be bought?

Others suggest using a 12v to 240v inverter to enable the use of the 240v smart battery charger (which I already have) to charge a 12v battery, but I am not going there!

Any helpful advice welcomed.

[rdfb2]

    this might help , i tryed doing itwithout split relay and blew fuse every time u started up .

[ffrenchie]

We use a 12v Machine Mart jump battery for camping and it has a charger from the mains and a 12v one for use on the car accessory socket. Maplin do a car to car charger/jump starter for £19.99 you might be able to adapt for your purpose.

[daldroflas]

this might help , i tryed doing itwithout split relay and blew fuse every time u started up .

Thanks for the heads up rdfb2.
The last thing I want is to keep blowing fuses so your info is valuable.

---

We use a 12v Machine Mart jump battery for camping and it has a charger from the mains and a 12v one for use on the car accessory socket. Maplin do a car to car charger/jump starter for £19.99 you might be able to adapt for your purpose.

Thanks ffrenchie. I have a similar jump battery device which I bought from Maplins a few years ago and could never get to supply a decent amount of charge or recharge properly from the car. Maybe I messed the battery in it up by charging and discharging it wrongly. I'm glad yours works well!
I might dig it out and try charging it up again.

That car to car charger device from Maplins looks like it could be just the ticket. I'll definitely look into that and might just buy one tomorrow.

[daldroflas]

The charge rate issue is something that concerns me after initially thinking the Maplins car to car charger would be ideal. It might deliver too much for a leisure battery.

I'll be giving Maplins a visit today to ask a few questions. They might have a solution. Either an easy to fit switch relay or a 12v power source smart charger.

[TonyM]

Thanks ffrenchie. I have a similar jump battery device which I bought from Maplins a few years ago and could never get to supply a decent amount of charge or recharge properly from the car. Maybe I messed the battery in it up by charging and discharging it wrongly.

I've got the Clarke Jumpstart 4000 & whilst it can be charged from a 12 volt socket it says in the instructions that it will shorten the life of the battery as the 12 volt supply can only charge the battery to about 50% of its capacity ,could well be the the same for the Maplin model.

[mikeB]

I had this problem with a caravan leisure battery I kept in ther boot of ther car so it could be charged up during the day. I ended up with a split charge relay connected directly to the main car busbar. It worked of course but the problem is that after a few minutes charging, the terminal voltage of the leisure battery rises and therefore the charge current falls, typically to about half an amp. So, to put back the 10 ampere hours you discharged last night you have to drive around for 20 hours. Clearly not a practical solution.

In my view the ONLY easy way is to buy a 12v to 240v inverter. These are cheap now and connect directly to the cigar lighter socket. They are usually rated at about 200 watts AC output so draw about 20amps from the accessory socket when running full power. However, with a battery charger you will almost certainly not be drawing more than 100 watts or so.

There are other advantages to this approach. You can use a smart battery charger, or a higher capacity charger, or even make your own to maintain a higher output voltage (say 15v) which is what is needed for short periods of charging. This is effectively what smart chargers do. Another advantage is that you can use the inverter for other purposes like charging your laptop, shaving, phone charging etc.

Machine Mart have them in stock and they work well and are not expensive. Give it a try.

[daldroflas]

Thanks mikeb and Tonym, I am checking out some inverters now.

---

My Ring SmartCharge+4 RSC504 is rated at 240V 0.8A input so this means a rating of 192W would be needed on any 12 - 240V inverter I think?

Output is 12V 4A max so 48W.

[daldroflas]

I have just ordered a Bestek 300W inverter from Amazon.
I'll post an update on how it performs.

Bestek 300W inverter at Amazon UK

[mikeB]

That sounds like a suitable inverter. I don't think you need to worry too much about high charge rates. If the leisure battery is, say, half flat (or half charged, depending on your outlook in life!) it will, when initially connected to a standard car charger, charge at about 4 to 8 amps, depending on the capacity of the charger. As the battery terminal voltage rises, and this will happen within a few minutes, the charge rate will fall, probably to 3 to 5 amps. Most inverters will handle this. The 300 watt version you have ordered certainly will. If there is a problem getting enough current out of your cigar socket then you may have to connect directly to the car battery, but this is unlikely to be necessary. The Berlingo socket has a 20 amp fuse.
In my experence these inverters are close to 50% efficient so, if you pump 5 amps into your 14 volt (not 12 volts while it's being charged) leisure battery, that's 70 watts. Your inverter needs to produce 70 watts but needs 140 watts from your 12 volt socket (because it's only 50% efficient), i.e. about 12 amps. So 12 volt connections need to be of good quality. If you are driving with the leisure battery in the car then you need not worry about overcharging. The battery will effectively look after itself. Every hour you are connected will put a lot back into the battery. BUT don't forget to disconnect when the engine is not running or you may end up with a flat car battery.

---