ALi-B

iTrader: (1)

I'm off on one of my projects again. This time its charging a 12v battery off a 12v car battery...WITHOUT the engine running.

Now I need 14 to 16 volts to do this properly. So the only way to do this is to up the supply voltage.

Basically all I want to do is top up my 100Ah portable bbq meats/beer -fridge battery so it'll last another day (currently get about 20hours out of it) without running an engine. Idea being to suck power off the car battery, which will then be recharged when we go out for a drive.

Problem here is I'd like to pull about 20 to 40 amps (300 to 600watts) to make it worthwhile (car has a 100Ah battery, so I could safely pull 40amps off it for an hour or two and still be able to start it, and give me over 12hours extra running time on the fridge (assuming 80% efficiency) .

This is where I'm struggling...I'm willing to throw £100-150 at this but it appears there is nothing out there that doesn't involve some self-assembly. Basically all I want is box with power connectors, but I can't find much in the UK that'll do it with that kind of voltage and current output.

Any ideas?

David Lock

Ask the guys at http://www.mdsbattery.co.uk ??

Or stop drinking

dl

scoobyster

Wouldn't you be better off bypassing the fridge's battery and powering directly from the car battery? As I understand it these 12v to ~19v dc laptop car chargers are some kind of chip based witch craft since you can't increase dc voltage with a transformer. I doubt you'll find chips of this nature that will handle the current you're looking for. Perhaps you could do it with a chunky inverter to 230v ac then transform back to 16v dc, but it won't be very efficient. Best of luck anyway.

ALi-B

iTrader: (1)

Thats what I did last year, but the fridge would have to be in reach of the car. And I have to use jumper leads to the battery as the cigar lighter only works with the ignition (which drains the car battery even faster due to the body electricals, plus insurance problems with leaving the keys in too etc).
And the fridge is heavier than the battery due to all the beer in it (although its manageable by the second day ).

You can get over 13volts DC (as high as you like) from a car battery with some magic; thats the dc-dc converter stuff - thats how most decent big wattage car audio amplifiers work (they step-up 12vDC to a +/-35v DC supply which allows teh amp to give out 70v peak to peak at the speakers ). But most things I've found are either low current, industrial/custom built to order or control chip minus the rest of the circuitry.

Already tried the inverter+battery charger...too much bulk and none of my chargers give out their rated output on a inverter power supply (i.e my 15amp charger only gives out 8 amps, don't know why, probably something to do with the battery charger's transformer not liking the inverter's simulated sine wave or something).

Generator was an idea...but all generators I've seen in the price range that have a 12volt (well, 13.8volt ) output only gives 8.3amps...useless and a waste of petrol....I'd be better off using a £20 15volt laptop car charger (apart from I've no idea if they have any current limiter as the current draw may fry it).

chris666

Why not fit a 100/120ah leisure battery in the boot & charge with a simple
split charger
then run the fridge from that
thats what ive got in my van also powers a 3000w inverter
when the engine is switched off the split charge is disconected so you cannot drain the car battery

simple split charge

JackClark

I tried finding similar so that I could charge my Macbook using the inflight adaptor. Couldn't find anything that was practical of within my price range.

legb4rsk

I don't know the technical bits but I used to have a smallish speed boat that had 2 batteries that could be switched over via big red switch. This was so you could charge either & for safety so you could use the other one to start the engine if one failed. It might be worth looking at a boat accessory place.