[jackdw]

After doing what I needed to ahead of schedule in my nearby city, I found myself wandering around, wasting 45 minutes until the bus home arrived, then I saw an electronics shop. I've wanted one of those mini fridges for ages but never saw a good deal. I went in, bought a mini fridge, got the bus home and plugged it in, it's great!

I went out to a festival a few days ago, brought the fridge along with it's car adaptor and left it on for a few hours running off the car battery. Luckily the car started.

How do you DIY a split charging system? Eg, a car battery say, in the boot, that charges from the engine, but doesn't drain the engine's battery when it runs flat.

Any ideas?

[StumpedTechy]

I don't know but I wish mfgrs of cars would get their act together and devise a smart car battery that cuts off supplying all electrical to any other part of the car once it drains close to where the car won't start on its own.

With the number of devices being powered by cars growing this problem can only get worse.

[Perfesser]

Check on J C Whitney (Autozone would probably have this, too) for a dual battery kit...it will be 2 heavy diodes on a big heatsink. I THINK the common side of the diodes will go to the alternator output and the ends of the diodes to the batteries. I haven't used one in a long time but they come with instructions. May also find this at a boat supply.

[jackdw]

Any ideas on how to make a home made one, or one out of cheap parts?
I'd rather not have a huge heatsink in the car...

[kiwiguy]

The problem with the dual diode arrangement using common cheap power diodes is that you "undercharge" both batteries. The 0.6v drop across the cheap silicon diode will lower the terminal voltage by that value, meaning neither battery will have much charge even though the alternator thinks they have.

Smarter systems do exist, using special (almost zero volt drop diodes) as above. Not cheap.

You can also do it using a relay that only connects the second battery when the key is on, (and also an isolating switch in the relay circuit otherwise a flat second battery will drain the good one from the moment you turn the key to start) but that essentially means you are charging 2 batteries is parallel, also not a good idea in practice. You need a relay + switch and a "low value resistance" (milliohms) in the charging circuit of the second battery to provide some small degree of isolation of current, to favour the main battery, as if you lose a cell in the secondary one the main one will discharge fast into it.

There have been many patents on reserve cell auto batteries that disconnect before you go flat, or maintain a switchable reserve but nobody is prepared to pay the price for the feature so they remain uncommon.

[jackdw]

Someone I know can do it very cheaply using wiring... Thing is, he lives a good 3-4 hours away and with the price of petrol as it is in England it's not worth it just for that.

Would a manual switch work, eg, turn it on when the 2nd battery needs to charge, and off when it's being used with the engine off?

[kiwiguy]

A switch would work, but again you end up with 2 batteries in parallel, which is not a "good idea" in any event.

Also imaging starting the vehicle with the switch accidentally "on", you could end up trying to extract a hundred amps through the switch from the secondary battery (if the main battery had minutely higher internal resistance. always a possibility), which would mean the switch would dissapear in a sheet of flame...

A 30 amp fuse would be essential, but then this could easily blow on charging current.

For safety's sake the best way is not the cheapest, but is depends of you consider the vehicle and it's electrical system "disposable" or not.

You used to be ale to but kitests here from an electronics store, but they seem to have disappeared now.

But a good detailed explanation is here.
http://www.geocities.com/harald_nancy/isolator.htm

But please, do not parallel lead acid batteries, but if you must, the details of the simple system is here:
http://www.geocities.com/harald_nancy/relay.htm

Even better, is where you can, install a separate alternator and keep the secondary system completely clear of the main battery. But that needs much under bonned mechanical work.

[jackdw]

Right, thanks for the links.

Looks hard, I'll speak to this person I know.

Thanks for your help all.

[kilowatt1]

Heres's what I would do. Buy a 12-volt deep cycle (marine) battery and a carrying case for it. Then I would go to an auto parts store and purchase a replacement cigarette lighter kit (or go to the local junk yard and find one)., a 30 amp in-line fuse kit, about 3 feet of #10 stranded copper wire, and a pair af alligator clips.

Drill a hole in the battery case and install the female end of the cigarette lighter in the case then connect the #10 wire to the lighter assembly and alligator clips, utilizing the in line fuse on the positive wire. Now when you wish to take the refrigderator with you all you have to do is set the battery next to the fridge, connect the alligator clips to the battery and plug in the adapter to the cigarette lighter. (This of course is assuming that the adapter for the fridge plugs into a cigarette lighter ).

When not in use, simply set the fridge and battery in your garage or wherever and connect the battery to a standard battery charger set to "trickle" charge to keep it ready for your next outing.

I have used this arrangement for fishing and camping trips and the marine battery has held a sufficient charge for up to two days depending on how often the refridgerator cycled on and off.

Just a thought.

Kilowatt

[jackdw]

That was something I was considering doing. This battery anyway, if it was car intergrated, would be portable. Just in case we were going somewhere where we couldn't park the car.

About this you suggest. I have a few old car batteries kicking about the garage, pretty good nick. I'll build a box out of wood and install the cigarette lighter etc (yes it does run off one), maybe two. If I put in two, would I need two 30 amp fuses?
Also, can household 30-amp fuses be used?

Thanks, I'm interested in this idea.

[kilowatt1]

Yes. I would recommend that each additional outlet utilize a fuse for safety sake.

[jackdw]

What cable though? House cable is 30 amp, I don;t have to use that do I?

What will do, desk light cable type stuff?

[kilowatt1]

No, you want have to use house cable. The auto parts store should have 30 amp cable or wire. If not, ask any electrician for a scrap piece of 30 amp stranded cable. You can also find it at home supply or hardware stores in the electrical section. The reason you want stranded copper conductors instead of solid is that the stranded is more flexible and will be more durable and easy to work with with this particular application. I wouldn't recommend anything smaller than 30 amp cable, although you might get by with 20 amp cable with a 20 amp fuse. The cable used for desk lights, etc. is usually only capable of or rated to carry a load less than 15 amps AC.

[jackdw]

I do have some house plug wire kicking about, I'll use that. It is however solid... I'll keep an eye out for standard but I don't think solid should be too much of a problem.

I will use 30amp. This sin't only for a fridge but who knows what else. Mini TV's, air pumps etc.

How about switch wise? I guess light switches aren't 30amp...

[kilowatt1]

You are correct. A standard single pole light swith is not rated for 30 amps. However, utilizing them probably will not be a problem with this setup. It sounds like a trip to a local auto salvage (junkyard) might be in order. You can probably get everything you need, especially a wiring harness which will have all the wires you need, along with switches, fuses etc. for very little or no money.