Seeking Dual Car Battery Installation Advice

Your best source for wire, connectors, fuse holders, etc. would be to maybe buy one of those amplifier installation kits. Get the heaviest duty one you can get.

a 30 Amp fuse seems to be a little on the small size as does the 10 gage wiring.

Check with a RV Shop on wiring size !!! or else use the same gage as the battery cables already on the car.

the 30 Amp fuse might be suitable for your load but not for the charging side of the circuit.

I am a little suspicious of a Dual Battery setup without Isolation Diodes.

Usually you would expect to be able to Start the engine using the auxillary Battery in an emergency, hence the need for heavier cables than 10 gage.

Yukio YANO

I'm no expert on this, but I do know that it's not as simple as just having two batteries that can be switched between each other. As Yukio says, there's normally a dual charge control box, which does indeed contain at least isolation diodes. My next door neighbour went through all this when he converted a van to an RV for touring round Europe. I think it would be wise to take professional advice from a reputable auto electrical shop on this one, or at least post the question on one of the auto repair groups that might have a genuine expert in this field.

Arfa

Arfa

The batteries also have a different charge rate.

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Good idea? Humm...

For what? Charging? Probalby not large enough. I'd suggest 4 AWG (both positive and negative, don't use the chassies for ground).

Define 'tight'...

If you are not going to use the switch, why bother installing it? You going to open the hood each time you use the battery? I doubt it. Also I suspect the switch, and your understanding of how it works, and what it does, is flawed. You should have a battery management system, not a simple switch.

BTW, Valmar's web site is dead so I can't look anything up there.

OK, but use proper installation practices in running the cables.

Bwah-ha-ha-ha... They won't know, or care. Don't waste your time.

Yes, you are going about it without considering everything. BTW, if this 'medical' equipment is life support/life critical, I'd recommend you get professional help.

For what? That dinky 10 AWG wire? It will blow the first time the battery needs a complete charge (and the 10 AWG wire would also fail).

"Arfa Daily" wrote in news:Ppuai.19611$ snipped-for-privacy@newsfe3-win.ntli.net:

Or check with an RV group.

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Years ago my family had a sailboat with dual batteries and my grandparents had an RV with a similar setup. In the boat, there was a switch which was wired to each battery with very heavy cables, it could select 1, 2, or both in parallel. Only the selected battery would be charged or drawn from. The RV had a similar switch, but charging was accomplished through an isolator which was nothing but a couple diodes potted with a large heatsink. Both were charged simultaneously but the switch selected which one loads draw from.

30A should be adequate for charging the battery, but it has to be heavy enough to handle both the charge current and the load current. Either way talk to RV or boat guys for solid advice on this one, it's a very common arrangement.

Separating the batteries poses problems. If the battery is cranking the the starter, you really can't use a fuse. Thats the way its been done for a long time. Ordinarily I recommed using batteries in parallel. Draining down a battery reduces life, and with two batteries, there is less sag, improving overall performance. With the widely separated batteries, using a diode steering unit is going to be mandatory. In any case, install a battery close fuse, unless it cranking the starter, and that lead should be as short as possible. There are race cars and other vehicles using a long starting lead, but its not common.

greg

Use a diode isolator. Avaialable at any rv store or auto parts store or target or wal-mart or K-mart or most any place else. Switches are a bad idea 'cause eventually, you'll forget and the car won't start. You also don't want to switch a dead battery across a charged one. Wears the swtich out big-time...or blows fuses. Depending on how fast you want to charge the battery, #10 may limit the current below what you want. Do the math on wire resistance. Don't forget that sharp metal cuts insulation. Grommet the holes.

Be aware that the hydrogen released during overcharge won't go out the floor vent. It'll accumulate in the top of the box. Rethink your vent scheme.

Are we having fun yet? mike

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With respect to you, your life and all the other answers here, keep a couple of things in mind:

Lead-Acid storage batteries contain HUGE amounts of energy, surrounded by nasty acid and generating explosive gases. Do not trust half-assed solutions or take half-assed advice (including mine) without several pounds of finely ground salt.

Then: Do the set-up right. Use a purpose-built automatic isolation system. This is one:

Another:

There are more than a few others.

Presumably, these systems are designed to do exactly what you want the way you want it. And, presumably they will not melt down you or the battery.

As to venting: Obtain one of the newer batteries with a collective vent to a hose that you may direct as you wish. My Volvo has the battery under the floor in the rear, my '87 VW camper has it under the right front seat, the other '99 VW camper has its second battery underneath the rear floor (right under the head of the bed). All of them are connected to such a venting system.

By the way, the battery cables on the Volvo and both VWs are massive, I would guess the metric equivalent of #4 stranded, at least as thick as my index finger. The second battery on the '99 camper is a marine- type and only to run the housekeeping systems while the engine is off. It charges off the engine as well as a built-in inverter if there is line-power available. Even though it does not serve as a back-up starting battery, the cables to it are massive as described.

Peter Wieck Wyncote, PA

And the venerable MGB has a pair of 6v batteries (in series) under the back package shelf...properly vented.

I think it's useful to go back to the OP which states that the second battery in his application is NEVER to be used for starting the vehicle. That's all well and good, but several posters apparently missed it.

Consequently, a '0' or '00' cable in his case is probably overkill. The cable would be sized based on the output capacity of the charging system on his vehicle vs. the expected voltage drop in the length of the conductor to the second battery. A look under the hood of most vehicles confirms that that the wire run from the alternator to the battery is rarely sized above 12ga or so.

That's not to say that a complete rethink of the OP's original setup is not in order. IMO, either of the above setups would be far superior to what he originally had in mind. OTOH, a properly engineered system could be designed to do exactly what he envisioned: provide a clean, capacious source of 12 volts DC to run *outboard* equipment...said source to be replenished by the vehicle charging system.

The OP apparently does not have the requisite knowledge or capability to design such a system, however. Unless that expertise can be purchased locally, I'd suggest he go with something like the above. What he wants is theoretically possible, but too difficult to design safely from a distance.

jak

When I read "medical equipment", the term "overkill" leaps immediately to mind. Followed by "reliablity" and "caution".

Peter Wieck Wyncote, PA

Granted, thus my recommendation for local examination of the situation and competent local help. This is not a case where a good determination can be made remotely.

jak