Charging two batteries?

Why would you do that. The car designer did a quite reasonable job of providing a battery suitable to serve the car for the normal life of a battery.

If you have some extra use such as camping then you need to go to proper separation of main and auxiliary and some of the Recreation news groups would provide answers.

John G.

People with RVs do it all the time, likewise tow truck operators. Marine batteries are frequently run that way - through a switch that allows either or both to start the motor - but while under weigh both are paralleled so they charge, then one or the other is running the accessories keeping one in reserve.

The downside, from my boating experience, is when one of the batteries has been abused it can sit there dragging down the good battery.

No expert on all forms of alternators . . . but the excited field style tends to be relatively bullet proof. The circuit usually uses a tickle charge from the battery to get the alternator pumping, but once it is, the main current for the field winding comes from an extra trio of diodes - so if the output is being dragged down, the voltage available to the field is also dragged down - so it tends to be current limiting.

That's why you should use isolation diodes.

Tom

--Understood. Yeah the application is a little nonstandard; nothing to do with an RV or other 'normal' app.

-- "Steamboat Ed" Haas : "Hold on! we're passing Hacking the Trailing Edge! : through the moronosphere!"

so long as both batteries were in good order - you might consider having a big diode to the +ve of the "spare" just to stop back feeding

- some kit I have used in the past has just this arrangement - a switch can then be used to short out the diode when the power from the spare battery is required back in the system - good idea to excercise the spare every now and then.

Non-standard? Doubt it. double battery installations are fairly standard these days. Basically, if you have anything you want to run off a 12V supply whilst mobile or moving regularly, then installing a second battery is fairly standard in all sorts of vehicles and "trailers".

Diodes are one method, relays are another. Isolation switches are commons.

Umm, jus make sure you get the correct type of battery. If you really just want to install a second spare car battery, then it doesn't matter. However if you are say touring and driving between places where you can for a day or two and need to run a frigerator, medical machine, ham radio, etc, etc, then make sure you get a proper deep-discharge battery. Also read the LeadAcid FAQ on how to look after them properly for a long life. Hint max 50% discharge at C/10 max rate.

AFAIK, "marine" batteries are a compromise between car battery and a deep-discharge and not a proper deep-discharge battery.

handle boat shock

I had a diesel GMC Suburban that came with two batteries in parallel, no isolation.

"Handle boat shock"? What does this mean? Is this some type of alien command like "Klaatu barada nikto"?

Bob

Sloppy editing. I was going to say that I understood marine batteries to have heavier plates to handle tinnies/tinnys bouncing around on the waves.

I don't know if I'd just throw in some isolation diodes and expect everything to be hunky-dory. A tenth of a volt drop represents a lot of amps to a battery under charge. But it all depends on the use - long recharge period and the diode probably matters a lot less.

If the goal is/was running accessories - I think a nice elegant solution would be a mosfet and active switch to isolate the battery used for accessories.

I know tow truck operators use two in parallel with no ill effects and plenty of military vehicles do the same. A relay is good for isolation too, but it may be impractical to use one on a starting circuit.

If you are planning on using diodes to isolate both batteries, I'd bump the voltage regulation up by one diode drop.

"Marine" batteries may be either deep-cycle or starting types. The difference between them and an automotive or other battery is primarily in the terminal type, and the fact that they often have a lifting handle, where a similar sized auto battery would not. I don't think there is any internal difference between a marine starting battery and an automotive starting battery.

Well, in all I've read, that is the first bit that made it clear that there are actually two different types of marine battery.