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I recently acquired very very large 8 and 6 volt batteries (1 of each) for a 14v power supply to run my car while at car shows. Before anybody says anything, yes, this will run my car alllllllllll day and then some.

Anyhoo, as the topic would suggest, my question is about charging. I only have a 6/12 volt 5 amp charger. This should work for the 6, but what about the 8? Is it safe to charge it at 12?

If not, what should I do? I've never seen an 8 volt charger before

 
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8v batteries require 16-18v alternators
16v-18v alternators are very very very very dangerous to use on a daily driver.
also, most amps cant quite support that as well

 
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These batteries will never be hooked up to my charging system. They get hooked directly to my circuit breaker for my amp which provides power to all the electronics in my trunk. They are only going to be used for putting beside my car at car shows to provide power for my amp and lighting, so I need to figure out a way to charge them.

 
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i would have just done dual optima yellow tops in the trunk thats what i'm working on.

 
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youre basically gonna have to feed a 9.6-10 volt or so floating charge to the battery to charge it
you have 2 options, find a charger that does 9.6-10 volts, or, get a standard 12v trickle charger and wire in some electrical components to drop the voltage.
you could either use very large resistors, or, a few large diodes, diodes yield a .6-.7 volt drop, so depending on the charger.
another option yet, is to build a charger

 
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But you aren't me, are you?

I don't think you could ever find yellow tops for as good a deal as I got these batteries for, and the ones I have are much more powerful than any Optima

 
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What about wiring the batteries in series to 14 and charging them that way?

What kind of voltage would I need to do that?