I've got a weird idea brewing for a temporary (for a parade) where I need to run an inverter off of 12v and could use a little direction:

I want to use my stock generator, add a battery isolator and a second 6v battery so I can then put the two 6v batteries in series and get 15 amps at 12v.

No, my vehicles are stock and I will not convert it to a 12v system.

 

So to make this you can only charge one battery. If you add a second battery then link the (+ to +) and (- to -) you will be able to charge both and have two 6 volt batteries and twice the amps. As soon as you tie the (+ to -) of the two batteries which you need to do to get 12 volts one battery will explode, most likely, unless you battery lead is small enough then it would just cook it and burn away.

Have you tried running the inverter off 6 volts depending on the circuit type it may still work.

Maybe just run a small honda generator in the bed? They are fairly quite.

 

The inverter has a low voltage detection feature that trips at ~10/11 volts, so 6v source is not going to work.

As for a gas powered generators, many parades in my area do not allow them as they are consider them a fire hazard. If not properly secured, they may tip over spilling gas near a hot engine or over a hot exhaust, and people may be tempted to refuel them while warm and all sorts of other no -no's.

Charging a single battery at a time sounds like a viable idea - I could even maybe add in circuitry to automatically change the battery being charged every few minutes to keep both somewhat topped off...

 

What about adding another 12 volt generator just for the parade?

OR my older brother has a 1952 F7 firetruck and it has a GIANT diode under the hood for a 12 volt inverter maybe you can rig something like that?

 

How long does the parade last? You might be just as well off having a 12v battery dedicated to the inverter. A good, fully-charged 12v should give 15 amps for several hours without a problem.

 

You could install a series-parallel switch. Very simple and straightforward.
6 Volt Series Parallel Switch 1119845-6V.

 

You could also install an extra battery and have it in series for 12 volts while only charging one of the 6 volt batteries as normal and if it starts to dim, swap the two?

 

A couple deep cycle marine or RV batteries should supply that type of current for most parade lengths. Must the draw be constant or can you shut power down periodically?

 

Any group 22 or 24 12 volt battery should be able to carry 15 amps for at least 3 hours, without hesitation. These are pretty standard auto batteries. A golf cart battery would last much longer. Could one be placed in the vehicle a dedicated to operating your inverter? What are you powering with the inverter? Is the 15 amps a constant or fluctuating draw?
The single dedicated battery seems a much simpler plan. But then, there has to be more to it than the simplicity of your original question.