this is a toughie so I'm putting it here.

a 12 volt car battery has 6 cells that are 2.1 volts each

a 6 volt car battery has 3 cells that are 2.1 volts each

question - is it possible to wire these two dissimilar batteries in SERIES to create an 18 volt (nominal) system, since technically it should show like 9x 2.1 volt cells, or will the 12 volt battery somehow overload the 6 volt battery and destroy both batteries and all the electrical in the car????

this question is in regards to looking for a poor mans 16v system(no alternator)

the other question is, how closely do the ratings of the two dissimilar batteries have to match in order to not cause problems.

 

The batterys in series would indeed be 18.9V and as long as you were charging them seperately out of the car then their match would not matter. The bigger question is how will the systems in your car react to such a high voltage.

 

Your fuel pump would certainly have the potential to crank out more volume, lol.

 

I verified that my ecu can handle up to 24 volts but the mfg recommended not going above 19.

the fuel pumps are a1000s and should be able to handle that voltage.

thanks guys for the replies

 

Yes, nominally they would produce 18V. However, under load, things change. For example, if you hooked a 1.5V alkaline watch battery in series with a 1.5V D-Cell alkaline, sure on a voltmeter(practically zero-load) it would read ~3.3 volts.. but if you put it under any appreciable load, along the lines of what a D-cell can handle(several amps), the watch battery will become a serious bottleneck. If the current capacity of one battery is significantly different from the other, you could damage the batteries, possibly catastrophically (overheating).
I'd say if you make sure the current producing capabilities are nearly the same, you could probably get away with it. But charge them individually...

This document gives some information about creating battery banks, and it warns to use identical batteries...
http://itacanet.org/eng/elec/battery/battery.pdf

I think a MUCH better option is to use three identical 6V batteries.

 

Both batterys need to have sufficient current capability but that does not mean that their capabilities need to match.

 

I would agree that if you're using a "normal" 6V battery and a "normal" 12V battery that you shouldn't have a problem. The issue of loading the different sized batteries and causing a failure would only apply if they were grossly different.

 

There is no reason why this shouldn't work so long as you charge the batteries seperately.

 

Batteries in series stack voltage but keep same amp rating. So if the capacity of one battery is less, it will discharge faster and possibly damage it. There is no problem charging them together with an 18v battery charger. You are essentially making one 9 cell battery as stated, and we all charge typical 6 cell batteries while they are in series...

 

light bulbs will be bright

 

How do OE style starters hold up to this dare I ask? Something else I will point out is that for most systems in the vehicle you can use a linear regulator and a transistor with a few passives to knock the voltage down to the level you want. Besides that you can also bump the alternator's voltage up by putting a Zener Diode in line with the field circuit and provided it lives it should output a higher voltage. YMMV on all of this naturally.

 

thanks for the input guys. im hoping to try it to see what kinda results i will get.

further questions.

with ls1 style coils, is there extra spark energy with additional volts?

 

just an idea, why not three matched 6v batteries

 

Yes there would be increased energy with LS1 coils. The coil/driver circuit should be getting a power equivalent of E^2/R. That means that if you go from 12V to 18V assuming the same DC and the same PW you're talking about 144/R versus 324/R watts of power from the coil. This neglects inefficiencies etc. Something else I will say you should consider is the life span of the coil. For the internal resistance of the coils themselves and the drivers built into the coil you will increase the power by this magnitude as well which is over 100%. I guess I'm saying keep a coupla spare coils around just in case. I would definitely not be running the same dwell times I would with 12V.

 

i already have a 12v battery lol.

 

great to know thanks. so how do i calculate what factor i reduce the dwell by?

 

The MS Tuning guide has a good how-to on setting dwell that is fairly good. I would strongly consider giving it a good read over. Besides that though I'd drop the dwell down 50% or so then if the engine is running rough increase it, but if it's running fine slowly decrease it until it started running rough and bump it back up. Don't forget about your PWM if the injectors you have are peak and hold, it could lead to burned injectors as well.

 

ok, researching if the injectors can handle the extra current.

 

I would still think you would have to tune the fuel curve differently.

 

More voltage will not increase flow, open is open, closed is closed, more voltage does not open them more. A 6 volt battery will usually have twice the cranking amps of similar sized 12 battery because each cell in a 6 volt battery is twice the size, so the 12 volt battery would run down faster. just one more option, get 2 8volt batteries. 8 volt batteries are becoming more common to upgrade 6 volt systems. Do you have a Interstate battery store near you. If you do I would ask them, they deal a lot with paralleling and series of batteries.