Dual Battery, No Alternator for Track Racing

[jeff denton]

While we're on the subject, can I get some expert advice on a battery situation? On our new limited sportsman car I don't want to run an alternator, and we are light enough we can easily carry a second battery. Not unusual, but, typically a dual battery setup is just two batteries connected in parallel.
I have to be different, or innovative. I want to use a cranking battery to run the starter only. And a marine deep cycle battery to run the ignition only.
My crew says I'm crazy, because nobody does that. I say it prevents ignition voltage drop when, for whatever reason, the engine has to be cranked more than a couple times per race.
I say it makes the most sense. They think I need two battery switches then, I say only one, one switch will simply ground both batteries at once to the chassis.
Any opinions? We use a DUI (Davis Unified Ignition) which is just a hopped up GM HEI. The tach could be fed by the starter battery. There are no other electrical gadgets.

[namba209]

Jeff, are you using an extremely heavy duty relay for the battery switch, to ground the battery? If so, then one switch, a DPDT, could isolate both batteries, depending on which way the switch was positioned. You could have the switch labeled "Start" and "Run", But it would take power from one battery or the other to operate both relays. If you want to run just one relay, then I would suggest using one side of the relay "NC" for the run side and "NO" for the start side, with a switch for the ignition, so the ignition is not powered with the relay de-energized. Hope this makes some sense.

[jeff denton]

No, stock cars aren't ever even that complicated. I'm talking about the rules-mandated "battery switch", just a big old rotary on/off switch. Both batteries' negative cable would go to one side, the other side to the chassis ground.
One battery's positive cable would feed the starter, the momentary contact toggle starter switch, and the tachometer power input. The other feeds the SPST toggle ignition switch and thus also the power terminal on the HEI.
A race car's electrical system is just about that simple, but I've blown my buddies' minds by suggesting we hook up the batteries a little differently.

[oldopelguy]

Naw Jeff, one switch will do just fine. Just get the one that has the alternator cut off switch built in, the 4-post one. Nothing in the running battery circuit will need more power than can be routed through the alternator terminals and the cranking battery can run through the main posts on the switch to the starter. One flick will interrupt both supply wires from both batteries.

[namba209]

Stephen's right, in that scenario. Just use the one battery switch to ground both batteries, run one positive lead to a momentary on switch to the starter solenoid from one battery. The positive lead of the second battery to a switch for the ignition and you can use either battery to power the tach. Super simple.

[wrench459]

Originally Posted by jeff denton We use a DUI (Davis Unified Ignition) which is just a hopped up GM HEI. The tach could be fed by the starter battery. There are no other electrical gadgets.

I like your thinking Jeff.
You can use the lead weights to your advantage.
A stock GM hei pulls 6 amps but I've never checked a trolling motor. I'm sure its over 6 amps. A tach draws in the milliamp range so why not let it run off of the deep cycle battery.
Does the track require a master kill switch at the rear of the car?
With a common ground dual battery system.. cant you bridge the starter b+ to solenoid.
Then run the ignition and tach on a different circuit or battery without an isolator???

[jeff denton]

Thanks guys, yes the circuits are just that simple, one big switch will isolate both batteries, as on any Caterpillar (often four huge batteries), and completely shut off ALL power to the machine by simply opening the ground.
Yes the rules mandate this switch! In the compact class, they want it right behind the driver's left shoulder.
In Limited Sportsman, they want it where it can be reached from either side, in both classes it must be in a big bright red spot, highly visible!
I had forgotten to ask the question this is really all about! Thanks for reminding (and answering), Dan. It was supposed to be "how well would a marine deep cycle battery power the HEI all on its own, without an alternator?" I never heard a spec for how much the ignition actually pulls, six amps is nothing! One size 24 deep cycle battery will run a trolling motor for hours, no problem, and it requires a 50 amp breaker when installed correctly. It should easily keep up good voltage to the HEI for 150 laps.
We're gonna give this a try.

[namba209]

Jeff, while I was doing my bike racing thing back in the 60s & 70s, I always ran a constant loss ignition setup. Think about how small the bike batteries are and running almost continually in the 10K plus RPM range and you'll get an idea. I usually ran from 20 to 30 laps a night, 3-4 races, with this setup and never missed a beat. So a marine/RV deep cycle should work just great. One thing, they don't like a quick hi-amp charger though, a trickle charger hooked up for a week between races should suffice to bring them back to fully charged.

[kwilford]

Originally Posted by jeff denton While we're on the subject, can I get some expert advice on a battery situation?

Um, not really "on the subject" of lightweight batteries for street use, so this has been split off to its own thread