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Mixing small and large batteries

Mixing small and large batteries

(OP)
I'm trying to help a professional race car team with their battery and starter motor system. Being a race car weight is critical, but the ability to start the engine is even more important!

The onboard battery is a small lightweight 12v AGM battery with a 26 amphour rating. The "slave" battery (used only for static starting in the pits or garage) is a "big" lead acid one as used on a big engine pickup truck.

The team has suggested they connect these in series (instead of in parallel) to give 24v "to spin the starter motor better". Would it? My battery technology knowledge is limited - what happens to the small battery if we do this? Would it damage it?

The starter motor is rated at 1.4kW with 12v. Obviously one would not use 24v on a 12v motor in a street car expecting it to last 10 years - but starter motors on race cars are rebuilt or renewed every year and they always carry a spare.

RE: Mixing small and large batteries

You would likely not damage the smaller battery but would likewise not help starting in the least because the little battery will have substantial internal resistance and not let lots of current pass thru it.

Your question really doesn't make clear what problem you're trying to combat so we can help.

Does the "cart" battery start the engine without problems?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Mixing small and large batteries

(OP)
Although this is professional team in a professional series it is not Formula 1 where teams spend millions to solve every little problem. The problem I'm trying to help with is simply to get a normal commercially available starter motor to spin the high compression engine faster. With the two batteries in parallel (ie. 12v) it still never seems super-good. Or as the team owner's email eloquently put it - I want it to go brMMMMMMMMMMMMMM not br-br-brmmmmmm.

RE: Mixing small and large batteries

If you do follow through on the concept of using 24 volts worth of batteries across the starter, then beware of allowing the 24 volts to back feed through the solenoid and start button wiring into the rest of the car systems. Obviously you will not be able to keep the ground side isolated, so you'll have to be very careful on the hot side to stay away from 12 volt wiring.

Back in the old days, some cars used to be 6 volt systems. Some were converted to 12 volts. Based on what I've read, it wasn't uncommon to just run the 6-volt starter on 12 volts.

It's still not recommended. Can you find a better starter? Perhaps from a diesel version of the same engine?


RE: Mixing small and large batteries

Your on-board battery will probably completely dominate the circuit through its internal series resistance. If it's on the order of 14 milliohms: http://www.power-sonic.com/images/powersonic/sla_b... That'll be 4x that of a conventional battery: http://www.power-sonic.com/images/powersonic/sla_b...

So, while you might have doubled the voltage, you would have increased the parasitic resistance of the battery by 4x. I would do a complete and thorough review of your jumper cable and other resistances before messing with the voltage

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RE: Mixing small and large batteries

Note, also, the series resistance appears to be somewhat dependent on discharge current.

Based on the discharge curves in the datasheet for the 26Ah battery, it might not even look like a battery at the discharge current required for a large starer motor.

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RE: Mixing small and large batteries

(OP)
The rest of the cars electrics are isolated and only connect to the small battery/alternator. The big slave battery feeds through a solenoid activated by the starter switch (and small battery).

RE: Mixing small and large batteries

Not sure who you're responding to, but I was referring to: "The team has suggested they connect these in series (instead of in parallel) to give 24v "to spin the starter motor better". " The gist of my posts is that you'll likely make it worse putting the small battery in series with the large. You'd be better off taking two large batteries and putting them in series. But, again, without a thorough understanding of where all the parasitic resistances are, you're simply rolling the dice.

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RE: Mixing small and large batteries

Perhaps the best thing to do would be define whatever problem you are trying to solve, and then people might be able to help solve it. Start with explaining what is wrong with the conventional starting arrangement and then go from there.

RE: Mixing small and large batteries

" Or as the team owner's email eloquently put it - I want it to go brMMMMMMMMMMMMMM not br-br-brmmmmmm."

What more needs to be said about the problem statement?

I would suggest measuring the voltage drop between the battery terminals and the starter motor terminals to see if cable and connector losses are significant. Connect the probes to the lead post on the battery and to the stud on the starter.

As others have stated, a small battery in series with the big one will only make thing worse. If larger cables won't help then you need a bigger battery (or two batteries of adequate size in series), or a more powerful starter motor. You could also consider an external booster battery for starting in the pits. It is pretty rare to need the starter motor during a race.

RE: Mixing small and large batteries

(OP)
I'd just like to thank all of you for your various inputs. We already use a larger starter motor and nice thick cables, etc. My main missing link was understanding what happens to mixing big and little batteries and your replies have educated me there. I quit working in engineering two years after graduating 53 years ago but the academic principles are still useful except when I come to chemical wizardry like batteries. I'm going to think about circuitry to use a big 16v slave battery (off car) not connected to the small on board battery - and hope the driver never has to restart on track!

A great forum and some great replies - thank you again - Derek

RE: Mixing small and large batteries

I'm pretty amazed your starter is only 1.4kW. That's like only 120A. Punny!

I'd look for a better starter. My two diesels both draw well over 1,200A starting. Shockingly a replacement starter motor I recently installed was a geared drive one, very unlike the OEM one, and the motor was easily a quarter the size of the direct drive OEM unit. It spins the engine to almost idle speed.

I'd check around for starter hacks your formula group uses since someone has probably 'fixed' your issue already with a starter motor mod.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Mixing small and large batteries

Look for a PM starter with gear reduction.
Pricey, but effective.

RE: Mixing small and large batteries

Are you using a specialty racing battery like a Braille or XSPower battery? If not, why not? Both make some really good light weight batteries for race type application. There really should be no need for a "boosting" battery if you pick the right onboard battery.

What make of vehicle/engine are you working on?

What kind of starter? If you're not running a permanent magnet gear reduction starter then why not?

No ability to start on track will appear to be a great weight saving option until the first time the starter is needed on track and the car goes down laps trying to get it push started or towed to the pits.

If there is a starter mounted solenoid then there is most likely a coil in the solenoid connected between the "start" terminal and the motor terminal. I doubt this type of solenoid would work right with different system and start voltages. You'd be reversing the current in the coil which could cause the solenoid to drop out.

Also, do consider the weight of all the cable and connectors you use for a certain configuration, not just the battery.

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