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Calculate battery capacity

R8RX

Member
I haven't decided on running an alternator in my R8 yet. Installing an engine from a Renault 17tl and there aren't room for the alternator without cutting some metal. Could move the alternator to the other end of the engine and drive it of the fan/crankshaft pulley though. But since this is a rallycrosscar it runs only a few minutes each heat. And therefore I am thinking of running it without an alternator. My question is if it is possible to calculate which battery capacity I need. Thinking of using a 30ah 450cca mc-battery. My biggest concern is the 2 small cooling fans I am going to use. Haven't had a problem running without alternator on other cars before, but that was with a beltdriven cooling fan, and a normal car battery.

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COL

Alpine A110
Would the fans be needed for a few minute long heat?

Are the fans on a thermostat?

What is the current draw of the fans and also of all the other electrical gear that you are running?
 

R8RX

Member
Would the fans be needed for a few minute long heat?

Are the fans on a thermostat?

What is the current draw of the fans and also of all the other electrical gear that you are running?
I don't know really. Radiator is mounted in the front, and coolant pipes are mounted under the car.

Thinking of just operating them with a switch.

Each fan draws 7 amps. Fans and starter motor have the biggest current draw I guess. And then you have instrumentation, brake light and wiper motor.

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Frans

1000+ Posts
I don't think you will have problems with that combination Two fans will draw 14 Amps, theoretically you will be able to run the fans continuously for an hour and then have another 16 Ah left for the ignition and fuel pump. I would suggest to have a spare and use that to jump start the car before going to the start line.

Frans.
 

R8RX

Member
I don't think you will have problems with that combination Two fans will draw 14 Amps, theoretically you will be able to run the fans continuously for an hour and then have another 16 Ah left for the ignition and fuel pump. I would suggest to have a spare and use that to jump start the car before going to the start line.

Frans.
Thank you :) Yes, never had any problems with running low on battery before. Used to charge the battery before each race. But as I said, that was without electrical fans. And as you say, a spare battery will be a fine backup.

I see you mention fuel pump. I've bought a new "original" mechanical one. This should be good enough for a twin carb setup?

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Frans

1000+ Posts
Yes that will be more than enough. Mechanical pumps are effective but they do get problems with heat and the fuel makes airlocks in a high temperature environment. But keep it and test first.

I would suggest that you wire the fans individually on two separate switches. You can then switch one fan only to see if the temps are constant and if the second is even required. If you then see that you do need two fans you can change the wiring and use one switch for both fans.

Frans.
 

COL

Alpine A110
I did some quick mental calcs and I reckon the 30 Ah will be fine for what you are going to do.

Remember not to discharge the battery to much as car batteries are not designed for this.

Have you thought about fitting a physically smaller alternator like Frans has done, may solve all your problems.
 

R8RX

Member
Yes that will be more than enough. Mechanical pumps are effective but they do get problems with heat and the fuel makes airlocks in a high temperature environment. But keep it and test first.

I would suggest that you wire the fans individually on two separate switches. You can then switch one fan only to see if the temps are constant and if the second is even required. If you then see that you do need two fans you can change the wiring and use one switch for both fans.

Frans.
That is good :) Yes, will use the mechanical one first and see how it works.

That was a fine advice about switches :) Hadn't thought about that.

Thanks :)

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R8RX

Member
I did some quick mental calcs and I reckon the 30 Ah will be fine for what you are going to do.

Remember not to discharge the battery to much as car batteries are not designed for this.

Have you thought about fitting a physically smaller alternator like Frans has done, may solve all your problems.
Thank you :) Yes, I know. Going to test the car properly before entering any race. So I have everything sorted out.

Yes, I have thought about fitting a smaller alternator. But will try without first :)

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speaksgeek

1000+ Posts
As a related but not absolutely direct comparison, I've run the race car full loss once. I have full EFI, electric power steering, the works. I got 10 laps in easy (approx 12 minutes) before I pulled the pin at around 10.5V. I know the main EFI required approx 12A to run continuously, the power steering ranges from 4-60A briefly.
I run a Lithium battery, roughly equivalent to 20Ah in lead acid. Much lighter, and they also tolerate a much deeper discharge (to a point, and then they don't) than lead acid. Also 3 times the price.
With you using carbs, mechanical pumps etc I wouldn't be too worried.
 

jaahn

1000+ Posts
Hi R8RX :)
Here is a tiny alternator that will fit anywhere. :) Very robust too.
It is used on small stationary engines and ride on mowers etc with small diesel engines. If you look you might find one at a repair place near you. At least while you are running there is something going in to the battery and keeps the voltage up to working level. At race type revs probably the full 14A.
https://www.ebay.com.au/itm/KUBOTA-...752946?hash=item1a7d627032:g:HJgAAOSwm-pZq6sC
Jaahn
 

R8RX

Member
Hi Jaahn :)

That looked like a clever little thing. Did actually find a couple of places here where you can buy these new. Haven't found the voltage regulator though. But it is triple the money compared to the one you found on Ebay.

Should be easy to make a bracket for it as well.

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PeterT

1000+ Posts
I also have a lithium battery. It's 420CA, weighs 1.7kg and has no problem turning over a 2L 12:1 engine. I bought a cheap, small 40A alternator off eBay. Unfortunately the regulator died soon after, so I fitted an external Bosch RE55 regulator.
 

JohnW

Too many posts!
I also have a lithium battery. It's 420CA, weighs 1.7kg and has no problem turning over a 2L 12:1 engine. I bought a cheap, small 40A alternator off eBay. Unfortunately the regulator died soon after, so I fitted an external Bosch RE55 regulator.
Wow, back to the future. RE55 no less... :) But why not?
 

JohnW

Too many posts!
I did some quick mental calcs and I reckon the 30 Ah will be fine for what you are going to do.

Remember not to discharge the battery to much as car batteries are not designed for this.

Have you thought about fitting a physically smaller alternator like Frans has done, may solve all your problems.
Frans' small alternators are very neat fittings, I can confirm. He could tell you exactly what he found that works so well for him.
 

jaahn

1000+ Posts
Hi Jaahn :)

That looked like a clever little thing. Did actually find a couple of places here where you can buy these new. Haven't found the voltage regulator though. But it is triple the money compared to the one you found on Ebay.
Should be easy to make a bracket for it as well. Sent from my SM-J600FN using aussiefrogs mobile app
Hi R8RX :)
Be aware that the alternator outputs AC so the regulator needs to rectify it first and then controlls by bleeding off the excess voltage through resistance in the heat sink. So nothing like a normal regulator on a car, but similar to how some motorbike systems work. Possibly you could fit one of them but why bother that one is priced OK.
Jaahn
PS when I was working on small engines I do not think I ever had to replace or fix one of those other than bearings damaged by pressure washers.:(
 

COL

Alpine A110
Frans' small alternators are very neat fittings, I can confirm. He could tell you exactly what he found that works so well for him.
I think it was a Hatachi, but anything from a small car would probably be ok
 

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