Two batteries system

[Renato]

Hello everyone,

I was thinking in using two batteries, one for starting and one for operaton. Some street cars use this sytem, but i don't know if anyone has already used this in a formula car.

What do you think??

ps.: I won`t remove the alternator!

[Renato]

Hello everyone,

I was thinking in using two batteries, one for starting and one for operaton. Some street cars use this sytem, but i don't know if anyone has already used this in a formula car.

What do you think??

ps.: I won`t remove the alternator!

[Jimmy_seven]

Why?

[Renato]

Some cars use this because when starting there's some voltage flutuation, and this could affect the function of the coils and injectors. Then, when starting one battery suply energy to the starter motor, and the other to the system. When the engine are running, the two batteries are charged together. Everything is controlled by an especific ECU.

[Jimmy_seven]

Ok but why do you want to do this with your car? Are you having issues starting it due to voltage fluctuation?
Seems to be to be a good way to make the car heavier and more complicated without adding much functionality.

[TorqueWrench]

I'm going to second what James said. Only time I would consider this is if you are having issues starting under the hot start conditions you see in endurance. Even then, I bet running one larger battery would be lighter than two smaller ones.

Only time I have encountered something like this is during testing when the engine was getting turned on and off five to six times a day without getting a good charge in between. Not really something you need to be concerned about for competition.

[exFSAE]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Jimmy_seven:
Why? </div></BLOCKQUOTE>

+1.

Batteries ARE certainly a recurring issue in FSAE.. but that's more just running out of juice from repeated deep cycles.

Having a jumper battery CONNECTION on the car, is a good idea. But why carry the jumper battery onboard?

[mech5107]

I don't think that dual battery setup isn't the right solution to your problems.

We are running a 10Ah bike battery on our cars.
Even the one with a big-single (600cc) starts instantly when hot and pretty easy when cold.

Probably your ecu trigger is not properly set.

If you are using a 4cylinder, i can assure you that even a small 7Ah battery can start easily the car.

[TMichaels]

I have also seen a lot of discussions regarding the battery size.
Some teams use very big batteries without a proper reason. Just calculate your energy balance and make some estimates when you lack data.
We used a 7Ah battery for years and were never anywhere near critical at competition. Just make sure that the battery is always fully charged and use a jump start connector for an external battery.
Oh and of course make sure that your battery never gets into deep discharge.

[Mike Cook]

I'm certain we could get away with smaller batteries but in my experience, the nature of a autocross car kills batteries quickly. Combinations of poor start up maps and also, that our actual start/run ratio is very high unlike a street bike that has plenty of time to charge the battery between start ups. My recommendation to any team is just to put a fresh (fully charged!) battery in the car before competition and it should be good to. We also build a charging port into the car and have a trickle charger on it while in the pits most of the time. Our ecu map has no problems starting the car when hot, and because of this we have never had a hard time restarting the car at drivers change (because it is charging for 10 laps before we come in). Even after letting the fan run for the 3 minutes, the car has always started back up. Most of the times teams have problems getting it started back up because they have issues with hot start ups.

It's amazing, but few teams seem to have a fuel map that can start well at cold, start well at hot, and the car will actual idle like it should. I know the idle thing doesn't matter much for a race car, but its brings a smile to my face every time our car just starts and purs and we see people running around the paddock with starting fluid and what not trying to get their stuff just to start.

[mech5107]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Cook:
I'm certain we could get away with smaller batteries but in my experience, the nature of a autocross car kills batteries quickly. Combinations of poor start up maps and also, that our actual start/run ratio is very high unlike a street bike that has plenty of time to charge the battery between start ups. My recommendation to any team is just to put a fresh (fully charged!) battery in the car before competition and it should be good to. We also build a charging port into the car and have a trickle charger on it while in the pits most of the time. Our ecu map has no problems starting the car when hot, and because of this we have never had a hard time restarting the car at drivers change (because it is charging for 10 laps before we come in). Even after letting the fan run for the 3 minutes, the car has always started back up. Most of the times teams have problems getting it started back up because they have issues with hot start ups.

It's amazing, but few teams seem to have a fuel map that can start well at cold, start well at hot, and the car will actual idle like it should. I know the idle thing doesn't matter much for a race car, but its brings a smile to my face every time our car just starts and purs and we see people running around the paddock with starting fluid and what not trying to get their stuff just to start. </div></BLOCKQUOTE>

Does you car start and idle from cold, without any throttle manipulation? How do you hold fast idle till it's semi warm? We have to keep the throttle open till it reaches 40-45 Celsius.

[Thrainer]

If you want to use two accumulators, it would make more sense to use one battery with high energy density and low power and one capacitor for short bursts of high current. I haven't tried this, though. We were using 4.6 Ah batteries the last two years with no problems.

[Hector]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by mech5107:
Does you car start and idle from cold, without any throttle manipulation? How do you hold fast idle till it's semi warm? We have to keep the throttle open till it reaches 40-45 Celsius. </div></BLOCKQUOTE>
Yes. It's starts up and idles with no manipulation.

It's all about programming your fuel and ignition tables. I don't know what ECU you use, but we use MoTeC. It has enrichment tables based on temperature that work very well.

Are you familiar with how chokes work on carb engines? They enrich the fuel mixture to allow for better running at cold temps. Most (hopefully all) ECUs have a way to enrich the mixture based on temperature.

Remember, whatever motorcycle you scavenged your motor from was originally able to start and run easily. If you're having issues, the problem is more than likely on your end.

Two batteries is definitely a waste of time, resources, and weight. As mentioned above, do an energy balance on your system to determine what maximum/average draw conditions are, and if your stator cannot keep up, make sure your (single) battery is large enough to supply the extra power for at least an endurance, probably more so that you're charging during testing.

We run as much power as anyone else at comp (electric fuel pump, water pump, fan, ECU, DAQ, etc) and Odyssey batteries have always worked well for us. Simple, fairly lightweight, easy to use.

[fixitmattman]

Only way I would go with a two battery system is if there's a small one on board, and a big cold start battery offboard you can disconnect. A warmed up running engine needs very little to get it going, only really need the big cranking amps for cold starts.

But this is assuming you have an engine that starts reliably without 20min of cranking. By the sounds of it and what I've seen at the competitions that's apparantly a tall order.

[TMichaels]

I second Hector: The same with our cars and Motec. The car starts up, cold or hot without touching the throttle and idles stable. We also managed this by fuel enrichment and slightly changed ignition angles at low engine temperatures.
I think nearly every available ECU should be able to do an engine temperature dependant fuel enrichment and ignition angle change.

Regards,

Tobias