# Seperate battery for headlights, turn signals?



## flashedarling (Jun 30, 2008)

Right now my design for ultimate range involves a LOT of battery weight. So I'm trying to find ways to get by with smaller batteries. Since I expect to be driving in the early morning to avoid traffic I will probably need to use my lights alot. 

Since going for Lithium Polymer or LiFeP04 batteries is too expensive for powering my entire system (since I'd need 4000 watt-hours I'd have to spend at least 3,500) I was wondering if I could buy a single 12v-15ah pack for powering the electrical system. This would also save me from having to buy a DC-DC adapter although I would need to charge it seperately. But under worst case driving conditions (as in using my lights alot) I save myself nearly 10 watt-hours a mile.

So bad idea? It seems that it is the way to if I want to squeeze out extra miles and am stuck with SLA AGM batteries for the rest of my system.


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## Andrew Carson (Jul 7, 2008)

Most of the builds i have looked over have a seperate battery pack for the lights,signals, etc. this is only connected to the main drive batteries at the charging port. This is also a better configuration otherwise everytime you step on the gas your lights will go dim from the pull on the batteries.... and in Florida atleast, you can get a ticket for your headlights doing that. Good luck!


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## Qer (May 7, 2008)

It's a tricky decision. If you have to drop a battery from the main pack to be able to get a battery big enough for the auxiliary systems it's definitely a loss, but if you can add a extra battery without weighting down the car too much you probably get more range even if the added weight will affect the range at least some. Downsides will be that you need a bigger auxiliary battery than if you use a DC/DC-converter and also that the headlights will be powered with lower Voltage which will mean less light, but it will mean a few percent better range, yes.

However, if you need those last percent so desperately maybe you've designed the car wrong? Shit happens and you can bet on that Murphy will get you stuck in a traffic jam or something that will deplete the batteries too much so you ALMOST get to work one morning. Perhaps it's better to put that extra weight on more lead in the main pack? Considering that peculiar effect (or whatever it's called) it might actually give you more range despite those 10 Watts.

Another idea is to automatically disconnect the DC/DC-converter when the pack-current peaks and only run on the auxiliary battery, because even if you have a DC/DC-converter you still need a small battery on the auxiliary end to avoid total blackout if you get problems and also as a starter battery for controller etc and you can as well take advantage of it since you need it anyway. That way you can lower the percolator effect (wait, don't tell me, I'll remember that word eventually!) when it's the most noticeable, ie when accelerating.


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## Qer (May 7, 2008)

Andrew Carson said:


> This is also a better configuration otherwise everytime you step on the gas your lights will go dim from the pull on the batteries....


Ehm. If you have a DC/DC-converter it will switch down your wildly variating 90-140 Volts (or whatever) to a nice, steady, non-dimming voltage no matter what you do with the accelerator.

If you connect the auxiliary system to the lowest 12 Volt of the main pack the head light will indeed dim, but if you connect the electricity that way I'd be so bold to say that the light is pretty dimmed above the neck-tie as well...


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