# Size for Auxilliary Battery?



## Coley (Jul 26, 2007)

This is probably the best place to put a marine deep cycle battery. 60 or 70 AH.

I use one and don't use a DC/DC, I just charge it once a week or so with a good smart charger.

Driving in the daylight hours, with no rain, I have had it go 2 weeks.


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## ZeroGasoline (Jul 30, 2008)

I'm using a 50Ah 12v marine battery from napa for my standard 12v power. I also do not use a dc-dc converter. I have a small 1 amp trickle charger that is turned on any time I plug the car in. That has seemed to keep it in good shape.

I would make sure you size your aux battery to supply at least 2x the electrical needs for your car for the maximum range you expect to have. When I sized mine, I calculated what the power draw would be if I turned everything in the car on and left it on for my daily commute. The last thing you need is for your headlights to start dimming cause you were driving home in the rain with the radio on. Since this battery will run your headlights, blinkers, taillights, rear defroster and the like, you simply cannot afford to cut any costs on this single battery - it just has to work, and it must work all the time.

I could be wrong, but I believe a dc-dc converter takes the place of a 12v aux battery.


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## Zemmo (Sep 13, 2007)

I have a small 12v AGM 33 Ah motorcycle battery for my 12 volt system. I have a smart charger connected to it which comes on every time I plug the car in. I did start off with a regular motorcycle battery and a DC-DC converter. I didn't like that combination and now I use the AGM battery/smart charger method.


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## dimitri (May 16, 2008)

I use $25 battery made for lawn mowers.... Its a smallest and cheapest battery on the shelf in Autoparts stores. Hmm, maybe its not the smallest, compared to motorcycle batteries, but its definitely the cheapest 

Works fine if the car is used on regular basis and has DC-DC. I had it drained too low while testing vacuum pump for several hours not realizing that main battery was disconnected, so DC-DC wasn't charging it up, but I quickly charged it back up and its good to go....

Hope this helps...


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## evcars (Mar 15, 2008)

What are the advantages of using a DC-DC with a small 12 volt battery vs DC-DC converter with no 12 volt battery?


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## Anaerin (Feb 4, 2009)

evcars said:


> What are the advantages of using a DC-DC with a small 12 volt battery vs DC-DC converter with no 12 volt battery?


Well, it means you can have your DC-DC controlled by your contactors, so it's only active when you need it. When you're not driving, the aux battery will keep things running (like the clock on your radio, for example) without having to have your full pack voltage live. It also makes for a smoother load on your DC-DC, so when your vacuum pump switches on there isn't a big spike in demand on your DC-DC converter, making it last longer.


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## Zemmo (Sep 13, 2007)

If you don't have a 12 volt battery to supplement the DC-DC converter, then you need to have a large DC-DC converter (more cost). If the DC-DC converter had a problem or if your battery pack had a problem (lost power on the pack side), you wouldn't have any 12 volt power. So no power for flashers or other safety things. I experienced this my self, the old 12 volt motorcycle battery died so I just took it out and only used the DC-DC. I had a battery terminal melt because of a bad connection and lost my pack voltage. So I was stranded without any power, and I had Texas traffic coming right up on me because of no safety flashing lights. I think just for a safety concern, a 12 volt battery is needed and for a cost concern, a 12 volt smart battery charger is cheaper than a DC-DC converter. Just my 2 cents.


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## dimitri (May 16, 2008)

Zemmo said:


> If you don't have a 12 volt battery to supplement the DC-DC converter, then you need to have a large DC-DC converter (more cost). If the DC-DC converter had a problem or if your battery pack had a problem (lost power on the pack side), you wouldn't have any 12 volt power. So no power for flashers or other safety things. I experienced this my self, the old 12 volt motorcycle battery died so I just took it out and only used the DC-DC. I had a battery terminal melt because of a bad connection and lost my pack voltage. So I was stranded without any power, and I had Texas traffic coming right up on me because of no safety flashing lights. I think just for a safety concern, a 12 volt battery is needed and for a cost concern, a 12 volt smart battery charger is cheaper than a DC-DC converter. Just my 2 cents.


The issue with battery and no DC-DC is that battery must be large enough to provide 12V power between charges. If you account for vacuum pump, lights, radio, etc. then suddenly you need a large battery for 12V which is not desireable in EV. On the flip side, DC-DC and no battery is not good either, so the best solution is a smaller 12V battery and DC-DC, IMHO.


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## Zemmo (Sep 13, 2007)

A DC-DC converter is not a battery charger. So if it was left on all the time, it would slowly cook the battery. You can have the DC-DC come one with the contactor which helps with that.

In the two years of driving my car, I haven't once worried about not enough power for my 12 volt side. I have a little display for my 12 volt side, so I always know what it is at. I charge my car every day, so that battery always stays charged. If you think about it, we only drive the car for maybe an hour (60 miles), so thats the only run time that you need to figure for the 12 volt battery. I took my car a step further and put in LED lights all around except for the head lights. So I did take a little pre-caution when going only with a 12 volt battery. But I will run out of traction battery way before I run out of a 12 volt battery.


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## engineer_Bill (Jun 24, 2008)

I used the original battery for my car a 35amp/hour starter battery. It provides all of the 12v power I need most of the time, but I have occaisionally run low when people leave the door open or hood up for a long time I have run out. A bigger AGM deep cycle or a dc-dc converter to give some insurance would be good. ( I've had to get an emergency jump when the pack was still full, but I didn't have the power to pull in main contactor).


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## swamplizard50 (May 16, 2009)

could a alternator be run off a drive shaft to charge only the auxiliary battery? this way it would have all the power needed for accessories and not be a drain on battery banks


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## Zemmo (Sep 13, 2007)

If its run off the tail shaft, then it is still a drain on the main battery pack. It was just first converted from electrical to mechanical and back to electrical. So it would still be a drain on the main battery pack. You could put a switch on it to only engage when on the brake and/or if the head lights are on. That seems work out pretty good for people. But if I was to have something regening off the tail shaft, I think I would rather have it be back to the traction pack. Get an alternator rewound to output pack voltage.


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## Grant_NZ (May 28, 2008)

Could you make a auxiliary pack out of lithium batteries and would it be an advantage or disadvantage?


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## Zemmo (Sep 13, 2007)

You can make a 12 volt lithium battery. It could save you a little weight but you would have to charge it properly just like any Lithium traction battery. I've seen people make a 12 volt lithium battery. I don't know how a lithium battery would do with a DC-DC converter constantly feeding it power.


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## Watt-a-mezz (May 20, 2009)

Zemmo said:


> If its run off the tail shaft, then it is still a drain on the main battery pack. It was just first converted from electrical to mechanical and back to electrical. So it would still be a drain on the main battery pack. You could put a switch on it to only engage when on the brake and/or if the head lights are on. That seems work out pretty good for people. But if I was to have something regening off the tail shaft, I think I would rather have it be back to the traction pack. Get an alternator rewound to output pack voltage.


 Ihad an alternator hooked up to charge a regular starting battery, but I changed it to a dc/dc converter.
Do you know who may be able to rewind an alternator for pack voltage? How well would it work for 96v? Take care, Watt.


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## Grant_NZ (May 28, 2008)

Zemmo said:


> You can make a 12 volt lithium battery. It could save you a little weight but you would have to charge it properly just like any Lithium traction battery. I've seen people make a 12 volt lithium battery. I don't know how a lithium battery would do with a DC-DC converter constantly feeding it power.


 
I take it something like this would work - 

http://www.made-in-china.com/showro...JoFyOCGpD/Sell-12V-100AH-Lithium-Battery.html


although as you say would it handle being constantly charged?


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## Zemmo (Sep 13, 2007)

You could make a 12 volt battery out of AA batteries. Of course, it would take a lot of them because they only store 2 Ah per cell. The reason an ICE car needed a big battery is because, it had to turn over the ICE to start it and run indefinetly because you can drive for unlimited time. For an EV, the heaviest load is probably the Head Lights. You don't have to have that cranking power any more when you remove the ICE. So then it is just a calculation of run time by what ever load your car has.


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