# Aux Battery Needed?



## jaspersk (Jun 26, 2008)

Frankencar said:


> I was wondering, because I've had a hard time finding info, if the aux 12V battery is really necessary? Would an EV work with just the DC/DC converter alone, or does that cause issues with the contactor/precharge business? Also, it seems the precharge resistor is something I overlooked in my purchasing. What should I look for, or can someone link me to a good one to buy?


It will work but it seems most would recommend against going without it. They will ask what happens if your DC/DC dies. I am not sure that is a valid argument as there are a lot of components in our EV's that don't have a backup.

Personally, I just got a small 12V sealed battery from a wheel chair. My local battery goes through their cores and sells anything that tests OK for next to nothing. I am using that. I don't like to leave me DC/DC on when the car is not in use. I feel like it is draining energy from the pack. I am not sure if that is a valid concern but it always worries me. My range was really bad once after letting my car sit for a week and I blame it on that. I use the SLI to keep the memory in my clock and radio. It is also there to make sure I have enough power to turn on the relay which turns on my DC/DC when I turn the key on.


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## Frankencar (Nov 17, 2008)

Sorry...SLI?


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## Tesseract (Sep 27, 2008)

I say always use a battery - even if it is just a small starting battery for a riding lawnmower - along with a DC-DC converter. The battery smooths out the voltage, provides much higher surge current capability, and generally results in a more stable electrical environment. Also, most advanced motor controllers will refuse to operate if the 12V system is too low in voltage (e.g. - the Soliton1 shuts down at 10V to protect the IGBTs from insufficient gate drive).

SLI - Starting, Lighting, Ignition... referring to the original single battery for said purposes in the vehicle.


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## jaspersk (Jun 26, 2008)

Tesseract said:


> The battery smooths out the voltage, provides much higher surge current capability, and generally results in a more stable electrical environment.


Good points! I agree.


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## Stunt Driver (May 14, 2009)

Have you looked at how much power you need from 12v system to drive from charge to charge? Because I don't plan on any dc-dc converter yet, as native Fiero's battery handles it al perfectly on its own.


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## Tesseract (Sep 27, 2008)

Stunt Driver said:


> Have you looked at how much power you need from 12v system to drive from charge to charge? Because I don't plan on any dc-dc converter yet, as native Fiero's battery handles it al perfectly on its own.


Hmmm... this is certainly possible, but your more advanced controllers monitor their 12V supply and shut down if it drops too low. This is because the gate drive to the switches will start declining at some point which leads to incomplete turn-on, which inevitably leads to their destruction. Do you really want to either risk getting stranded by the side of the road because you forgot to charge up the 12V battery, or else blowing up your controller because it's too dumb to realize it doesn't have enough supply voltage to work properly? Even a small - say, 15A - DC-DC converter is better than nothing at all!


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## Stunt Driver (May 14, 2009)

Makes sence, I just don't have this problem on my fork-lift / contactor combo

For that reason I think smaller and so cheaper DC-DC power supply will work. There are options for 10-15 bucks that take anywhere from 8 and up volts on input and give solud 12v output, just just a few hundreds mA. No idea how much is needed just for megaexpensive controller, but would do such separate power supply option powered from same 12v battery.

And don't forget to charge it, as you don't forget to charge you traction pack. Just have them on the same power line.


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## Frankencar (Nov 17, 2008)

So if I go with, say, a lawnmower battery, how many cold-cranking amps do I need to close the contacter? I am using a Albright SW-200CW-3 12 VDC.


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## EVfun (Mar 14, 2010)

You should have a small 12v battery. My EV Buggy does not, relying only on the DC to DC converter. 

Without a DC to DC converter...

1. You can't even park at the side of the road with your hazard lights flashing.
2. Turning on your headlights is likely to freak out a Zilla controller. Headlights have very low resistance (nearly a short) until the filament heats up. It knocks the neon lights off for just an instant on my buggy (the old Curtis doesn't care about a 12v system.)
3. Your DC to DC converter has to be large enough to handle the situation where every possible load is on simultaneously. 

A little 16 to 22 amp hour Zappy scooter size battery (a common size, about 3 by 7 inches by 7 inches tall) doesn't cost much and is all you need. I just finished the little inner fender well mounting bracket for the Zappy battery (mine actually says "Zappy" on it.)


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## Tesseract (Sep 27, 2008)

Frankencar said:


> So if I go with, say, a lawnmower battery, how many cold-cranking amps do I need to close the contacter? I am using a Albright SW-200CW-3 12 VDC.


About 5 CCA! And this is just a momentary peak - the actual current to operate the contactor is less than 1A, IIRC.

So, the contactor doesn't draw enough to even worry about. It's the other 12V devices in the car that you need the battery for, like the wipers and headlights.


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## Frankencar (Nov 17, 2008)

Tesseract said:


> About 5 CCA! And this is just a momentary peak - the actual current to operate the contactor is less than 1A, IIRC.
> 
> So, the contactor doesn't draw enough to even worry about. It's the other 12V devices in the car that you need the battery for, like the wipers and headlights.


Ahh, good to know.


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## dtbaker (Jan 5, 2008)

Frankencar said:


> I was wondering, because I've had a hard time finding info, if the aux 12V battery is really necessary? Would an EV work with just the DC/DC converter alone, or does that cause issues with the contactor/precharge business? Also, it seems the precharge resistor is something I overlooked in my purchasing. What should I look for, or can someone link me to a good one to buy?


You really DONT need an aux battery in most cases... just a 300+ watt dc-dc. I have been running a curtis 1440 that puts out around 300-350 watts, and it has been fine running lights, wipers, fan, vac pump all at once. I am going to go slightly bigger (500 watts max) when I upgrade my battery pack voltage just to be safe. I found pretty great prices on Chennic.com, but they come direct from China, so communication is a little bit of an issue, not bad though. They delivered in about 10 days for just $99 including shipping. Only problem was that the packaging was beat to heck and the in-line fuse holder on switch wire was smashed.... easy to replace.

I have my dc-dc wired as 'always on' and feeding 12v juice right to the main fuse block just like it is a battery. I wanted power for lights, radio, clock to be available pre-key-on.

If you DO go with an aux battery, make sure your dc-dc puts out like 13.5-13.8 v, not 12.0, or it won't charge the battery!

the precharge resistor is pre-installed in many main contacters, mine was from kta-ev.com and it was set to go...


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## peggus (Feb 18, 2008)

jaspersk said:


> It will work but it seems most would recommend against going without it. They will ask what happens if your DC/DC dies. I am not sure that is a valid argument as there are a lot of components in our EV's that don't have a backup.



But safety critical components does and should have backups. The emergency flashers is one such system. On all cars I've worked on it's been on a separate circuit, dedicated filaments and separate flasher relay.

And I believe being able to run the flashers for 30mins is a legal requirement in most places.

The existence of single point failures is never a valid argument to add more.


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## Duxuk (Jul 11, 2009)

I'm using a 2Ah burglar alarm battery wired through a emergency flasher switch. I only need to charge it every 3 months and with the clever switch I don't need a seperate relay. With my 10 watt indicator bulbs it might even run for half an hour.


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## brainzel (Jun 15, 2009)

I'm not sure if I understood everything right, but this is a "normal" schematic diagram I want to use (fuses are not plotted):










I will get permanently 13,8V out of the DC-Converters direktly to the battery (to charge) and parallel to the electric loads.

Is there anythings wrong?
Do I have to include diodes or something or is it just fine?


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## CrunchTime (Feb 13, 2009)

I've been considering this option, but I was thinking this circuit (brainzel's) would be trickle charging (bad), rather than float charging (good)? I guess if the DC-DC is only connected to 144V on key-on or mains-charging, rather than permanently, it won't be such a problem though...

I suppose another concern is whether the other stuff in the 12V circuit can handle 13.8V?


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## Qer (May 7, 2008)

CrunchTime said:


> I've been considering this option, but I was thinking this circuit (brainzel's) would be trickle charging (bad), rather than float charging (good)? I guess if the DC-DC is only connected to 144V on key-on or mains-charging, rather than permanently, it won't be such a problem though...
> 
> I suppose another concern is whether the other stuff in the 12V circuit can handle 13.8V?


Normal voltage for a car is about 13.8 Volt today so no, no problem at all. In fact, this is a very strong reason to use a DC/DC to begin with, head lights etc will be much weaker if you run them on battery only since you then will get somewhere between 11 and 12 Volt only.

However, a backup battery to the DC/DC is still a good idea I think, since otherwise your car will be completely dead if the DC/DC stops working. It's a bit like if the alternator gives up, you can still drive home on battery only.


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## brainzel (Jun 15, 2009)

CrunchTime said:


> I've been considering this option, but I was thinking this circuit (brainzel's) would be trickle charging (bad), rather than float charging (good)? I guess if the DC-DC is only connected to 144V on key-on or mains-charging, rather than permanently, it won't be such a problem though...


Maybe there is a charge-controller needed or is this no problem?
My thought was, that in an ICE car, the alternator does exactly the same to the battery.


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## TX_Dj (Jul 25, 2008)

The battery will tend to self-regulate its charge off the DC converter, so long as the DC converter can provide a float voltage. If the battery wants 3A to charge at 13.8v, the DC converter will supply that 3A.

Most of them already act somewhat like a charge controller, in at least so much as they only output as much current as is demanded, give or take.

Then again, I'm no expert.


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## EVfun (Mar 14, 2010)

Right TX. If you connect a lead acid 12 volt battery to a power supply between 13.2 and 13.8 volts it will float charge. When it's full it will draw well under one amp to stay full. If you take some amp hours out of the battery it will charge more quickly up to about 90% charge then charge ever slower as it approaches full. Charging at this level doesn't require the charge to be stopped because the battery can take it for 4 to 10 years before the amp hour capacity start to seriously decline.


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