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Discussion Starter #1
I am looking at using a 12 volt wet cell motor cycle battery - 12AH

I see that all of the EV's have full size 12 volt batteries but I don't have the space and I don't want the weight -

Can anyone tell me why if at all this is a bad idea?

Also another important question - the DC to DC converter is supposed to keep the 12 volt battery charged up, I get that but what is stopping the battery from getting over-charged????
 

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All the auxillary functions get powered from the 12V system, in addition to all the relays and main contactors. If the 12V lets go while driving or charging, then the relays open and will arc and damage the precious metal surfaces of the main contactors and/or weld them.

If you need to change a flat tire on a dark and rainy night, then you might want to have some reserve available for hazard lights and wipers.

The typical lead acid battery is not designed to operate with much discharge; they are best kept full all the time or will sulfate and degrade. So you need a voltage regulator to keep it charged up but not overcharged, if the DCDC doesn't have that feature then one will need to be added. A 12 Ah is probably only good for 3Ah or less before it needs to be recharged.

Failure of the 12V aux battery in an EV causes much worse problems and potential damage than in an ICE car; that is why the OEMs use full size starter batteries for the aux.
 

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Discussion Starter #3
Hey thanks for your input

So basically your answer is base on expected damage due to a sudden failure of the DC to DC converter -

I don't get the bit about the arcing - When I shut the car off by removing the key the DC to DC cuts all power to the relays anyway ?
 

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Hey thanks for your input

So basically your answer is base on expected damage due to a sudden failure of the DC to DC converter -

I don't get the bit about the arcing - When I shut the car off by removing the key the DC to DC cuts all power to the relays anyway ?
I use a lawn tractor battery with 450 cca for the aux.
I keep it charged with a 70 amp alt.
It only charges while the car is moving but I have not had any problems with the setup in the 15+ years I have been running it.

I think what kenny means is if you lose aux. power with the electric motor under load it can cause electrical arching on the contacter and pitting on the points.

I have had the contact on my car open while under load in the past.
Not because of aux. battery but the switch on my emergency brake.

When I first built my car I took off with the emergency brake on.
Didn't go but a tenth of a mile before I noticed it but it was enough that I decided to put an over ride on the on off switch by running the ignition through the e-brake buzzer switch.
Now the car will not go if the e-brake is on.

Great idea and worked fine for many years.
The emergency brake handle is right under my right elbow so I got in the habit of resting my hand on the handle while driving.
The extra weight on the switch eventually caused it to lose contact and shut the power to motor off while driving.
I took several shut downs before I finally figured out what was happening and fixed it.

I checked the contact points and they look to be in good shape.
This is not a recommendation to intentionally open the contacts under load.
 

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With you being in New Zealand, what do the certification rules say about power reserve? You need to get your pipsqueak battery sizing past the certifier.

If you don't have room for a proper sized battery, there's no reason you couldn't put batteries in more than one location if you need the reserve.
 

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Discussion Starter #6
oddly enough the rules say that you are not required to have any 12 volt battery ????

"a separate battery or electrical supply management system that provides
priority to the power supply needs of the vehicle’s safety equipment,
such as lighting equipment, brakes, and windscreen wipers;"


of course I will have a battery and I will probably use a small car battery

https://www.lvvta.org.nz/documents/standards/LVVTA_STD_Electric_and_Hybrid_Vehicles.pdf
 

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I'm in NZ and I'm using 8 Headway 16Ah cells is a 2P4S configuration - but I don't have a DC-DC so I just charge them up at the same time as my main battery
 

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Discussion Starter #8
Duncan please call me - or text
i would love to swap ideas and speak to a fellow kiwi EV builder -

021 2522510
 
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