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Old 10-03-2014, 03:18 AM   #20 (permalink)
Join Date: Nov 2013
Location: Australia
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Originally Posted by Astro View Post
I haven't estimated the weight of the 12v accessories battery as i am still trying to decide if i will use a small motorbike battery or some form of LiFePo4 battery. I am leaning towards the motorcycle battery as it will not require as much monitoring or charge / discharge control.
Is self quoting allowed?

I just measured the parasitic draw on the accessory battery with car all off.
I wanted to know how quickly a small motorcycle battery would be discharged with the parasitic load.
The current draw is 5.27mA.
So about 55 days to flatten a 7Ah battery to dead flat.
Maybe count on a month before the voltage drops too low to be useful for energising the contactors. If the contactors can be energised then the DC-to-DC converter will fire up and the car will be good to go and the accessory battery will start charging.
If that is correct then i should be able to ditch the massive 12.6kg (27.8lbs) standard car battery and replace it with a battery that takes up about a quarter of the space and saves over 10kg (22 lbs) in weight.

I will have to check the regulations to find out how long the accessory battery is required to continue without input from the traction battery via the DC-to-DC converter.
Checked, it has to be able to power the hazard lights flashing for a minimum of 20 minutes.
So 6 globes. (rear 2 x 21W, front 2 x 21W and side 2 x 5W) 94watts or 7.8A at 12 volts
50% duty cycle (flashing) lowers it to 3.9A averaged.
So 20 minutes should only need about 1.3Ah from the 7Ah battery
Looks like it may be doable.
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