12V standby battery

[arber333]

Hi

I must have discharged my 12V AGM battery too many times. It is dying on me. New AGM 35Ah 12V battery is at 110€! Also i noticed they have quite slow charge rate of only 10A. This makes for dangerous charging level if cell is discharged too low. I bet this is the reason my poor AGM is toast.
Since prices of LiFe cells are slowly lowering I am wondering if that would be my alternative. Does anyone have any suggestion? Please bring it on...

A

[bexander]

For what purpose do you have the 12V battery?
Does it supply power to the main contactors and once they are on the vehicle 12V is supplied by a DC/DC from the mains pack or does it supply power during the whole operating time of the vehicle?

In my car I use a very small pack consisting of 4pc A123 LiFePO4 2,5Ah cells together with a cheap ebay BMS. It supplies power to the main pack BMS when the car is parked but the BMS goes to sleep after 15min so very low consumtion in this state. When the car is powered on it powers the main contactors and then the DC/DC takes over. This pack can handle high currents for short periods of time, if needed.

If you have higher consumtion I would go with a higher capacity pack.

[Kevin Sharpe]

How cold does it get in your location? We fit lead-acid batteries in our commercial applications because they can always be charged even at extremely low temperatures.

[arber333]

For what purpose do you have the 12V battery?
Does it supply power to the main contactors and once they are on the vehicle 12V is supplied by a DC/DC from the mains pack or does it supply power during the whole operating time of the vehicle?

In my car I use a very small pack consisting of 4pc A123 LiFePO4 2,5Ah cells together with a cheap ebay BMS. It supplies power to the main pack BMS when the car is parked but the BMS goes to sleep after 15min so very low consumtion in this state. When the car is powered on it powers the main contactors and then the DC/DC takes over. This pack can handle high currents for short periods of time, if needed.

If you have higher consumtion I would go with a higher capacity pack.

I use 12V battery to start the car, actuate windows, radio and sustain BMS, emergency lights and stby systems.
So... you use smallish 12V pack and switch on DCDC at the first key position?
But still everything on standby inside my car consumes some 0.2A continuously.
Also my battery is already beat up and it just folds in about 24hrs.
I connected 2A 12,6V power supply so battery doesnt drop voltage, but that means constant drain on my HV system.

A

[bexander]

In my car I only have two key positions, on and off. On results in contactors on and DC/DC starts up. The battery only powers the car for a second or two. 12V 2,5Ah and 300g is a tiny battery but does the job. The current consumption in off position, after bms goes to sleep, is less than 100uA.

A standby consumption at 0,2A results in 4,8Ah used every 24hrs. A 35Ah battery will be empty in less than a week.
Is there any way you can reduce the standby consumption? I think if you can it will increase the life of any new battery.

[Doig5710]

Yea I'd focus on getting the draw off down, not EV specific but on cars we'd get through the workshop current draw needs to be below 0.040A for everything but have let a few euro vehicles go with up to 0.060A. Some cars used to take up to 30-40 mins to go into sleep mode.

[jon volk]

I got tired of having to plug in both to charge the HV battery and a tender for the 12v. My VCU is live 24/7 so I just wrote a function to turn on the DCDC for 15 minutes if it drops below 12.2vdc. I haven't had to tend to the 12v battery in 5 months.

[Isaac96]

Nice. I've got a tender connected to the J1772 for a little trickle when charging, but that way sounds much simpler.