Recharging the 12V AGM battery under the hood.

[JoulesThief]

How does the AGM 12V battery that runs all the peripherals and accessories in your car recharge? How do you keep it top charged? Is there also a generator or alternator that recharges it during driving, or does it recharge with the main high voltage traction battery recharge at the same time? What about when you DC charge?

 

[miimura]

All of the energy for the car comes from the traction battery. All electric cars have what they call a DC-DC converter. It is a power supply that takes the high voltage from the traction battery pack and produces voltages appropriate to charge and maintain the 12V battery. I don't know the exact spec of the DC-DC in the e-Golf, but most EVs have converters that can produce over 100A @ 12 VDC.

Whenever the car is OFF, the traction battery pack is isolated from the car's wiring. The 12V battery must be in good condition in order for the contactor (relay) inside the battery pack to activate and pass the high voltage to the car. You can tell where the high voltage DC is in the car because the cables and connectors must be orange. The e-Golf has a relatively quiet high voltage contactor, but you can hear it if you listen for it. Others make a much louder thunk or click when they activate.

Here is the list of things that get or put power directly to/from the high voltage traction battery pack. Everything else gets power from the 12V battery, which is charged from the traction battery by the DC-DC Converter.

- Traction Motor Inverter/Controller
- On-Board Charger
- Air Conditioner/Heat Pump Compressor
- Resistive Cabin Heater (cars without heat pump)
- DC Fast Charge Port (large pins below J1772)

That's just off the top of my head since I have not seen a schematic of the e-Golf. Some other EVs use HVDC for things like electric power steering.

 

[mfennell]

- Resistive Cabin Heater (cars without heat pump)

Taking this out of context - the SEL must have a resistive element too, right? Heat pumps lose efficiency much below freezing, if I understand it correctly. My father in FL has a heat pump in his home and much below freezing and it turns on the element.

I've only been down to about 30F so far and have not seen anything that indicates massive draw for heat.

 

[miimura]

- Resistive Cabin Heater (cars without heat pump)

Taking this out of context - the SEL must have a resistive element too, right? Heat pumps lose efficiency much below freezing, if I understand it correctly. My father in FL has a heat pump in his home and much below freezing and it turns on the element.

I've only been down to about 30F so far and have not seen anything that indicates massive draw for heat.

There is a component under the hood near the firewall that is fed by HVDC and has what looks like coolant pipes coming out. It is present in both the LE and SEL, so you may be right. I haven't seen a schematic of any automotive heat pump setup, so I'm really not sure.

 

[REM]

My LEAF's 12v only charged from the traction battery when the cars was on and not plugged in. I assume it's the same with the e-Golf-- does anyone know for sure?

 

[miimura]

My LEAF's 12v only charged from the traction battery when the cars was on and not plugged in. I assume it's the same with the e-Golf-- does anyone know for sure?

It should charge the 12V any time the traction battery is connected to the car. That includes active charging and remote climate control. Some EVs monitor the drain on the 12V and wake up the car to recharge the 12V at other times. I don't know the exact behavior of the e-Golf when not driven for extended periods.