I’m installing a stereo in a 1999 e4 and can not find a source of constant power for the station memory and clock. Any ideas?
I’ve considered both of the following but don’t feel is it safe to have multiple sources of power and grounds.
- Directly connecting to one of the main batteries
- Installing a small 5ah VRLA battery under the dash.
I can’t find the post, but at least one member just used a NiMh or NiCad rechargeable 9V battery along with a resistor to drop the voltage to a voltage appropriate to charge the 9V battery. Seems like the best solution I’ve read of, so far. So, I thik it would be wired from 12V supply to resistor to one pole of 9V battery and from other pole of 9V battery to memory wire from radio. Elegant solution, and doesn’t require you to carry around a big lead acid battery for such a tiny electrical load. Ohm’s law can be used to calculate the proper value for the resistor. You might also want to put a diode inline as well.
You could buy one of those two way battery switches that are used on boats to switch between two batteries. When the car is parked and being charged you could be on a battery and once you are moving you could switch to the 12 volt power supply that normally runs the radio. The nominal power used to keep the memory powered would be insignificant to the battery you used to power it so it would not cause that battery to become more run down than its mates. The switch usually has three positions, left (1), right (2) and both.