An R107 that will not start after sitting for a week usually has a parasitic drain, an aging battery or both. The drain test takes about half an hour with a multimeter.
What is normal?
Every car draws a little current with the key off for the clock and the radio memory. On an R107 the normal drain is small, typically a few tens of milliamps or less. A drain much higher than this will flatten a battery over days or weeks. Use the figure in the factory manual if your car has one.
The multimeter test
- Make sure the battery is fully charged.
- Turn everything off, close the doors and trunk (or hold the switches closed so the lights stay off) and remove the key.
- Set the multimeter to measure amps on the 10 A range first.
- Disconnect the negative battery cable and connect the meter between the cable and the battery terminal.
- Wait several minutes for any circuits to go to sleep.
- Read the current. If it is high, switch to the milliamp range only if the reading is well within its limit.
- Pull fuses one at a time and watch the reading. When the reading drops, you have found the circuit.
A good multimeter makes this easy: automotive multimeters on Amazon.
Common drains on an R107
- Trunk light staying on because the switch is misadjusted.
- Glovebox light staying on.
- Radio or an aftermarket amplifier wired to constant power.
- Alternator diode: a failed diode can drain the battery. Test by disconnecting the alternator’s main wire during the drain test. See alternator.
- Clock: a faulty movement can draw more than normal. See clock.
- Interior lights with a bad door jamb switch.
- Relays sticking on, such as the fuel pump or blower relay.
Battery condition
An old battery may not hold a charge even with no drain. Have it load tested. A good battery at rest should read about 12.6 V fully charged.
Battery disconnect switch
For cars that sit for long periods, a battery disconnect switch on the negative terminal is a simple way to stop any drain. Remember that the clock and radio settings will reset.
Battery tender
A smart battery maintainer keeps the battery charged when the car is stored, and is the best option for winter storage. See storage.
Quick answers
- Can a bad starter drain the battery? Only if it is sticking on, which you would notice.
- How long should a battery last? Several years with proper care.
- Should I disconnect the battery for winter? A maintainer is better; a disconnect switch is the next best.
- Is a high drain dangerous? It can mean a short; find it.
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