How to check for battery drain and battery health

997s are known for draining their batteries quite quickly due to a combination of relatively high idle drain and small batteries. It is recommended to have the car on a battery conditioner when sitting still for more than a week or so. Since the cigarette lighter plug is always connected to the battery (even with the ignition off), this is a convenient way to charge the battery. I have a Ctek MXS 5.0 with extension cable and the cigarette plug adaptor for this purpose.

If your battery goes flat within just a few days, something is wrong. There are two possible causes:

  1. Something draws a lot of current even when the car is “asleep”, i.e. off with locked doors.
  2. The battery has a reduced capacity and is thus bad.

For both checks you’ll need a multimeter. Fluke is the premier brand, but they are expensive. Cheaper multimeters should do fine. You’ll want one that can measure higher currents. 10A is common, but many cheaper multimeters can only do these for a limited amount and possibly unfused. If you work alone, you can make life much easier for yourself by getting a multimeter that has Bluetooth. I’ve got an Owon 18B that has a great feature set for its price point – auto range, fused 20A input, and Bluetooth for connecting to an iOS or Android device.

Battery drain

For measuring battery drain, the multimeter needs to be placed in series. Disconnect the negative pole on the battery and wire the multimeter in between. It needs to be set to current (amperage) measuring mode, using the 10A/20A input. Then let the car think that all doors and the frunk are closed by manually closing the latches, and lock the doors. The car should now go into sleep mode quite quickly, and the multimeter should then show the idle drain. It’s explained very well in this video:

My car draws about 2.4A with the doors unlocked and open. I’ve read that 40-50mA is pretty normal for the idle current, but my multimeter shows even less – that’s a good sign. Should your car exhibit a high current draw, you’ll need to draw the fuses one by one until you find the circuit that causes the draw, and then identify the source. As the fuse compartment is in the driver’s footwell, Bluetooth on the multimeter will make this much easier.

Battery health

If there’s no excessive idle drain, but your battery still goes flat quite quickly, chances are that you battery is in poor condition. This can be established my measuring voltage (electrical tension). Voltage is measured in parallel. Both idle voltage and voltage under cranking are good indicators of battery health, as explained by ChrisFix in this video:

My car sits around 12.8V with the car totally asleep. Cranking voltage can be a bit difficult to measure if you car starts quickly, but this appears to be about 10.7V, which is fine. The “ignition on” voltage of 12.2V is a bit low in my opinion, so this might be an indication that the battery isn’t totally fresh any more.

Voltage with car asleep
Voltage after unlocking doors
Voltage with ignition on
Voltage with running car, alternator is working

Another indicator of battery health is the internal resistance. I’ve found these guidelines:

  • < 5 mOhm: Very good condition
  • 5-10 mOhm: Good condition
  • 10-30 mOhm: Poor condition, but possibly revivable
  • >30 mOhm: Very bad condition

My multimeter would not give a resistance reading for my 997’s battery, which could be taken as a good sign, but it also possible that even 30 mOhm would be outside the multimeter’s measurement range. With a dedicated battery tester it should be possible to measure this correctly.

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