ELECT-26, Excessive Battery Discharge Rate


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General Information

All 924s, 944s, and 968s use a BCI Group 41 size battery. That means that the dimensions of the battery are 11- 3/16" L x 6 - 7/8" W x 6 - 7/8" H. For a full list of BCI Battery sizes click here. Most 944 batteries now days are rated for 50 to 70 amp-hours (AH). That's means if you have a 50 AH battery, it should be able to discharge 1 A for 50 hours without falling below 10.5 VDC terminal voltage. That's considered about the minimum cranking voltage for a battery.

The normal battery discharge rate for a 924, 944, or 968 with the ignition turned off and the key removed from the ignition should be less than 60mA. If you current draw is more than 65 mA, you should start looking for a problem.

So, working the math, if you have a 50 AH battery and your battery discharge rate is 60 mA, the battery should last for about 30 days and still be able to crank the car. Realize that this will change as the battery gets older and it will also be different under varying weather conditions to which the car (and battery) may be subjected.

Checking Battery Discharge Rate and Identifying Causes of High Battery Discharge Rate

  1. Disconnect the battery positive lead.
  2. Connect the digital multimeter between the battery positive lead and the battery positive terminal.
  3. Select the multimeter to the ammeter function and turn the meter ON.
  4. Check the multimeter reading less than 65 mA (.065 A). That's your discharge rate.
  5. If the discharge rate is greater than 65 mA, start removing and then reinstalling fuses one at a time. When you remove each fuse, write down how much the discharge rate of the battery decreases.
  6. After that, try removing and installing relays one at a time and write down how much the discharge rate drops for each relay.
  7. Once you've completed this, identify all of the locations where you saw a significant decrease discharge rate. Using the factory wiring diagrams if necessary, determine all components supplied by those fuse/relay locations. Disconnect components from those circuits one at a time until you identify which one is the cause of the high current draw (as indicated by the ammeter).

Clark's Garage 1998