Battery Checks
Visual and electrical procedures for checking 944 battery condition, state of charge, charging, and jump-starting, including the critical warning about boost charging and DME damage.
Denne prosedyren er ikke oversatt ennå og vises på engelsk.
Introduction
A visual inspection is the important first step in assessing battery condition. If the visual checks reveal nothing obviously wrong, proceed with electrical testing.
Visual Checks
-
Inspect the battery case for cracks.
-
Check the battery terminals for corrosion (white or rust-colored deposits on posts or clamps).
a. If corrosion is present, loosen and disconnect the battery clamps. Clean the battery posts and clamps with a wire brush. b. After reconnecting the terminal leads, apply petroleum jelly to the posts and clamps to slow future corrosion. Felt anti-corrosion rings for battery posts are also effective.
-
On 944s, the battery cover is important for minimizing corrosion and protecting the battery tray.
-
Check for bulges in the battery case — a bulging case indicates a failing battery.
Battery Condition Check
The most reliable method is to have the battery professionally load-tested at an auto parts store. Alternatively, the state of charge can be estimated by checking the no-load open-circuit voltage with a quality digital multimeter (accuracy of 0.5% or better). Disconnect the battery positive lead before measuring.
No-Load Voltage vs. State of Charge
| No-Load Voltage (VDC) | State of Charge |
|---|---|
| 12.75 | 100% |
| 12.45 | 75% |
| 12.25 | 50% |
| 12.05 | 25% |
| 11.85 | 5% |
Temperature correction: If ambient temperature is below 21 °C (approximately 70 °F), add 0.021 V to the reading for every 1 °C (1.8 °F) below 21 °C.
Another method is to check specific gravity with a hydrometer, though most sealed batteries do not permit this.
Replace the battery if any of the following apply:
a. 0 VDC after a 6–8 hour charge — indicates an open cell. b. Approximately 10.0–10.6 VDC after a 6–8 hour charge — indicates a shorted cell. c. Battery will not hold at least 12.45 VDC after a 6–8 hour charge at 13.8–14.6 VDC.
Quick check: If the interior lights are extremely dim or will not illuminate, there is insufficient charge to crank the engine.
Charging
The best charging method is a slow charge using an automatic constant-voltage charger. Charge for 6–8 hours at 13.8–14.6 VDC. Charging below 13.8 VDC or above 14.6 VDC shortens battery life significantly.
Jump Starting
The battery positive terminal on the 944 is located close to the windshield frame, making it difficult to attach a jumper cable clamp without the clamp contacting the frame.
One effective approach: clamp needle-nose vise grips onto the battery positive terminal and attach the jumper or charger lead to the vise grips.
Connect jumper or charger leads in the following order:
a. Dead battery positive b. Good battery positive c. Dead battery negative (preferably a ground point away from the battery) d. Good battery negative (again, preferably a ground point away from the battery)
Warning: Never use a battery charger's BOOST or fast-charge feature when connected to a 944. The resulting voltage spike will damage the DME computer.