Battery question

[dabuddha]

1998 Mercury Grand Marquis. I've owned the car since 2005 and had a remote start system installed back in 2005. This car was my daily driver up until August of this year. Since then, I've driven the car once a week to keep it all kosher but I've been having problems with my battery dying on me. One thing to note, the battery had been completely discharged a couple of times back in Jan/Feb 2011 because of a bad alternator. I got advanced auto parts to warranty the battery out and I got a new one this weekend.

My question is, how can I test to see if there's a drain on my battery when the car is off? I'm wondering if the remote start or something else might be draining the battery. I might not have noticed it previously since I drove the car daily so it'd get charged by the alternator.

TIA!

 

[Raizinman]

It sounds more like you just don't drive the car enough. Remember, a normal alternator doesn't really put out much amperage until you get to around 2500 or more RPM. If you keep starting the car, driving it a short distance, it's likely you're not charging the battery enough. Is it possible that you use the remote start and just let the car idle for a period of time? At around 800 or so RPM, the alternator puts out about 5-10 amps, which is enough to run the car, but not really enough to charge the battery.

 

[dabuddha]

I'd usually drive it down to the 7 eleven (about 5 miles away) and back (after getting a slurpee of course!) But what's strange is the battery would get drained within the week after sitting in my driveway. I've never had that happen with my other cars (I came back after 2 months from a trip to India and my grand caravan started up just fine).

 

[PottedMeat]

you can try this

1. turn off everything you can
2. copy your radio presets or whatever may be forgotten
3. disconnect the positive cable from the battery
4. put multimeter into 10A range ammeter mode and attach the positive wire to the battery, the negative wire to the disconnected wire
5. read ammeter ( if you blow the ammeter fuse - something more than 10A there's definitely a problem ) the off draw should be very little

 

[dabuddha]

you can try this

1. turn off everything you can
2. copy your radio presets or whatever may be forgotten
3. disconnect the positive cable from the battery
4. put multimeter into 10A range ammeter mode and attach the positive wire to the battery, the negative wire to the disconnected wire
5. read ammeter ( if you blow the ammeter fuse - something more than 10A there's definitely a problem ) the off draw should be very little

Ahh I'll try that. So basically the multimeter will go between the battery and the car and ideally, there should be very little to none coming from the battery?

 

[PottedMeat]

Ahh I'll try that. So basically the multimeter will go between the battery and the car and ideally, there should be very little to none coming from the battery?

yeah, the multimeter goes in series. it shouldn't be 0, because at the very least some control unit is running like security or keyless entry.

oh and make sure you're in 10A DC current measuring mode and disconnect any accessories.

 

[Raizinman]

You left out a step. Make sure to pull the clock fuse and the hood bulb, otherwise, you will be getting a false reading of a load on the battery. Also, make sure the doors are all closed. Then if you still have a load on the electrical, you to isolate which circuit, start pulling fuses until the multimeter reads zero. Then you will know if the drain is coming from the brake light circuit or wiper circuit, etc. etc. Most common drains are bad diode in alternator and glove box light staying on.

 

[disappoint]

You can test for more than 10 amps if you want to. Though I agree it shouldn't draw more than 10. To test for more than 10 amps use a high power low resistance resistor in series and check the voltage drop, then use ohms law to calculate the current flowing through the resistor.