Battery keeps going dead

[1slow95]

I let my car sit for about 2 weeks and the battery is almost completely dead. I measured the amps being drawn with nothing on and it was 90ma. When I pulled the fuse for the radio/clock/etc, it dropped to 13ma. What should I look for next?

The battery is brand new, but I drained it trying to start the car with a bad starter. Im gonna start it charging in the morning.

 

[HISSIN50]

13 mA is outstanding! I would chalk it up to perhaps a not fully charged battery and the sitting for 2 weeks withoutstarting.

Even the 90 mA is very good. An SPDT relay draws more than that (common in aftermarket alarms).

I would just start the car more often, use a trickle charger, etc........

I dont know how you are going to drop more than that (the puter is said to use 10-20mA for the KAM).

Good luck.

 

[1slow95]

Well that's good to know. So Im guessing it's more likely to be a charging problem then? I've noticed the voltage guage has been reading pretty low. I ran 4ga wire to the battery in the trunk, could that be too small? Bad ground? Bad alternator?

With the car off, I measured a .08 volt drop from the battery to the alternator.

 

[HISSIN50]

I would have run a larger cable for the battery, considering the 130 amp alternator and voltage drop that can occur. But that has nothing to do with the drain, unless the drain stops with the alternator disco'd from everything (suggests bad diode in the alt or the smallest short ever).

If your charging numbers look good at cold idle, hot idle (full load and no load), then the alt should be ok. I have not done a trunk battery on a stang so I am not sure what is acceptable for voltage drops over that length. Your less than a tenth of a volt drop looks excellent to me (the old rule of thumb is to stay under 1/2volt-drop for primary cables).

I am not sure where else to look for such a small draw. The KAM might be what is left............

Good luck.

 

[1slow95]

My thinking now is that maybe it's not something drawing the battery down, but maybe it's not charging properly.

At idle with nothing on(radio, fan, etc.), the guage looks a little low, but still good. With the lights, fan, and radio on, it drops to the "N" in normal at idle.

 

[BlueOvalStangGT]

Something is draining my battery as well. My battery gauge reads at like the R in normal so I guess that means its charging fine. Let me know what you find so I can look at mine as well.

 

[HISSIN50]

Mike, you really gotta grab your meter and check the specs. The stock gauge is just too inprecise to interpolate IMHO.

With the mods you have done, testing at various locations (i.e. the battery) can be helpful.

Good luck.

 

[1slow95]

Ok, I got the battery charged and re-installed. I checked voltage at 12.57 at the batter and 12.49 at the alternator.

Now for the interesting part. With the car running and all acc. turned off, I measured 14.7 volts at the alternator, and 13.7 at the battery. By turning only the fan on it dropped to 13.6 at the alt. and 12.5 at the battery. I didn't check it with the radio or lights on, but I figure it's probably draining the battery instead of charging at that point.

I came up with 3 likely culprits here, which one should I address first?
a, the 4ga battery wire is too small
b, the 8ga ground wire is too small
c, all of the above

On a side note, I don't recommend the UPR trunk mount kit. The box is nice, but the cables suck, and I don't think it's worth $129 for just a box.

 

[HISSIN50]

Yep, I dont like those voltage drop numbers. Personally I dont feel that 4 AWG is near enough for a trunk kit. 1/0 would have worked better. And IMHO always use a ground cable at least the same as the power cable. Remember that power flows from ground to positive. If the ground is weak, all the positive cables in the world cant help.

Please do note that I have NOT done a trunk mount on a stang and dont have personal experience with it. So if others disagree with what I said and they have done the relo', go with them.

You could run another set of cables in parallel if you wanted to. I dont know what the easiest and most cost effective solution is.

Sorry Mike.

 

[tmoss]

Yep, you need better cabling. 1/0 for both + and - would be good. If you have subframes, connect the - to the back of the subframe and then take a length from the front of the connector to the block. The + has to go all the way naturally. Run it inside the subframe connector to protect it. If you don't have subframes - add them!