My 1998 Chevrolet Suburban has been a faithful trooper since we purchased it right before our 3rd child was born back in November 1997. We had dead battery issues a couple of times because the kids would turn on a dome light or reading light and it would drain the battery overnight.
| To combat the issue, we just leave the dome override switch “off” and it has pretty much solved the problem. I recently put a new battery in the vehicle only to have it go kaput withing about four months. I checked the operation of the alternator and charging system and everything seemed fine. The replacement battery lasted about two months. What the heck is going on?The other day, I decided to see what the current drain was when everything was turned off. Disconnecting the battery cable and installing my trusty Fluke 77 digital voltmeter, I could see about 3.5 amps discharge for about 3-5 seconds that would drop to ~1 amp a few seconds more it was down to 300mA and by about 10 seconds it would sit at 150mA. I couldn’t find a spec for what the discharge should be but the most articles said no more than ~250mA and probably much less, given |
that it was older and has only minimal “brains”. I found another website that said I should probably see no more than 25mA. Time to hunt…
Under the Hood
I raised the hood and installed the ammeter. I pulled the major fuses, one at at time and the fuse labeled “Lights” and rated at 50amps would stop the discharge when removed. All the other fuses had no effect on the current drain. They might cause a momentary spike when re-installed, but it always settled back to ~150mA. Hey! We’re getting somewhere. I couldn’t believe that there was a single fuse for all the lights at 50amps. Shouldn’t there be a fuse per load? Perhaps one for head lights, one for tail, one for brakes etc?
Who Says Owner’s Manuals are Useless?
The owners manual says there is a second panel. I opened it up and started pulling fuses. On the third one the discharge dropped to 0! The courtesy lights and such were the culprits, maybe… I continued checking all the fuses and there were no others that had any effect on the current drain.
During this ritual, I could see where the “experts” that installed our cell phone base station connections (Charger, antenna & microphone) had replaced the fuses in the wrong spots and had little pieces of copper wire poking out of the fuse block. Basically all they did was strip the wire, place it across the fuse slots and then push the fuse back in. Lovely! These are the same guys that used really long screws when they installed the stand/mount. Fortunately they didn’t hit any wires, cables or hoses running alongside the transmission, but I did have them replace the screws with shorter ones so it would be less likely. They’re the same guys that installed the antenna cross polarized (no wonder there was no signal improvement when the phone was in the holder.) Loss dB = 10log10(Cosθ)2 while the formula says that I should have had infinite loss, that would probably only be true under very controlled conditions. In fact, the phone did “work”. I guess the next time I have an expert do an installation I’ll insist on watching over their shoulder… Oops, I digress.
OK, I re-read the manual for what was in that circuit, supplied by that fuse. The dome lights were off, if I turned them on, the drain went to over 3.5amps. OK, not them. Power mirrors, moving the switches has no effect. (Ignition is off) Next to check is the lights that illuminate the mirrors on the back of the sunvisors. What’s this? The driver’s side cover has a broken hinge but it shuts ok… can I verify the light is turning off as it closes? I couldn’t see under the cover as it has a lip that goes over the edge. OK, I’ll open it all the way and press on the switch. Bingo!! that’s it. I took the lights apart and was unable to remove the #74 wedge base bulbs… What I need is one of those Chinese finger puzzles designed for an infant. Further dis-assembly exposed the wires, snip one and fold it back problem solved. Mirror re-assembled and we’re nearly done. Now to jump the car and let it charge the battery.
So the no-load current drain appears to be less than 5mA.
One thing that might be of use to you is how to approximate the state of charge of the battery. Knowing that a fully charged battery is 12.6v, here’s a quick and dirty thumb rule: 0.2V/25%
| Open circuit Voltage | Charge |
|---|---|
| 12.65 V | 100% |
| 12.45 V | 75% |
| 12.25 V | 50% |
| 12.05 V | 25% |
| 11.85 V | 0% |
These voltages and percentages are approximate. The only way to truly measure the state of charge is by measuring the density of the electrolyte. The voltages above are at 20C(68F) and should be adjusted -0.022V/C. Again these are approximate numbers.
Feb 12, 2010:
Update: After figuring out and correcting the problem, I replaced the battery. Fortunately, it was under warranty so it cost me nothing but the time to remove/install and drive to from the auto parts store. It’s been nearly three months now and nary a whimper from the battery. Starts first time every time and vigorously.
May 5, 2010: Happy Birthday #1 Son! The hinge on the mirror cover broke completely so we threw it away. The battery is going great guns and we’re a bunch of happy campers…
