450 SL Battery/Alternator

[450sl4me]

My SL won't start! I'm pretty sure it's my battery, but don't want to lay out the cash until I'm a little more certain. I put a volt meter on the battery while the car is running. It's charging at about 13.8 volts w/ no accesories running, 13.5 w/ ac, stereo, & lights on. It started having trouble starting about three months ago. It did this if I let it sit for 2-3 weeks between starts. Then it was if I let it sit for a week, now it won't start even right after I drive it. What should the voltage be while charging? I always thought you should have 14+. Even if I jump start it, I have to let wait 1-2 minutes before it will start even w/ it hooked up to my truck. What do you think??? ANy help would be greatly appreciated!

Chris

[G-Benz]

A good charging system should see 14.4V at the battery. 13.5V is kinda marginal, but it will do the trick.

The clue here is that you are having trouble starting. Your charging system does nothing for you here, but your battery needs quite a bit of cold cranking current to perform this.

Even if the battery reads a nominal voltage, a dead cell here or there won't allow it to maintain a proper charge.

I'm betting on the battery. If the charging system was at fault, it would suck a battery dry within a few days of driving...even a new one.

Waiting a week or two to start up the car wouldn't make any difference.

[kip Foss]

You can find out your problem for about $1.00. Go to Wal-mart or the like and buy a hydrometer. They are hanging on a small rack in the auto section. They are about the size of a fat pen and measure the acid density of the cells. They look like an antifreeze tester in that they have 5 small plastic balls that float at different concentrations of acid.

Charge your battery over night with a 2 amp or so charger. Check each cell. All cells should should float an equal number of balls-at least 3 and better yet 4 to 5. If one cell floats noticeable less balls then you have a bad cell and the battery is destined to fail. Scrap it and spring for another.

Also get an amp meter to test the amp drain on your battery with all accessories shut off. There should be a small drain for something like the clock but it should only be in the tenths of an amp. If you have more you have a ground. Check out which circuit it is on by pulling out fuses until the amp meter returns to zero. This will tell you which circuit has the ground. Next, find the ground.

[cbdo]

The hygrometer is the best method for determining a lead-acid battery's state of charge, but as a quick and dirty alternative you can use the voltage when the battery's at rest (ideally no charging for at least a half-hour), or under mild load (eg parking lights on). I keep the following chart taped to the back of my VOM. Also, check the voltage with everything at least nominally off, and then turn on something like the parking lights and check again; if you see a significant drop or an increasing drop after a minute or two, the battery's low, and if this happens after you've just charged it overnight, it's a large paperweight.

Battery volt/charge chart

% charge----------------volts
0 ---------------------- >12.6
75-100 ---------------- 12.4-12.6
50-75 ---------------- 12.2-12.4
25-50 ---------------- 12.0-12.2
0-25 ----------------- 11.7-12.0
0 -------------------- <11.7

The source was official, I just can't remember where I found it.