If you've been thinking about replacing the battery in your Bimmer because it won't hold a charge anymore but you don't want to spend forty-plus dollars for a new one, it may be possible to revive your old one. The process seems to work best on batteries that have sat unused for long periods of time, such as cars in winter storage or motorcycles.
When confronted with a year-old BMW R90S motorcycle that wouldn't start, I found that a new battery (German Varta) would set me back 75 dollars. Since very few batteries would fit in the space provided, I decided to try to save the old one.
Assuming most BMW owners already have a battery charger (trickle will do), you will also need a battery hydrometer and, depending on battery size, enough battery acid for a refill. This can be purchased at an auto parts store for $2-3 for 1 1/2 gallons. Sears makes an inexpensive hydrometer with specific gravity readings which is better for this purpose than the floating ball type.
Let's first look at the way a battery works. In the conventional lead-acid battery, there are two kinds of plates present. The positive plates are made of lead peroxide; the negative plates of spongy lead. As the battery becomes discharged by putting a load across the + and terminals, the sulfuric acid combines with the lead peroxide and lead to form lead sulfate and water. This lowers the specific gravity of the electrolyte (or floats less balls). Charging the battery reverses this process by returning the sulfate to the solution and releasing hydrogen and oxygen. This also raises the specific gravity of the electrolyte. But if the battery is inactive for long periods of time without being charged, a condition known as a "sulfated" battery may result. The lead sulfate will group into large crystals and may eventually bridge across and short out one or more cells in the battery. (For every cell you lose, the battery loses 2 volts. Twelve-volt batteries have 6 cells; six-volt batteries 3.)
So, the first step in attempting to revive your battery is to put it on a trickle charger overnight. Then, using rubber gloves to keep the acid off your hands, pour off the acid in the battery. Where you put it is your problem. I poured mine on a rock driveway, then hosed it down to dilute it. Don't pour it on concrete as the acid will eat right into it. Refill the battery with tap water and tap on the battery case firmly to dislodge the lead sulfate crystals, but not so hard as to loosen the plates. Pour off this liquid and keep repeating until the fluid comes out clear.
Now, the next step would seem to be refilling the battery with new acid, but DON'T DO IT YET!!! There is still plenty of sulfate on the plates which must be driven back into solution and drained off or the addition of new sulfuric acid electrolyte will raise the specific gravity too high and eat up the plates. So just keep adding water, trickle charging, draining, refilling, and checking the specific gravity until it will get no lower (or the fewest number of balls will float). I did this 5 or 6 times till I couldn't get a lower reading.
Now you may add the new acid, being careful not to overfill. Trickle charge and the job is complete. If it works, you've saved quite a sum. If not, you're only out the cost of the acid and maybe a hydrometer. Anyway, you should have bought that trickle charger before and had it connected to your Bimmer's battery last winter while it sat waiting for spring to arrive. Then that nasty old lead sulfate would never have formed in the first place.
Author: Tim Dorr
Extreme caution should be observed whenever you are working with battery acid. Even though you wear rubber gloves to protect your hands be sure not to touch any other part of your body with the gloves unless they have been thoroughly rinsed off with cold water. Also be very careful not to spatter any acid on your clothing Tech Ed.