To fill your battery you need a funnel and some distilled, not tap, water. You can get it by the gallon at most grocery stores. Slowly splash small amounts into the funnel until the point that it is just visible not leaving the overflow, which is approximately at the bottom of the cap when it's in. The reason you can't use tap water is the minerals and negative ions like chlorine and flourine interfere with the chemistry within the battery. To tell you more than you probably ever wanted to know, what is going on inside your battery is the following: The battery is composed of alternating + and - plates. The + plates are composed of lead oxide (PbO2) and the negative plates of pure lead (Pb). The battery is filled with sulfuric acid (H2SO4) at a specific gravity of about 1.26 or so. When you demand power from it (discharge), the SO4 from the H2SO4 solution combines with the pure lead plate to make PbSO4, and the remaining H2 from the solution combines with the O2 from the positive plate to make water (two molecules). All of this extra water lowers the specific gravity of the solution and thus the charge of the battery. When you charge the battery, the reverse occurs. The SO4 is forced from the negative plates (back into solution), O2 back to the positive plates. Most of the H recombines with the SO4 to reform H2SO4, but some of it escapes as hydrogen gas, hence the explosion hazard and the ultimate reason batteries die eventually. The specific gravity goes back up as well. So, after you have filled it and charged it, and be careful as the faster you charge a battery, or the more charge it needs, the more likely it is to release the hydrogen ( which is why you have shaved some time off of your battery's life) so make sure it's charged in a ventilated area and there is no ignition source (smoking, sparks from charger leads, etc). Once you have it all charged up, you can check the specific gravity of the solution and see that it is OK. You can get a tester anywhere that works a lot like an antifreeze tester. As far as maintenance free batteries go, I don't like them. They last half as long as these batteries (periodic addition of water replenishes some of the H's lost), and they have shorter reserve times. The only reason to use one is if it's buried under other components as in new cars. Plus, these batteries serve as a little reminder that your car needs you and your diligent maintenance to survive. You haven't harmed your battery as much as you might think.
1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick