Manufacturers introduced the 6-volt, sealed lead-acid battery as a maintenance-free and safer alternative to a regular lead-acid battery. The concept of a sealed battery is good, until you suspect that the electrolyte fluid level has fallen. Regular lead-acid batteries are simple to top-up -- just remove the cell caps and fill the reservoirs using distilled water. If you find your sealed lead-acid battery isn't charging properly, you have a few options. Get a replacement free, if it's under warranty. Purchase a replacement, if it's out of warranty. Or, break the seals to check the fluid levels to see if you can fix it.
Put on protective goggles and lightweight gloves before attempting to fix your 6-volt, lead-acid battery. You will be accessing the fluid in the cells which contains sulphuric acid.
Disconnect the negative black battery cable from the 6 volt battery using a wrench. This ensures the electrical supply to the vehicle is completely isolated.
Look on the top of the battery and you can see a sealed plastic plate centrally placed, running from one end of the battery to the other. Under the plate are three cells caps. The plate usually has a label saying that removing the plate invalidates the warranty, so only proceed if your battery is out of warranty.
Break open the plastic plate. The method depends on the way it is fitted, so you need to work out the most appropriate way. The plate is usually fixed using one of two methods; it simply has strong adhesive tape over the top, or the plate is securely glued in place. Go to step 5, if the plate is held in place using tape, or go to step 6, if the plate is glued in place.
Use a sharp knife and cut cleanly through the tape around where the plate joins to the main battery casing. Use a flathead screwdriver to pry under the edges of the plate and you find it starts to lift off. Remove the plate using your fingers to reveal the three inner cell caps.
Use a sharp knife and very carefully slide it around the edge of the plate to try and break the glue seal between the plate and the casing. You may need to do this several times, but each time you will find the knife goes between the plate and the casing a little deeper. Once the knife slips between the plate and the casing, use a flathead screwdriver and slide it in the gaps you have made with your knife. Twist the blade of the screwdriver, so you start to pry the plate upward. Repeat this in different positions around the plate and the plate starts to come away. You need to be patient as it can take a while. Don't struggle too hard as you may snap the plate. It will eventually come lose. Remove it using your fingers to reveal the three cell caps.
Use a screwdriver and pry under the cell caps. Twist the screwdriver to release them, and then remove the caps using your fingers. Try not to break the caps during the process as you will need to get replacements, which may be difficult.
Check the fluid levels. The fluid needs to reach the maximum marker on the side of the cell. Pour distilled water into the cells up to the maximum marker. Don't exceed the maximum marker since there needs to be room for expansion.
Attach the cables from your battery charger to the terminals on the battery. The red cable attaches to the "+" terminal and the black cable attaches to the "-" terminal.
Set your charger to slow or trickle charge. Fast charging will not fix your battery. Turn on your charger and let it charge for at least 12 hours; 18 hours is even better. Check your battery after about six hours; you see fine bubbles rising to the surface in the three cells. This is a good sign and means the battery is charging.
Turn off the charger after the prescribed time. Remove the battery cables from the battery terminals.
Place a little adhesive sealant around the edges of the three cell caps. Push the cell caps into place using your fingers. Put adhesive sealant around the edge of the top plate, then put the plate into position and press down firmly using your fingers. You may find it easier to put a weighted object on top of the plate for a while to hold it in place while the adhesive dries.
Reconnect the black battery cable to the "-" terminal on the battery. Use a wrench to tighten the clamp.