Solid State Voltage Regulators For 3 Brush Motorcycle Generators


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12 To 6 Volt Battery Conversion
A re-VOLTing experience

    The problem of finding a battery to fit many early motorcycles can be a tough one. A very limited number of 6 volt motorcycle batteries are being manufactured today. The dimensions of those that are rarely fit the dimension of an antique motorcycle’s battery box. The result is that you usually resort to running a smaller (sometimes much smaller) than normal battery just to have something in the battery box. Running a smaller battery than normal creates two problems. First, because of the lack of battery amp/hour capacity, your lights will not be as bright as they should or will grow dim sooner than normal. Secondly, your generator is much more likely to overcharge the little battery, severely reducing its life expectancy.
    An easy and economical way to make a 6 volt battery that will come very close to filling your battery box, is to convert a 12 volt battery. Dozens of different sizes are available from your local motorcycle or power equipment dealer. All you need to do is select a suitable donor. The resulting 6 volt battery, while extremely functional, wins no awards for original appearance. For this reason the conversion is best suited to machines that use an enclosed battery box. It can also be used with a replica battery shell or hollowed out original battery with a little ingenuity.
    How are we going to do this you ask? Well its not alchemy, but it’s almost as valuable as turning lead into gold. Essentially, you’ll be sawing a perfectly good, brand new 12 volt lead/acid battery in half to make a 6 volt! All motorcycle type batteries are divided into cells. Each cell produces approximately 2.1 volts. So actually a 12 volt battery produces about 12.6 volts - cut in half you’ll end up with a 6.3 volt battery.
    A few words of caution first. Only use a new battery that has never had acid in it. Battery (sulfuric) acid is nasty stuff that can cause serious injury to you and your motorcycle. Use only a lead/acid battery, not use a “gel” or any other super-duper modern type battery. Whenever you’re handling acid, always wear approved safety goggles, rubber gloves and protective clothing. Work in a well ventilated space, preferably outdoors.

    The first thing you need to do is measure the dimensions of your battery box or case. You will be selecting a battery that when cut, will yield about 3-3/4 cells that will fill the box as completely as the original. The fourth cell is where the battery will be cut and the plates removed. This allows some flexibility in fitting the battery to the dimensions of the case.

Donor battery showing approximate location of cut

    You will want a new 12 volt battery that is almost as tall as the box (leaving a little room for a battery pad on the bottom and connections on top) and width to just fit inside the box. The battery’s length should be just a little less than twice the length of the box or shell, so that when cut through the fourth cell it will be the correct length to fit inside.
    With your battery box dimensions in hand, go down to your friendly neighborhood motorcycle shop and ask to look at their 12 volt battery charts. These will show the dimensions of each battery as well as the terminal types and locations. Keep in mind that the new battery’s length is divided into 6 equal sized cells, you should select one that when cut near the outer edge of the fourth cell will render the 6 volt size you need. It needs to be cut near the outer edge of the fourth cell to leave room for the new terminal connection on the interconnecting link.
    Be sure to tell your dealer not to put acid in the battery. New batteries are usually shipped with a sealed container of acid, make sure you get it along with the battery. While the dealer may think he is doing you a service by filling and charging it, in reality he will have ruined it for your purpose.
 Now that you have your battery on the bench, look at it carefully. One end should have a vent on it.     This is to allow gasses to safely escape when charging and in the event of a spill, allow acid to drain safely away from the battery box. It makes no difference whether you cut off the positive or negative end, but you must have the vent on the part you will be using. Take a marker and carefully draw a line where you want to make your cut. Double check your measurements. Remember the old carpenter's adage -"measure twice, cut once".  Gently but firmly hold the battery in a vice (or if your assistant trusts you, hold it firmly by hand). Get your hand saw out and carefully cut the battery in two. The plastic shell will cut easily as well as the internal lead connecting link.

A hand saw cuts through the battery with ease

    Voila’!  You now have two batteries. Inside you will see alternating layers of plates remaining in both halves. Set aside the 2-1/4 cell part, you won't have much use for it. The lead plates will be attached to the interconnecting link by small tabs. Take a pair of wire cutters and carefully clip the lead plates away from the link.
    If you made the cut at the outer edge of the fourth cell, there should be about 1/2" - 3/4" of interconnecting link protruding from the wall of the third cell. The shape and exact location of this link may vary with manufacturers, but all batteries will have it. Drill a 3/16" hole for a #10 screw through the protruding link. This will be where the new terminal lead is attached.
    Looking at the freshly cut end, you should see a ventway that runs the length of the battery across the top. Your saw cut right through it. This ventway must be sealed at the cut end to prevent acid from leaking out. Mix up a little epoxy and carefully fill the opening. Use just enough to seal the end. Too much and it will seep into the third cell, ruining it.
    Now for the terminal leads. A two conductor trailer wire connector is an easy way to make a quick disconnect. The one I use is rated at 20 amps, as is the inline fuse holder. This doesn't mean use a 20 amp fuse.  Rather, use a fuse rated just a little over your motorcycle’s maximum electrical load or generator's maximum rated output, whichever is greater. For example my 101 Scout's Splitdorf generator puts out a maximum of 5 1/4 amps, which is a little more than what the headlight and taillight draw. A horn can draw anywhere from 2 to 5 amps depending type, so be sure to take this into account. In this case I decided to use a 7.5 amp fuse.
    Solder a #10 ring terminal to one end of the fuse holder lead. Feed one of the quick connect leads through the unused fourth cell filler hole and solder it to the other fuse holder lead. Use shrink tubing to insulate all soldered connections. Solder a terminal to the remaining quick connect lead in order to attach it to the battery's other terminal. Connect your new leads with new hardware and coat the terminals with battery anti-corrosion gel. Solder the other half of the quick connect to the bike's hot and ground wires, making sure they match the polarity of the battery’s leads.
 You may recall that when you first cut the battery in two, there was some dark powder or sediment in the bottom of the battery cell. This was residue from the insulating plates. As time goes on, the plates will continue to deteriorate primarily due to vibration, leaving this residue. Before adding the acid, remove all the caps and turn the battery upside down and gently shake the residue out of all the cells. This will help extend the life of your new battery.

The completed 6 volt conversion

    You are now ready to add the acid. Take the battery outside to a safe location. Put on rubber gloves and safety goggles. Follow the manufacturers instructions for filling the cells with acid and charging.
 For less than about $30.00 you have a new 6 volt battery that should last several seasons if properly care for. Happy trails.

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