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A few remarks on batteries.
-Lifespan. A battery on a classic motorcycle will only last a couple of years. It should be charged and discharged with a current of about 1 Amp. In the real world this never happens. The battery is  losing its power in storage and then the bike is taken for a ride, often with a half ass electric circuit.
-Amps. “Haven’t you got a battery with more power?”No. A certain size of battery can only hold so much current. This is directly related to the amount of lead that fits the box.
-Gel batteries? No doubt they are very good and useful, but you cannot squeeze a decent size in a decent way into a British battery carrier.
-Smaller battery? “Nice and cheap, I always come home before dark, I’ve got this clean chopper and they’re is no room for a big one...” Whatever, but in my experience people with a small battery suffer more than average electrical misery. The dynamo cannot discharge its current and a little battery is too small a buffer.
-Ventilation. Every cell needs a vent hole  which may leak a bit of acid as well as hydrogen. Remove all caps and stickers from vent holes. Be sure not direct the vent tube to your chrome.
-Cracked black rubber battery? Melt it shut with a soldering iron.
-Cracked plastic battery? Bondo it with car body filler.
-Acid. Fill with 37% H2SO4, sulphuric oxide. Sorry we can't send it in the mail without melting postmen.
-Acid level will drop after a while. Then top up with distilled water.