Hi, Just a bit of info that may save you buying a new battery, Bike batterys generally fail due to going flat then not taking a charge the basic science is, as you use a lead acid battery calcium comes out of the electrolyte and attaches to the plates, this is soft at first so if you put it on charge the calcium goes back into the electrolyte If the battery is left discharged to long the calcium hardens on the plates and stops them conducting so will not charge If you put them on a regulated power supply at about 100ma 14v for about 3 days it will dissolve the calcium and you have a very good chance of good to go It will not recover a 25 year old dinosaur
I’ve just resurrected the battery out of my VW bus that deep discharged over winter. I’ve got a cheep Chinese charger with a repair mode. I put a meter on it and observed what it was doing, basically what you said, except it was pulsing between 12-14v. I guess that speeds up the process. The other way you can revive deep discharged batteries, if you have an arc welder - they put out a load of current at about 20-30V. A few short bursts can decalcify the plates but this is more stressful on the battery and can actually shorten its life. Works though if the battery doesn’t have some other internal fault.
