Ride them all more often... Seems like there should be a way to gang them together and run off of one battery tender. Don't ask me how, but it seems like that should be doable. Maybe someone else will chime in and actually be helpful
Yup. I try to pull all of our bikes (and cars) out of the garage once month. Crank them and let them run til operating temps. My grandfather told me when I was a kid, 'your vehicle needs to move 21 feet every 21 days or it will deteriorate just sitting there'.
That will surely help but old motorcycles don't generally have great charging systems. They were marginal even when new and some are not made to bring a flat battery up to charge. You should keep the battery charged up or you could overheat the stator.
I really wish that I could ride them more often, but there isn't enough time.
We have about 20 rideable bikes at a time at the shop, so I am looking for a system that will help preserve the battery systems with the least amount of effort.
Take the batteries out, sit them on your work bench or where ever and hook them all together in parallel so they remain as 12 volt and then hook a charger up so it does all batteries at once. I do this with car batteries if I have a few cars that are not being used and plus a spare battery. The charger is hooked into the same point as the workshop radio, so when the radio is on while I am tinkering away the batteries are being charged and if I have finished up in the work shop and can still hear the radio I know to turn it off (My workshop is under my house) and this way the batteries do not get boiled by forgetting to turn the charger off. It is surprising how much you have the radio on and the batteries are always getting a good constant charge which is what they like.
Pulling this thread back to the top.
If you gang charge a few batteries, don't leave them connected after you have finished charging them. No two batteries ever have the same voltage or potential. If one battery shows 12.4 Volts and another battery shows 12.2 Volts, the 12.4 Volt battery will try to balance the voltage between the two batteries. The 12.2 Volt battery will leech from the other battery. If left this way for a while, both batteries will eventually decrease in voltage, till they aren't able to start anything.
Same thing with storing a battery on a cold, concrete floor or in the basement. The bottom of the battery is cold and the top of the battery will be warmer. The acid inside the battery will be cool at the bottom of the plates and warmer at the top. Now the plate will try to balance the voltage difference between the top and the bottom of the plate. This difference is measured in millivolts but it will deplete the battery over time.
Always store a battery on a bench or a shelf where temperatures are more consistant. Use a piece of wood to set the battery on, to isolate it from a metal shelf. The shelf acts like a heatsink.