Main Motorcycle: 06 Thruxton
Join Date: Aug 2008
Location: Winter Park, FL USA
Other Motorcycle: '69 Bonneville
Extra Motorcycle: '71 Bonneville
The rough rule for charging most 12V batteries is 13.8V-14.2V. Remember, this is rough rule so check with the manufacture's recommendations. However, it's been this way for as long as I can remember, it's just what 12V batteries need to maintain themselves.
If the charging system will maintain that level then you are good to go. In other words, if there is that much voltage that finally reaches the battery after everything else, the battery will be maintained. Unfortunately, these old bikes were never intended to be ridden with the lights on all the time (all appliances on the bike will siphon away some power) so eventually the charging system will "get behind" and need some outside help.
Once you have determined that the voltage to the battery is insufficient (that is, based on a realistic assessment of the capabilities of the system) then you will need to start checking through out the system to try to identify what needs to be "fixed" to return the system to it's normal level of performance.
To check the status of the battery (ie. it's state of charge) check the battery voltage with everything turned off (static charge). Then, turn ignition and all the lights on (engine not running) and see what the voltage drops to. This is a crude "load test" and will give you an idea of just how much power is stored in the battery. If you do this when the battery is relatively new and fully charged you get a base line to compare to for the future.
If the static charge drops under load, from say 12.5V to perhaps 10V you can feel pretty confident that the battery is in fairly good shape.
If the static charge is 10-11 V that suggests that it is in a state of discharge, and, if you just charged it, let it set for an hour or so, and this is all you get, then either the battery is starting to fail or however you charged it didn't really do the job.
If the static charge is good but under load it drops to perhaps 8V or lower, that suggests that the battery may not be up to the task.
These are just some rough rule checks that you can perform with very basic tools. If you find any of this helpful that is all I have to offer.
Last edited by BirdoPrey; 11-18-2012 at 04:27 PM.