The Reserve Capacity (RC) of the battery that fits my FLSTC is rated at ~52 minutes. RC is a BCI Standard. RC is the number of minutes that the battery in any 12 volt vehicle can deliver 25 amperes for; without falling below 10.5 volts. A Chemical Engineer would cringe but . . . at that point you're at roughly 50% State of Charge (SOC). A general rule of thumb is that if you multiply the RC in minutes by 25 and you get a number called amp-minutes. In this case 25 X 52 = 1,300 amp-minutes. If we assume a worst case situation that your battery isn't 1/2 discharged . . . that it is at 0% SOC then 1,300 X 2 = 2,600 amp-minutes have been consumed. If you were to charge the battery at one ampere for 2,600 minutes that should bring it back to ~100% SOC . . . or close enough for gov't work anyway. Bottom line is that 2,600 divided by 60 minutes in an hour is slightly less than 44 hours. So-o-o-o-o If you put a 1 amp trickle charger on your battery for almost two days no matter what the SOC was when you started it had better be at or pretty darned close to 100% SOC now . . . and if it isn't . . . you need a new battery because continuing to charge the battery at this point isn't doing anything except boiling off the electrolyte. Use a 2 ampere trickle charger and it's 22 hours . . . a 10 ampere trickle charger is slightly less than 4 and a half hours . . . Trickle Chargers are also called "Constant Amperage Chargers' so it makes the math fairly easy. The large Battery Chargers that roll around on wheels that are used by Service Garages are Constant Voltage Chargers and if you use one of them the math gets a bit more complicated . . .

 

I think also you need to know what type battery you have when talking about battery chargers. I found this web site http://www.batterystuff.com/tutorial_battery.html#3

newer Harleys have AGM batteries Older have wet cell

 

A trickle charger is not a good thing unless it shuts itself off.You are much better off with a tender because of the float circuitry.They maintain the batteries at the perfect voltage and the newer ones have a feature that helps to clean the battery plates.