| One question that I wonder about. It specifies the power supply | should preferably be in the 10.5 - 15 volt range,
By preferably it means absolutely. If you get out of this range, the charger will turn off and start beeping very loudly.
| with ability to deliver 13 amps while maintaining 12 volts. This | seems to be asking it to strain a bit to effectively charge 12.8 | volt lead acid batteries. Am I "nit picking" here?
You only need 13 amps to charge a battery with lots of cells at five amps. Charging it at a lower rate, or fewer cells, will use less power.
If you're charging less than 8 cells from 12 volts, then the actual current needed will not be much more than the rate you're charging at. So if you're charging at one amp, you'll not be using much more than one amp from the battery.
| I guess the more applicable question for my use is - will either my | 12.5 volt, 3 amp regulated supply or my 15 volt, 6 amp regulated | supply suffice for most needs?
Yes.
| The 15 volt, 6 amp supply CAN be varied to a lower regulated supply | voltage
15 volts might be too much ... better lower that.
| most likely candidate, wouldn't it? Finally - when will I ever need | the stated 13 amp supply capability, if the only lead acid batteries I | will be charging are rated at 12.8 v at 6 AH?
No. I'm not sure what the maximum safe charge rate for those would be, but I doubt it's over 2 amps. 3 amps would probably be ok for that.