To reduce voltage drop from a charger, when a diode is inserted, the diode
should be a shottky type diode. If 3 amps maximum current is sufficient, a
1N5822 will work fine. Those diodes are available at Radio Shack. Voltage
drop is about .3 volts.

In many cases, a "protection" diode is not needed when charging a system.
They simply prevent current from going the wrong direction when the charger
is not connected.

Solar panels may or may not require them. Many circuits show a diode in
series with a solar charging panel. The theory is that it prevents current
from reversing direction during darkness. In many cases, the diode is not
necessary and the instruction books are guilty of simply copying a circuit
without understanding it. The reason diodes are often not necessary in
solar panel charging systems is that the panel itself is a diode. I always
check with a ma meter, just to make sure.

I installed a 100 ma (.1 amp) panel on a friend's motorglider (no electrical
charging system or starter) and it has charged the battery for over a year
with no problem. He is an occasional user and not a contester, nor does he
fly many hours at a time.

If you consider a 12 volt storage battery as being fully charged at about
14.2 volts and unusable for your purposes at 12 volts, you can consider the
stored power available pretty much as a straight line with 14.2 as 100% and
12 as 0 %. That means that when the voltage is 13.1 volts, you have 50%
power available. Batteries, both rechargeable and non-rechargeable may
expect the voltage to go below 12 volts to achieve the rated amp hours, so
if 12 is your minimum, you may need to de-rate your battery. The
manufacturer should have a graph showing the actual capacity
available with the maximum and minimum voltage you have in your electrical
system.

Colin