Consider the throttle load for battery voltage calculation

@DrTon @sjwilks This change considers the internal resistance of the batteries for charge calculation.

@DrTon I'm aware of your capacity PR and want to integrate it as well. However, while this change allows us to get closer to the lower limit (and hence warn at the right time) the charge based approach (without a smart battery) requires still a voltage based sanity check/limit.

So while this PR does not improve accuracy, it helps to not warn the user too soon because of high throttle events.
After 3.4V voltage drops much faster and deep discharge (3.2 is deep enough!) is very bad for battery health. Default parameters should be tuned for safe flight. With 3.2V copter may crash after 3 seconds after first warning if user forgot to charge battery, it may be not enough to land.

Of course it depends on battery model, but I would like to stay on safe side with 3.4V. If user wants to kill the battery, he can change parameter.

The problem with voltage drops on high thrust should be solved by internal resistance compensation I think, that you are already trying to do.