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.