A ggod multistage external regulator will enable maximum bulk charge for as long as the battery temperature and voltage permit. These sensors are a vital part of the functionality which an internal reg just doesn't have.
The ext regulator will then step down the current to the field windings as the battery nears full charge - or as the battery temperature increases. There are "smarts" in these regulators which*means, for example, that a battery that is at 90% will get a fast charge at bulk for a few moments before the regulator reduces the current. An internal reg will not*allow this.
The max capacity of the alternator is only used in the first*phase of bulk charging, which is a relatively short period. It's in the subsequent phases*(both bulk and absorption) when the ext VR manages the charge current. These phases are*usually only 50 to 70% of the first phase and last much longer, so the size of the alternator is not the total answer.
However, an internally regulated alternator will drop the charge rate much more quickly as it operates on voltage, as seen at the alternator rather than *measured at the battery, so the maximum capacity of the alternator is used for an even shorter time, so becomes less relevant. Also there is no temperature compensation, which not only protects the battery, but ensures the charge current is maintained to the highest*rate the battery can accept.
As a battery accepts a charge, it gets warmer and the*higher the temperature, the greater the charge it can accept. So*without some limits,*thermal runaway*can occur. A*clever charger will actually increase the maximum bulk charge rate as the battery temperature rises to the optimum (preset) temperature. It will not allow the battery to exceed this temperature. When charging with our DC genset, the current typically starts at ~70A and then*steadily rises to ~90A before beginning the step down process.*Ours is a 24V system.
Hope this helps, I'm sure FF will*provide any necessary corrections.
-- Edited by Bendit on Tuesday 9th of August 2011 07:05:25 PM
-- Edited by Bendit on Tuesday 9th of August 2011 07:34:22 PM