The Sterling units are well designed, quality products. We are using the Pro-Reg D with a high output Delco style alternator (way cheaper than a Balmar). The Pro-Reg D requires modifying the internal regulator
wiring inside the alternator housing. It bypasses the internal regulator and controls the alternator field voltage to achieve a longer and higher output 4 stage charging
current profile. It includes microprocessor control, is programmable for different battery types, and also monitors alternator and battery temperature. Cruising and anchored out for several days, recharging a large 440 Amp Hour battery bank (at ~ 20% discharge) takes no more than an hour per day of engine time.
The Sterling Alternator to Battery Charger works by creating an artificial voltage drop in the field
current, "tricking" the alternator into a higher output profile. It does not require any internal modification of the alternator. I have no personal experience with this unit but can see in the Sterling info that it has a large heat sink and requires significant
cooling, which no doubt means it is not as efficient as the smart regulator units.
Balmar offers a 4 stage smart regulator system matched to their excellent line of premium priced alternators.
My only caution with the Sterling systems, particularly the Pro-Reg D, is that they do require knowledgeable installation. The
manuals provided require some interpretation by an installer with a good foundation in DC charging and
wiring. Sterling is a small company in
England -- getting an answer to a tech question can be a challenge.