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The question they is why is your alternator not achieving close to its max output? It could be a problem with the alternator...

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There might not be anything much wrong with the alternator. I agree with Joe (Captainslarty). The rating published for an alternator is the cold rating. As soon as the thing warms up, they can produce far less than the headline figure.

Limiting factors are: 1. Battery internal resistance (which rises as they accept charge and which some people call the acceptance rate.) 2. Size of alternator (and how warm it is!) 3. Size of cable between alternator and battery.

Asuming that the cable size is appropriate (in the positive and return circut) and that there are no poor connections, the discharged 390 Ah battery bank will accept a lot more than 25 amps when it starts charging. The figure will reduce as the batteries charge up. We have a similar size battery bank and it accepts 40 or 50 amps at first.

However, I reiterate, there might not have been much wrong with the 55 amp alternator. It's just being asked to do too much, gets warm and self limits. The alternative suggested sounds fine. Just check the cabling and connections!

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“When you discover that you are riding a dead horse,
the best strategy is to dismount.”

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