[sailingdog]

Just curious, why do you need three battery banks. You'd be much better off with just two. If you're planning on using an 8D for a starting bank, you're not thinking too clearly. You don't need an 8D to start most small sailboat diesels. You'd be far better off getting a group 31 for the starting battery and using the three 8Ds as a single house bank.

OF course, a lot of this depends on what kind of boat and engine you have. It would probably be wise to give all the relevant information when asking questions in the future. I'd also recommend you read the POST in my signature to help you get the most out of your time on sailnet.

There are a couple really good reasons for not splitting the housebank into multiple banks as you're suggesting. First, is the Peukert factor. If you have two single 8Ds as a battery bank, you're going to get less usable hours from them than if you have the exact same loads on a single bank of two 8Ds. This is because the load is higher relative to the bank size when the batteries is split into two separate banks.

This is the same reason a battery has a 5 hour and a 20 hour amp-hour rating, and the 20 hour rating is always significantly higher than the 5 hour rating. The second is that the batteries are going to be harder to maintain if they're in separate banks.

Second, if they're in the same bank, they'll get roughly equal usage and undergo equal charge/discharge cycles. If they're in three separate banks, they'll each have different use and charge/discharge cycles...

As for charging... what I'd recommend is connecting all the charging sources—alternator, ProMariner, etc—to the house bank and then connecting a BlueSea ACR, or DuoCharge or EchoCharger to keep the starting bank topped off. There's really no reason to charge the starting bank separately.

I'd point out that the ProMariner 1240i is really quite undersized to handle THREE 8D batteries, which will create a battery bank of 600 amp-hours or so. Generally, the charger should be sized at least at 20-25% of the bank size. This is more so the case if you're using AGM batteries, which have a very high current acceptance rate during the bulk charging phase and could easily draw enough to shorten the life of your alternator, and possibly the ProMariner, depending on the charger design. A bank of THREE 8D AGM batteries may have an acceptance rate of over 200 amps during bulk charging if you've got the capacity to supply that much current.

However, without more specific information, such as what alternator you have, whether it is internally or externally regulated, what engine you have, etc., it is hard to say more.