I have two Group 27 Sears DieHard Dual-Use batteries on board, one aft and the other in the forepeak, that are charged through a Guest 2612 two-line charger when at dock and through the alternator on the diesel through an "Off-1-2-All" battery switch. Last season I noticed a regular drop in the electrolyte level and an inability to retain a charge in the forward battery (the house battery). The aft battery that serves the engine never has a problem. I replaced the battery thinking that it might be defective, but to no avail. So every month I had to add half a gallon of distilled water to it. How I can avoid this?
Tom Wood responds:
I am a little confused, but that’s not out of the ordinary. My Guest catalog shows that the model 2612A is a small, portable, single-stage, ten-amp charger with alligator clips on a four-foot cord for one battery. It would charge two batteries, of course, given enough time and the fact that the battery switch was left in the "All" position. My confusion stems from the fact that you state that the aft battery is a dedicated engine battery and the forward one runs the house, but I can’t see how they are isolated. Unless your electrician added a charging isolator to the alternator output line and a second one to the Guest output line (which is highly unlikely), I would say that you do not have isolated banks.
What I guess is happening to your forward battery is that it is being over-utilized and under charged, resulting in a very quick death due to sulfation. One-half gallon of water a month is an unbelievable amount for a Group 27 battery, indicating that the plates are at least 50 percent dry—and when the plates dry out just once, the battery is forever dead. The fact that the battery will not maintain a charge only reinforces this belief, since a badly sulfated battery will refuse to hold a charge, throwing off any energy given to it in the form of excess gassing and water usage.
The first thing you should do is make a schematic of the system as-is so that you understand fully how it works. Check Guest's manual to guarantee that it is a dual-output unit as you believe and that it is wired correctly. I think the root of the problem is that you think your electrician isolated the batteries when in fact he did not, leading you to undercharge the forward battery. The second thing would be to take a good digital voltmeter and measure the charging voltage at both batteries—once with the alternator charging and again with the Guest doing the work. With the long run forward, an inadequate wire size could be giving you a substantial voltage drop to the forward battery. A voltage reading would also ensure there is no fault in the charging circuits to either battery and that the Guest is not over-charging or under-charging. Whatever the outcome, you can bet that the second battery is shot so don’t put a third one up in the forepeak until you can figure out what the real problem is.
And, if you need to purchase a voltmeter for this work, several are available right here at SailNet.