Some background. My boat came to me with a dual alternator charging system, installed by the previous owner. It has the regular 50-amp alternator in the usual top left position but it charges only the gel-cell start battery. The second alternator charges the house batteries and has its own regulator that is switched on with two switches. One breaker on the DC panel puts 12V power to the regulator and there is a switch on a separate panel in the nav station that connects the field wire between the alternator and the regulator. The engine sill on the left was carved out to accommodate the second alternator. The PO expanded the house batteries to four 105-AH AGM batteries in two banks. AGM batteries can take a large charge current when they are discharged to 50%. I see 60-70 amps initial charge going into each bank, using the nice battery monitor he also installed. So, this 190-amp rated alternator earns its keep despite the loss of horsepower while underway. All the electronic nav gear is hooked to the house system and I can keep the house batteries at full charge until we anchor, thus giving us the maximum life when not on shore power charging.
The house alternator was removed when I began the engine work (Oil in Coolant thread). I connected the house alternator up a few days ago, but with reverse polarity. When I turned on the house battery switch there was smoke and crackling noises. The smoke and noise came from a small component that is connected across the alternator terminals. Turns out that is a Motorola MR2535L transient voltage suppressor. So, off to the local alternator rebuilder for repair. It had been rebuilt 12 years ago, with a new rotor, bearings and brushes, for about $250 by the same outfit.
Before taking it in I cracked it open just to see how complicated it was and found it quite easy to work on. The brushes still had a lot of life in them and the bearings were nice and tight. But, with the burned up device (unknown device at that time) and the probability of some rectifier damage, I took it to the repair shop. The damage was two blown rectifiers and they offered to replace them, clean it up, new brushes and bearings, and test it for $460. I expected some inflation :0, but that was too much so I declined the extra work and they replaced the bad rectifiers and re-assembled and tested it for $120. They had no idea what the burned up device was and just gave it back to me.
I cut the shrink tubing off of the device, fearing it would break into a million pieces, but it didn't. There is a crack in the case, but the component number is quite readable. The MR2535L is obsolete, but I have the spec sheet and will be able to find a suitable replacement. Why is it there in the first place? I think that the high output alternator could throw a big DC voltage spike when it's switched into the system with partially depleted batteries. That's an educated guess, now that I know what the device actually is. It burned up with the reverse polarity because it can only withstand micro-amps of forward current flow. It may have protected the rectifiers from more damage by sacrificing itself.
I will be making some new markings on the alternator terminals to ensure this does not happen again.
The house alternator was removed when I began the engine work (Oil in Coolant thread). I connected the house alternator up a few days ago, but with reverse polarity. When I turned on the house battery switch there was smoke and crackling noises. The smoke and noise came from a small component that is connected across the alternator terminals. Turns out that is a Motorola MR2535L transient voltage suppressor. So, off to the local alternator rebuilder for repair. It had been rebuilt 12 years ago, with a new rotor, bearings and brushes, for about $250 by the same outfit.
Before taking it in I cracked it open just to see how complicated it was and found it quite easy to work on. The brushes still had a lot of life in them and the bearings were nice and tight. But, with the burned up device (unknown device at that time) and the probability of some rectifier damage, I took it to the repair shop. The damage was two blown rectifiers and they offered to replace them, clean it up, new brushes and bearings, and test it for $460. I expected some inflation :0, but that was too much so I declined the extra work and they replaced the bad rectifiers and re-assembled and tested it for $120. They had no idea what the burned up device was and just gave it back to me.
I cut the shrink tubing off of the device, fearing it would break into a million pieces, but it didn't. There is a crack in the case, but the component number is quite readable. The MR2535L is obsolete, but I have the spec sheet and will be able to find a suitable replacement. Why is it there in the first place? I think that the high output alternator could throw a big DC voltage spike when it's switched into the system with partially depleted batteries. That's an educated guess, now that I know what the device actually is. It burned up with the reverse polarity because it can only withstand micro-amps of forward current flow. It may have protected the rectifiers from more damage by sacrificing itself.
I will be making some new markings on the alternator terminals to ensure this does not happen again.
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