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post #2 of Old 07-15-2013
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Re: m25 alternator question

Originally Posted by SSBN506 View Post
Hi everyone,

Bit by bit I am starting to understand my bigger boat. I have a question about my alternator and how and what they output.

I have a 1986 Catalina 30 TRBS with the m25.

I just put new house batteries in my boat the old ones were in very bad shape. I put in two 232 AH 6 volt on bank 2. On bank one I have a 70hh 12 volt starting battery.I also have a clipper bm-1 battery monitor.

When I installed the 6 volts the battery monitor after doing the load test to calibrate the batteries showed them at 80% and 12.7 volts.(or very close to that) I connected my changer set it to 6 amps and the BM-1 registered the 6 amps change as 6 amps so working thus far.

I then started the motor after disconnecting the charger to se how many amps it would do. the amps wet to about 7 to 9 and the bm-1 told me at that rate it would take 7 hours to get to full charge. Should it not put more in when the battery is at that level? When I switched back to my starting battery it went down to like 2 as it was fully changed so it is seeing resistance.
Two things:

1. A battery monitor is only accurate when it has been properly calibrated. Over time, they become less and less accurate unless re-calibrated.

2. Your new batteries may be nearly fully charged, so both the battery charger and the alternator are putting out about the same low amperage.

New batteries won't deliver their rated capacity until exercised (discharged and re-charged and, sometimes, equalized). I'd suggest you leave them on charge for a few days to make sure they're fully charged, then recalibrate your monitor.

After using the batteries, i.e., discharging them a few times to about 50% SOC and recharging them fully, re-calibrate the monitor again.

While they are 50% discharged or so, measure the voltage output of your alternator (should be 14.4 or above for flooded or AGM batteries) and measure the amperage output as well. That should give you a much better estimate of how well your alternator and regulator are working.


Last edited by btrayfors; 07-15-2013 at 10:40 AM.
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