Motor not charging battery?

m9000m9000 Posts: 2,038 Captain
Spent a week in the Keys and noticed the voltage on the wet cell, deep cycle battery serving the starboard engine, GPS, sonar and radio was declining as the day wore on (12.2- 12.3 from a start of 12.6). We spent most of the days trolling at 1,000-1,500 rpm, which has always kept the battery charged. The voltage displayed on the GPS was identical to the voltage recorded at the battery terminals. I got through the week by charging the batteries on the built in charger each night.

Now at home, I ran the engines and put a voltmeter on the battery nuts on the engine block. The normally functioning port engine showed 13.4 volts before cranking (port engine battery also was 13.4 on terminals), 13.7 at idle and 13.8-13.9 at about 1,000 rpm. The starboard engine showed 12.40 before cranking (the starboard battery is a little discharged and showed 12.40 on the battery terminals). At idle, starboard battery was 12.18 and at about 1,000 rpm 12.16.

The batteries are wet cell. When I noticed the problem, I checked the fluid level in both batteries. The starboard battery (deep cycle) was low and the top of the lead core was barely exposed. I added distilled water and put it on the charger overnight. The battery only came up to 12.5 volts 15 minutes after removing the charger and was 3.5 years old, so I bought a new battery. The new battery lost charge over the day just as the old battery did (12.8 to 12.4 over the course of a day).

I’ve done a little reading on charging issues and it appears that there are 3 options assuming no bad connections, shorts or other wiring issues: stator, rectifier or regulator. I have ordered a shop manual and will try to trouble shoot the problem for fun, but will let a pro handle the repair. The engines are 2005 200 HPDI with about 650 hours and they have been flawless.

For you guys who understand the charging system, what is the likely problem?


  • SangsterSangster Posts: 85 Greenhorn
    Sounds like the alternator isn't putting out enough volts if it was only measuring 12.1 volts at idle and 1k rpms. An ideal voltage while running is approx 14V.
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