Dual Batteries/electrical iasue.

[mklearl]

I'm having issues with my batteries draining. I have dual batteries set up on a 1-all-2 switch. I had an issue last year with the batteries which were 4 years old so I replaced them last season. Every time I would go out to the boat this the batteries would seem to get weaker and weaker. I have 2 dual.purpose batteries which I switch off using one one way and then the other on the way home to get them noth charged up.
Yesterday I went out and didnt even get a click. Voltage said 10.7 volts. Called towing company who jumped me. The engine spun the starter but the starter didnt engage. Tapped the starter with a hammer and turned the key and it fired up. I ran it for about 45 minutes about half throttle and wide open for a few. Gauge said 13.2 volts. I threw the anchor out and swam off the beach for a couple hours. I had no electronics on.... just sat on anchor. When I went to restart it the battery was down to 10.7 volts again. So I would originally think it would be bad battery but for both of them to only be a year old and both to go bad seems unlikely. Would it be an alternator issue? The alternator seems to be working since I have 13.2 volts when running. Any thoughts?
lastly, I mention the starter because I am eating through a starter per year. I'm not sure why but I'm sure this is a seperate issue but figured I'd mention it since it is a part of the electrical system.
Thanks in advance.

 

[alldodge]

working since I have 13.2 volts when running

You should see around 14.5V running. I think your ALT has an issue and is draining the bat. Disconnect both bats completely and charge them on a shore bat charger. Find out which one is closest to 12.6V after being removed from charger for an hour.

Connect that battery up to the boat at start. Alternator should show 14.5V and if it doesn't then the ALT is the issue

 

[mklearl]

Awesome. Thank you.

 

[mklearl]

Both batteries were bench tested and were fine as was the alternator... what am I missing here?

 

[alldodge]

Both batteries were bench tested and were fine as was the alternator... what am I missing here?

Don't know how were they bench tested?

When connected to the boat what is the voltage being produced by the ALT?

 

[KD4UPL]

13.2v is too low. It should be over 14v, maybe 14.2 to 14.4. You're not getting a good charge.

 

[dingbat]

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Failing starters are notorious for draining batteries.

Your “dead battery”, tap, now starts is the classic failure scenario. Leaking current to ground....

Replace the starter and your problem will go away

 

[mklearl]

Sorry.... bench tested the alternator. Load tested batteries. All ok. Alternator guy suggested a bad alternator ground... there was no direct ground from the alternator ground terminal to the block. I wired one on and put in a new starter... so far so good. 14 volts with no load measured directly at the batteries. Chart plotter/fish finder says 13.3 volts.

Why would a failing starter drain batteries? Once the starter has started it still draws power?