Onboard charger ( 3 bank) question

[agereety]

I have a Minn Kota MK315 onboard charger. It's a 3 bank charger mounted under the lid to the battery storage in the bow area of my 190 Bay. Today, when I went to connect the extension cord - one of the 3 banks had a red light ("check connection") light on - while the other two banks appeared to be charging. In the manual it states when this light comes on - you should disconnect the charger immediately, so I did.
We looked at all of the fuses and checked for continuity- everything seemed alright.
All the connections were fine, and previously - the charger worked fine. The manual also said that the light can come on when a battery has less than 8 volts. I checked each of the batteries: one was around 6 volts, one was around 11 volts, and one was around 9 volts. Is this normal for 3 brand new Optima blue top sealed batteries to have such varying volt levels?
Another bit of possibly relevant info. is: I was on vacation this week in Maine with no power to the dock, so I ran the trolling motor for several days without charging them. I ran them almost dead. These batteries are seperate from the starting battery, so I am assuming that they are not connected to the alternator charging system like the starting battery is.
For now, I am charging the battery with 6 volts on a separate charger (after having disconnected everything else). Hopefully, that will bring it above 8 volts, so I can hook them back up to the onboard charger. I would appreciate any insight that anyone may have regarding this scenario. Thank you. - Andrew

 

[pamarine]

Charging issues

For the sake of the batteries' life-spans, it it not advisable to discharge them below 50% capacity except in an emergency. Also, be very careful what type of charger you have attached to the low-voltage battery. Optima batteries are spiral-wound Absorbed glass mat, as therefore have almost no electrolyte vs a traditional flooded lead-acid battery. If the charger does not have a tightly monitored voltage and current output (say more than a 5% tolerance), the health of the battery will be negatively affected as well, since the smal amount of electrolyte will evaporate out of the mat. That being said, it is unusual, but not unheard of, for a new lead-acid battery of any technology to be damaged when purchased. The only way to know the true health of the battery is to have the battery load-tested. While you can do this yourself it is far easier to take battery to any automotive or marine battery retailer and have them test the battery on a electronic load-tester. The simplest of these will give you state of health and state of charge. The more sophisticated wil be able to give you cycles per charge and a printout of the results to keep for your records. Hope this helps.

 

[190steve.s]

agereety, I also have the optima batteries and as pamarine(he is an excellent resource!) said you can get duds. I had to return one as soon as i got it. You are right to separate the one to bring it up. I was without an onboard charger for a while and my batteries were drained. I trickled charged at 2 amps for 18 hours to breathe new life into mine, only then could I increase to 5 then 10 amps....I switched to optimas after my normal deep cycles froze. I never did know which went bad first,the old onboard or the batteries

 

[agereety]

Update on the 36 volt battery situation

After I disconnected the battery in question and put it on a trickle charge - it charged up fine. Then, I reconnected the battery in series with the other 2 , hooked up the 3 bank onboard charger, and all is well. Each of the 3 batteries is around 12.7 or 12.8 volts now. I must say that I am a huge fan of this trolling motor setup. It runs all day long, and never appears to lose power. I went with the smaller batteries, so I could squeeze all 3 in the battery storage area, and I now have no doubts about their abilities.
I also got the foot pedal with the cord, and it's kind of quirky, though. Soon, I will start playing with the copilot remote. I had the opportunity to use one of these copilot remotes in another boat, and it was extremely responsive. - Andrew