Charging large battery banks, such as this dedicated bow thruster set-up, can be problematic
One of our readers sent in a query just yesterday regarding a rather large battery bank on a Niad sailboat and the fact that some of the batteries keep going dead. Check this out:
I have a customer with battery problems. The boat is a Niad 44, maybe 7 yrs old. There are 6 Lifeline Group 31 batteries in 3 banks of 24v. The 3 battery banks are each wired to a common bus bar of pos and neg. The pos, thru an on-off battery switch. 12 v needs are taken off a 24-12 convertor. There are no power drains off any battery. In each battery bank of 2 batteries, 1 battery is failing. Testing was done with a Midtronics tester, 250cca on a 600cca battery. Lifeline refused to accept the results (they say the tester is junk) so I confirmed the results with a full charge and resting voltage test. Charging is directly to the pos buss, with a Balmar 24v alt or Mastervolt charger. The batteries are only 1 year old. These are the 4th set of batteries. Like everyone, they probably don't get a regular full charge. Voltage drop is minimal. My question is would it be better to wire the 3 banks in parallel to make 1 large bank, with 1 single pos and neg to the bus bars. Would it promote more even charging? I don't want to replace the batteries without making some change to a system that's not working. What would you recommend, and why? Thanks for your help.
Well without actually seeing the set-up in person, I can't give a definative answer here, but experience certainly tells me that Alan is thinking on the right track when he says that maybe combining all the batteries into one singular bank might be better.
There are several things going on here that may be causing the problem, which I believe to be nothing more than an imbalanced charge regimen. The 24-12 volt converter, and multiple banks and switching complicate things considerably.
In any application as complex as the one Alan describes, with many terminal connections and even a voltage shift going on, the risk of uneven draw and uneven charging is just too great. I believe that is what is actually going on here and the net result is that the batteries that are at the weakest point in the system are suffering from a continual undercharging situation, causing pre-mature failure of the otherwise great batteries.
Oh, and I categorically do not agree with the "Midtronics" testers are junk statement either. I think they are the best available. Alan has confirmed that his tests aren't lying to him.
My advice? Apply the KISS principle here and I think the problem can be solved.