A house and starting battery mystery

On our boat, we have ten house batteries and one starting battery.  The house batteries are Trojan brand T-105 6V batteries paired to make what is essentially five 12V batteries.  They are divided into two battery banks one of 6 and one of 4.  Our starting battery is a Deka Group 31 deep cycle starting battery.  We recently replaced all the batteries which cost about $149 each if you can get them for a good price.  Our starting battery also runs our bilge pumps.

The evolving mystery is this….twice in the last year, our starting battery has been too low to start our diesel after being away from our boat for about 6 weeks.  We have 7 solar panels, a wind generator (this was turned off) and were plugged into shore power when this has happened.  We have three barrel switches that select the charging method for the batteries, the bank that the house operates on and one isolates or connects the house and starting batteries.  Normally the start battery is isolated.  According to Nigel Calder this setup is optimal for a larger boat with two battery banks.  The catch is that you need to use “Shottky” diodes to protect your batteries from discharge.  These diodes essentially act as a “one way” valve for current to flow into the batteries with a slight drop in voltage (between .15 and .65 by most accounts).  The simple fix has been to switch the batteries to “all” and let the house bank top off the starting battery.  After about five minutes all is back to normal.  I just don’t want to count on this when we are away from shore power.

I can’t imagine a circumstance where our bilge pump would run enough to discharge our starting battery and normal self discharge for this battery should be a much longer time than this. Even at 80 degrees ambient air temperature the discharge time according to many sources is well beyond 8 weeks.  Normally the self discharge rate is 4% to 6% per month.  I’ve checked most of the connections for excessive heat and found nothing yet.

I plan to do some more checking and troubleshooting or replacement of our diodes and I will post more as it develops…comment for suggestions are always welcome…..

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I'm the average sailor...converted from a landlubber of course. While I was born with a love of the ocean it has evolved into a love of sailboats and other vessels that ply the open seas and connected waterways. I am probably like most of the people who (hopefully) will read our blog with the exception that we are now doing what we always dreamed of doing and I hope to help others do the same. I am NOT any of the following...a naval architect, a marine surveyor or connected commercially to any builder, distributor or boat sales organization. My opinions are generally my own, although influenced by many years of research.

One thought on “A house and starting battery mystery”

  1. Interesting enough I’m also replacing both my house batteries and starting battery. Although they are both AGM, and the house bank has been being charged either by the solar panel, or shore power, or the engine running, it failed more quickly and more completely than did the start battery ( a much smaller/less voltage capacity battery)

    One point to consider, if you have the batteries on “all” and have one battery go bad, it can draw down the rest of the batteries, the same as when you have one cell in a wet cell battery fail, which will compromise the whole battery . 🙂

    good site !!

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