Short Battery Life on a Cruising Boat Fixed

JHH5-12213A year and a half after we installed a new housebank on Morgan’s Cloud, and embarked on a field test of AGM batteries and the care regime that Justin Godber at LifeLine Batteries helped us develop, we tested the batteries. Here are the results..short version, it works.

Our Usage

First off, let’s put the results in perspective by talking about our usage. After installing the batteries we spent a total of six months in a marina with shorepower available and the other 12 months out cruising.

When we were in the marina, we cycled the batteries to about 20% discharge once a day (mainly powering our diesel heater) because we turn off our shorepower when we go to bed—the hum of the isolation transformer and the whirr of the charger fans is very irritating. A comparatively light use.

Once we got out cruising our batteries got a real workout with at least one 50% discharge every day and sometimes two.

So our batteries have seen some 180 20% discharges and about 400 50% discharges. They have also been abused by three 100% discharges for testing purposes (not recommended).


We ran a discharge test and were pleased to find that both batteries showed just about 90% of their original capacity left, which is a huge improvement on the life we got from the two sets that both died after about 400 50% discharge cycles.

While there is no knowing for sure, I would guess that, based on this test, we are going to get at least 1000 50% cycles and possibly as much as double that, before replacement—pretty impressive.

What Changed?

With the previous two sets of house batteries that failed prematurely, we had done all the usual things that conventional wisdom dictates, including installing a three stage alternator regulator and battery charger, and never discharging below 50%.

This time we took a much more active approach as documented in this Online Book, including reprogramming our alternator regulator from the factory defaults, equalizing our batteries once a month where possible, and manually managing our shorepower and alternator charging.


This has been an interesting and rewarding process that we have drawn the following conclusions from:

  • The standard wisdom about battery care is flawed and, on a sailboat that spends most of her time far from shorepower, will result in premature failure.
  • Most charging equipment, including the fancy three-stage stuff, that claims to automatically take care of your batteries, won’t.
  • There is a huge amount of absolute rubbish published and rumoured about battery care, and the “professionals” are often the worst offenders in this regard.
  • Most staff in boat yards are almost totally ignorant about proper battery care, but that does not stop them having opinions.
  • You don’t need a lot of highly expensive or complex gear to take care of your batteries properly.
  • If you follow our relatively simple “Eleven Steps to Better Battery Life” you will do fine.
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John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 18 years, as co-editor/publisher of AAC.

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