This is the full text of the question and answer on battery management that appears in the June 06 Any Questions

Dear YM,
Our cruising habits are to spend 5 days a week away from marinas. After 2-3 days our batteries have used 70-90amps. With a 60amp alternator and battery capacity of 280 amps this take hours of noisy engine charging. Starts charging at 30amps, drops after 1st hour to 25amps and progressively gets slower. To make matters worse in addition to our 80 litre fridge we want to instal into the existing top opening cool box(approx 50 litres)an additional fridge/freezer. To avoid the complications of solar/wind generation would adding an alternator management system plus buying Lifeline absorbed glass mat (AGM) batteries reduce our charging time by 50%? Or would you recommend another solution?
Alastair Locke, Harrogate

PS. Other info if needed -yacht is Wauquiez 40 pilot saloon based in Brittany, engine volvo 55hp, battery monitor xantrex, batteries are 2x140amp plus engine 80amp. Alastair,

Nigel Calder replies:You are solidly up against the limiting factor in a cruising boat’s DC system which is the charge acceptance rate of batteries – the amount of charging current a battery can safely absorb at a given state of charge. This varies somewhat according to battery technology and ambient conditions, but in general at a 50% state of charge a battery can safely absorb a charging current that is around 25% of the battery’s rated capacity, expressed in amp-hours, but thereafter the charge acceptance rate falls off fairly rapidly as the battery becomes more fully charged. At the 80% charge level, the charge acceptance rate is around 10% of battery capacity.
You have a 280 amp-hour battery bank. Let’s say you discharge 90 amps from a full charge. The batteries are now at around 70% of full charge. The nominal charge acceptance rate is around 15% of rated capacity, i.e. 42 amps, but you are only seeing 30 amps. One possibility is that the batteries have lost some capacity so now the actual capacity is less than 280 amps. However, the numbers you are giving me also fall within the possible range, so the batteries may be OK. The question, of course, is how to make the batteries do better than this!
You have several options:
1. Add an external multi-step voltage regulator. This will enable you to force feed the batteries the maximum charge rate they can sustain without damage. You may not be able to easily add such a regulator to the existing alternator, in which case you will also need a new alternator – an expensive upgrade, but well worthwhile for serious cruising.
2. 2. Discharge the batteries more deeply before recharging. The deeper the discharge the higher the charge acceptance rate and therefore the shorter the time required to put back a given amount of energy. However, the deeper the discharge, the shorter the battery life. A good rule of thumb to maximize system performance without excessive battery damage is to discharge to 50% of capacity and recharge to 80%, with a charge to 100% at least once a month (this is necessary to prevent something known as sulphation).
3. 3. Increase the size of the battery bank. Let’s say you double the battery bank size, but continue your current practice of discharging to around 70% of capacity. Where you are now putting in 30 amps, you will be able to put in 60 amps (assuming the alternator will sustain this), in which case you can put back the energy you need in half the time.
4. 4. Fit gel-cell or absorbed glass mat (AGM) batteries in place of wet cells. The gels and AGMs have a higher charge acceptance rate, and so will accept a given amount of energy in a shorter period of time.
5. 5. Add a wind generator and solar panel. Whatever output you get from these can be directly deducted from what you need to get from the engine-driven alternator.
For serious off-the-grid cruising, I recommend all of the above!