Batteries | Sailing Blog - Technical Hints and Tips - Sailing Television


Continuing from earlier blogs on Electrical Budget I have been working on how to fit in all the batteries we need. From the budget calculations we would be using 130amp-hours (130ah) when hanging at anchor and 200 when passagemaking. Two basic assumptions to calculate battery capacity needed are as follows...
  1. Plan on recharging batteries once a day
  2. Do not discharge your batteries more than 50 percent on a regular basis
This means you calculate the capacity of battery you need to be double the budget numbers we did. To provide 200ah of capacity we should plan a 400ah house battery bank. Then we would be able to discharge it to 50% with one day power use while passage-making.

I have an additional criteria for batteries - at anchor it would be nice to be able to stay at anchor for 2 days without starting the engine. If we have found a sweet little anchorage we often decide to stay a day at anchor and then leave the next day. I hate having to fire up the engine in that rest day in a peaceful cove just to be able to run the fridge!

To have the power to run the ship for two days at anchor we would need 130 per day X 2 days = 260ah X 2 = 520ah capacity.

Now the tough part. Where to put all these wonderful batteries?
  • Many boat manufacturers do not plan sufficient batteries for a boat that will be used for long-term cruising
  • Batteries are quite heavy and should not put the boat out of balance
  • Lead-acid "flooded" batteries must be vented - this restricts where they can be put
Here's the original battery compartment with 4 standard flooded batteries - 3 X 110ah = 330ah for the house bank plus one 110ah to start the engine.

Luckily with the Southerly 42 there is sufficient room in the original battery box to place 3 of the Mastervolt AGM batteries to act as the house bank. We can fit in 2 of the 220ah large batteries (equivalent to the American 8D size) and one 160ah. Total capacity will be 2 x 220 + 160 = 600 ah. Only small changes to the compartment will be needed. The only problem is that this compartment was also meant to house the engine starting battery, so we needed to find another home for this. Mark at Northshore suggested a place under the floor where a battery box could be built for the 90ah starting battery.

The best part of this plan is that the additional 94kg of batteries will go in the exact center of the boat where they will not affect trim.

The decision to go with AGM batteries makes sense from a couple of reasons. First, the AGMs can be recharged faster than conventional flooded batteries. This means shorter running times for the engine or generator. They do not need water adding on a regular basis so we will not have to be going into the compartment and checking them every few weeks. And their sealed design means the extra battery box does not have to have such care for the venting and ease of access.
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