Battery Capacity Revisited | Sailing Blog - Technical Hints and Tips - Sailing Television

Battery Capacity Revisited

Well we have lived on Distant Shores for almost 1 and a half years now and I thought it was time to revisit the issue of Battery Capacity.

How did my original electrical budget work out? How do the Mastervolt AGMs work? Did we use more or less than planned?

The budget was fairly close. Of course it is always hard to judge in advance what total use would be and there are going to be changes based on where and how you cruise. Luckily our Mastervolt power system includes a battery capacity monitor so we have been able to see total power used and judge how we are doing.

Basically our budgets were fairly close... most of the time. I said we would need 130AH while at anchor. This is true if we anchor and swim/read/explore etc. The difference is if we get out the computers and do a few hours of work! On Two-Step we didn't do that often since there wasn't really space. But with Distant Shores we have the space and occasionally have a day at anchor where we catch up on work, emails, and even some rough editing on the television show. The computers are power users and put us over our budget. Running a typical laptop will use 3-4amps so running both of ours for 5-6 hours is a big 40 addition to the budget.

At sea was where I had thought we would be closer to the line with a budget of 200AH per day. This has turned out to be close. I think it is around 220 in practice. Luckily the boat is very easily steered and the autopilot uses less than Two-Steps pilot to keep her on course. But the instruments are fairly heavy users. We have two plotters, the big E120 down in the nav station and the Raymarine E80 out in the cockpit. Those both use a fair bit. They are wonderful devices and well worth it but I didn't budget correctly for them. The E80 draws roughly 2 amps and the E120 about 2.5. Then I discovered that it wasn't really the plotters themselves, but the backlighting. So at night when we dim them the draw is almost 1/2. At any rate, the budget for the ship at sea is really around 220 AH. This is really a bit close to the line with the batteries we have.
Our ships battery bank includes 2 big house batteries. David at Mastervolt wanted us to use 3 - totaling 675 AH of house capacity. I decided the 42 couldn't accommodate all three since the third would have had to go in a separate custom battery box. So we have two totaling 450AH power and on long passages we have to run the engine more than once a day. In practice we run the engine for an hour each morning and evening.

If we were to do it again I would find some way to follow David's recommendation and fit 3 of the big Mastervolt AGM.

How have the AGM worked out? Well these Mastervolt AGMs are my first experience with the batteries and they are brilliant. AGMs charge quickly, do not require maintenance, do not vent fumes and do not discharge themselves when left for a while. All in all they are great. On Two-Step we used to be checking our batteries, filling them with water regularly and cleaning up the spills. The AGMs just sit there and look just the day they were installed.

Note the black thing that is stuck to the left hand battery... this is a temperature sensor. To look after any battery is important to having them give good performance over a long life. The temperature sensor ensures that the big 100Amp battery charger we have is not over heating the batteries, and that they are being charged at the correct rate.

Got to run! Next time I will report on my recent visit to the Northshore Yard (builders of our Southerly 42).
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