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islander bahama 24
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Personally I like the 12 volt 50 amp3 bank marine charger on the boat ( old centaur unit)
I am going out on a limb here but it may be that he actually has a 3 bank 12 volt charger and miss reading as a total of 36 volt charger not three bank 12 volt

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3,841 Posts
We are on the hook all summer, similar to your intended use.

...The Group 27 is 115 Amp Hours... ...I need to use the microwave...
...the inverter often shuts down due to low voltage if the battery bank is too small. An inverter supplying 1000 watts is consuming 100 amps DC. A single group 27 battery can only sustain that for a few minutes.
Mitiempo beat me to it, but I can add some real world experience. We have a small, low wattage microwave on board and a Xantrex 2000 true sine wave inverter -- the $1,500 kind. It's great having a microwave, we love it. The low-wattage ones aren't great at making popcorn but good with everything else.

But you need the batteries to drive it and a group 27 won't work so well -- only when fully charged and for maybe a minute or 2.

Last year we replaced our 330 AH Gel cell bank with a single 165 AH Lead-Acid 4D battery. Using a microwave before was simple and the 330 AH bank supported it with ease, even with the 120 amps coming out of the bank (as measured on the Victron battery monitor). But now with the 165 AH bank, the inverter starts beeping about a low supply voltage. Starting the engine relieved this condition, by the way, and picked up exactly half of the load; only 60 amps came out of the bank. Bottom line is that a 300 AH battery bank is needed to support a microwave.


Funny thing is that the microwave sucked down only 65 amps when powered by a cheaper, square wave inverter. And then the microwave worked horribly. You really need a pure sine wave inverter to drive one. The Xantrex MS200's pure sine wave AC means the microwave uses 120 amps, and then it works great. Remember there are inverter losses. It was a full 120 amps (measured) for this small microwave:

So the most expensive part of the microwave-on-a-boat equation is the inverter ($1,500) which you would likely not need otherwise.

A standard kitchen toaster pulls 160 amps (measured), worse than the microwave, by the way. it worth the expense of a mppt controller the pwm comes with the panels
As soon as you think you have limited space, the MPPT makes sense. As soon as you need to spend money on mounting the panels, the MPPT makes sense.

Note the MaineSail would likely have gotten zero charging on those overcast days if he didn't having a PWM or MPPT controller.

For a 40 quart Engel the specs are .7 amps as a fridge and 2.5 amps as a freezer. In other words about 17 AH/24 hours as a fridge and 60 AH/24 hours as a freezer in 77F ambient temps...
Good insulation around your firdge/freezer is a must.

We also do extra cooling when the battery bank is topped off, and we keep pure water in the bottom of the fridge to store the "cold" (often as partial ice). This way the batteries aren't needed at all to run the fridge/freezer until about 4:00 AM. We use a circuit I designed to cool to a lower temperature when the sun is shining.

Hope this helps.


8,586 Posts
I have a question what do you mean by a massive lead acid bank mine is just over 400 ah and honestly don't expect to use more than 50 or 60 ah per day with 200 watts solar is it worth the expense of a mppt controller the pwm comes with the panels
I ran 240 watts if solar into 270 amp hours of batteries without a controller for two years without a problem.
Just wire them straight into the batteries and see how they go.
If you are only needing to charge 60 amp hours per day then why spend $500 on something that will make you get to 100% half an hour earlier?

Living aboard you get to know your boat intimately. It wont take long before you work out what works or not....
(Like today theres no wind and overcast all day so I know I cant have my freezer on and the fridge too... So I moved stuff into the freezer and wont go shopping today. Its all flexibility)

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