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Old 21-09-2015, 08:44   #1
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Battery bank size - small boat, big trip

Hey guys,

So with the solarpanel installed the boat is finally ready for a proper house-battery! The main question remaining is, how big do we need it to be and how big do we want it to be...? I know every boat is different and it's best not to discharge your battery under 50% of its total capacity. With most all of the boats I can find electricity allowances of online being bigger and having more gizmo's than mine, I'm having a mighty hard time estimating what kind of battery I should fit... With the following in mind as far as consumers are concerned and knowing the only recharging will be via a wall-fed charger (not often) or the 255W solarpanel permanently mounted on the stern, what would you suggest? For some inexplainable reason I'm stuck with 200-250Ah in my head, and initial calculations seem to support that, but I have no real-life data on the matter. We have a batterymonitor, but it has little use until we can take the boat out for longer cruises WITH all the electrical stuff working...

5 small interior lights (LED)
navlights (all LED)
HF radio
VHF radio
autopilot (to be used barely as we have windvane)
chartplotter
depthsounder
speakers/radio
charging for ipad/ipod/computer

No fridge, no electric bilgepumps, no electric waterpump, no waterheater, no microwave, no electric heating, no radar, no electric windlass, no bowthruster,... Pretty spartan little boat
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Old 21-09-2015, 09:03   #2
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Re: Battery bank size - small boat, big trip

With that setup, I think it hardly matters.

The usual criterion, to my mind, would be that you need to be able to run your refrigeration (the biggest consumer by far on most boats) for 24 hours on 25% or less of the bank's real (not nominal) capacity, but you don't have refrigeration, thus you've got no real critical continuous loads other than VHF and electronics. How many amp/hours does it take to run your electronics for 24 hours? I bet not more than 60 at most?

I would think that 250 amp/hours * 12v nominal would be very adequate.
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Old 21-09-2015, 09:06   #3
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Re: Battery bank size - small boat, big trip

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Originally Posted by Orchidius View Post
Hey
I'm having a mighty hard time estimating what kind of battery I should fit...

The manuals for many of your gizmos will tell you how much current they use when turned on.

-Chris
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Old 21-09-2015, 10:01   #4
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Re: Battery bank size - small boat, big trip

It's easy enough to figure out how much everything uses (and I have done that, I've got an excell spreadsheet for that), but how long will you use everything for? The chartplotter for example, you'll want it on when navigating close to coasts, but on longer passages it won't be on at all, same with the depthsounder. The SSB doesn't consume much when it's in standby, a little when receiving and a hell of a lot when it's transmitting, but how many seconds of transmitting do you think you'd use on such a trip...?

That's what I mean by real-life data, because the paperwork might give you an idea, but will surely bite you in the ass come departure day.

As far as refrigeration goes, we do have an icebox that we retrofitted, but won't count on it when stocking up on food and won't "need" it along the way. Since we're 20-something backpackers we're not used to such luxuries anyway , grandma just happened to have this one laying around doing nothing and it just happened to fit like a glove into a cabinet.
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Old 21-09-2015, 10:25   #5
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Re: Battery bank size - small boat, big trip

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It's easy enough to figure out how much everything uses (and I have done that, I've got an excell spreadsheet for that), but how long will you use everything for? The chartplotter for example, you'll want it on when navigating close to coasts, but on longer passages it won't be on at all, same with the depthsounder. The SSB doesn't consume much when it's in standby, a little when receiving and a hell of a lot when it's transmitting, but how many seconds of transmitting do you think you'd use on such a trip...?

Yeah, good points, but I can't help you with that part of it... and maybe only you can figure that stuff out for your situation.

FWIW, our navigation electronics are always on when we're underway. About half of our electronics suite is always on when we're at anchor. Both fridges always on. Et cetera. Obviously different from your approach.

-Chris
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Old 21-09-2015, 12:11   #6
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Re: Battery bank size - small boat, big trip

You want more than you need rather than not quite enough. You do the math.
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Old 21-09-2015, 12:16   #7
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Re: Battery bank size - small boat, big trip

Two x group 31 would be plenty. Maybe a separate Group 24 for starting.
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Old 21-09-2015, 12:40   #8
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Re: Battery bank size - small boat, big trip

That's a big solar panel. I think you will be fine with 2 100 Ah batteries. How are you for weight and space? Bateries are heavy and expensive.
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Old 21-09-2015, 13:14   #9
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Re: Battery bank size - small boat, big trip

I know they're expensive, that's why I don't wanna go overboard on it... The 230Ah ones I've been able to source are 61kg. I'm not gonna lie, it's gonna be a bitch to store and secure them, but not much I can do about that. Anything lead-acid is going to be similar. The storage spot is right next to the engine; as low as I can get em, but not quite on the centerline.
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Old 21-09-2015, 17:14   #10
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Re: Battery bank size - small boat, big trip

Girlfriend and I just did three weeks on my 24 footer.

We have a 100w panel for charging, that's it.

We have a bd-35 compressor fridge which draws about 35Ah a day, being in Scandinavia where the ambient temp in the boat was rarely more than 20C.

All nav gear, chart plotter, depth sounder, and radio (with ais, nmea2000 network) draw about 1.4 amps combined.

We have a tp22 tiller pilot, which draws about 4 amps when actuating, but it's duty cycle is about 25-50 percent (huge approximation as it change a lot based on weather).

We only motored about 30 mins a day, (average)

All of this, we could comfortably handle on an 85Ah battery, without going below 12.4V open circuit, so about 60%. This was quite sustainable.

As an aside, do everything you can to avoid shading on the panel. A little bit (5-10%) shade can bring the output down by 80%.

With 200+ Ah, you will be golden, even with a fridge (which I highly recommend, even as a youngun myself)


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Old 22-09-2015, 08:50   #11
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Re: Battery bank size - small boat, big trip

I strongly recommend an automatic electric bilge pump
Bill


Quote:
Originally Posted by Orchidius View Post
Hey guys,

So with the solarpanel installed the boat is finally ready for a proper house-battery! The main question remaining is, how big do we need it to be and how big do we want it to be...? I know every boat is different and it's best not to discharge your battery under 50% of its total capacity. With most all of the boats I can find electricity allowances of online being bigger and having more gizmo's than mine, I'm having a mighty hard time estimating what kind of battery I should fit... With the following in mind as far as consumers are concerned and knowing the only recharging will be via a wall-fed charger (not often) or the 255W solarpanel permanently mounted on the stern, what would you suggest? For some inexplainable reason I'm stuck with 200-250Ah in my head, and initial calculations seem to support that, but I have no real-life data on the matter. We have a batterymonitor, but it has little use until we can take the boat out for longer cruises WITH all the electrical stuff working...

5 small interior lights (LED)
navlights (all LED)
HF radio
VHF radio
autopilot (to be used barely as we have windvane)
chartplotter
depthsounder
speakers/radio
charging for ipad/ipod/computer

No fridge, no electric bilgepumps, no electric waterpump, no waterheater, no microwave, no electric heating, no radar, no electric windlass, no bowthruster,... Pretty spartan little boat
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Old 22-09-2015, 09:03   #12
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Re: Battery bank size - small boat, big trip

I would add a small Honda 2000 or equal generator
on my Morgan 27 (I have no solar) and 480 ah on one bank and 120 ah on other bank
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Old 22-09-2015, 09:08   #13
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Re: Battery bank size - small boat, big trip

+1 on a separate start battery with an automatic charging relay. This way you know the start battery is always at 100% even if you accidentally draw the house bank down. One less worry at 3AM. A smaller group 27 or 24 size will be plenty big for this and ideally can be placed close to the engine to reduce the loss in the cables.

This way the house bank doesn't have to be sized to be able to start the engine when it's down 50% (or more). So two deep cycle grp 31's (200 ah total) will be fine and don't weigh too much.

And while you have to be careful mixing battery types, an AGM starting battery works fine when charged with a flooded house bank. AGM's like the Optima deliver a lot of cranking amps.
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Old 22-09-2015, 09:18   #14
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Re: Battery bank size - small boat, big trip

Quote:
Originally Posted by Orchidius View Post
The SSB doesn't consume much when it's in standby, a little when receiving and a hell of a lot when it's transmitting, but how many seconds of transmitting do you think you'd use on such a trip...?
While it is true during transmit a fair amount of power is being drawn, probably not as much as your thinking. Typical 100 watt transmitter is about 60% efficient so for 100 watts expect about 165 watts draw. I know that sounds like a lot but you have to remember this is SSB and not CW, AM, or FM that is 100% duty cycle. So an hour of usage instead of consuming 165 whr, would consume about 1/3, and if a normal conversation you will listening 1/2 the time. 1/6 of 165 whr is only 27 whr, a rather small drain.
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Old 22-09-2015, 09:27   #15
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Re: Battery bank size - small boat, big trip

I think you will be fine, until you get a fridge.
I think I'd go for two golf cart batteries for a house bank and a small 12V start battery for your starting bank, although I admit I don't think multiple banks are essential, I can manage a single fuel tank, why can't I manage a battery bank?

On edit, what I don't know is how much power do you get out of that panel in Belgium?
A lot less than in the Caribbean in Summer I'm sure, so what may be your limiting factor may not be battery bank size, but ability to re-charge it.
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