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Old 02-05-2021, 20:16   #1
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AGM Starting Battery

I have a question a about the starting battery for a 38' crusiing boat with a 40Hp Yanmar.
The previous owner had a SeamasterGold MFM 70 for the starting battery -- CCA 750 @ 0F/-18C and amp hrs 85. This is described as a maintenance free battery -- maybe a gel? Retails for around Aus$200.

I am keeping the boat on a mooring. I have a solar panel but the mooring is surrounded by talls trees. So solar charging in patchy.

I have been away from the boat for 2 weeks and found the battery was dead (10.7v). Since I can only get to the boat every second weekend I am thinking of putting in an agm. Lifeline have one that would fit at CCA 550 @ 0F/-18C and 45 amp hrs. Retails around Aus$480.

Would that be adequate for my engine? Next size up is CCA 650 and 75 amp hrs but would be a difficult fit (would need to sit it on its side). Also about 60% higher in price.

The house bank consists of 4 x 6v 232 amp hr golf cart batteries new in 2020. They seem to be working well.

Would appreciate your thoughts/ideas. Thanks in advance.
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Old 02-05-2021, 20:25   #2
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Re: AGM Starting Battery

It doesn't take much to start a small Yanmar, so that battery should be fine.
More important is to figure out why your starter battery died after only two weeks.
I assume your solar charges the house bank. Is there a relay that then charges the start battery?

Even without a charging source two weeks is too fast. There should be no load on the start battery, so only internal loss. I'd check with a current meter what loads are on the start battery.
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Old 02-05-2021, 20:42   #3
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Re: AGM Starting Battery

No need for an AGM if you can get to your boat every fortnight or even every month.

As Paul posts, a 40hp Yanmar doesn't require a very high starting current and any MF starting battery above say 500 CCA will suffice. These should be under or around $200 Aussie.

Paul is also on the money regarding checking why the existing battery went flat so quickly.
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Old 02-05-2021, 20:44   #4
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Re: AGM Starting Battery

I've got an Optima Red Top AGM 45 ah start battery on my boat starting a 30hp motor. It has no other directly connected source of charging aside from the standard alternator. I chose the Optima because the battery is mounted in the engine bay and is exposed to engine heat, which is supposed to be ok with these batteries. The engine rarely takes more than a few seconds to start and the battery has been flawless in this role. I'd almost suspect this battery could easily start a 40hp, but if not there will be one in the range that would.


I do have an isolation switch for the battery that disconnects it from everything, including the engine and I switch this off whenever the boat is left for a while. There's been periods measured in months using this setup and the battery hasn't let me down yet.
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Old 03-05-2021, 00:08   #5
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Re: AGM Starting Battery

Thanks for the helpful replies.

I have contacted the previous owner who tells me the solar panels don't charge either bank unless batteries are switched on. To charge start battery switch needs to be set to 1 or both. Is this usual?

It was always my practice (on my last boat some 10 yrs ago) to switch batteries off (bilge pump was wired directly to house bank). So this explains why there was no charge from the solar panels.

Should the solar panels be wired differently? I was always taught that batteries should be set to off when leaving the boat for any extended period due to short/fire risk.
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Old 03-05-2021, 00:12   #6
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Re: AGM Starting Battery

Quote:
Originally Posted by Nyssa View Post
Thanks for the helpful replies.

I have contacted the previous owner who tells me the solar panels don't charge either bank unless batteries are switched on. To charge start battery switch needs to be set to 1 or both. Is this usual?

It was always my practice (on my last boat some 10 yrs ago) to switch batteries off (bilge pump was wired directly to house bank). So this explains why there was no charge from the solar panels.

Should the solar panels be wired differently? I was always taught that batteries should be set to off when leaving the boat for any extended period due to short/fire risk.

I agree with you, my solar is set up to charge direct to batteries, unswitched but of course fused at the batteries.
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Old 03-05-2021, 00:29   #7
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Re: AGM Starting Battery

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Originally Posted by Uncle Bob View Post
I agree with you, my solar is set up to charge direct to batteries, unswitched but of course fused at the batteries.


Same for me: solar connected directly on buss bar of service bank. Engine battery isolated, only charged by either alternator or land-linked charger at pontoon. Both bank switches are off when off the boat.

For the battery capacity, I have a 70Ah for 20y-old 56hp Yanmar, flawless start within less than a sec. with a 8y-old closed LA battery. Id guess your 45Ah option should be fine.
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Old 03-05-2021, 00:52   #8
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Re: AGM Starting Battery

Quote:
Originally Posted by Nyssa View Post
............
The previous owner had a SeamasterGold MFM 70 for the starting battery -- CCA 750 @ 0F/-18C and amp hrs 85. This is described as a maintenance free battery -- maybe a gel? .........
No, it isn't a gel, it is a just flooded lead acid sealed maintenance free (SMF) battery.

More info on SMF here https://www.centurybatteries.com.au/...smf-technology
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Old 03-05-2021, 04:55   #9
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Re: AGM Starting Battery

Quote:
Originally Posted by Nyssa View Post
I am keeping the boat on a mooring. I have a solar panel but the mooring is surrounded by talls trees. So solar charging in patchy.

Since I can only get to the boat every second weekend I am thinking of putting in an agm. Lifeline have one that would fit at CCA 550 @ 0F/-18C and 45 amp hrs. Retails around Aus$480.

Would that be adequate for my engine? Next size up is CCA 650 and 75 amp hrs but would be a difficult fit (would need to sit it on its side). Also about 60% higher in price.

AGMs don't self-discharge all that quickly, so this might be a good use case for AGMs.

Which exact Lifeline starting battery are you eye-balling? What is your engine and what are it's minimum CCA requirements?

Who else sells AGMs in Oz? FWIW, I lean slightly more toward Odyssey for starting and dual-purpose AGMs... usually more cranking amps per size. Maybe Northstar might be similar...

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Old 03-05-2021, 22:50   #10
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Re: AGM Starting Battery

The Yanmar manual doesn't specify CCA. It only specifies min capacity 80AH.

I went ahead and bought a Century Dual Force AGM CCA 750 AH 90. Good for both starting & cycling. Not cheap -- about Aus$400. Very similar to the Odyssey Chris mentioned.

Still trying to fully understand the solar set up but it seems to be wired into the battery switch which I will need to change.

Many thanks for your replies.
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