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Old 24-01-2022, 13:30   #1
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Advice needed: position of battery bank and safety

Hi fellow sailors, I am preparing to replace the entire electric system of my sailboat and was hoping someone here with expert knowledge about batteries (I have just started learning about the topic..) could help me answer a question about safety and the positioning of my battery bank.

Currently the batteries (start battery and house batteries) are positioned in the bilge, just below the floor boards. Obviously, if one should be unfortunate, have an accident and take in water quickly these batteries are positioned above the bilge pumps...but the volume of water would obviously be filled and, in the process, drowning the batteries a lot quicker than if the batteries were located for example under the berths.

In terms of cabling / distance to switchboard / control panel etc I would prefer to put the batteries (lead AGM, not lithium for now..) under the berth next to the engine room. I have heard that batteries produce gas while charging. Does this represent a risk in terms of 1) someone sleeping on that particular berth while the engine is running / the battery is charging? 2) the short distance between the engine and the battery?

Hope someone can help me address these questions / suggest solutions / confirm whether I am worrying about something that is not really an issue..
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Old 24-01-2022, 14:17   #2
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Re: Advice needed: position of battery bank and safety

They do produce some gas. It seems to be minimal, as most battery banks do not seem to have had any resolution to the gas issue. I have never heard of a boat exploding due to battery gas, only propane.
Even purchased battery boxes are not entirely enclosed. (how do you get the cables out?)
Many boats have banks under a quarterberth.
If you want to be nice and safe, build the box under the berth with sealed corners. The top plywood is enough especially with the cushion over it and you want a bit of air to be pulled in. . A small vent tube/ flexible tube (1"? 2"? Vaccum cleaner hose type) could be constructed to vent out somewhere high, like the aft coaming. A rise of 3 feet in the tubing creates a surprisingly strong "chimney effect" pulling air from the box.

I dont think AGM really vent any gas do they?
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Old 24-01-2022, 15:23   #3
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Re: Advice needed: position of battery bank and safety

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I dont think AGM really vent any gas do they?
No, they don't, until they fail. And then they do.
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Old 24-01-2022, 21:20   #4
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Re: Advice needed: position of battery bank and safety

Hey Bacchant, I too am in the process of rewiring and installing completely new battery banks, for which I have done considerable research. You have not given a great deal of information about your proposed battery installation. So, how many batteries and of what size? Separate house and starter banks? What kind of charger(s)?

I am planning to install a house bank of four 6-volt deep cycle batteries and a separate starter battery. This will be replacing a house bank of two Series-31 deep cycle batteries and a starter battery. The four 6-volt batteries represent a combined weight that is pushing over 260 pounds (123 Kilos), so appropriate placement is important from a balance perspective. I had thought of placing them in the bilge, but the space was too tight for easy maintenance and I too had concerns about flooding. However, from a weight and balance perspective the bilge was ideal. So, you may want to consider the impact on your boat's balance if you relocate. I will be locating mine under a main-cabin settee, in a locker which is as low down and central to the boat as possible.

Lead-acid batteries can emit hydrogen gas when charging, but the quantities are small if you are using a proper charger. If you are using an old-fashioned charger that boils off the electrolyte, well, then things can be different. A small vent to the open cabin in the upper corner of the battery compartment should be sufficient to safely disperse any gas discharged. I will be installing a ProNautic 12-40P charger which has sufficient capacity and smarts to keep my batteries going for a long time.

One important thing to consider: for lead-acid batteries the optimal orientation is transverse, across the beam of the boat. This does a better job of keeping the plates covered with electrolyte while heeling, as opposed to a longitudinal orientation along the fore-and-aft line of the boat. I did some rough calculations for the 6-volt batteries. On a longitudinal (fore-and-aft) orientation, the top corners of the lead plates get exposed starting at about 15 degrees of heel, presuming that the cells are all fully topped up. If they are not, then exposure occurs with even less heeling angle. On a transverse (across the beam) orientation, the tops of the plates do not get exposed until you reach about 35 degrees of heel, which no sailor willingly sustains. Exposure of the lead plates has implications for amount of charge available, as well as sulfation and battery longevity.

My starter battery, which is lighter and has infrequent loads with minimal charging activity will be located in its own box within the cockpit locker/engine space. Regardless of position, all batteries will be well-secured so that they cannot shift or tip over. I learned that lesson very early in my sailing career.

I hope that this has provided some food for thought. Good luck on the project.
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Old 25-01-2022, 07:22   #5
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Re: Advice needed: position of battery bank and safety

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Originally Posted by PerfectPirate View Post
Hey Bacchant, I too am in the process of rewiring and installing completely new battery banks, for which I have done considerable research. You have not given a great deal of information about your proposed battery installation. So, how many batteries and of what size? Separate house and starter banks? What kind of charger(s)?

I am planning to install a house bank of four 6-volt deep cycle batteries and a separate starter battery. This will be replacing a house bank of two Series-31 deep cycle batteries and a starter battery. The four 6-volt batteries represent a combined weight that is pushing over 260 pounds (123 Kilos), so appropriate placement is important from a balance perspective. I had thought of placing them in the bilge, but the space was too tight for easy maintenance and I too had concerns about flooding. However, from a weight and balance perspective the bilge was ideal. So, you may want to consider the impact on your boat's balance if you relocate. I will be locating mine under a main-cabin settee, in a locker which is as low down and central to the boat as possible.

Lead-acid batteries can emit hydrogen gas when charging, but the quantities are small if you are using a proper charger. If you are using an old-fashioned charger that boils off the electrolyte, well, then things can be different. A small vent to the open cabin in the upper corner of the battery compartment should be sufficient to safely disperse any gas discharged. I will be installing a ProNautic 12-40P charger which has sufficient capacity and smarts to keep my batteries going for a long time.

One important thing to consider: for lead-acid batteries the optimal orientation is transverse, across the beam of the boat. This does a better job of keeping the plates covered with electrolyte while heeling, as opposed to a longitudinal orientation along the fore-and-aft line of the boat. I did some rough calculations for the 6-volt batteries. On a longitudinal (fore-and-aft) orientation, the top corners of the lead plates get exposed starting at about 15 degrees of heel, presuming that the cells are all fully topped up. If they are not, then exposure occurs with even less heeling angle. On a transverse (across the beam) orientation, the tops of the plates do not get exposed until you reach about 35 degrees of heel, which no sailor willingly sustains. Exposure of the lead plates has implications for amount of charge available, as well as sulfation and battery longevity.

My starter battery, which is lighter and has infrequent loads with minimal charging activity will be located in its own box within the cockpit locker/engine space. Regardless of position, all batteries will be well-secured so that they cannot shift or tip over. I learned that lesson very early in my sailing career.

I hope that this has provided some food for thought. Good luck on the project.
Is "transverse" with the long side of the rectangle laying port-starboard, or forward-aft?
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Old 25-01-2022, 07:54   #6
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Re: Advice needed: position of battery bank and safety

Long side is port-starboard.
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Old 25-01-2022, 15:50   #7
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Re: Advice needed: position of battery bank and safety

On a sailboat I want my batteries low. If the boat floods that is a much bigger issue! With a good charger you get very little gassing. If the area is. If the area is big enough to sleep in I do not think gassing would be an issue. I am no expert but with some reading, and some good equipment, I managed to squeeze a set of Rolls FLA house batteries for 20 years on a mooring and without solar.
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Old 25-01-2022, 16:05   #8
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Re: Advice needed: position of battery bank and safety

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On a sailboat I want my batteries low. If the boat floods that is a much bigger issue! With a good charger you get very little gassing. If the area is. If the area is big enough to sleep in I do not think gassing would be an issue. I am no expert but with some reading, and some good equipment, I managed to squeeze a set of Rolls FLA house batteries for 20 years on a mooring and without solar.
It's a good point. Weight low is good.
If there are bilge covers directly over, it's easy to check the batteries too.
Under the berth you have to remove items, remove cushions, remove lids. All that stuff is big and in the way.

Everything is a compromise on a boat.
The OP's using AGM batteries though, so no checking much.
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Old 28-01-2022, 11:04   #9
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Re: Advice needed: position of battery bank and safety

Thanks for a lot of useful feedback, greatly appreciated! I will make some changes to my plans..
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Old 29-01-2022, 03:36   #10
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Re: Advice needed: position of battery bank and safety

I had 12vt house and start batteries (flooded lead/acid) in the bilge area under the cabin sole on my 12m Joshua, BUT had a 12vt battery under the nav seat as an emergency power source for the SSB in case the bilge got flooded. The emergency battery was connected to the solar/alternator charging system with diodes, but also had an isolating switch to completely isolate it from the other bats if they were compromised.
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