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Old 04-12-2009, 14:45   #1
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Ventilation of Battery Compartment

I'm thinking of using my starboard cockpit locker as a compartment for two 6v flooded cell batteries. Knowing that these flooded cells will release hydrogen during charging, I want to provide some passive ventilation for this locker so as to avoid the build up of this potentially explosive gas. I have read in various places that only a small hole is needed in a battery compartment and that it should be placed as high as possible in the compartment, since hydrogen is lighter than air.

A local West Marine employee (who is also a liveaboard) suggested that I install a clamshell vent or possibly even a cowl vent on the cockpit coaming. I told him my only worry was that this would provide a point of entry for water into the cockpit locker in the event that the cockpit itself took on some water in a following sea, etc.


Would be grateful for some voices of experience on how I should approach this ventilation issue.

Thanks,
Roscoe
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Old 04-12-2009, 14:50   #2
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I don’t have any vents and my 2 group 27s are under the port settee near the companionway.
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Old 04-12-2009, 15:18   #3
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There's little utility in venting your battery compartment to the interior of the boat.

ABYC E-10.7.10 states:
A vent system, or other means, shall be provided to permit the discharge from the boat of hydrogen gas released by the battery.

Vented batteries emit potentially explosive gases under charge conditions. Therefore it is important to calculate the amount of ventilation required for your battery compartment.

The CONSERVATIVELY required number of air changes per hour (A) is given by the following formula:

A = (0.045 x N x I) / V

Where:
N =
Number of cells in the battery
V = Volume of compartment in cubic metres
I = Charge rate in Amperes

This formula will give the number of air changes per hour required during bulk charge conditions.

On float charge, the amount of gas emitted is approximately 1.5% of that liberated whilst on boost charge, and under most circumstances, this will be dissipated by natural ventilation, and will not present a hazard.
However, itís recommended that the bulk charge condition is allowed for at the design stage to ensure the appropriate decision on ventilation requirements is made.

Although Valve Regulated Lead-Acid Batteries require little ventilation under normal operating conditions, it is good practice to apply the formula to calculate the number of air changes required to achieve minimum risk under battery fault or failure conditions.
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Old 04-12-2009, 18:49   #4
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What you are saying, Solitude, corresponds to what someone else has told me. He said that he has had flooded cells in his cockpit locker for 24 years without a venting system. He said that the hydrogen escapes from the gaps around the edges of the locker hatch, but only because he does not have a sealing gasket around the edges.

From what Gord is saying, however, in order for a boat to be ABYC compliant, it must have some sort of venting - at least enough to correspond to the air changes per hour (A) as determined by the stated formula: A = (0.045 x N x I) / V

If ventilation of the battery compartment is a must (in order to avoid getting dinged in a survey or, worse, to avoid blowing up the boat), then the question still remains:

What sort of venting is the most safe, that is, the least likely to allow for the entry of water into a cockpit locker?

Thanks for your helpful advice,
Roscoe
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Old 05-12-2009, 07:11   #5
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I am in complete agreement with Gord.

Further, I see no way to meet your design criteria regarding a passive system; you cannot passively move enough fresh air through your system.

A relatively simple solution would be to install a goose neck in the vent path and then use a voltage sensing module that would turn on a brushless pancake fan when the batteries are being charged. This system would evacuate from the compartment overboard through the gooseneck and clamshell. Contact me by private message if you need a source for the module to control the fan.

AGMs (Valve Regulated Lead Acid or VRLA) batteries are inherently safe in this regard as the gas is recombined within each cell and the only event that would cause a gas discharge from the battery would be charging at an excessive voltage that would cause the cell relief valves to lift. There is at least one manufacture of AGMs that has included a chamber on the top of the battery to collect any gas that is generated and vent it through the 1/4" hose barb nipples provided in the chamber.

I regularly dissuade owners from installing FLA batteries in living spaces because most of them do not want to go to the trouble of properly providing a vent path for the explosive gasses.

Hope this helps.
Charlie
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Old 05-12-2009, 07:54   #6
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Quote:
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☞ I am in complete agreement with Gord. ☜
... I regularly dissuade owners from installing FLA batteries in living spaces because most of them do not want to go to the trouble of properly providing a vent path for the explosive gasses.
Hope this helps.
Charlie
Explosive & Corrosive gases.
Even if they don't explode, as it requires a very narrow range of concentrations (upper & lower explosive limits), they WILL attack your upholstery fabrics & other materials, and creat an unhealthy living environment.
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Old 05-12-2009, 08:36   #7
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Excellent point Gord.

Charlie
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Old 05-12-2009, 09:36   #8
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Anyone doubting Gord's observation can simply place a piece of paper near the battery bank. Place a second piece elsewhere. Compare the two pieces a couple months later. Now, consider what long-term effect that can have on fabric, electrical wire, and lung tissue. Or don't.
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Old 05-12-2009, 09:41   #9
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Thanks Charlie. So the gooseneck in the cockpit locker would serve as an anti-siphoning device. The clamshell would then be mounted somewhere on the coaming. Is that right, or by "overboard" do you mean that it would be mounted somewhere on the topsides?

Roscoe
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Old 05-12-2009, 10:12   #10
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By "overboard" I am implying that you provide a path for the corrosive and potentially explosive gasses that vent, well, overboard and dispersed in the atmosphere with no mechanism for these gasses to return in hull. Analogous to a properly designed propane locker but the other way round as propane is heavier than air and hydrogen is lighter than air.

Hope this helps.
Charlie
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Old 05-12-2009, 10:59   #11
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This is another argument for a non-permanently-hooked-up (portable) battery charger(s) on smaller boats (less than 27' or so) where a permanent battery venting system may be impractical. The lazarette/cockpit hatch is simply opened when the charger is hooked up and charging, and closed when it isn't.

Also, it is interesting that many of the same folks which have AGM's inside their cabin are the same people claiming that a lack of dedicated vents in a lazarette battery compartment may introduce hazardous gasses into the cabin. Newsflash: AGM batteries vent too, although not as much as regular lead acid, they still vent. And if they're tucked away under your v-berth or saloon, then you've been sucking on hydrogen gasses also, particularly when sleeping. In other words, a regular lead acid battery in a lazarette separated from the cabin likely introduces the same amount of hydrogen into the cabin as AGM battery(s) located inside the cabin do.
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Old 05-12-2009, 11:31   #12
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Quote:
This is another argument for a non-permanently-hooked-up (portable) battery charger(s) on smaller boats (less than 27' or so) where a permanent battery venting system may be impractical. The lazarette/cockpit hatch is simply opened when the charger is hooked up and charging, and closed when it isn't.
There are a lot of liveaboards on 27' boats in my neck of the waterfront, so this arbitrary limit seems high. If the "portable" battery charger is a marine rated model then this idea is OK. If it is an automotive battery charger it has no place on a waterborne vessel and can be downright dangerous.

Quote:
Newsflash: AGM batteries vent too, although not as much as regular lead acid, they still vent.
Sorry NotSure, you are incorrect on this point. Read my post #5 in this thread.
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Old 05-12-2009, 11:58   #13
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... AGMs (Valve Regulated Lead Acid or VRLA) batteries are inherently safe in this regard as the gas is recombined within each cell and the only event that would cause a gas discharge from the battery would be charging at an excessive voltage that would cause the cell relief valves to lift. There is at least one manufacture of AGMs that has included a chamber on the top of the battery to collect any gas that is generated and vent it through the 1/4" hose barb nipples provided in the chamber...
AGM and Gel Cell batteries can indeed out-gas hydrogen. Proper charging usually doesn't pop the vent caps open, and normally hydrogen and oxygen are recombined in the cells.
However, during overcharge and/or equalization conditions, the internal pressure of the gasses in the cell could exceed the common 1-2 PSI limit (the best AGMs vent @ 5psi), allowing the vent seals to out-gas hydrogen.
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Old 05-12-2009, 12:03   #14
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There are a lot of liveaboards on 27' boats in my neck of the waterfront, so this arbitrary limit seems high. If the "portable" battery charger is a marine rated model then this idea is OK. If it is an automotive battery charger it has no place on a waterborne vessel and can be downright dangerous.


Sorry NotSure, you are incorrect on this point. Read my post #5 in this thread.
Simply because you post something doesn't make it true. Both gel and agm batteries 'vent'. Just not near as much as regular flooded lead acid.
To wit: 'By keeping the moist with electrolyte, gas recombination is more efficient (99% AGM). This leads to fewer incidents of exploding batteries than either of the 2 types above' (agm, flooded lead acid).
Battery Type Comparison
And this comes from the mouth of an AGM battery vendor, not exactly an impartial source when it comes to battery venting. I would posit that the 99% efficient number is the low end of the venting spectrum for agm batteries...it's likely more probable to be around 95% or so.
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Old 05-12-2009, 12:11   #15
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AGM and Gel Cell batteries can indeed out-gas hydrogen. Proper charging usually doesn't pop the vent caps open, and normally hydrogen and oxygen are recombined in the cells.
However, during overcharge and/or equalization conditions, the internal pressure of the gasses in the cell could exceed the common 1-2 PSI limit (the best AGMs vent @ 5psi), allowing the vent seals to out-gas hydrogen.
Correct. Also note that many high quality chargers used on boats include an automated equalization function, and many such chargers are high amp fast chargers, meaning that the agm's connected to them are likely gassing regularly when connected to such chargers.
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