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Old 06-04-2016, 16:24   #1
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Battery Venting Revisited

Lead-acid batteries (of all types) emit hydrogen gas.

H is highly explosive above 4% concentration in air.

The consensus view seems to be that lead-acid batteries require external venting to avoid a dangerous buildup of H inside the cabin. As I was reading about this recently, I had some doubts that venting was really necessary, at least for my purposes (~30' boat with <200AH battery bank), given how little H is actually produced, relative the volume of air in the cabin. So I found this calculator from a battery supplier.

I plugged in information for 2 x 100AH lead-acid batteries, and assumed a very conservative 600 ft3 of internal volume for the cabin. The result was that the batteries would emit .81 ft3 of H per hour while charging (less/none when not charging).

Thus, preventing H from reaching a 1% concentration in the cabin (that's a very conservative margin of safety, as ignition occurs only at 4%) requires a complete air change every 442 minutes. To achieve this rate of air change would require a fan pushing 1.36 ft3 per minute. For a point of reference, a typical 3" computer fan pushes upwards of 30 ft3 per minute.

Whenever a boat's hatches are open (which is presumably most of the time in tropical climates), there's a certain amount of passive air movement. I don't know how to calculate this, but it strikes me as highly improbable that it would be less than 1/30th the air movement provided by a 3" computer fan. Another point of reference; the typical air change rate for a well insulated house in winter (i.e. doors and windows mostly closed) is 1 air change per ~150 minutes. Would a boat with mostly open hatches (probably in a windy part of the world) have 3 times less passive ventilation than a sealed house?

So, my semi-scientific conclusion is that, at least for a 30' boat with a <200AH battery bank, external venting is not necessary, provided the batteries vent into the main cabin (as opposed to being enclosed in a locker), and the climate is such that hatches are frequently opened.

Thoughts?
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Old 06-04-2016, 16:44   #2
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Re: Battery Venting Revisited

Those calculations probably apply in a dedicated battery room with an old style 'dumb' charger with pressure fed water replenishment. The site does not reference a standard or describe the calculation.

I disputed, with data, the need for a $2million dollar charging facility on a large military shipbuilding program based on some very spurious risk analysis. There were no flooded lead acid batteries only AGM and VRLA but that is another story altogether.

With modern chargers you wouldn't get anywhere near 1% H2.

The natural convection in the boat would be sufficient to dissipate any H2 buildup. H2 is a very light and very small molecule.

CO and Propane are far greater risks than battery gassing. Gasoline is insidious in vapor form and will snake its way around inside like an alien life form.

Remember H2 production is proportional to electrolyte (water) consumption. How much water are you consuming.

Where your calculations by weight or by volume?

Note the calculator site includes an add for a H2 detector. Standards applicable to the design of hazardous environment and confined spaces tend to specify a minimum number of air changes and not a detector and alarm as the risk control.

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Old 06-04-2016, 17:59   #3
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Re: Battery Venting Revisited

Quote:
Originally Posted by leftbrainstuff View Post
How much water are you consuming.
None

...because I have no batteries.

...because I have no boat.



Still in the planning stages, trying to minimize the number of thru-hulls.

Quote:
Note the calculator site includes an add for a H2 detector. Standards applicable to the design of hazardous environment and confined spaces tend to specify a minimum number of air changes and not a detector and alarm as the risk control.
The H detectors are pretty expensive...

If it were you, in my hypothetical 30' boat with <200AH (smart charger), would you bother?

P.S. While we're at it, you mentioned CO...

I'm interested in using a multi-fuel camp stove as my primary cooker.

...precisely to avoid the hassles/dangers associated with propane.

Based on this study (the only relevant data I could find), a stove of this type burning kerosene (dirtiest of the fuels) would create 1000ppm CO in a 14 ft3 area in 4 minutes. That means that for my hypothetical boat with a conservative 600 ft3 of interior space, a 1 hour burn would cause a concentration of 358ppm in the cabin, which is very far from fatal, but would cause unpleasant symptoms after a couple hours exposure.

The question is: given the passive venting of the cabin, what ppm level would actually be reached at peak and how quickly would it dissipate? Obviously I would have a CO detector, but it'd be nice to know in advance if this is feasible because, if not, I have to come with some other cooker arrangement.

My guess is it'd be fine; the CO would clear before it became an issue.

Thoughts?
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Old 06-04-2016, 18:22   #4
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Re: Battery Venting Revisited

I think leftbrain nailed it. Check water consumption regularly, especially with new system. Fix the charging problem if you're losing a lot of water.

Also keep in mind the batts on your boat may not be in the main cabin area. Mine are tucked up inside a lazarette.

Good luck man!
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Old 06-04-2016, 18:46   #5
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Re: Battery Venting Revisited

Quote:
Originally Posted by laika View Post
I think leftbrain nailed it. Check water consumption regularly, especially with new system. Fix the charging problem if you're losing a lot of water.
That seems like the simplest solution.

I like simple.



Quote:
Also keep in mind the batts on your boat may not be in the main cabin area. Mine are tucked up inside a lazarette.
Not sure where I'll put mine.

Weight won't be an issue (only about 100lbs). Seems the priorities would be (a) keeping them away from water, (b) easy access, and (c) keeping wire runs as short as possible. Given the floor plan I have in mind, a (vented) galley cabinet might be a good option, but then they're right next to an open flame...

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Good luck man!
Thanks!
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Old 06-04-2016, 19:21   #6
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Re: Battery Venting Revisited

Years ago shared space with a mechanic. One morning he asked me to check a car's battery with my drop tester.

Three of us were leaning in to see the tester's needle when I touched the posts with the probes, BANG!

Mechanic hadn't thought to tell me the car had just finished charging and that there was a hole in the case

Blew the top off the battery & our faces were splashed with acid - we ran for the water hose expecting blindness, disfigurement etc...

Didn't even sting - no pain, no burned or stained clothes - nothing.

This obviously isn't a recommendation to be reckless, just one surprisingly non-deadly experience.
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Old 06-04-2016, 20:31   #7
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Re: Battery Venting Revisited

I have been in the presents of several battery explosions ,in two cases poor connections at the battery post were being repaired, one case the battery was being grossly over charged the third, was unexplained but all were most likely very low on water considering the lake of fluid afterwards .In the overcharging case the smell from the battery had gone from the typical rotten egg smell to a bleach like smell then blew up?????
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Old 07-04-2016, 06:29   #8
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Re: Battery Venting Revisited

Quote:
Originally Posted by KISS View Post
... The question is: given the passive venting of the cabin, what ppm level would actually be reached at peak and how quickly would it dissipate? Obviously I would have a CO detector, but it'd be nice to know in advance if this is feasible because, if not, I have to come with some other cooker arrangement.
My guess is it'd be fine; the CO would clear before it became an issue.
Thoughts?
You can't safely rely on passive/natural ventilation.
Carbon Monoxide Poisoning: Ventilation (AEN-209)
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Old 07-04-2016, 08:06   #9
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Re: Battery Venting Revisited

The problem is that you are looking at overall gas concentrations in the entire cabin.
It's very easy to get a localized build up of gas, hence the need to have a big air flow, in the hope that this would dilute any localized build up.
A bit like a sledge hammer for a fly.
A computer fan in the box housing the battery, plus a natural air flow with a hatch open would be better, provided the fan is explosion proof. (I have no idea if this is the case or not.)
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Old 10-04-2016, 13:39   #10
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Re: Battery Venting Revisited

While charging. The hydrogen gas build up in the cells is a problem. In an open space there is little problem. In a small un-vented battery locker there is some problem. I witnessed a battery explode due to smoking a cigar over the charging battery. Destroyed battery. Another a fool used a match for light, igniting gas in battery, badly burned eye, with some loss of vision, as well as destroyed battery. Don't underestimate the harm that can be done with a spark at a charging battery. But, most ventilated spaces will be fine.
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Old 15-04-2016, 11:21   #11
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Re: Battery Venting Revisited

Quote:
Originally Posted by espresso View Post
The problem is that you are looking at overall gas concentrations in the entire cabin.
It's very easy to get a localized build up of gas, hence the need to have a big air flow, in the hope that this would dilute any localized build up.
A bit like a sledge hammer for a fly.
A computer fan in the box housing the battery, plus a natural air flow with a hatch open would be better, provided the fan is explosion proof. (I have no idea if this is the case or not.)
A general computer fan is not going to be intrinsically safe or explosion proof unless you are paying a shed load. However, modern fans are less likely to create sparks as they don't uses bushes anymore so the chances of a fan created spark are greatly reduced.

When you also consider that nothing else electrical onboard is IS or EX or EEX rated then the fact the fan isn't is neither here nor there. If you have managed to get 4% H2 in the cabin volume then turning on a light, your mobile phone ringing or even the VHF receiving could cause a big BANG. Even static electricity could do it.

I'd be more concerned about petrol vapours as the LEL for petrol/gasoline is only 1.2% in air. Butane is 1.8% and Propane 2.1% with Methane (Natural Gas) at 5% in air.

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