Battery Venting

animatt
animatt Solar Expert Posts: 295 ✭✭✭
in Off Grid Solar & Battery Systems #1
Looked for specific on here but see many just rough guess on what venting is okay.

I have 8 crown cr-235's batteries 235ah at 20hr rate(6v).

Wired 2 strings in parallel and will have 24v @ 470ah.

Initially will have very small solar array(464w). Few months later will get more panels to put it into a good charging territory. But batteries will be moved when this happens and venting will not be an issue.

This is all in a motorhome. So I went with shorter batteries as finding space for larger batteries are difficult. Anyway have these 8 batteries.

They are mounted under a bench seat in the motorhome. I am still sealing the space off from the coach. There were other places to mount batteries, but this is more theft proof. I made a hole out of the side wall of the motorhome using a 2" hole saw bit. I am thinking of putting in a flanged 1.5" pvc piece there with a screen end to prevent insect infiltration.

I also bought some water saver caps from NAWS . The idea was to limit gassing.

So is there any more definite guidelines for venting?

I mention above the small solar array basically the batteries will not be charged very fast. Current array would only produce about 3% of AH in optimal conditions. Also discharge will usually be low. Something like a converter freezer to fridge a few minutes an hour. Some days there maybe microwave usage for several minutes.

If batteries are getting low (down 25% from full or after several days to get back to full charge) I would run a generator for the bulk phase of charging. Probably till batteries have 90% of charge. This will be a faster charge rate but at times when out gassing is not a big deal.
Charger is only 35 amp. still less than 10% of ah rating of batteries.

Anyway main questions is what venting is adequate here. Are there any more definite answers on this. Like per AH have a certain inches ^2 of vent area. Then it using water saver caps cut the vent area in half?

thanks
matthew

If I have time later I will take a picture or battery setup.
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Battery Venting

    Water Miser caps only "filter out" droplets of electrolyte to drip back into the cells--They do not reduce/impact gassing at all.

    It is really hard to say how well your airflow will work. Hydrogen tends to leak out of any area through gaps, wood, etc. pretty easily--But if the "roof/lid" is plastic, the gas can collect at the high spots.

    You should have air flow through the space to mix/move the gases out (high/low vents, large vents). Or use a fan when the battery charger is turned on (some folks use a voltage controlled switch--I.e., turn on fan(s) when the battery voltage is over ~14.0 volts (gasing voltage is a few tenths of a volt higher).

    The other issue is the acid "mist"--good air circulation the acid fog out of the box/off the battery connections and keeps the sulfur smell out of the living spaces.

    I am sure there is a recommendation for XX CFM for every 100 Amps of charging current--But a small computer fan with good venting should be OK.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #3
    Re: Battery Venting

    I used Zephyr vent. It was pricey. It is in fact assembled from pieces of PVC pipes with a low power fan (2.4W) inside. I could've assembled it by myself for 1/3 of the price. It works well. When the box is closed, there's no smell. Of course, you cannot smell hydrogen, but I would assume it follows the same path.

    Here's the information from Zephyr. It contsains some CFM guidelines:
    Lead-acid batteries produce hydrogen gas while charging. But if the battery box is left open to vent gas in cold climates, the batteries get too cold and battery capacity is significantly reduced. A vent that solves this problem is especially important when battery boxes are placed in basements, garages and sheds. When heat rises in the structure, a low pressure area forms around the box and gases vent into the structure. The Power Vent controls battery box venting, removing hydrogen gas while reducing cold air infiltration into the box. The Power Vent contains a gravity-operated damper that normally stays closed. When connected to a voltage-controlled relay, the fan operates only when the batteries are being charged and blows gas vapors out. Designed for battery banks under 2200 Ah and charge rates under 125 amps. Fan can be operated from the auxilliary relay on the Xantrex SW inverter, from the auxilliary relay of an Outback FX inverter (use 12 volt fan for all Outback inverter voltages) or by a oltage controlled switch (sold separately) The 12 or 24-volt unit uses 3 watts and pushes air at 6 CFM with a 360 degree maximum change of direction.

    Dimensions (12 & 24 V): 4" diameter x 7.25" with a 2" PVC pipe socket on the inlet and outlet. The 48-volt unit uses 6 watts and pushes 8 CFM with a 360 degree maximum change of direction.

    Dimensions (48 V): 4" diameter x 10" long, with a 2" PVC pipe socket on the inlet and a 3" PVC socket on the outlet.

    You would need abother hole - for intake - at the bottom of the opposite side of the box. Otherwise air will not move.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #4
    Re: Battery Venting
    NorthGuy wrote: »
    I Of course, you cannot smell hydrogen, but I would assume it follows the same path.

    Well, hydrogen will rise very aggressively, while the other gasses and mists will not.
    A high level vent upward to the outside will get rid of hydrogen even without any fan or lower level air inlet. What you want to avoid is a sealed "cap" over the battery space which can concentrate the hydrogen gas as it rises.
    The main advantage of the powered vent system is to keep acid mist and odors from your living space.
    The fan in the Zephyr design serves to push open the gravity-closed damper and carry the heavier gasses along with the upward flow.
    SMA SB 3000, old BP panels.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Battery Venting

    There are always the questions of do you want to pressurize the box or evacuate the box (fan on inlet vs outlet).

    Pressurizing the box will better mix the air and keep the fan away from the acid mist (a bit longer life). Will also force air out through seams in box, possibly into living space.

    Evacuating the box will pull fumes/air into the box through inlet vent (and screen) and seams of box. Helps keeps smell out of living area. Air mixing is usually less, so where inlets/outlets are place are a little more important (fan "stirs" air and mixes better if blowing into space to vent).

    Your choice. Either should work (differences are small), but evacuation is better at keeping smells out of the living space.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    #6
    Re: Battery Venting

    You guys are always so fast to respond. thumbs up.

    The 2" hole I drilled it at the very top of the box exiting the side wall. There maybe 2mm above the vent. Easily solved by packing lid out if that is a problem. Or just moving hole up a smidgen. I have a hole in the bottom of the box for 4/0 cables. While it is not on the other side of the box I was thinking at such low flow rates the hydrogen would leave the box pulling hydrogen from the other side of the box. This hydrogen would pull air up. and this air would pull in new air. Obviously not optimal but should happen. I could put a few smaller holes on the bottom other side of the box. Stuff like for for battery monitor.

    I was unaware of water miser caps not saving gas. Will post a photo later tonight.

    matthew
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Re: Battery Venting

    Generally, the exit vent should be larger than the entrance vent (warm air expanding, etc.)... The smaller vent is going to be the limiting factor.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #8
    Re: Battery Venting
    animatt wrote: »
    I was unaware of water miser caps not saving gas. Will post a photo later tonight.
    Some vent caps contain a catalyst which can at least partially recombine the hydrogen and oxygen and drip the water back into the cell. Those are more expensive than the simple mist collecting caps and will wear out after awhile (depending on how heavily you use them by over gassing and what impurities are in the air around them.) They are most often used with forklift batteries where overcharging is common and adding water is expensive labor.
    I am not sure which type you are looking at.
    SMA SB 3000, old BP panels.
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  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    #9
    Re: Battery Venting

    The "cheap" ones NAWS stock. http://www.solar-electric.com/batwatmiscap.html

    Say caps can be left on while equalizing and descriptions says nothing about saving off gassing.

    I will be happy with decrease water usage. Was hoping they would help with gassing as well. Should have looked better.

    matthew
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #10
    Re: Battery Venting
    animatt wrote: »
    Was hoping they would help with gassing as well. Should have looked better.

    You made the right choice.

    The water misers do a good job of keeping the sulfuric acid mist from escaping and corroding your battery cables. The mist also settles on the battery case and becomes conductive, meaning that it will become a parasitic load on your batteries. They reduce water consumption only to the extent that the mist contains water.

    The catalyst caps are expensive, get ruined by the mist, and must be removed during heavy gassing (just when they are most needed) or they will overheat.

    The water misers are made in three heights... the taller they are the better they work. NAWS does not carry the tallest ones... I ordered mine from the manufacturer.

    Forum member Blackcherry04 has advised that if you have regular battery caps, you should aim the vent away from the battery cables. That is good advice, but even better is to contain the mist with water misers.

    interesting to read: http://www.solarpaneltalk.com/showthread.php?7937

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    #11
    Re: Battery Venting

    Was a decent read. I was looking for a more definitive solution to his problem, but as you are all probably accustomed to definite answer to the problems are sometime not found, and mostly not reported.

    With such a large battery bank(2040AH at 48v) those vents seem too small to me. 3 - 3/4 vents that were long. My current 2" hole is about 2.5" long. With vent all done will probably be down to about a diameter of 1.5-1.6" and about 3.5" long.

    That single 1.5" should have much better venting than that person 3 -3/4" tall vents(plus no input vent). That single 1.5" having roughly 1.88x cross sectional area and roughly 1/45th the internal surface area of tubing(less restrictive flow)(4 inches of 1.5" vs 3 - ~10ft of 3/4 sections"). But not sure comparing with a system with issues is a great idea. But I guess at 11.5% battery size and much more favorable flow conditions I may try just adding a 2" inlet on the far side of the bank, with 1.5" outlet.

    Anyone know of a cheap relay to controller a 24v pc fan but at only when batteries are at high voltages? The zephyr setup is too big.

    Or maybe the Morningstar 45a mppt controller has inputs for that. I will have to look at that.

    Matthew
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  • YehoshuaAgapao
    YehoshuaAgapao Solar Expert Posts: 280 ✭✭
    #12
    Re: Battery Venting

    I went overkill on my venting. 2 5" server fans (Delta AFB-1212M) each with a short 8" 5" duct going out the wall. Its not so much for the hydrogen, but to vent out any heat build up in hot 110+ degree phoenix summer days.

    http://www.pbase.com/greatinca/image/149182173
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