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DIY Power Tanks and Battery Ventilation - help please


H22A

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Hi

I am in the process of designing and building an 85Ah leisure battery based portable power tank.

I plan to use a two tier plastic wheeled toolbox, with the battery,wiring and sockets in the bottom tier, with storage for eyepieces and other paraphernalia in the top.

I am taking inspiration from a number of previous threads on SGL, and in particular the design and wiring of this one (admittedly this is only a single tier unit):

http://stargazerslounge.com/topic/154685-power-tank-build-help/

and finished here:

http://stargazerslounge.com/topic/155962-power-tank-build-complete/ 

However, one area that I am unsure of, and doesn't seem to be covered in any detail in the builds, is that of battery compartment ventilation.

I get the impression from the majority of builds that they are intended to be used "with the lid shut", rather than open to the elements in the field - dew, cold temperatures etc.

Also, a number of builds have integrated a cigar lighter charging socket, again suggesting that an intelligent charger, CTEK for example, could be left connected to the device to maintain the battery whilst the lid was shut.

To me this raises the issue of what is adequate ventilation - especially given the presence of switches and relays within the same enclosure, thus potentially increasing the spark risk.

During charging, a battery has the potential to produce hydrogen and sulphuric acid gasses.

The battery I just bought from Tayna came with strict instructions to remove the battery from it's sealed bag immediately after delivery and to do so outside in the open air due to potential gas build up.

So, really I am asking what constitutes adequate ventilation?

Taking the example of Andy's build linked above, would a number of additional holes, say the same size required for a cigar socket but covered with a wire mesh (for tidiness), be sufficient to allow gasses to not build up.

SGL doesn't seem to be inundated with "my power tank exploded" threads, so maybe this is a non-issue, If anyone has any thoughts on this subject it would be appreciated...

Cheers

Paul

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Thanks Dave - but I'm specifically looking at lead acid wet batteries.

I think however that I may have realised there is a simple answer to my question.

When unpacking my battery, the instructions said to remove two small red vent plugs from each end of the battery prior to use.

In theory I should be able to insert a couple of tubes into these vent holes and run them outside the compartment... solving the venting issue!

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I would imagine that any box you make will not be 100% airtight and I would also imagine that a charging battery, assuming it is being charged correctly, would emit much hydrogen at all, Im sure if there was any danger then things like caravan/boat battery boxes would have to have ventilation holes in them.

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Lead acid batteries do emit hydrogen, but primarily when charging. If you have your battery in a sealed box you should open the box to allow the hydrogen to escape. Then again this would only really be an issue if you where to go near the sealed box, full of hydrogen, with a naked flame or spark generator. There will be a small amount of hydrogen emitted during normal use but not so much that you need to have / keep the box open. It would do no harm to have a couple of holes 1 - 1.5 inches in diameter on either side of the box  to prevent a buildup of gas. Overtime the gas will have a corrosive effect on anything in the immediate vacinity i.e. within the box

I used to have a job where I would get a large motive power battery, used in fork lift trucks, after it had just come off charge and use a large hacksaw to saw the top off the battery. It used to be quite funny as sometimes the metal blade would contact across a couple of the terminals and you would get a large bang as the hydrogen exploded. People also don't realise that Sulphuric acid actually tastes quite sweet - sometimes you get slight splashes of it as you were cutting the batteries open.

Please do not try any of the above at home. I was wearing protective overalls and safety glasses as I was working as a Product Development Engineer for a lead acid battery manufacturer . :grin:  I must say lots of my ties has small holes in them.

Ian

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Thanks for the replies.

To be on the safe side, I will include a couple of holes.

Agree that it wouldn't be airtight anyway, but a bit of extra venting won't do any harm.

Not sure that my idea of using a vent pipe would work in this instance anyway.

What I thought were vent holes that I could insert a tube into, are the holes that contained what Tayna refer to as "transit plugs".

So have no idea what purpose they served, or whether they would be the path that the hydrogen took to escape from the battery.

Ian, that sounds so dangerous, don't worry I won't be attempting to replicate!!!

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Halfords sell clear plastic tubing and plastic adapters that slide into the transit plug holes, route any gasses given off to the outside of the case to stop build up .... another thought is does the hydrogen given off while charging contain any traces of sulphric acid? if so would you really want to put EP's with delicate coatings in that enviroment/proximity ??

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Probably no exploding leisure battery stories on SGL but there are a few of them elsewhere if you search. Have a look at this thread http://www.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/Exploding-Leisure-Battery/30580/

I recently went down the LB route and my battery has a venting plug at the side. I do just to be safe remove one of the caps on top whilst charging but that probably is not required. My battery has 2 cigar sockets coming from the terminals and is housed in a proper battery box, the interesting thing is the design of the box has a raised lid with two protruding edges that are designed to ventilate if the lid is left on. Personally when charging the lid comes off and as mentioned I do actually remove one of the caps as a precautionary measure.

It seems to me that its just a few basic safety tips and it may be good housekeeping to ensure the battery is well ventilated at all times charging or not. How much is enough is unclear, but whilst charging I would take a guess and say the battery should have plenty of well ventilated headroom in a well ventilated area. Also reducing ,removing ,or separating any source of possible ignition makes sense.

One thing from your post suggests you would want to keep the lid on whilst charging? Maybe that might not be best practice if it really is not necessary. The other thing is when my battery is charged I would not leave it plugged in trickle / maintenance charging but prefer to charge as required or once a month to top up.

Well that's my battery life story....

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I take the top box off my one when charging it. Think you have seen the one I finished up using the two tiered box. Battery in the bottom half, and the electronics in the top one (and can still have space for stuff... 

You don't have to leave them charging all the time...just put a reminder on your phone once a month to throw the charger on to keep it topped up...at least thats what I will do.

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Thanks, will definitely get some adaptors for the plug holes and vent to exterior, certainly no harm in taking this precaution.

A good point re the lens coatings too, so another reason for keeping the two halves of the toolbox seperate. It may be overkill, but it's no hassle.

One of the reasons that I went for the CTEK charger was the fact that it is intelligent enough to leave it connected to the battery.

I have taken the circuit diagram provided in original link, and adapted it for my purposes (mainly the inclusion of a relay switched power supply).

I don't know much about Elecontronics (CSE grade2 many many years ago), but to me it looks logical!

Any comments or glaring errors would be appreciated!

BatteryBox

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