Advice needed regarding circuit with SLA batt in tin shed

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You should NEVER charge a lead-acid battery in a sealed enclosure
because of the hydrogen it could evolve during charging. never mind
the "sealed", they all have vents to allow hydrogen they can\'t
re-absorb into the electrolyte to escape.

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I\'d seriously consider burying it.

[selective snip]

Isn't going to drop *below ambient*, as per your OP

Assuming this ambient heat is caused by direct sunlight, use PV panels to power the cooling (and maybe contribute to the charging). They can offer a bit of shade for the shed, too.

TCM

I can't really add to the others' advice here, other than, "ventilate the shed" and "shade the shed". What's it cost for a bit of corrugated metal or sheet metal to put a "false roof" over the shed? "45-50 degrees celcius" is really quite warm - what's the outside air temp. when it's like this?

I also like that other poster's suggestion about using photovoltaics, which could serve double duty.

Good Luck! Rich

Need some advice!

I am designing a circuit for a small tin shed that requires backup power from a 12V SLA battery. It's an alarm with added features such as LED lighting and other additional features. All the circuitry will be in a "IP65 Sealed ABS Enclosure" (Size HB-6132) from Jaycar Electronics, which has pretty thick walls, and inside the box will be a small 12V SLA battery that will be kept on float charge (for backup) and the PCB's to make it all work, including the power supply/charging circuit.

My concern is the heat in the shed which may affect the SLA's performance. I read that the batteries should not be stored/charged in high temperatures. I'm sure that it won't reach the same temp inside the box if it's sealed and unvented, but I will probably need to vent the box because of the components that generate some heat - or is it not essential?. Inside the box will be some aluminium sheeting surrounding the walls as heatsinks. On a hot summers day, the temperature inside the shed can reach 45-50 degrees celcius quite easily, as there is a lack of ventilation.

- What do you reckon? Should I keep the box unvented to keep the battery cooler than the shed's ambient temp? Should I vent it and maybe use a fan (when it starts getting exceptionally warm)? Other suggestions?

Note that the float charger will have temperature compensation to keep the voltage correct.

Any help will be much appreciated.

Thanks, J

In article , John Fields wrote: [...]

I solved this once by making a little manifold that was glued over the valves of the SLA and ran to the outside through a very small tube. A drop of water can't enter because its is a small tube with the end blocked by the SLA.

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kensmith@rahul.net   forging knowledge

In article , Jason S wrote: [...]

A "thermal diode" heat pipe may help in your situation. It is just a copper tube partly filled with water and the rest is water vapor. When the bottom is hotter than the top, the water boils in the bottom, condences in the top and rains back down to the bottom. When the top is hotter, noting happens. You can use this to connect and disconnect a top mounted heatsink depending on which is better to do.

[...]

You could shutdown the charger totally when the temperature passes some point, if you can be sure that the temperature won't stay that high too long. An SLA won't self discharge much in 12 hours. This could save you some heat.

How long does the SLA have to last? Even with a fair amount of abuse, they last a year.

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kensmith@rahul.net   forging knowledge

In article , budgie wrote: [...]

Or use an aluminum box, perhaps based on a finned extruded tube. You can get a sealed box with a good thermal connection to the outdoors this way.

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kensmith@rahul.net   forging knowledge

ergo you have s ource of heat within the box.

Why are you sticking it all in an IP65 enclosure?

Ventilation - with or without a fan - isn't going to get the electronics and SLA temperature below the surrounding air temp. Short of a cooling system, that isn't going to happen, so the best you can aim at is to minimise the contents' temperature differential above ambient air. Heat sinks inside the box won't help. What will help?

1. forget the box.

-or-

1. vent and fan

-or-

1. connect the inside heatsink to an external one with some decent heat conductive system, and put a fan on the external one.

-or-

1. Peltier cooling of the contents/internal_heatsink

Seriously consider a decent charger. Just a float charger isn't doing the best for the SLA. Google for the appnote for the TI/Unitrode UC3906 charge controller. There is probably still a kit based on this at Jaycar.

hi budgie, thanks for the advice. my replies are marked below with : *** [msg]

*** No particular reason. I like the size, shape, colour, and because its very strong. I dont want a weak and ugly box! Why do you ask? *** what do you mean 'forget the box'?

*** I thought you said ventilation - with or without a fan - isn't going to drop the internal temperature.?! *** Hmm, Peltier cooling sounds interesting. I will have to read more about this. Sounds expensive to run. *** ok, but the charging method is a separate issue. I'm more concerned about the heat at this stage =)

Thanks John. Yes i thought about the venting issue for the battery just after I posted my original message. It doesn't sound like there's a positive inexpensive solution available for my problem, is there? I mean put it this way - you are a 'Professional Circuit Engineer'... If you were to design a circuit for a shed that can reach high temperatures during the summer, and the circuit required 12V backup power, what sort of system would you design?? Maybe I should just forget about having a circuit in the shed, problem solved =( I didn't think it would be this hard.

J

If you can, dig a hole in the ground and put the battery box underground. As long as you can keep the water out, it will last longer because it will stay cool. Maybe you could use a length of wide sewer pipe with a cap on the lower end to stop the water/mud getting in, and lower the battery down the pipe. The rest of the electronics could be in the box you suggested above ground because it would probably be less severely affected by heat. Another thing: I don't really trust sealed lead acid batteries unless they are the right way up, with the vents at the top. I'm not sure if it really matters but I put them that way up.

Chris

You could have a MOSFET in the charging path to the SLA and shut it off. This way, the SLA would be able to take over if the power failed but would not draw charging current when things are hot.

Remember the MOSFET has a substrate diode.

From DC supply-------+-------- To load ! !!- -----!! P-MOSFET !!---- To SLA

You could use an N-MOSFET if you add a higher voltage supply at pratically zero current.

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kensmith@rahul.net   forging knowledge

They will do bugger all if the box isn't vented !

How do you think a heatsink works ? It doesn't just make the heat *vanish* you know.

Sounds like you're making a pressure cooker to me.

Graham

You can't keep anything cooler than ambient temp ( without a peltier device or other active cooling ). That's why it's called ambient temp.

Graham

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Geez, I guess you missed my earlier suggestion to bury it... ;(

Because an IP65 enclosure is going to prevent heat dissipation by natural ventilation. The buildup iof heat in a sealed box will be horrific - it's an oven!

See above. Putting the goodies in a box will increase their heat problems.

See Chese machine's reply too. Vent and fan can at best get the temp in the box down to near the ambient air temp.

Might add some small cost. But it does provide an effective avenue for cooling a sealed enclosure's contents. Heat pipes can provide heat exchange with the outside of the box, but again will be similar to vent+fan i.e. can't get temperature below ambient air.

The charging method may well contribute more heat to the innards (the SLA).

Hi Chris, actually I was laying in bed this morning thinking of a very similar idea to yours! Digging a hole shouldn't be too much of a problem because we have cement tiles around 20 x 10 inches laid as the shed flooring. Better the battery be on the colder side rather than the hotter side (in summer), although I'm not really sure just how cold it *is* underground! I've read that batteries don't like cold either. If I go ahead with this idea, I'll have to run conduit down into the ground from the circuit box (which is on the wall)... so that the wires can't be snipped very easily, and also adds general protection anyway from little accidents. Thanks!

J

Thanks Rich for the suggestion. But I'm sure the inside temp wouldn't be

*that* much cooler if I went with that option? Could turn out to be quite expensive too. J