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[Gaffers]

One for the experts.

I an in the process of installing some raspberry pi powered services around the house, some of which must run on a battery in case of power failure.  For this I have a 10W solar panel connected to a 60Ah leisure battery through a regulator.  I am going to put the battery and regulator in the loft with the 12v feed entering in to the cabinet where I have my Pi farm and install the panel on the south facing wall.  initial tests are positive and the voltage remains fairly stable but I have a second one available to add in parallel if needed.  All good there.

My worry is that in the event of a regulator failure the battery *could* be overcharged leading to a release of hydrogen and oxygen in to the loft.  Now, without hurting anyone's feelings we all know how that can turn out.  So I looked in to a sealed battery box with a vent I can pipe outside to safety but found nothing suitable.  Nor can I find any online advise about installing leisure batteries in your home, either there is nothing to worry about or it is a closely guarded industry secret.

Am I being overly cautious?  Or will I have to look at building my own airtight box from MDF with all the necessary glands and vents?

[minifreek]

The escaping gasses will vent down wards - not upwards, as the gasses would be heavier than air thus using gravity they go downwards....

Would be better to install the battery in a sealed battery box that can vent downwards instead of having to escape upwards then downwards with possible ignition sources...

Im with you on the extreme case scenario but I reckon it could be overcautious with the regulator from the solar panel, might be better with some sort of antisurge device fitted...

The second battery I would keep on a trickle charge and only connect it when the need arises, otherwise the regulator failure could take out both batteries, then you would be left without any power whatsoever to the Pi boards that require the 12V supply...


I could be talking shite but it makes sense to me

[Bigron]

Monsieur Guffer, I'm intrigued by your project!
My first thoughts are that you need over-voltage protection and the easiest way to provide this is by means of what is known as a "crowbar" circuit - fairly standard in power supply design for me in a former life.
The idea is that the output from your regulator is monitored usung a zener diode of a rating just higher than the normal PSU output voltage and if said output voltage rises the zener fires an SCR (silicon controlled rectifier)  that is wired across the PSU output, blowing a fuse.
As you will have battery back-up, you don't want the full charge from the battery dumped into the SCR - you WILL have a fire then - so inserting a diode in the line will prevent this.
If that makes sense to you and meets your needs, I'm happy to PM you a suitable circuit.

Ron.

[Marks DTM Calib]

Well contrary to the above, I am pretty sure the krauts proved many moons ago that Hydrogen is lighter than air so it would head upwards.

I must say that the solar panel sounds a tad small for such a big battery.....as you will be lucky to average 2-4W out of that thing.

As for casings, it ideally wants to be double skinned given the install so maybe a cooler box would work ok with a suitable vent added to the lid area.

[05omegav6]

Have a rummage here...

http://www.vetus.com/electricity-on-board.html

Accessory section includes battery boxes, charge relays etc

[minifreek]

@MDTM - Yes you are correct, the Hydrogen will vent upwards - I dunno which gas I was thinking of that would vent downwards.... either way its still a flammable gas and would need to be kept away from a possible source of ignition...

[Kevin Wood]

I would say that a 10W solar panel can give its' all to a 60Ah leisure battery and the battery won't flinch at all, regulator or not! As said, though, that panel will probably struggle to power one Pi 24/7 let alone a farm!

It would be sensible to at least contain the battery in case something happens whether it be gassing, electrolyte leakage or an electrical failure. I would say a cool box will contain leakage pretty well, you can get batteries with a hose barb on them, which will probably be a suitable way to vent any gases produced during normal charging. You might want that in a metal cabinet in case something more spectacular happens to the battery, though.

Of course, take every precaution to ensure that an electrical failure somewhere can't cause the battery to be shorted. Fuse it conservatively, and as close to the battery as possible!

Given how frugal a Pi is, and the expense and trouble you need to go to power one "off grid", is it worth it?

[Gaffers]

I like the cooler box idea, easy to vent, air tight and cheap

Initial tests in a less than ideal position maintained the voltage for a week at 12.8v whereas the week prior the voltage drop was down to 12.2 without the panel.  I hear what you say, would slapping the second panel on (taking it up to 20W) fair better?




The project is essentially a farm with:

* monitored security alarm - this is the only one that requires to be battery powered as I need it to work regardless of power supply.  Monitored alarm with web interface.  Send me alerts wherever I am in the world.  Very low CPU consumption thus more on the lower end of the PI energy requirement scale.

* home automation - Zwave, needs no explaination but I shall be doing my own code to have the lights follow people around the house and to monitor the soil humidity and water them when required.

* 'Jasper' the butler (under development) - uses a the Jasper voice recognition engine to interface with a google calendar to know what is happening each day and gather content from multiple sources (such as the Met office API for weather forecasts) and push them to me verbally or by email dependent on calendar content.  So the idea would be:

    - Alarm with agenda for the day, weather, traffic on any route I am likely to take, etc
    - reminder to put the bins out the night before (and whether it is recycling week)
    - reminders for birthdays, mother's day, etc a couple of weeks in advance

* Barley PI - an automation project for my brewery.  It should remove the requirement for all but 2 interventions in the brewing process (identifying the end of the first runnings and thus starting the sparge and then stopping the sparge at the minimum liquor gravity)  Ideas on that welcome providing they dont break the bank


* Web and Media server.

* VPN Server (running on Odroid U3 as it has more memory and a separate NIC and USB bus)

Each room will also have a pi and touchscreen with a web interface for the house (alarm, media, house control, etc)

[Kevin Wood]

So you really only need to cover it for power cuts?

(the Pi plus whatever networking hardware it needs to communicate with the outside world)

I reckon the consumption of the Pi will be insignificant compared to giving it comms throughout a power cut.

You might be better off just giving it a backup power supply using a small gell cell, pretty much as I did for my central heating controller (early lashup illustrated!):

https://dl.dropboxusercontent.com/u/26795734/Photos/OOF/HeatingControl.jpg

[Gaffers]

I like that setup, but I have these batteries going spare so I want to put them to good use.  I am currently conducting tests on the 3g module as a backup connection for the internet.

The calculations I have done show that a 10W panel is just about ok for one Pi, so if I slap on the second I should be safe.  I have a monitor though so I shall keep an eye on it.

[steve6367]

From experience on the boat, 10W into 60AH will never overcharge it, even without a regulator.

Any chandlery will sell you a plastic box with suitable drain.

Steve

[Kevin Wood]

From experience on the boat, 10W into 60AH will never overcharge it, even without a regulator.

Any chandlery will sell you a plastic box with suitable drain.

Steve

Yep, especially if you have a Pi hanging off it consuming a couple of watts, 24/7. You'd be best off eliminating the regulator and the losses that go with it, unless it's a boost/buck job that can actually make more efficient use of the solar panel output than dumping it straight across the battery.

But.. IME a 10 Watt panel is probably rated to generate 10 watts absolute maximum at the equator in bright sunlight, and probably not when the terminal voltage is constrained to the requirements of the battery. Especially once a couple of seagulls have dropped their load on it. A Pi will take at minimum about 2 watts when idle, so 48 watt hours per day - nearly 5 hours consistently, every day, of maximum output, ignoring that the battery won't be 100% efficient in storing it for the hours of darkness.

[TheBoy]

Even heavily used (and I've thrashed some of mine to within an inch of their lives, such as run OOF from one for a few days), a pre Jan 15 Pi will pull at most about 2.5W, but in reality if fairly idle, nearer 0.5W.  The new quad core, 1Gb ones pull a little more.

Its the interfaces and adapters you plug in that pull all the power.  You'll find the 3G dongle is right on the ragged edge of USB caompatiblity, sucking 2.5W on its own.

[Gaffers]

The B+ takes 1.21 according to their site, so lets say 1.5 for sh!ts and giggles so that's 36 Wh per day and 252 Wh per week.

assuming the panel generates 5W @ 16v for 5 hours a day = 1.56 Ah /day = 10.9 Ah per week = 131 Wh @ 12v

2 Panels = 262 Wh per week a slight surplus  based on what I think are very conservative winter figures (or not)

Edit:  that's before inefficencies are taken in to account, but hoping that my conservative figures have cancelled them out

[Gaffers]

Whether it's right or not, let's give it a spin and see how it fairs.

Now, to find a cheap cooler box