DIY 12v power system for garage

[Jon Parker]

I'm considering building a basic 12v power system for my garage comprising a car battery, a solar panel on the outside to keep it charged, and a 12v outlet on the exterior wall and probably one on the inisde also. Going to be used for running the air pump mainly when pumping up bike and car tyres, paddling pools etc.

I intend to use a 3-position switch to enable me to easily switch the battery positive to the solar charger, or over to the 12v outlets, and to prevent the outlet from receiving power when the battery is connected to the charger. I'll need some sort of weatherproof box for exterior wall outlet of course. I expect I'llneed to put a suitable fuse in the positive line, around 6 inches from the terminal, 10 or 20 amp should be fine I think.

Does that all make sense? Any suggestions?

Jon

[Andy Burns]

Even assuming you've got a 'free' 12V battery, what you spend on 3-way switch, 2x sockets and a weatherproof enclosure will probably cost more than occasionally running the car and plugging the air pump into the cigar lighter ...

[Andrew]

You are assuming the because the OP has a garage that he also has a car.

[newshound]

Because you don't have mains out there? You may find you need a larger solar panel than you might expect to keep the battery charged, particularly in the winter. Don't expect one of the 12 inch square ones to do much for you. DAMHIK.

I used 12 volt batteries to provide lighting to an off-grid stables, and also run electric fencers when required. Have one of those little 12 volt LED voltmeters plumbed into the system (on a switch) so that it is easy to keep an eye on the battery state of charge.

[Jon Parker]

Not overly bothered about cost. Just something I've been meaning to get round to, and suddenly I seem to have a lot of free time...

Maybe not a car battery, perhaps a motorcycle or similar size.

[Andy Burns]

Well, he did mention pumping up car-tyres.

[Harry Bloomfield, Esq.]

It happens that Jon Parker formulated :

There is no obvious need to switch the power between charge and the sockets?

If I were considering doing this, I would be installing a mains powered

12v (13.8v) SMPSU - do you not have any mains power in the garage?

[Martin Brown]

Do the sums very carefully before spending any money! Unless you are seriously off grid it will almost certainly be cheaper and much less hassle to buy a mains charger and a pair of SLA's to use in rotation. Solar panels work in summer but allow batteries to be ruined in winter.

That is the solution I opted for my greenhouse watering system and for powering my telescopes. The batteries last about 5 years or so.

You don't need to switch the solar panel out of circuit. They are quite high impedance and tolerant of a dead short across them. Battery voltage will be a bit high in full sun but most stuff for nominal 12v lead acid use is expecting about 13.8v anyway.

You will need a schottky diode in series with the solar panel (unless it already has one) to stop leakage at night. Be sure to include a low voltage automotive fuse very close to one terminal. And make sure that nothing can ever short out across the battery terminals.

[Chris Green]

Or running mains power to the garage (if not there already) and getting a 12v power supply.

[tabbypurr]

Get rid of the switch then it'll work. Just connect it all together. Battery fuse should be chosen to suit the wiring in use. If you use 2.5mm T&E you can use 30A fusing - you could even re-use an old Wylex fuse box if you want.

NT

[T i m]

I have that here for some 12V flouros in the workshop.

Anything 'exterior' is going to need to be weatherproof to avoid corrosion. It 'might' be better to just leave it at internal and have a temporary extension if required?

Ok.

Why (or are you talking of an additional 'charger')?

See above.

It needs to be a bit more than the worse case of the load and less than the capacity of the cable. ;-)

Sort of.

Yes. The problem with the above solution is that there is no protection for the charge, or the battery being over-discharged. So, I'd be tempted to use a charge controller that is sufficiently rated to cover the load and the solar panel.

That way, you won't cook the battery during a run of sunny days (depending on the size of the panel and battery) or run it past say

50% discharge whilst pumping up that big pool on the first sunny day after a run of dull ones.

eBay has quite a selection and a PWM or better, MPPT model of the load of peak load ratings you mention aren't that expensive.

Just make sure you use reasonably heavy cables on the output / battery connections (and the solar panel if you have the option).

At 12V, every ohm counts. ;-)

Cheers, T i m

[T i m]

<snip>

+1 ;-)

I've done that ...

But isn't the outcome a function of all the components?

eg, If the OP's use is likely mostly in the summer (except tyre inflation probably but that's rarely a sustained load), and that's when there is the most sun, if he had a big enough panel connected though a charge controller to a good enough battery, why wouldn't that be as good as all those solar powered roadside signs that seem to be able to cope all year round?

And would that typically run every day, all year round?

That should be easy if you are anywhere near a big city in the UK and need a cleat mild night when it's not raining! ;-(

Whilst I'm not suggesting that a pair of 'portable' batteries wouldn't also be a viable solution, it might not be as convenient (money / spec being no object) than a decent solar solution?

I think they key here *is* having the right spec gear and so insuring you have way more (charge specifically) capacity than you typically need, including in the winter.

Cheers, T i m

[Martin Brown]

You need a very big solar panel and a smart charge controller to protect the battery from overcharging if you want something to do real work.

Those solar powered "please go round the dangerous bend" signs are fine in midsummer but invariably dead in the water on cold frosty winters mornings and trash a set of batteries every winter. There is one not far from me on a sharp bend where a car entered someone's living room.

It was midsummer so the sign would have been working.

[T i m]

I'm not sure about 'very big' but you certainly need 'big enough under the worst conditions'. ;-)

Really? Can you cite any evidence for this being a general not local / specific issue please?

Erm ... so nothing to do with the sign?

And irrespective, *if* such signs were failing in the winter then I would say they were poorly designed?

We were using the 'two sealed LA batteries being cycled on a mains charger' solution to power an electric fence around a rabbit hutch. They were 17Ah batteries and we were probably having to swap them out every week.

We 'upgraded' that to a 5W solar panel and PWM controller and depending on the weather, the run time went up to about 3 weeks per battery. Also, should the battery voltage drop below a preset level, the output would be disconnected, protecting the battery from over discharge.

And this was with the panel being put in a very poor location (where it only saw the sun for a short slot in the morning) and the load being on *every day* (well night, typically dusk to dawn, that could be quite a few hours mind winter).

The OP's worst case usage pattern is likely to reflect the best sunshine (pumping up paddling pools). Cycle tyres typically get the most air in the spring and car tyres could be at any time, but only for relatively short bursts (excuse the pun). ;-)

Cheers, T i m

[Martin Brown]

It is a problem with the UK's high latitude and low sun in winter so probably applies to the entire country. I expect the signs work fine at lower latitudes that get some decent amount of winter sunshine.

+1 but it reflects how they fail round here. North Yorkshire isn't really all that sunny in winter and fogs linger in the vale of York.

If he only uses it in the summer then it might be a cost no object solution to the alternative of straight battery swap.

[T i m]

<snip>

Sure, I appreciate we aren't ideally placed for the best solar harvesting but we do get some. As long as that's more than something is using (overall) then it can be a viable solution, especially in non-grid situations.

I expect they work most places they are designed for.

<snip>

Understood. So, I can see how a generic sign tested on average UK sun exposure might fail under specific circumstances.

<snip>

If he doesn't actually need power down there for most of the year and is intending to use solar anyway then it needs to be usable when he needs it. So that could be (as suggested elsewhere) just a small panel and matching battery (nothing else). Not optimal but could be sufficient (depending on usage pattern).

If he only needs it very infrequently he might as well take the battery down there as required (that can be charged indoors)?

Cheers, T i m

[Brian Gaff (Sofa 2)]

What sort of battery will you use? Will there be a back up charger for mains for the winter? Brian

[Brian Gaff (Sofa 2)]

Maybe he has a garage where the car is actually the last thing that goes inside it. Brian

[newshound]

We used to have a "private" speed sign on the approach road to work. It would have been well designed and maintained, and it worked fine all the year round. It did have quite a big panel with an unobstructed view of a lot of sky.

[T i m]

And that's the thing isn't it, it would have to be designed (both it and it's location) to work with the energy available at that location and in 'worst case' scenarios (little sun for a time, lots of usage for a time, battery / panel efficiency reduction over time etc).

Anything less, causing the unit to fail down to lack of energy is simply down to poor design limitation considerations.

So, if you have the OP's garage 12V supply situation. If you know how much power the air pump takes (easy to read off the plate or measure with a DMM), and the longest time you might run it for in a particular period, you know what the battery will need to supply that energy and therefore what size panel you might need to be assured you can cover both that usage and any parasitic drain created by any charge controller.

Or, that with a big enough battery and a matched panel (that wouldn't do the battery any harm by overcharging in the 'best' conditions), just a straight panel <> battery config (that the OP is not going to never going to discharge the battery beyond say 50%) then he should be good to go. ;-)

Cheers, T i m