Protecting Hardware on Unstable Power Sources?

psuedo_samurai asks: "Later this year, I will be returning for a visit to a small 3rd world country in Africa. I was lucky enough to travel to the country before, and the last time that I went I was able to bring four fully functional computers that I donated to a local high school, to provide a small computer network for teaching purposes. I had loaded Red Hat Linux with Open Office and a multitude of free goodies onto the systems and everything was working well. The equipment I brought back with me survived for about 12 months, but eventually fell victim to power surges, brownouts, blackouts, and so forth. On my return, I will be better prepared and am planning on setting up 8 computers, this time around. However, I am still stuck on how to best provide either a battery backup (aside from lugging UPS's along with me) with automatic shutdown and/or AVR on the cheap. Does anyone have any good references, experience, or suggestions on how to over come the challenge of running a computer network in a country where the power fluctuates wildly and multiple outages in week are not unusual occurrences?"

Comments

419?

[tepples]

I will be returning for a visit to a small 3rd world country in Africa.

I hope this doesn't have anything to do with an advance fee scam someone sent to your e-mail, does it?

Inverter + charger

[wowbagger]

What you need is known as an off-line UPS, which is nothing more than a battery charger, a battery, and an inverter.

The charger charges the battery, the inverter runs from the battery - if the line voltage spikes, the battery charger takes the hit. If the line voltage sags, the inverter draws power from the battery until power is restored.

You can take along just the inverter if you can count upon getting batteries and chargers for them at your location.

The charger MUST be able to put out more current than the inverter will draw - so for a 400W computer system your charger will need to be able to put out about 40 amps at 12VDC.

The other advantage to this approach is the ability to run off a battery string charged by photovoltaic panels.

Lastly, if you can find somebody with the skill, you can get a replacement power supply for the computer that will take 12VDC to make the voltages for the computer (usually 5VDC, 12VDC, and 3.3VDC) - this will eliminate the inverter (at the downside of using a non-standard power supply for the computers.)

Re:Inverter + charger

[LWATCDR]

Actually Hackaday had just what you want.
http://www.hackaday.com/entry/1234000583030620/
Okay it is not exactly but pretty close. Would make a good project.
Back in the old days IBM mainframes used an interesting power conditioning system. It was an electric motor that turned a big flywheel that was then connected to an generator. A very effective if not efficient way to decouple you computer from the power lines. Of course the often added a diesel motor and clutch to the mix. Power drops clutch connects the engine to the flywheel, generator combo and instant UPS.

Laptops?

[fm6]

Unless you have deep pockets, you're stuck with off-the-shelf technology. That doesn't leave you with a lot of options, since standard computer hardware is mostly designed for situations where there is a reliable power supply. (Non-standard custom-built hardware is much more flexible -- and much more expensive.) This goes even for UPSs, which are meant to support clean shutdown, not extended replacement for regular power.

The off-the-shelf system that doesn't need a reliable power source is an old-fashioned laptop. You can buy obsolete laptops very cheap. Many are too big and bulky by todays standards, but will fill in nicely for a desktop system when power comes and goes.

Why not a UPS?

[PurpleFloyd]

For your requirements, it seems to me that a UPS is pretty much the only solution. However, it might not be so difficult to bring one with you, if you can obtain lead-acid batteries while you are in Africa (probably not so terrible, although I don't know how removed from civilization you will be). If weight is a concern, you might buy some small UPSes in the US that have the features you need, remove the batteries here, then ship them to Africa and buy new batteries locally; most UPSes have fairly standardized lead-acid gel cells, and it might be possible to hack in larger batteries if you can't find precisely what you need once overseas. One caveat: if you're going for weight savings, the large transformers inside UPSes are still quite heavy and might make this option less attractive. Still, it's worth investigation.

Another idea might be to roll your own. This would involve buying a large-capacity 12V power supply capable of running from whatever voltage/frequency combination you'll have in Africa and a high-capacity inverter here in the US, shipping them to Africa, and using them to create a UPS together with car batteries purchased locally. If you have some hardware and driver experience, you could probably create an automatic shutdown system without much trouble, and might even be able to create an automated battery-maintenance system which delivers a slightly higher charge every few months to prevent sulfation (discussed in the linked article). This setup has the advantage of being a true UPS: it will provide power filtering and protection up to the point where either the battery goes dead or the battery chargers take one for the team and catch on fire. Also, it's incredibly scalable; with enough batteries, you could run a computer room for weeks without power.

File server and semi-thin clients

[isj]

Install a file server that is protected with an UPS. Configure automatic shutdown of it.
Put the client PCs behind a surge protector (but not on an UPS). Make them boot from the file server drives.

That way the storage/filesystem is reasonably protected and is not smashed everytime there is a brownout. And you don't have to spend the money for a large UPS for the client PCs.

Re:ghetto UPS

[itwerx]

Car batteries aren't normal lead-acid batteries.
Car batteries actually are "normal" lead acid batteries. So are deep-cycle batteries. The only differences that are ever present in any type of lead acid battery is 1 - the physical structure of the plates (grid vs foil), 2 - the venting system (sealed or not) and 3 - the acid (water vs gel).
There are other minor variations. E.g. car batteries have extra ribs of plastic which help to support the plates so that they don't crack loose when you hit a pot-hole. So-called deep-cycle batteries have extra heavy-duty bus bars between the plates to handle the power and help act as a heat-sink.

you risk reversing a cell any time you discharge below 80%
Cells don't risk reversal until they *completely* discharge. Even a damaged cell that discharges earlier than the others will retain its polarity as long as the other cells have any energy left in them. There's no way a lead acid cell will reverse if it still has at least 20% charge (maybe you meant to say that above?) Even then it is highly unlikely to reverse until well below that.
Not to mention that unless the cell is physically damaged so badly that it can't be charged at all, the reversal will be undone as soon as it is charged with the correct polarity.
To get to your point about reduction in voltage, if a cell were truly reversed and somehow stayed that way even through a charge cycle (not sure that's even possible unless you purposely disconnected it and charged it separately with a reverse bias) you would actually see a 4v reduction as it would not only be failing to contribute it's own 2v but that reversed 2v would effectively cancel out 2v worth provided by one of the other cells.
In the case you're describing I would assume that the battery was charged (or discharged) at a higher rate than it was designed for and the plates warped from the heat causing the cell(s) to short out.

I'd bet that the reduced capacity of your batteries just became easily noticable after a few years.
This is actually due to sulfation. Even a sealed battery will have impurities present in both the lead and the acid compound. An unsealed battery is especially vulnerable. Sulfation occurs when the sulfuric acid reacts with the impurities and leaves deposits on the plates. Eventually enough of the surface area is covered that the cell ceases to be functional. This is also why a car battery (or any battery designed for a physically abusive environment) will have a fairly large gap between the bottom of the plates and the bottom of the reservoir. The sulfation in a car battery tends to get knocked loose and settle to the bottom where it shorts the plates out when it accumulates to a sufficient depth.

Anything else I can help clear up about lead acid batteries? :)