Project time…
One of the things Ive been wanting for a while is a dedicated lighting system for when the power goes out. Ideally, it is to be utterly and completely ‘normal’ looking. I’ve got a cabinet-type nightstand that would house a battery, charger and, if necessary, an inverter, quite nicely. What I was thinking of doing was picking up a 12v trouble light and swapping it out with the innards of a normal table lamp. Then I’d simply run the wires to the battery inside the nightstand and , ideally, run it off of the battery. Now, the problem is, I’m very fuzzy on battery/electronics. As I understand it, since I’d be charging this battery indoors I would want a sealed UPS-type battery so that no hazardous fumes/gases would be given off during the charging process? Is this correct? The other question is can I leave the battery hooked up to a charging device constantly so that it charges to the proper voltage and then maintains it? Ive seen several chargers that charge batteries and then ‘float’ them, keeping them topped off as necessary. This seems to be what Id be looking for..something I could plug into the wall and know the battery is always at its peak…and then when the power goes out I can operate the light off of this completely charged battery.
Or would doing such shorten battery life or short the battery itself?
Well, there are UPS systems out there that are fine for indoors, I have one uber one hooked up to my computer right now. So charging certain types of batteries should not be an issue. (Notably any sealed or dry cell battery would be fine. Old school lead/acid batteries like they used to put in cars would be an entirely different deal altogether.)
Now I’ve got a battery tender on my garage queen of a car, and once it reaches a certain level it simply maintains that charge. It doesn’t seem to have had any degenerative effect on the battery. (Much less so than when before I got that tender and hardly ever drove the car.) It plugs into the wall, just like you are mentioning, and has two modes, trickle charge, and maintain. It even has status lights to tell you which is going on.
Sportsman’s Guide has a Vector 1.5A trickle/float charger that I use for this purpose. My friend and landlord has built three different UPS type devices similar to what you are describing. I got a bunch of big UPS batteries and gave him some to build these. Each one has two 120AH 12v batteries with float chargers and inverters (he and I also use vector inverters). When the power goes out, the switchover is done manually.
As far as dedicated emergency lighting is concerned, I’d recommend AC 13W power compact fluorescents. They’re more power efficient in terms of lumens output per watt, and they’re much cheaper than 12v fluorescents. You could power a lot of ’em with a 400W inverter.
What Heath said. The small float charger works well to keep a charged battery fully-charged, but the big batteries really required a good charger to get them charged in the first place.
Battery size is the main question you need to answer. If you know what kind of load you’ll be drawing and how long you want it to run, you’ll get an idea of what you need. For example, if you want to be able to run a 15 watt fluorescent for two nights (say 8 hours/day), then you’d need to be able to supply about 15w/12v=1.25 amps for 16 hours, which is 1.25×16=20 amp-hours. This ignores the inefficiencies of the inverter.
My own goal was to run specific sets of components for two days, which takes care of most of our local power outages. You might want to get more off-line battery time than that, but if you do, you should also consider solar chargers to help extend battery life.
A 40 amp-hour sealed lead acid or gel cell is about 6x7x8 or so. I’m using one of those to run my cordless phone and reading light, as well as two 125 amp-hour batteries to run the TV/radio and the computer respectively. I don’t know what real-life time I’ll get with it, as I’ve only had to use it once, and that was for a very short time.
I haven’t searched for batteries online, but I’d probably start with Google and look for sealed lead acid or gel cell batteries that are specifically designed for UPS applications.
Wouldnt it be more efficient to use lighting components that are already 12v to prevent any loss of power through conversion from DC to AC?
I plan on running just one appliance: a light. The only things I’d need to power are a perimeter security system, a radio scanner, a 10/11 meter radio, and some lights. However, in this particular case, I am wanting to simply have one lamp in my main living area that I’d like to keep lit. The battery shop across the street has a 70ah hour battery I was looking at, figuring that I’d want to have lighting for about eight hours an evening for several days.
What sort of formulas are there for calculating things related to this? For example, how do you calcualte optimum charging rate for a battery of given amp/hour? Calcualting gauge of wire to use? Run times? Etc. I know its a pain in the ass, but do you happen to know of any links that might be suitable? Preferably with instructions for those of us who are fairly ignorant (but willing to learn) on these things?
The 70ah battery should be more than adequate.
Use the 12v light if you don’t want to run an inverter. I also installed a Radio Shack 12v cigarette-lighter type outlet on all but my computer power stacks so that I can run things like my 12v maglite charger directly.
However, having 120v ac available gives you additional options. I’d recommend having an inverter connected, even if you choose to run the 12v light directly. You may find a use for it in the middle of an outage.
As far as equations go, P=EI is the DC power law, where P=power in watts, E=voltage in volts, and I=current in amps. Batteries are rated in amp-hours, so if you know the draw in amps, multiply that by hours to get the required amp-hours. Conversely, if you know amp-hours and amps, hours=ah/I.
That’s assuming discharge of the battery to the rated discharge voltage. Expect to get less efficiency, but the only way to know for sure is to run the system and find out.
For wire sizes vs current, the issue is one of heating. Electrical codes usually provide safe limits. I searched google for “wire sizes current” and got what looks like a bunch of sites that would cover the issue. The 12 Volt.Com seems like a good reference.
When we run low-voltage wire indoors for 12-v lighting systems, it’s 10-gauge stranded. P=I2R=VI, so when you drop the voltage down, you need to run more current for the same power output, and since you run more current you dissipate a lot more heat in the wire.
And stranded’s just easier to bend. For a 12-v system, 10 gauge is fine, anything larger is just overkill, unless you’re talking about considerably long runs. R=resistivity*(length/area), so if you’re talking about a few dozen feet, best to get a thicker wire. But if he’s just running a lamp, again, 10 should be fine.
I plan on running just one appliance: a light. The only things I’d need to power are a perimeter security system, a radio scanner, a 10/11 meter radio, and some lights.
Just as a general efficiency thing, if you want to run these things when the power goes out, you’re better off just increasing your backup-generator capacity than purchasing additional batteries and keeping them charged off your AC.
The other goodies (alarm, radio, etc) have their power needs met elsewhere. This particular case is for just one item – a lamp. As an aside, the draws of the scanner and perimeter alarm are pretty minimal…
Generator is in the future but for now I want something quiet, portable, stand-alone and low-profile to provide me with illumination for my main living area.
Wouldnt it be more efficient to use lighting components that are already 12v to prevent any loss of power through conversion from DC to AC?
AC compact fluorescents run through an inverter are more efficient in terms of light output relative to 12v incandescent lights run directly off a battery. The losses through the inverter aren’t all that great, but it does increase the complexity giving another component to fail. However, a battery-powered inverter will give you the ability to run other AC-powered components for a while without having to fire up a generator.
Sealed gel cells don’t typically deal with being overcharged gracefully. Make sure the charger is not overcharging the battery. (If the battery gets hot, it will vent hydrogen and is being seriously overcharged). Low amp trickle chargers at the correct voltage with a high battery voltage cut off should do the trick. A small solar cell (like used on cars to keep the batteries topped off) would likely work nicely.
UPSes often wind up being very cheap with the various rebates online. Most of them (APC, etc) use Anderson Powerpole type connectors, making it easy to swap an external battery into the circuit or you can just cut the connectors off for added runtime.
You’ll want to fuse the battery cable, near the battery terminal to reduce the risk of shorts. Unfused gell cells can dump a lot of current with a short, enough to burn the insulation off the wires and depending upon amp hour capacity and wire size, set things on fire. Inline fuse holders are cheap insurance.