Funny how life works.
Yesterday morning, out of the blue, the power went off.
I didn’t even know about it.
Until, that it, my lovely bride and brother-in-law wandered into my office and asked: “Did you know the power was off?”
I gave a gesture around the room: My super-computer (in the midst of an upgrade to an SSD) was still on. So were all four monitors that display what’s going on via the two dual HDMI video cards. The music was on. Zeus the cat was laying half up against the 750-watt electric heater *(which was on low). My halogen desk lamp was on. The scanner was still displaying blue ready for scanning. The duplex laser printer just had a page coming out of it.
“No, how would I know the power’s off?”
This, my friend, is the joy of a huge, massively over-built power system. I seem to remember there had been a single “beep” from the computer, router, and satellite uplink UPS power system a few minutes earlier. But the light hadn’t gone out.
Still, even though my office was warm and cozy while the rest of the property was getting more than 3-inches of rain, I picked up the analog old-style wired phone and called the local power company and reported the outage, although what most people may not appreciate is that while yes, Smart Power Meters may slowly morph our genetics, the Smart Meter would also stop reporting and power companies know that when blocks of meters go off-line together, there’s an outage.
Checking for Dangerous Backfeed
I turned off the halogen lighting and the heater. They’d been on since morning and I didn’t really need additional heat or light. and had Elaine follow me out to the power center.
I held up the 7-watt light in a plug-in light socket – one of those $2 parts that you can find at Lowes which is useful to see when an outlet is powered up, or not.
“Even though I don’t need to do this, I do it anyway,” I said, as the light was plugged into an outlet box labeled “Inverter 1 grid side” and then into “Inverter 2 grid side.”
As it should, the light stayed out.
“Do I need to know how to do that?” Elaine wondered.
“No, I just do it whenever we are at home to make sure the grid-interactive part of our system is still dropping the grid like it should. “ A note to the system log followed. “Tested system for no backfeed” and the date.
What’s backfeed and why does it scare the hell out of power company linemen?
Suppose you have a generator and the power goes down and you haven’t spent the money to put in a real transfer switch to physically disconnect your home from the grid in event a wire goes down somewhere down the road a ways.
People often times will plug a generator into the closest outlet in their home and if they are lucky, that leg of the 240 VAC power coming in will bring up a TV and the fridge and maybe some lights in the house.
Few bother to think about the downed line. A generator that’s plugged into house wiring will feed power back down the line to where it is broken.
It’s enough to kill an innocent lineman.
So apparently, Universe arranged our Thursday to make a major point about real back-up power systems.
NEVER NEVER EVER BACKFEED.
Now, about that Real Emergency Power System…
Hers is the shopping list I would go with for a really simple system:
Renogy 100W Mono Starter Kit: 100W Solar Panel+20′ Solar Cable+30A PWM Charge Controller+Z Bracket Mounts This will set you back $185 at Amazon. For another $150, add a second RENOGY 100 Watt 100w Monocrystalline Photovoltaic PV Solar Panel Module 12V Battery Charging as their 30-amp charge controller should handle the power of both cells.
Then you just need a good to great deep cycle battery and an inverter: Power Bright APS1000-12 Pure Sine Power Inverter 1000 Watt continuous / 2000 watt Peak 12 Volt DC To 120 Volt AC which is about $280 bucks.
Inverter School 101
There are two kinds of inverters: The cheapo kind are “modified sine wave” and the way these work, if you look at their output on a scope is it looks like those stepped pyramids in Mexico, rather than a smooth rise. It’s a stair-step kind of thing.
The pure sine wave is a better choice if you are planning to use a television, microwave, or radio gear because those “steps” may cause microwaves to hum oddly, televisions to have oddities to their pictures, and ham gear to hear “hash” on low bands.
But they are less than half the price of pure sine wave and if the batter is fully charged, you can microwave popcorn with either variety.
Battery Capacity 102
Now we need to talk about your battery capacity.
The most important thing to know is scientifically called the Peukert Exponent. Old Wilhelm (going from memory) came up with this law in the 1800’s. What he discovered was simply this: The faster you discharge a battery, the lower its effective capacity is.
For the science nerds, the details are in Wikipedia:
Manufacturers rate the capacity of a battery with reference to a discharge time. For example, a battery might be rated at 100 A·h when discharged at a rate that will fully discharge the battery in 20 hours. In this example, the discharge current would be 5 amperes. If the battery is discharged in a shorter time, with a higher current, the delivered capacity is less. Peukert’s law describes a power relationship between the discharge current (normalized to some base rated current) and delivered capacity (normalized to the rated capacity) over some specified range of discharge currents. If the exponent constant k was one, the delivered capacity would be independent of the current. For a lead–acid battery, however, the value of k is typically between 1.1 and 1.3. It generally ranges from 1.05 to 1.15 for VRSLAB AGM batteries, from 1.1 to 1.25 for gel, and from 1.2 to 1.6 for flooded batteries. The Peukert constant varies according to the age of the battery, generally increasing with age. Application at low discharge rates must take into account the battery self-discharge current. At very high currents, practical batteries will give even less capacity than predicted from a fixed exponent. The equation does not account for the effect of temperature on battery capacity.
Although the new Xantrex 84-2031-00 Link Pro Battery Monitor ($260) user guide doesn’t get deeply into a discussion of Peukert exponents, they are terribly important and the reason we built the forerunner product (the original eMeter at Cruising Equipment which was also branded the Link 10 for then sister company Heart Interface, before the whole shebang was rolled up under the Xantrex label) was to make the best battery “fuel gauge” possible.
The BIG lesson we wrote on page 25 of the old Link 10 manual (that you can still find here) is on page 25 where this handy-dandy chart appears. (I wrote the manual and in this part was explaining that most battery capacities (marine and RV deep cycle type) are rated based on their 20-hour discharge rates. So our typical Group 27 100 Amp-hour wet cell should support a 5 amp load for 20 hours before crossing the 10.5 volt threshold which is where a 12-Volt battery is considered dead.
Well, you can see that if you are running a 100 amp load, the battery capacity is effective cut to less than half!
There is so much more information about batteries in the old Link 10 manual (like our discussion of how to solve for Peukert’s exponent and do the calculations is on page 39 of the manual) with discussions like Charge Efficiency Factors and so forth, it really is worth a ready because it’s about the best battery course out there.
Still, I have to agree, somewhat, with the vastly simplified product documentation Xantrex provides on its newer product. Do you really need to know how Peukert’s exponent is derived? I mean like any more than you need to understand how signal strength of the RF envelope arriving at a cell tower’s receiver scores link quality and compares it to the same digital signature from nearby towers?
Sadly, our technology is leaving us behind and becoming magic to most people.
Nevertheless, one of these days, I will even be adding a Link Pro to my system because the Outback Power Systems MATE monitor on our system doesn’t tell us anything (except voltage and charge current) about our battery bank.
I wouldn’t be worth a damn as a technology business geek if I didn’t have this compulsive need to know…
The Professional Grade Battery Installation
A lot of readers will have problems with batteries if they start stacking up golf cart batteries (6V type) in order to build a bigger capacity battery bank.
You simply can’t screw up two six-volt batteries in series. BUT if you have four, or more, you can screw it up and here’s why:
There is a wire from the + side to the other + terminal highlighted in yellow and a wire from the – terminal, too.
These wires are very small “resistors” electrically speaking, so the lower pair of batteries will do a bit less work than the top pair of batteries. Rinse and repeat for 6 months and you will have four batteries will along the road to perdition.
By connecting your + and – connections at opposite corners, do you see what happens?
Both pairs of batteries has an equal amount of wire resistance in it.
The bottom string has the left yellow wire resistance while the top string has the right wire’s resistance.
Which *(if you make your cables equal length for these connections, which they should be) will result in vastly improved battery life.
I am amazed at how many people in the battery business don’t know this one…or, if they do, they don’t tell people about it.
But, then again, what do we say around here? Oh, yes…Everything’s a Business Model.
Like I said, more in the Peoplenomics archives on solar, but this ought to at least get you buying by the watt, and if you are a dry-camper or anyone who uses lots of battery capacity (like transoceanic sailors) where life depends on batteries to run the water-maker, then the Link Pro of any of the kilowatt-hour integrating amp-hour/capacity meters we designed in the late 1990’s are still worth their weight in gold.
Some day I’ll tell you how I got my “Skunk Works” patch…or no, I can’t. But I can tell you that the outfit I was with really knew their poop when came to DC power systems.
Ad of the Week
Out of Pro Sound News – the lectrosonics.com ad. Not only does their ad feature the headline “Post Apocalypse Wireless Mics…shown being worn by over-sized cockroaches, but their tag line rocks, too:
“Made in the USA by a Bunch of Fanatics.”
Keep them in mind…that’s the kind of kick-ass ‘tude that makes it in the world, any more.
Ya’ll come back Monday when we will return to the light-hearted frivolities. Including reader comments on this series and some field reports from actual DC power users on their learnings along the way.
Meantime, write when you break-even,