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Brushless Low Maintenance Weatherproof 12-48 VDC and up Output over 1 KW

Energy Systems & Design

P.O. Box 4557, Sussex, NB, E4E 5L7, Canada • Tel: (506) 433-3151 • Fax: (506) 433-6151 website: www.microhydropower.com • email: [email protected]

Test setup: The regular meter panel (visible from the easy chair). Clockwise from upper left: analog voltmeter tells battery voltage at a glance, E-Meter amp-hour meter, analog meter for 12 V circuits, ammeter for 12 V use (lights and little stereo), ammeter for DC input into Trace SW4024 inverter, ammeter for 24 VDC use (Sun Frost), and ammeter for the array output.

Test setup: The regular meter panel (visible from the easy chair). Clockwise from upper left: analog voltmeter tells battery voltage at a glance, E-Meter amp-hour meter, analog meter for 12 V circuits, ammeter for 12 V use (lights and little stereo), ammeter for DC input into Trace SW4024 inverter, ammeter for 24 VDC use (Sun Frost), and ammeter for the array output.

JT\ recent issue of Home Power A \ magazine restates a common opinion that "lead-acid batteries can last ten to twelve years if maintained properly" (HP74, page 149). I've frequently been told that my twenty-year-old bank of C&D 2-volt cells must be about to crash, or that they are somehow unfit to rely on.

I am trying to reuse a resource that is otherwise simply going to get wasted. There are quite few of these C&D batteries out there, surplused from backup service with the telephone companies. I recently called the manufacturer in Pennsylvania to try to determine the model and capacity of these batteries. They had me count the plates, and determined that we have KCT-720 cells. They are rated at 720 AH at the eight-hour rate, or 882 AH at the twenty-hour rate. New, they cost about US$600 per cell.

Turning Off the PVs?!?

After long considering the possibilities, I decided to run a drawdown and load test. In spring we often have more energy than we can use in our solar-electric home (which has never had a backup generator or propane appliances). At noon on Thursday, the first of April, 1999 (not an insignificant date), with the E-Meter reading 100 percent, I threw the switch...both figuratively and literally.

With only battery power (PV panels disconnected), we began the countdown. How far would I go? How far could I go? Which would crash first—my batteries or my courage? Or maybe...just maybe...we could draw down to the target 20 percent state of charge, run a load test, and then charge back up to 100 percent without mishap.

Energy Guzzlers

At our normal usage of approximately 1 kilowatt-hour (KWH) per day, it would take over two weeks with no input at all to drop to 20 percent state of charge. I did not want the batteries sitting in a discharged state for that long. The goal was to draw them down in less than a week. So we had to shift from our normal lifeway to being energy guzzlers. We began to use electricity like there was no tomorrow. This was somewhat difficult with our energy-conserving appliances.

We washed all of our clothes. We pumped water out under the yard trees. We left lights on. After twenty-four hours, we had used 118 AH, and the E-Meter read 86 percent. We upped our power usage. We drained, scrubbed, and refilled our wood-fired hot tub (500 gallons; 1,900 l). Jennifer vacuumed the house and washed the rugs and bedding. We used the hotplate to pop popcorn, and then to cook lunch.

By Monday morning, after four days, we had used 617 AH, and the E-Meter read 21 percent state of charge. The batteries had not crashed...yet. And we hadn't lost our nerve.

Load Test

We were ready for the load test at 20 percent state of charge. We turned on all the lights in the house, and then loaded the inverter with the freezer and the deep-well pump to test whether the voltage would hold up to keep the Trace SW4024 inverter on for all essential services.

When this proved successful, we added the 1 KW hotplate to the load. As the last kernel in our pan of popcorn popped, the voltage held at 21.3 under a 104 amp load on the severely discharged batteries. The Trace never missed a hertz-beat. A successful load test!

Recharge

We shut down all the loads, and ten minutes later at 8 AM, the voltage had recovered to 23.4. A beautiful sunny day had presented itself to begin recharging the batteries. We switched the array back on and started charging at 10 amps. It took four variably sunny days to reach a 100 percent state-of-charge reading again. In the two years since the test, the batteries have performed normally (including supporting our home during the dark days of two winters).

The results of this test seem to raise a number of questions. What is the difference in lifespan between a truly fully-charged battery and a chronically undercharged one? How do we define and achieve full charge? And how long can high-quality flooded lead-acid batteries last?

Still Life with Batteries

Our normal habits of charge and discharge may contribute to battery longevity. As we have a subsistence farm, the year-round pattern is seasonal and fairly predictable. From mid-January through spring, we have more electricity than we can ever use. The Trace C40 controllers allow the voltage to reach 29.4 V, and hold it there for two hours. Then the batteries are floated at 28 V for the rest of the charging day.

After a month of daily full charging, the battery will hold at 28 V with less than one amp. When our battery holds this high a voltage with this low an input, it is a well-charged battery. This conditioning treatment prepares it for harder work ahead.

As serious irrigation begins in (gasp) May or (usually) mid-June, the battery is cycled daily to at least 85 percent, and occasionally as low as 50 percent (if we expect to be away a day or more). Four or five thousand gallons (15,000-20,000 l) a day are metered out through an automated drip irrigation system for garden vegetables, fruits, seed crops, shelter belts, and the green lawn that is our firebreak.

We begin to have excess energy again as the weather cools in September, and the surplus continues until winter storms begin to roll in off the Pacific. From early November through early January, we have to make careful use of the available electricity. When the state of charge drops below 85 percent, we begin to limit discretionary use: the VCR, the big stereo, power tools, air compressor, vacuum cleaner, refilling the hot tub, and such.

Lance, in front of the PV array that energizes the Barker home. The Barkers have never had propane appliances or a generator!

Lance, in front of the PV array that energizes the Barker home. The Barkers have never had propane appliances or a generator!

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