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Meter, meter

on the wall...

Therese Peffer

Finally, the last piece fell into place. I’d grown used to my pretty blue photovoltaic module out on the

grass soaking up the sun’s energy and filling up a car battery. I’d installed a charge controller, fuses, and a circuit breaker, but one thing was missing. I didn’t feel comfortable about unplugging from the Home Power Office & Power “grid” until I had meters — a voltmeter and ammeter — in place.

I knew the panel was charging the battery. The regulator has LED lights, so I know when it’s “regulating” (shunting power from the battery when it is full). But just how many of those flowing electrons are going in and out of the battery? What is the battery voltage?

Why Me(ter)?

So why have a round-the-clock, 24-hours-a-day voltmeter on a battery? I suppose I could continue to use a multimeter to measure my battery’s voltage every so often. After all, it’s 12 Volts — not going to change much, right? Boy have I learned a lot about batteries! I don’t have the space or expertise to discuss battery “state of charge” here (Richard promises to write an article for the next issue). But I learned that the voltage of a battery varies depending on whether it is being charged or discharged. The voltage indicates the state of charge, that is, how full the battery is or how close it is to its capacity (measured in Amp-hours). And the voltage will vary as the temperature varies. If your battery’s voltage differs from the voltage of a “healthy” battery under the same conditions, you may have a “sick” battery. So constantly monitoring battery voltage is vital to a renewable energy system.

The ammeter I installed to watch how much electricity was going in or out of the battery. I was curious how many amps of current my panel produces when it’s cold or very hot, during a day that’s overcast or bright and shiny. I also wanted to see how much current some of my loads draw (a few lights and a small stereo).

Photovoltaic module

DC circuit breaker

12 Volt DC loads

Voltmeter

Back to the Basics

regulator

11 16

–+

– +

Ammeter

–+

20 amp fuses

The Trailer Power System

Take a meter, please

I decided to buy analog meters rather than digital. Digital meters are more accurate, but I find analog meters easier to read at a glance. My analog voltmeter will give accuracy to a tenth of a Volt — good enough for me.

Another decision was to buy surplus meters that I had to alter rather than ready-made meters I could just plug in and use. Richard assured me this would be a cheap and easy solution and it was! I bought two meters from C&H Sales. One was a 0–1 milliAmp DC meter, with a scale from 0–10, and cost $2.95. The other was a 50–0–50 microAmp DC meter (Nullmeter) that cost $4.95. Both meters are ruggedized and can stand to get moved and jostled — great for a portable system.

I used the milliAmp meter to build a expanded scale voltmeter. The meter is mounted in my small plywood power box. Small (#22) copper wire travels from the meter’s positive and negative posts to the battery’s posts (see diagram above). A typical 12 Volt voltmeter might measure 0–16 Volts. But a lead acid battery’s voltage should never go below 11 volts. The ten divisions of the milliAmp meter were perfect for modifying the meter to a voltmeter with a scale that reads 11–16 Volts (see sidebar next page).

The 50 microAmp meter I used as an ammeter. It is also mounted in my power box. The positive and negative leads attach across a shunt connected to the negative post of the battery. This way I can monitor current coming into and out of the battery. The meter didn’t have any numbers on it, so I decided that the scale would be –10–0–+10 Amps. The amount of current through the meter depends on the resistance of the meter and the shunt.

A shunt is basically a piece of metal with a known resistance. The ones you buy are rated in terms of Amps and Volts, and have bolts for easy attachment. Use Ohm’s Law (Voltage = Current x Resistance) to figure the resistance of a shunt. For example, a 500 Amp-50 milliVolt shunt has a resistance of 0.0001 Ohms (Ω) (0.050 Volts divided by 500 Amps). The current rating tells you the ampacity of the shunt, or how much current it can take on a continuous basis.

+–

12 Volt car battery

Home Power #35 • June / July 1993 91

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