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Ran D. St. Clair · 03-05-2010, 06:21 PM

Trust But Verify (Is that an oxymoron or what?)

I built my electrical monitoring system. It has 4 each 3 1/2 digit LCD (Liquid Crystal) displays that continuously monitor and display the following:

1. House Battery DC Voltage (12V Nominal)
2. House Battery DC Current (Total current irrespective of where it goes)
3. Battery Charger Current
4. Accessory Load Current (DC loads via the two each 12VDC branch circuits including lights, fan, heater, refrigerator, and any other 12V DC appliances)

I did not feel the need to measure the AC current or voltage. The AC voltage had better be 120V otherwise it is broken. The AC current will be roughly proportional to the DC current into the inverter. If I want, I can calculate that by subtracting the battery charger current and the Accessory Load current from the battery current.

The more important issue is my battery current, which gives me a direct idea of how long my batteries will last, or how long I will have to run the generator to charge them up again. I set the polarity on the battery current so positive is current going into the batteries (charging) and negative is current going out of the batteries (discharging). The battery charger current and accessory load current displays are both set for positive only readings since the current will only ever flow in one direction.

The 3 1/2 digit LCD Digital Panel Meters I used are available from MCM Electronics:

http://www.mcmelectronics.com/product/72-8100

They draw very little current (1mA) and the digits are over 1/2” high for easy reading. They only cost $6.79 each plus shipping.

The displays read from -1999 to +1990 and you can put the decimal point wherever you want. The full scale reading for the battery voltage is set to 19.99V though it will clearly never get that high. The full scale house battery current is set to 199.9A which is theoretically possible. Likewise the battery charger current is set for 199.9A, though it will never go above 90A. The Accessory load current is set to 19.99A full scale, and the actual current could theoretically exceed that value, but it seemed a better choice than 199.9A which it will never even get close to.

I realize that most of you aren’t electronics engineers, so I won’t bore you with every technical detail. It might be useful to have an idea what is possible though, and to know that it is actually fairly easy to do.

Measuring the battery voltage is really simple, a 9V regulator chip, a couple of resistors, and a pot (variable resistor) for calibration. You can probably buy this ready made somewhere, though I didn’t research where, and just hook it up to the battery with a couple of wires.

Measuring current is a little more challenging, especially really large currents like 200A coming out of your battery. There is a simple way to do it though. The big heavy 2/0 wire from the battery positive to the inverter is also effectively a resistor measuring 0.00028 ohms. Put another way, with 200A running through it, it will have 56.6mV (0.0566V) across it. All I did was amplify that voltage to make the display read 200A There is no need for special current measuring resistors, or expensive inductive couplers, the wires that are already there will do the job. To be fair, this method may not be super accurate, but I don’t need super accurate. The other current measurements were made using the same technique, but the scale factors are different.

The system requires 8 wires from various points like the battery positive terminal, ground, Inverter positive terminal, etc. None of these wires carries any significant current so they can be as small as is convenient.

I will post pictures and schematics under key word “Stealth” but for the moment the web site won’t let me log on. If anyone knows how to fix that please let me know, because I have tried all the normal things.

To be continued….

03-05-2010, 06:21 PM	#167
Ran D. St. Clair Senior Member Join Date: Apr 2009 Posts: 212	Trust But Verify (Is that an oxymoron or what?) I built my electrical monitoring system. It has 4 each 3 1/2 digit LCD (Liquid Crystal) displays that continuously monitor and display the following: 1. House Battery DC Voltage (12V Nominal) 2. House Battery DC Current (Total current irrespective of where it goes) 3. Battery Charger Current 4. Accessory Load Current (DC loads via the two each 12VDC branch circuits including lights, fan, heater, refrigerator, and any other 12V DC appliances) I did not feel the need to measure the AC current or voltage. The AC voltage had better be 120V otherwise it is broken. The AC current will be roughly proportional to the DC current into the inverter. If I want, I can calculate that by subtracting the battery charger current and the Accessory Load current from the battery current. The more important issue is my battery current, which gives me a direct idea of how long my batteries will last, or how long I will have to run the generator to charge them up again. I set the polarity on the battery current so positive is current going into the batteries (charging) and negative is current going out of the batteries (discharging). The battery charger current and accessory load current displays are both set for positive only readings since the current will only ever flow in one direction. The 3 1/2 digit LCD Digital Panel Meters I used are available from MCM Electronics: http://www.mcmelectronics.com/product/72-8100 They draw very little current (1mA) and the digits are over 1/2” high for easy reading. They only cost $6.79 each plus shipping. The displays read from -1999 to +1990 and you can put the decimal point wherever you want. The full scale reading for the battery voltage is set to 19.99V though it will clearly never get that high. The full scale house battery current is set to 199.9A which is theoretically possible. Likewise the battery charger current is set for 199.9A, though it will never go above 90A. The Accessory load current is set to 19.99A full scale, and the actual current could theoretically exceed that value, but it seemed a better choice than 199.9A which it will never even get close to. I realize that most of you aren’t electronics engineers, so I won’t bore you with every technical detail. It might be useful to have an idea what is possible though, and to know that it is actually fairly easy to do. Measuring the battery voltage is really simple, a 9V regulator chip, a couple of resistors, and a pot (variable resistor) for calibration. You can probably buy this ready made somewhere, though I didn’t research where, and just hook it up to the battery with a couple of wires. Measuring current is a little more challenging, especially really large currents like 200A coming out of your battery. There is a simple way to do it though. The big heavy 2/0 wire from the battery positive to the inverter is also effectively a resistor measuring 0.00028 ohms. Put another way, with 200A running through it, it will have 56.6mV (0.0566V) across it. All I did was amplify that voltage to make the display read 200A There is no need for special current measuring resistors, or expensive inductive couplers, the wires that are already there will do the job. To be fair, this method may not be super accurate, but I don’t need super accurate. The other current measurements were made using the same technique, but the scale factors are different. The system requires 8 wires from various points like the battery positive terminal, ground, Inverter positive terminal, etc. None of these wires carries any significant current so they can be as small as is convenient. I will post pictures and schematics under key word “Stealth” but for the moment the web site won’t let me log on. If anyone knows how to fix that please let me know, because I have tried all the normal things. To be continued….