View Single Post
Old 08-25-2009, 05:32 PM   #51
Ran D. St. Clair
Senior Member
 
Join Date: Apr 2009
Posts: 212
Default

I know I left my electrons here somewhere…

Lets talk electrical monitoring systems. Ideally the electrical system should have tons of margin and you would never have to even think about how much current is being drawn or whether your batteries are sufficiently charged. Everything would be automatic and the generator would start itself.

Clearly that is not the reality of my world. I need to be able to keep track of things and make sure they don’t get out of hand. Perhaps in time things will fall into a pattern and I will develop a good feel for what I need to do to keep everything happy, but I still need some basic information to help develop that feel.

There are lots of super cool and expensive systems out there with remote readouts and programmable alarms etc. They are probably all better than what I have planned. I intend to install low cost digital panel meters at various locations to measure voltage or current or both. Measuring something like house battery voltage is easy. Just hook up the meter with an appropriate pot to calibrate it, and possibly a small regulator to provide power and you are done. It is always on, but it draws a trivial amount of current so who cares. The only issue is where to place it for convenient readout.

Measuring DC current is only a little harder. Even the heavy duty wires I plan to use will have some voltage drop, especially at 100 Amps or more. It’s just a matter of measuring the voltage drop across the wires and calibrating the meter accordingly. These units are commonly available with 0.2V full scale input. If I have to I can make a little op-amp circuit to boost the voltage, but I suspect that won’t be necessary.

http://www.futurlec.com/Panel_Meters.shtml

So what kinds of things do I think I need to know?

1. House Battery Voltage
2. DC Current from the Battery Charger
3. DC Current into the Inverter
4. DC Current to the internal 12V appliances.
5. Net Battery charge/discharge current (can be calculated from #2, #3, and #4 above)
6. AC Current from the Inverter (can be approximated based on #3)

At a minimum the house battery voltage should be viewable from inside the living quarters. The DC current from the battery charger, to the appliances, and into the inverter would be very useful as well.

You may note that there is no such thing as a battery gas gage on my list. Certainly such things are possible but they are notorious for being wrong. It is possible to integrate the net current into and out of the battery over time, and knowing the size of the battery and a fixed starting point like when the battery is fully charged, to estimate the current state of charge. The estimation errors add up over time though, so you have to keep resetting the fixed starting point. Some circuits will attempt to figure all this out automatically just by monitoring the battery voltage and current, but they are easily confused. I don’t plan anything so sophisticated. I will simply keep an eye on the battery voltage under nominal load, and also on the charge current when the battery charger is running.

I have purchased a clamp on ammeter that can measure both DC and AC current. It’s also a voltmeter and a few other things, but mostly I needed the ammeter. It’s an Extech 380941 for about $240.

http://www.extech.com/instrume.../310_399/380941.html

http://metersuperstore.com/Mer...ch_Type=AND&Offset=0

It might seem a bit pricy, and compared to a cheap multimeter it is, but there are many more expensive makes and models for measuring DC current, and having to insert a shunt resistor into a high current line is a big hassle. I have the unit now and it seems to work well enough. I set up a 20A current loop with a regulated bench power supply and it measured 20A so I guess it works as advertised. Speaking of the specifications: CAT III, 300V, AC Amp Max Resolution: 200A / 10mA, DC Amp Max Resolution: 200A/ 10mA, AC Volts: 400V, DC Volts: 400V, Jaws: 0.9", Resistance: 400 Ohms, Frequency: 10kHz, Display Range: 4000, Other: Min/Max, Hold; Auto Zero, Leads, Battery and Case Included.

To be Continued…
Ran D. St. Clair is offline   Reply With Quote