Solar Array Battery level monitoring with the Elk M1?

[HoustonFirefox]

I know we have some sharp characters on the forums so I thought I would throw this out there to catch anything I might have missed.

Scenario:

1) I have a 3.1kw 12v solar array feeding into a bank of deep cycle batteries (2.1k ah total at 12v)

2) Using the Elk, it would be nice to monitor both the battery levels as well as the voltage from the solar panels (12v, 24vdc open circuit).

Obviously shooting 2,100 amps of DC current into the Elk would probably be a bad idea so I figured I'd check here to see if someone is already doing this.

The solar panels (under load) hold pretty steady around 13.2 volts but can shoot up to 24volts if the charge regulator is disconnected so having 3,100 watts of power (129 amps at 24vdc) shoot into the Elk would probably not be wise either.

Somehow Isolation circuits come to mind not to mention the look on the face of Elks engineers when the unit is sent back for "repair"

Gentlemen, this is a highly charged topic so any advice would be appreciated (sorry for the pun)

Thanks!

 

[BraveSirRobbin]

Well, I'll take the first crack at this.

Since the voltage can go as high as 24 volts you could add a current viewing resistor array (two 1Kohm resistors in series) to reduce that down (to half), have the Elk measure voltage as an analog zone across one of those resistors, then use a Y = mX + b generated equation set get the overall voltage (in this simple case, just double the value).

You can reference my Guide for Analog to Digital Conversions for further info.

There are two main problems that I see trying to do this with an Elk. One is the Elk inputs have a pull up resistor that can possibly influence this reading. Two is the Elk has only an eight bit conversion capability, meaning that you would divide the 12 volt range into 256 'steps' to determine the measurement resolution.

Also, there is really no way to read the value directly from the Elk. You could write some rules and note when the system reaches some critical values though.

I would think you would also want to look at a history (graph) of the voltages so you can gauge the regulator's performance as a function of your load. You would also get a handle on how low the voltage goes during the non-charging times.

Just some thoughts...

 

[politics123]

Monitoring voltage of panels and batteries has dallas one-wire written all over it. The Maxim-IC site has a white paper for battery charge state monitoring, and while you could also measure voltage from the panels, an even easier option is buying one of hobby-boards solar sensors, and calibrate it against your power generation.... you could get a close approximation for $50

Having it in the ELK is only as good as what you can do with it... I'm assuming writing info to the keypads/screens and/or some basic automation? For example, when battery charge state indicates bbatteries at 75% capacity, disable all non-essential circuits. When capacity is at 50%, turn on relay that starts my generator?

 

[HoustonFirefox]

Thanks for all of the help guys.

Where I'm trying to go with this is to get trend data of generation versus usage. Although I've never been able to run the battery bank dry, it would be nice to know how much reserve capacity I'm using for the reason Politics123 stated above. If it's getting way down, then having some non-essential loads shut down.

In addition, it would be nice to have the Elk display (on it's keypad) the charge state of the battery as a percentage (of course, 100% being fully charged). It might also be cool to have the same percentage for the panels as a percentage of full sun/no sun generating capacity.

I'll have to look into the one-wire circuits, can't say I've examined that route yet!

If it's home automation, this board is the best! Thanks again guys.

 

[mdiehl]

Thanks for all of the help guys.

Where I'm trying to go with this is to get trend data of generation versus usage. Although I've never been able to run the battery bank dry, it would be nice to know how much reserve capacity I'm using for the reason Politics123 stated above. If it's getting way down, then having some non-essential loads shut down.

In addition, it would be nice to have the Elk display (on it's keypad) the charge state of the battery as a percentage (of course, 100% being fully charged). It might also be cool to have the same percentage for the panels as a percentage of full sun/no sun generating capacity.

I'll have to look into the one-wire circuits, can't say I've examined that route yet!

If it's home automation, this board is the best! Thanks again guys.

 

[mdiehl]

Thanks for all of the help guys.

Where I'm trying to go with this is to get trend data of generation versus usage. Although I've never been able to run the battery bank dry, it would be nice to know how much reserve capacity I'm using for the reason Politics123 stated above. If it's getting way down, then having some non-essential loads shut down.

In addition, it would be nice to have the Elk display (on it's keypad) the charge state of the battery as a percentage (of course, 100% being fully charged). It might also be cool to have the same percentage for the panels as a percentage of full sun/no sun generating capacity.

I'll have to look into the one-wire circuits, can't say I've examined that route yet!

If it's home automation, this board is the best! Thanks again guys.

Just curious, as a user of Solar PV myself you might find something like this very useful for what you're trying to do with system monitoring...

http://www.thesolarbiz.com/Controls_and_Metering.htm


This type of system monitor is widely used in Solar PV systems and has been around for quite some time. These monitors typically use a .25 Ohm "standard" shunt, and should be able to get most every item you want as far as your system info. Some I think, also offer "outputs" such as RS-232 for use with a PC etc. for even more flexibility.

They'll also give you "depth of discharge" information, which as you probably know is critical to long battery life, i.e. 50% recommended maximum depth of discharge and 20%-30% discharge during typical use.

Out of curiosity, how are you using your system?