We have been looking into, and installing solar powered nodes as part of our local mesh. In the process I have picked up a few bits of information on charge controllers I thought might be worth sharing.
First off, (for those completely new to the topic) a charge controller connects the Solar (photo-voltaic - PV) panel(s) to the battery and controls the charging of the battery. Charge controllers often have an additional output for connecting a load - like a radio. The power to the load comes from the battery, not directly from the solar panel.
There are two basic technologies for solar chargers: PWM (pulse-width-modulation) and MPPT (maximum power point tracking). There are several good references on the web that describe, in detail, the differences between the two and I won't go into that here. However the main differences for this discussion are:
PWM is usually much less expensive (typically $10 to $50 for 10 to 30 amp controllers) and are around 10% less efficient. PWM controllers rarely have extra features like external interfaces of remote meters. MPPT controllers, on the other hand, start at $70 and average in the $150 to $300 range for small capacity (10 to 30 amps) controllers.
If you are looking to have a 100% off-grid installation, one feature you will need is low-voltage-disconnect. This will cause the controller to disconnect the load when battery voltage drops below some pre-set level. On better systems this level can be set by the user, otherwise it is fixed, usually around 11.5 volts. This feature protects your battery from being damaged by being drained too low. However, there is a problem if the controller has low-voltage-disconnect but does not have the low-voltage-reconnect feature. Assuming your controller is configured for all-day (24 hour on) operation, and the load is turned off because of low battery voltage, unless you have low-voltage-reconnect, you must physically reset the controller (I.e. a site visit is required). For controllers without a load connection, or without low-voltage-reconnect, there is a work-around for this problem, connect the load directly to the battery through a low-voltage disconnect switch. These are commonly available online for between $10 and $30. Just make sure you pick one with automatic reconnect.
Besides the increased efficiency, there is one other advantage of MPPT controllers: many come with an interface for an external meter of computer. Thus, if your controller has such an interface, you may be able to connect it to a small computer (eg. a Raspberry-Pi) and be able to monitor the solar system remotely.
My next step is to look into hybrid solar/mains power to allow the battery to remain fully charged off the mains, biut switch over to solar charging if mains power is lost. I would be eager to hear from anyone with experience in this area.
First off, (for those completely new to the topic) a charge controller connects the Solar (photo-voltaic - PV) panel(s) to the battery and controls the charging of the battery. Charge controllers often have an additional output for connecting a load - like a radio. The power to the load comes from the battery, not directly from the solar panel.
There are two basic technologies for solar chargers: PWM (pulse-width-modulation) and MPPT (maximum power point tracking). There are several good references on the web that describe, in detail, the differences between the two and I won't go into that here. However the main differences for this discussion are:
PWM is usually much less expensive (typically $10 to $50 for 10 to 30 amp controllers) and are around 10% less efficient. PWM controllers rarely have extra features like external interfaces of remote meters. MPPT controllers, on the other hand, start at $70 and average in the $150 to $300 range for small capacity (10 to 30 amps) controllers.
If you are looking to have a 100% off-grid installation, one feature you will need is low-voltage-disconnect. This will cause the controller to disconnect the load when battery voltage drops below some pre-set level. On better systems this level can be set by the user, otherwise it is fixed, usually around 11.5 volts. This feature protects your battery from being damaged by being drained too low. However, there is a problem if the controller has low-voltage-disconnect but does not have the low-voltage-reconnect feature. Assuming your controller is configured for all-day (24 hour on) operation, and the load is turned off because of low battery voltage, unless you have low-voltage-reconnect, you must physically reset the controller (I.e. a site visit is required). For controllers without a load connection, or without low-voltage-reconnect, there is a work-around for this problem, connect the load directly to the battery through a low-voltage disconnect switch. These are commonly available online for between $10 and $30. Just make sure you pick one with automatic reconnect.
Besides the increased efficiency, there is one other advantage of MPPT controllers: many come with an interface for an external meter of computer. Thus, if your controller has such an interface, you may be able to connect it to a small computer (eg. a Raspberry-Pi) and be able to monitor the solar system remotely.
My next step is to look into hybrid solar/mains power to allow the battery to remain fully charged off the mains, biut switch over to solar charging if mains power is lost. I would be eager to hear from anyone with experience in this area.
You may want to look at Flinthills Radio. http://ki0bk.no-ip.com/~pwrgate/LLPG/Site/Welcome.html
I started running his LLPG and Solar controller for my radios this year. I have a 100W panel, an MFJ 25amp Mighty Lite, and 2 12ah batteries for my "go gear".
Whether a MPPT makes sense depends on several things:
1) The peak power voltage of the PV panel and how it compares to the battery float voltage. If the peak power point isn't much above the float voltage, the panel will already be near its peak power point with a PWM controller and you won't gain much more from the MPPT.
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2) How much the outside temperature (and that of the panels) varies. The peak power voltage of a PV panel depends strongly on temperature, increasing when it is cold, so by letting the voltage rise to that peak point with a MPPT you could get a useful boost in the winter when days are short.
3) How deeply you discharge the battery every night. A discharged battery has a low terminal voltage, and a non-MPPT (e.g., PWM) controller will force the PV panel to operate at that low and inefficient voltage when it recharges in the morning.
Thanks
Well, that part shouldn't be hard to understand -- if you don't get enough sunlight, you won't generate enough power!
But seriously, without a MPPT you will get less out of your panel precisely when it's most needed -- to recharge a discharged battery with a low terminal voltage.
A PV cell (or panel) can be modeled as a bench power supply with both current and voltage limiting. To get the most power out of the panel, you want to operate it right at its current and voltage limits simultaneously.
These limits aren't static; the current limit increases with sunlight and (very slightly) with increasing temperature, while the voltage limit increases with decreasing temperature. The job of the MPPT is to find this point and operate the panel there, assuming you actually need that much power, and it is quite likely not the voltage on your battery or its load especially if the battery is heavily discharged.
If you really want a reliable off-grid power source, especially on a mountain, you should consider adding a wind turbine. They tend to produce when PV panels don't, like at night or in the winter. They also need a controller, and some PWM-type PV controllers can be also used with wind turbines. But while you can just unload a PV panel when you don't need its power, doing that to a wind turbine risks overspeed damage. So instead of configuring the controller to open the connection from supply to load when some output voltage setpoint is reached, you configure it to shunt in a dump load (typically a resistor) to pull the output voltage back down.
There are probably MPPT-type wind controllers as well that optimize power delivery regardless of wind speed and battery voltage, but I don't know about them. I suspect they're very different from MPPTs designed for PV panels since the model of a turbine generator is very different.
I am interested in hearing your thoughts.
Thanks
Richard - wb6tae
For the time being, we have settled on the EPSolar Tracer series of MPPT charge-controllers. These relatively low cost (<$70 on Amazon) devices not only seem to do a good job, but they also feature a serial port for an optional external meter. The meter is of dubious value for a remote site. But, there has been some excellent work done in interfacing this controller to a Raspberry-Pi and developing software for accessing the data and delivering it over a web interface and graphing performance. (images below).
This is not a simple turn-key project, though there is an Instructable on building the interface. The software is a little more complex and requires some shell knowledge. I can provide more information if there is interest.
CAVEAT: This is very much a work in progress. But, contributions are welcomed.
Richard - wb6tae
May I have more details on the EPSolar controller? I see that there are the Tracer-1210RN/1215RN with lights and knobs, and there are the 1215BN for inside-rack use. Which one are you using?
73 from Dan at K7MM
I am not sure the indoor/outdoor designation makes much difference as for this application most these will all be mounted in some sheltered location, like inside a battery box. However there are other differences to consider:
- The BN series are the more recent models and probably better overall charge-controllers.
- The BN series appears to require an external meter or PC to perform initial programming.
- The RN data communication is via a modified MODBUS protocol and is not officially supported while the BN uses true MODBUS with official support and software. The RPi interface for the RN series is strictly DIY.
- The RN series is probably easier to integrate in a homebrew remote monitoring system. But, the BN does have good support for their proprietary monitoring products.
- The BN offers more operational data and allows tuning of some aspects of the charge process.
As I noted, for us the price difference was important. But, if we could have afforded it, the BN was probably a better choice.Hope this helps.
richard - wb6tae
A popular auction site has the 1215BN and MT50 bundled for around $110.
73 from Dan at K7MM
I purchased one of these Tracer charge controllers. Would be interesting in leveraging your RasPi implementation to avoid re-inventing the wheel. Connect up at - ae6xe at arrl.net ?
Joe - AE6XE
I don't know if its PWM or MPPT, I highly suspect PWM personably because it doesn't specify anything to the contrary but I can't say positively one way or the other.
In theory I like the thought of it, charge via a wall outlet as your 'topper' charge and keep the unit up overnight floating with solar during the day to peak it to highest bulk and avoid hitting your battery as much.
I don't know how well this will work long term in practice, but it is kind of the sort of solution I was thinking might be nice in certain locations that have access via an Ethernet line and at same time you wanted a solar resilience.
Though I'll admit most the sites I think I'm going to spec out im more looking like I'm just going to put a float charger on the site and call it 'done' since they have AC power, put a big enough battery for a few days and then if disaster strikes I have a few days to make the rounds and do repairs (add more battery, swap in a generator, etc)