Charge controllers and set points
Tom Vineski
This month I thought we might review the role of the charge controller or regulator in the solar battery charging system. It's one of those BOS (balance of system) components that we take for granted and may not think about until something goes wrong as has happened to several fellow RVers on a forum I monitor.
In one case the house batteries were overcharging and the alternator/isolator system was suspected. Only after battery replacement and complete frustration did it become apparent that there was no charge controller in the system allowing the one solar module over time to overcharge and ruin the batteries. In another couple of instances, an older "relay" style controller stuck closed and "boiled" dry the batteries.
Charge controller function
What's the charge controller supposed to do? Basically, it's pretty simple. The charge controller’s purpose is to prevent reverse flow of electricity when the solar module voltage drops below that of the batteries (at night) and to prevent overcharging of the batteries when they are "full.”
Charge controller types
The earlier controllers regulated the flow of energy by switching the flow on and off, sensing when the preset battery voltage was achieved and then simply opening the circuit. SolarGuard & the Trace C30 are examples. These may still work well, but are inefficient, wasting a lot of power and don't provide the best charging regimen for the battery. But, they are relatively inexpensive.
The other type is known as Pulse Width Modulation (PWM) controllers which reduce the current flow gradually while maintaining the voltage. These may work in several ways or "stages."
- Two-stage controllers: These generally hold battery voltage to the charge set point and then reduce voltage to a lower voltage to finish the charging.
- Three-stage controllers: These allow the maximum energy flow from modules to battery to about 90% of full charge (bulk charge) then taper off the current (absorption charge) until batteries are nearly full then drop both voltage and current to the final "float" charge and maintain it for the balance of charging or until a load or demand is made of the battery system. At this point, the controller will automatically allow the energy flow to increase to carry the load or begin again the three-stage charging cycle.
Some controllers add an "equalization" option, either manually or automatically, to the controller. This is a time- and voltage-controlled overcharge of the batteries (wet cell, lead acid only) which stirs up the electrolyte, drives off sulfate deposits from battery plates and attempts to "equalize" all cells in the battery to the same capacity. (More on this at another time when we discuss batteries.)
- The latest introduction to three-stage charge controllers is the Maximum Power Point Tracking or "MPPT" controller which under certain conditions will actually boost the charge current available to the batteries by converting excess voltage to current (amperage) when batteries are discharged or modules produce over rated output. (We'll do a separate discussion of the MPPT controllers next issue.)
Which is best? It depends... and we'll look at that more definitively in a future column.
Charge controller set points
Charge controller set points are determined by the battery manufacturer and are generally the same depending on type of battery e.g. flooded lead acid, AGM (absorbed glass mat) or gel. These set points will vary not only by battery type, but by battery temperature. You may notice battery capacity and charge set points are generally listed at 77 degrees F/25C.
Many charge controllers will also have a "temperature compensation" included either as part of the controller itself or with a remote sensor that connects directly to the batteries. Basically, the higher the battery electrolyte temperature, the less voltage it takes to fully charge a battery; the colder the temperature, the more voltage it takes. This is a very useful feature if you find yourself in areas of wide temperature variation where batteries are outside or in uninsulated locations. It not only ensures batteries are fully charged in colder temps but can reduce water loss due to evaporation in hot, dry areas in the summer.
Typical set points for 12 V lead-acid batteries at 77° F (25° C) (These are typical, presented here only for example.)
- High limit (flooded battery): 14.4-14.6 V
- High limit (AGM): 14.2-14.4 V
- High limit (sealed gel battery): 13.8-14.0 V
Temperature compensation for 12V battery:
- +/-.03 V per ° C deviation from standard 25° C or range from 2.74 volts per cell (16.4 volts) at -40 C to 2.3 volts per cell (13.8 volts) at 50 C.
This column appeared in the June 28, 2010 RV Lifestyles ezine. Subscribe to our free ezine at the signup page.