Charge controllers are a key part of the system. Their job is to charge the battery bank as quickly as possible without over doing it and ruining the bank.
Lead Acid batteries are peculiar beasts. They love to be charged and if possible each and every cycle should be brought back to 100% state of charge.
Charging them causes a chemical reaction to run “backwards”. But chemical reactions are affected by temperature. That means that more voltage (think of that as “push”) is required at low temperatures.
Trojan, who are a respected battery maker, now recommend 14.8 volts for their jars to become fully charged.
The simplest charge controllers were simple “on off” devices. They simply pass the voltage from the panels to the battery bank. The bank determines the voltage. As the bank approaches 100% full the controller would keep the charging circuit switched off for longer and longer periods of time, slowing the charging rate.
We have a finite time frame in which to charge a bank. Generally the figure used is 5 hours per day. While it is true that charging may begin shortly after dawn and continue to dusk, not much energy is moved into the bank. Most of the power will be harvested in the 90 minutes before solar noon, and the 90 minutes after solar noon. It is true that solar may take care of the parasitic loads, and that leaves any “extra” power to do a low and slow charge of the bank.
More sophisticated controllers may use PWM which stands for pulse width modulation. When the battery is low all the harvest may be poured into the battery bank. As the bank passes 85% state of charge the pulsing may begin. The pulses get shorter and shorter and faster and faster so that energy is still being presented at the acceptance rate of the battery.
There is another type of controller that uses an MPPT design. That translates to “maximum power point tracking”. A good MPPT controller may out perform a PWM if conditions are favorable. However, this extra performance increases the cost of the device. When panels were expensive in cost per watt, going MPPT was a no brainer. Today it may be cheaper to simply add an additional panel. There is still a place for MPPT, especially if there is no more room on the roof for more panels.
So what must a good controller include in its design?
– adjustable voltage set points for the various types of charging
– a temperature compensation probe on the battery bank
Any controller that has those features will work better than one that does not. My current favorite is a 40 amp controller from Grape. It is quite inexpensive and can be “programed” from a cell phone using a blue tooth connection.
40 amps refers to the output of the controller. If we use 12 volts as a nominal value, a 40 amp controller will function well with up to 480 watts of panels. But remember, panels are often 17 volts. The “extra” volts will be ignored by the controller, so the “real” answer becomes 17 volts x 40 amps, or 680 watts of panels. However the “harvest” would be as if there were only 480 watts of panels (at 12 volts).
If you are willing to use propane for cooking and cooling 480 watts may essentially eliminate generator use.
Here is a link to the Grape controller which is sold by Home Depot!
If you wish to have more solar harvest, then either a higher capacity controller is needed, or the voltage needs to be doubled to 24 volts. That would mean about 1360 watts of panels could be used. The additional equipment needed would be a 24 volt DC to 12 volt DC converter, capable of handling the regular 12 volt loads in an RV.
1360 watts puts us within “striking distance” of being totally off the grid while allowing some use of a roof air conditioner.
The biggest capacity controller I’m aware of is from Magnum and is rated at a whopping 100 amps. However that is a bit of a swindle as it can’t do that at 12 volts. More typically the maximum is about 80 amps with controllers from several makers, such as Outback, and Victron. Be prepared for sticker shock.