Charge controllers are extremely useful devices. As part of a standalone power system, they make sure that batteries are not overcharged and damaged over time. However, the very nature of this process, providing an even and mild stream of current to a battery or batteries, leads to waste. Some energy is "lost" within the charge controller and here is one way to reclaim that lost electricity.
Letís say that we are using a photovoltaic array that provides 6 amperes in full sunlight and that we have one battery that has a capacity of 225 ampere/hours at 12 volts dc. There is a charge controller between the PV array and the battery. My experience has been that at almost all levels beyond 0% state of charge (SOC) the charge controller will not let anywhere near 6 amps through to the battery. This is the controllerís regulation process. We can verify this by hooking an ammeter inline just before the battery. In full sunlight, it would not be unusual to find an ammeter reading of 4 amps. This means that only 4 amps is making it to the battery when the PV array is producing 6 amps. This is precisely why when you touch the charge controller it is warm. Those 2 amps (difference between 6 amps and 4 amps) are being burnt off as heat at the charge controller.
Reclaiming Lost Electricity
The following method is a very simple, albeit not the best, way to recapture at least some of this energy. I usually use two, older 12 vdc batteries that I have lying around that may not be in tip top shape. In addition, you can use another charge controller with these two batteries, or purchase a 15 volt, voltage regulator. I have never seen a 15 volt regulator, so I bought several 5 volts regulators at Radio Shack. Putting them in series yields 15 vdc. Keep in mind that the Radio Shack 5 volt regulators are only rated at one amp. If you want a 3 amp rating you need to put three of them in parallel to get 5 vdc at 3 amps, and then hook three of these threes in series to get 15 vdc at 3 amps.
The idea is to attach the regulators (or charge controller) to the PV array before any other charge controllers or equipment and feed this 15 volts to the two 12 vdc batteries wired in parallel. During the day, these two additional batteries will grab some of the 2 amps that was previously being lost. Using a charge controller that is adjustable is more precise than using the 15 vdc regulators. An adjustable charge controller will allow one to change the charging voltage, hence changing the charging current which could be useful at times. If it is a somewhat cloudy day and the PV array is only producing 3 amps, you want all that energy going to your main battery banks (because it will take the full 3 amps), not the reclaiming batteries.
Using the Reclaimed Power
Now that the reclaiming batteries are charged after a day in the sun and the sun has set, we need to pour this energy into the main battery banks. I will leave the method up to you since I have used several different ways to perform the transfer. Essentially, the two reclaiming batteries are to be disconnected form the system, wired in SERIES to get 24 vdc and then this power is to be sent through a CHARGE CONTROLLER (I suggest using the one that was originally part of your system) to your main batteries. Now, the reclaiming pack is acting as a 24 vdc source of power just like the PV array does during daylight hours. You may wire the 24 vdc of batteries in PARALLEL with your PV array to achieve this, but remember to put BLOCKING DIODES inline so all power runs through the charge controller and not up and into the PV array where it would be wasted as heat energy.