MPPT Controllers - Volts versus Amps

Volts and Amps:

Solar panels vary in their output. A solar panel is used to supply battery charging power to a 12 volt electrical system. But, the actual voltage and power output of a solar panel will vary greatly, depending upon how bright the sun is and the angle of attack to the solar panel. If it's dark out, you'll have a zero volts output. If the sun is hitting it at a perfect 90� and is nice and bright you will be receiving maximum voltage, which may be in the area of 18 volts, depending upon the brand and model of solar panel.

Now batteries don't care about voltage whenever they are being charged. You need to have a higher voltage than what's in the battery or else it won't charge. Conversely, if the battery voltage is higher, it won't charge but will instead discharge. What batteries really want is amps. Batteries give off amps when they are used so you just need to refill them with more amps - pretty much like gallons of fuel in a gas tank. Your battery may be rated at 220 amp-hours. This means that you can withdraw 220 amps and it will be dead in 1 hour. Or, you can draw 22 amps and it will take 10 hours to draw it down.

A common electrical law is that amps times volts = watts. Watts is the measurement of true power. The reciprocal of this formula is that watts divided by volts = amps. Remember, amps is what we want and the more, the better.

Let's just assume that the panel we'll use in these examples is a 120 watt panel rated at a maximum of 18 volts. If this panel is receiving maximum benefit of the sun it will be putting out it's maximum rated voltage of 18 volts. 120 watts divided by 18 volts = 6.67 amps, therefore 6.67 amp can be put into a 220 amp-hour battery per hour. At this rate it will take about 33 hours to recharge a totally dead battery. However, once the sun dims a bit, the voltage may drop to 14 volts. 120 watts divided by 14 volts = 8.57 amps, therefore it will now only take around 25 hours to recharge that same battery.

What we find is that lower volts allows for higher amps. But, if the voltage drops too low (beneath that of the battery) then charging will no longer take place.

We also need to have a way to regulate the charge coming into the batteries. Pumping 18 volts into a 12 battery will tend to boil the electrolyte and you'll be adding water all the time. It may also send that higher voltage charge down line and you may have issues and failures with some of the more sensitive electronics in your RV. Just as important as excessively high voltage is low voltage. In the evening there is no sun so the solar panels aren't putting out. When your batteries are charged up they will have power. Solar panels have a phenomenon called "dark current". Dark current will turn your solar panel into a load rather than a source and your batteries will discharge through them. Pretty much solar charging in reverse.

Charge Controllers:

To handle these tasks we add a Solar Charge controller into the system. The charge controller mounts between the solar panels and the battery bank. In it's basic form it's a voltage regulator. It won't allow the voltage to get too high in order to protect the batteries. It's also an electronic check valve. It won't allow backflow of current so that your batteries won't discharge by night. A good charge controller will also regulate the charge in 3 stages - bulk, absorption, and float. The bulk charge state dumps it in fast. Once the battery voltage starts to get close to where it should be the absorption stage kicks in which tapers off the charge so that the battery plates can absorb the charge rather than allowing excess voltage to boil the electrolyte. Finally, the third float stage kicks in to maintain the battery at a minimal trickle charge level.

The latest type of solar charge controller is the MPPT, or Mean Power Point Transfer charge controller. This latest version of controller can actually increase the usable power output of a solar panel. MPPT charge controllers utilize electronic circuitry to do some fancy voltage regulating. They will regulate the charge voltage to be just above whatever the battery needs. By doing this, they can trade volts for amps. As we saw in the previous examples, charging at lower volts gives more amps, which is what we really want. If a battery is down to 10 volts, it only needs 10.2 volts to push a charge in. When a solar panel is cranking out 18 volts we are wasting 40% of that potential charge because 120 watts divided by 18 volts is only 6.67 amps. But the MPPT controller will lower it's output to 10.2 volts. 120 watts divided by 10.2 volts = 11.76 amps, which is a sizeable increase in output capacity. As the battery recovers some of it's charge the voltage will increase. The MPPT controller will raise the voltage in steps to keep it just ahead of the battery voltage. On the average, MPPT charge controllers offer a 30% boost in performance, which is like having a free solar panel that isn't really there. Their obvious benefits, with no drawbacks make it a no brainer of a choice. They are available at any of the major solar panel suppliers.

Submitted by Mark Quasius - 2/22/06
 

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