Charge Controllers

Re: Charge Controllers

Exactly what makes an MPPT better than a PWM in chrage controllers?

Is an MPPT controller good even with a small 75 to 200 watt system

tsp,

The PV array operates at close to the battery voltage in bulk charge mode when a “PWM” controller is used. If the array is delivering, say, 5 A, and the battery voltage is 13 V, then the controller will deliver 5 A at 13 V to the battery bank. In bulk charge mode, PWM controller’s are ~99% efficient.

When an MPPT controller is used, the controller (sort of) searches for the PV array voltage associated with the maximum current available. Any voltage difference between the array’s maximum power voltage (Vmp) and the battery voltage is converted into additional charging current, less the controller’s internal operating losses. For example, if the controller finds the Vmp of the array above to be 15 V, and the array is delivering 5 A, and the controller’s efficiency is 94%, then the controller will deliver (15 V x 5 A x 94%) / 13 V = 5.42 A to the battery, a charge current gain of 8.4%.

MPPT controllers show their most benefit when the battery voltage is low and the PV array voltage is high. Accordingly, winter operation typically realizes the most benefit. But, when the battery voltage is relatively high, and the array voltage is relatively low, such as in the summer, then the MPPT benefit is reduced, if not effectively eliminated.

Note that the benefit of MPPT in an off-grid system is pretty much limited to bulk charge stage. Absorb and float stages are current limited operations, so MPPT is effectively non-operation during these stages. Also note that while manufacturer claims of “30% more power” are mathematically correct (usually high morning voltage times low charge current), the controller will essentially never deliver a 30% more energy over a full day.

I’m a big fan of MPPT technology. However, it’s my view that an MPPT controller is usually not cost effective in a “small” system with a PV array of less than 300 W – 400 W STC. One issue is that the controller’s operating losses reduce the MPPT benefit. Another issue is that the additional cost of an MPPT controller might be better spent on a higher power PV array along with a PWM controller. But, if you’re starting out small with the intent of expanding your system, then an MPPT controller may well be something into which you can grow.

HTH,
Jim / crewzer

Re: Charge Controllers

Bill,

I was afraid the DC-DC topic would show up. It's true that most MPPT controllers include the useful DC-DC down conversion feature ("high" voltage array to "low" voltage battery bank). Personally, I consider this to be a feature separate from MPPT. For example, the Blue Sky 2000 and 2512 MPPT controllers do not include DC-DC down conversion.

This feature has both advantages and disadvantages. As you indicated, high voltage arrays allow for smaller AWG wire and/or longer wire runs with lower voltage drop. However, controller efficiency suffers a bit as the span between array voltage and battery voltage is increased.

Another advantage of DC-DC down conversion is that it allows for use of "high" voltage arrays in hot climates where "regular" array voltage might drop too low for target battery charge values.

HTH,
Jim / crewzer