To Power Point Track Or Not

An Eccentric Anomaly: Ed Davies's Blog

As mentioned in a previous post I have a few small PV panels out in the garden feeding an old battery via a Morningstar TriStar MPPT 60 charge controller. With the experience of playing with this controller I'm having some second thoughts about how worthwhile MPPT still is as PV panels get cheaper.

Hearing of two of these failing in a short time also helps to undermine my resolve. [Update 2013-07-24: …and another one bites the dust.]

The basic function of an MPPT is to make the best use of the higher voltages available from the PV panel when the sun is bright while at the same time getting some power out when it's less bright by doing a DC-to-DC conversion from the optimium PV voltage at that moment (maximum power point — Vmp) to the battery voltage. If the battery's charge is low its voltage will also be low so the MPPT also charges more efficiently then. A secondary advantage is allowing the panels and the wiring from them to be operated at a higher voltage than the battery's thereby reducing the current and therefore the losses in the cables (or allowing thinner cables).

The alternative is to use either a PWM or a diversion-mode charge controller which operates the panels at the battery voltage. Leaving enough “headroom” for some charging to happen when the PV voltage is low and the battery voltage is high means that when the batteries are low or the sun is bright the PV will be wasting a significant amount of voltage, and therefore power, as the PV panels are essentially constant current devices — there's very little increase in output current from them as the voltage drops. The excess is wasted as heat in the panels.

Still, the MPPT has a noticeable amount of self-consumption, just over 2 watts so about 50 Wh per day, so it's worth doing some arithmetic to see if it's really worthwhile. On a long sunny day it clearly is from the simple energy received point of view.

On a dull winter's day, the critical case where optimum performance is required off-grid, it's less clear cut. Suppose that there's half an hour's sunshine available in the day. With 1600 W of panels (a plausible amount for a 24 V battery bank with the charge controller's 60 amp limit) the MPPT will produce 1600 W × 0.5 h = 800 Wh and consume 50 Wh for a net gain of 750 Wh.

Suppose the panels and batteries are reasonably well matched so that the battery voltage is 85% of the medium-bright sunshine Vmp and, because we have a much cheaper charge controller, we can afford an extra 200 W of panels. Daily production will now be 1800 W × 0.5 h × 0.85 = 765 Wh.

The gain is small and rather sensitive to the assumptions we make but still wiring the panel strings to the batteries with just a blocking diode, using a diversion mode controller to stop the batteries getting fried, wins us:

Another aspect to consider is that it's possible to tune the voltage of the battery to match the PV panels in use by selecting an appropriate number of cells. There's no particular need to match the traditional 12, 24 or 48 V lead-acid batteries as long the battery voltage stays within the input voltage range of the inverters (or other output device) over a useful proportion of the charge range.

To play with this sort of thing, amongst others, I've just got a set of LiFePO4 cells (10 cells of nominally 3.2 V, 20 Ah each — so 8 cells are 0.5 KiWh (512 Wh)):

I have to say I didn't have Outtasight's nerve to buy a set of 400 Ah cells straight away. Gulp. (And I also think he's being quite “courageous” in his videos waving his hands around high current devices like these whilst wearing what looks like a metal watch strap and a metal ring.)