Imagining the right panel, controller and battery set-up.

[bluegoatwoods]

If I could have my way I would charge a 12 V, 100 Amp hour life po4 battery off of a solar panel at about .4c. In this case that would be 40 Amps.

So I'm picturing a 100 Watt panel in strong sunshine. The panel is outputting 100 Watts. So 100 Watts divided by 12 Volts would seem to indicate that this panel is delivering 8.3333 Amps to the battery. (My PWM controller is wasting perhaps 30% of that? Yeah. But at least I can account for this and beef up the system accordingly.)

So if I connected five panels in parallel this system ought to produce 8.3333 Amps times 5. For a total of 41 and a half Amps. Or something very close to that. I've forgotten the values I calculated to the right of the decimal.

If each panel had a 20 Watt controller, then I don't see that part of the system being particularly stressed. And those five controller outputs can be paralleled into one battery input, is that right? This would give me that .4c charge rate.

I suppose I'll go ahead and take controller loss into account. I'll add another panel (!) for that. So I've got six panels producing 8.33 Amps each for a total of (practically) 50 Amps. Nice! But multiplying 50 Amps by .7 estimated controller power loss gives me the disappointing total of a 35 Amp usable charge rate.

Wow! I might as well give it up right now. It was a fun thought experiment. But realistically it's not much more practical than a Lead Zeppelin.

I was going to ask if my reasoning was correct or had flaws. There's no point now. Though if there is some flaw and it really is more practical than I think it is, then I'd be glad to hear that.

 

[sunshine_eggo]

If each panel had a 20 Watt controller, then I don't see that part of the system being particularly stressed. And those five controller outputs can be paralleled into one battery input, is that right? This would give me that .4c charge rate.

This is where you lost me. Did you mean 20A?

 

[bluegoatwoods]

Yup! That's it. A 20 Amp controller. Not 20 Watts.

Those darned units. You've got to watch them carefully or the wrong one will step up and steal the spotlight.

 

[TacomaJoe]

Not to ruin your thought experiment but isn't hydrogen gas corrosive to lead?

My 100W panels are more like 20+V and closer to 5A output. I have 300W of solar (flat mounted) on my trailer and I ran some numbers in May and got about 100A pumped back into my battery in a day of good sunshine. My panels are in series with a MPPT controller.

Why the 0.4C charge rate? We're probably both at about the same latitude, so I would guess that you would have similar results.

 

[Lt.Dan]

(My PWM controller is wasting perhaps 30% of that? Yeah. But at least I can account for this and beef up the system accordingly.)

The PWM controller shouldn't waste that much power, even though a lot of people think they do. MPPT controllers are cheap enough though, why not get a single, bigger MPPT (40-50a), with better efficiency, fewer wires/connectors etc....

I suppose I'll go ahead and take controller loss into account. I'll add another panel (!) for that. So I've got six panels producing 8.33 Amps each for a total of (practically) 50 Amps. Nice! But multiplying 50 Amps by .7 estimated controller power loss gives me the disappointing total of a 35 Amp usable charge rate.

Typical MPPT controller loss is in the 95%+ range. So if you put 5x 100w panels in place, and account for your 75% output from smog, imperfect angle to sun, etc, you are at 375w, then multiply by 95% MPPT efficiency to get 356w.

You can charge a 1280Wh battery from 0-100% in around 3.5hrs. Still pretty good. No reason it wouldn't work.

 

[bluegoatwoods]

Why the 0.4C charge rate? We're probably both at about the same latitude, so I would guess that you would have similar results.

I have heard that li po4s should not be charged at a rate exceeding .5c. I figured .4 is fast enough for me, but with a bit of safety margin.

 

[bluegoatwoods]

The PWM controller shouldn't waste that much power, even though a lot of people think they do. MPPT controllers are cheap enough though, why not get a single, bigger MPPT (40-50a), with better efficiency, fewer wires/connectors etc....


Typical MPPT controller loss is in the 95%+ range. So if you put 5x 100w panels in place, and account for your 75% output from smog, imperfect angle to sun, etc, you are at 375w, then multiply by 95% MPPT efficiency to get 356w.

You can charge a 1280Wh battery from 0-100% in around 3.5hrs. Still pretty good. No reason it wouldn't work.

Really, I was wandering off into the weeds giving that speculation about PWM power loss. I know practically nothing about either type of controller. But I have heard that PWM lose about 25-30%. I have also heard that they are more weather resistant than MPPTs. This is appealing to me. So my guess is that I'll choose PWM. But that's tentative. I'll need to learn more before I have any definite preference.

A 3.5 hour charge time is pretty tempting. Right now I'm getting from about 20% to full on a 1280 Wh battery using grid current and a 20 Amp charger. 3.5 would be nice. And it's heartening to think it's possible. I'm glad.

Now I guess I've got to decide whether I want to look after and manage 5 panels plus associated hardware and wiring, or if I'd rather just buy more batteries to rotate them from 'in service' to 'charging'. If I did that, then I could tolerate a 10-12 hour charge time. And if I added panels later, then I could harvest and store quite a bit of electricity.

I ordered a pretty basic 200 Watt panel and controller set yesterday and I'm expecting arrival in a few days. Then I'll be able to setup and start measuring my solar collection abilities. That'll be cool.

 

[sunshine_eggo]

Really, I was wandering off into the weeds giving that speculation about PWM power loss. I know practically nothing about either type of controller.

PWM work by shorting the panel directly to battery. Panels must be matched to the battery voltage (typically about 1.5X the NOMINAL battery voltage, 12V, 24V, 48V, etc.). This forces the panel to operate at battery voltage instead of the panel Vmp. It modulates voltage by connecting/disconnecting the panel from the battery thousands of times a second. Since the panel is shorted, most PWM can't regulate current, so in addition to matching panel voltage to battery, the panel current can't exceed the charge controller.

MPPT are basically a fancy DC-DC converter that convert high voltage/low current DC (PV) to low voltage/high current DC (battery). They have an algorithm that varies the draw from the panel until it find the right combination of voltage/current that results in peak power (typically very near the panels' Vmp). Most MPPT allow you to place multiple panels in series for higher PV voltage and lower curren. There is a very small efficiency penalty due to the larger conversion, BUT that is usually more than offset by decreased losses in the wiring, especially if long runs are involved.

Allowing PV to work at it's optimal power voltage, Vmp, MPPT are about 30% more efficient by the ratings; however, the reality is that most PV cells are > 25°C due to solar heating, so they're Vmp is reduced from spec. PWM do not see any penalty due to this heating because they are already notably lower than Vmp. MPPT see this penalty because the PV Vmp is reduced, so peak power is reduced. Thus, "real world" MPPT numbers are about 10-20% better than PWM for typical conditions.

Winter can be very interesting with MPPT as Vmp INCREASES with colder temperatures. Winter performance of MPPT is notably better than PWM.