BS 2000e MPPT Charge controller questions

westbranch

I went looking for a posting or 2 about PWM CC’s and MPPT that I read lately by Crewzer, but can not locate them so …

I just made some fine tuning adjustments to my Blue Sky 2000e CC plus I added a BTS.
Before doing so I reread the manual to make sure I ‘had it right’.
The battery was not fully discharged, the preferred state for setting the current boost (MPPT), but was at near full charge for setting the charge voltage set point. Ambient temps were at +2 C and a good steady wind so nice cool panel temps for near rating output. Setup went ok, and then I started thinking about what was actually happening to volts and amps…
What is the significance of setting the current boost (MPPT) value Voc – Vmp? (21.6 – 17.2, 4.4 in my case). If the charge voltage value (14.38v) is set, will the unit not use any and all voltage above this amount for the MPPT boost?
When the panel was not producing at max values (low sun, 10:00), it appears that MPPT kicks in? I believe I observed the latter this morning as the MPPT function LED was on at 12.8 (battery) volts with low amps (1.0+-). Is this possible/normal? (Start voltage was 12.57)
Please correct me if wrong, I thought that the MPPT would provide all available volts up to the max charge point (14.38v) then the MPPT would start and do its magic and add more amps…but this system does not seem to be working to this pattern???

Thanks
Eric

BB.

Re: BS 2000e MPPT Charge controller questions

Eric,

I don't know the details about your controller... But the way MPPT works...

Assuming that your battery voltage is below the controller's set point...

The controller is trying to put as much current into the battery as possible. The amount of energy being put into the battery is:

P=I*V... using some examples, 12.8 volts and 1 amp (at the battery) so

P=12.8 volts * 1 amp = 12.8 watts...

If your Vmp=17.2 volts, then the current from the array would be:

I=P/V=12.8 watts / 17.2 volts = 0.74 amps from the solar panel...

The above does not take into account the controller's losses--but, basically the MPPT controller is able to take high voltage/low current (from the solar panel) and transform it into low voltage/high current (at the battery).

In this example, the battery is being charged with an additional 0.26amps*12.8volts=3.3watts additional power because of MPPT...

In real life, there are controller losses, so the advantage at low power (like this) is quite small, and is consumed by the controller electronics.

However, where it really helps is in two places... The first is compared to a PWM controller and cold days... Vmp rises by a few volts on a cold day, so this difference can be "Transformed" into additional power to charge the battery (for an MX60 60 amp controller, this requires, roughly a 400 watt array minimum to make it worth while). If you have a standard "12 volt panel" (~17.5v Vmp) and live in a warm climate, the advantages of the more expensive MPPT controller are not that much.

For example a 17.7 Vmp panel may have a TC of -0.08V/°C, so a 25°C run a freezing (0°C) would give an increase in voltage of:

0-25°C * -0.08V/°C = 2 volts increase or Vmp@0°C=17.7v+2v=19.7 volts

The other place this is useful is you can have a large array of panels and instead of running them at 17.5 volts, you can run them at 100 volts+ (for example). This allows you to either run the wires from the panels longer distances or use smaller gauge wire because of the lower current:

1,000 watt panel / 17.5v Vmp = 57 amps
1,000 watt panel / 100v Vmp = 10 amps

In the above mythical example, we have reduce the current to 1/6'th of the original level--so now we can either send the 6x farther (with the same wire losses) or use much smaller (and cheaper) copper wire between the solar panels and charge controller...

-Bill

PS: I should add, as the battery charges, its voltage will rise. Once the battery voltage hits the Controller's voltage limit, the controller will start cutting back on the charge current going into the battery... At this point the MPPT (maximum power point tracking) does not matter any more as the controller is no longer using all of the energy available from the solar panels.

crewzer

Re: BS 2000e MPPT Charge controller questions

Eric,

These might be some of the posts for which you were searching:

http://forum.solar-electric.com/showthread.php?t=960&highlight=PWM+MPPT

http://forum.solar-electric.com/showthread.php?t=605&highlight=PWM+MPPT

http://forum.solar-electric.com/showthread.php?t=342&highlight=PWM+MPPT

http://forum.solar-electric.com/showthread.php?t=68&highlight=PWM+MPPT

HTH,
Jim / crewzer

westbranch

Re: BS 2000e MPPT Charge controller questions

thanks guys I have read the links and am still digesting the info.. also reread the NAWS info page and it is all coming together, slowly.

One question remains:

What is the significance of setting the current boost (MPPT) value Voc – Vmp? (21.6 – 17.2, 4.4 in my case).

I didn't seem to find this setting mentioned in anything I have read. did I miss it?

Thanks

Eric

BB.

Re: BS 2000e MPPT Charge controller questions

I have not read the manual for your controller--But in my little experience, I have never seen a Voc-Vmp setting anywhere...

There are several different ways that controllers estimate the Vmp point... One (earlier?) controller would measure the Voc voltage and just multiply by XX% to a generic estimate for Vmp... Probably close enough for small applications.

Another method is the controller does a "scan" of the output once every few minutes. The controller raises and lowers the Panel Voltage (by decreasing and increasing the solar panel current it draws) and maximizes the P=I*V equation results.

And some controllers do this maximization up to 60 times per second (was first done with Grid Tied inverters)... You would have to ask Solar Guppy (or one of the other designers here) about how they do that and ensure Imp*Vmp is maximized so often (if allowed by his NDA)...

This is not a simple exercise--Changing sun conditions, clouds, shade all make this a non-trivial problem to calculate and obtain consistent maximum power results.

Since Vmp (and Imp to a lesser extent) is temperature dependent, programing a fixed value in the controller would not be a great idea as panel temperatures are highly affected by sun, wind, and ambient temperatures. Having the controller search for the maximum Vmp*Imp is the best.

-Bill

Solar Guppy

Re: BS 2000e MPPT Charge controller questions

Your programming into the unit what the mppt should be. Its doesn't track but uses % if VOC for the mppt. Your programming what your panels vmp are it then uses that ratio ( VOC/VMP ) when it does the periodic VOC check.

This is the most basic of "mppt" unit on the market and technically is not a power point tracker, it works by a percentage of VOC ( user entered ) and had no "power" ( Voltage * Current = Power ) tracking at all.


If you using all the same type of panels and don't having shading issues, its works reasonable well, maybe with 10% of the true vmp. Solar panels vmp do change with the irradiance so this is at best a coarse mppt unit

westbranch

Re: BS 2000e MPPT Charge controller questions

thanks Guys, its getting clearer.;)

Eric