Unexpected MPPT behaviour when reaching maximum power output of panel

[Doug]

Hi All, I realise this is a better forum to post an MPPT question than my previous post!

I have an Epever Tracer 2206AN MPPT controller, a cheap 160W solar blanket that is not performing anywhere near spec - not my main focus here, but possibly related, and a 12V LiFePO4 200Ah bank and alternately a car battery. The MPPT behaviour is disappointing, I have noticed the panel voltage varying from 14 up to 20V, so the DC-DC converter is working with the panel voltage above the battery voltage. Under clear sky full sun though the panel voltage drops down though, not much above the battery, and therefore providing similar output to a cheapie PWM that came with the solar blanket. The current is similar on either side of the DC-DC converter, i.e. it's not doing much! Could it really be that the max power point for this particular panel is so low, or is this normal for cheap panels or even the MPPT under performing also?

If there is high cloud I have noticed the MPPT running the panel at higher voltage and there is a greater delta over the DC-DC converter, but with only a small drop in light, if the panel output drops below about 1.5A, the MPPT gives up and stops tracking. This is a known issue with the Epever, it behaves like a PWM at low light levels with the panel voltage only just above the battery voltage, same as the PWM controller, and I knew this before purchase, but I wasn't expecting to hit those levels so frequently. It could be that this particular charge controller will work better with a higher power or better performing panel, staying out of that low level give up range! I was intending on connecting a few panels in parallel which is why I got the 20A model, I just wanted to try one first.

So is it correct for the MPPT to reduce the panel voltage under full load? I was expecting the panel voltage to hover around the 18V mark, with lower current, but voltage lowered and current boosted into the battery, reaching the power output limit of the panel.

My previous post is here, with similar text to above as well as more on the panel, but I'm hoping the panel issues are unrelated to this MPPT issue:

Weird MPPT behaviour with an under performing solar blanket

Hi All, I have an Epever Tracer 2206AN MPPT controller, a cheap 160W solar blanket that is not performing anywhere near spec - not my main focus here, but possibly related, and a 12V LiFePO4 200Ah bank and alternately a car battery. The MPPT behaviour is disappointing, I have noticed the panel...

diysolarforum.com

Cheers,
Doug

 

[gnubie]

It's been covered many a time on the forum, MPPT controllers like a bit of head room to play with. With Vmp around the sort of place I'd expect a 12V class panel to be, 16-18V, that's only a volt or two over the potential battery voltage. Epever controllers need at least 2V over battery to charge. Slice and dice that small operating window further by adding in a psuedo-PWM mode and you can see it's going to drop out of MPPT at the slightest excuse.

The answer is to provide higher voltage to the controller, put two panels (blankets) in series.

 

[Doug]

Ok, that's very helpful, I was expecting a higher Vmp so thought there was enough headroom for the DC-DC converter, but obviously not. I'll have a look when the battery is a bit lower voltage in the first instance, but I was always intending on getting more panels, so I'll focus on that. I was originally thinking of having panels in parallel, but perhaps as you say series may be better as long as I have some bypass diodes to handle shading. Sorry for missing the other posts on this topic, I did look through a lot before posting but obviously not enough. I know the series vs parallel stuff is well discussed.

I am pleased that every time I've calculated the efficiency of the DC-DC converter, it's been around the 98-99% figure, matching or above the advertised spec which I didn't think was possible.