Solar Install; the on-going saga

What is MPPT an why do we need it?

A typical solar panel is configured as a string of 36 x ~0.5V Solar Cells.




Which produce 19Volts at full load.
While our battery bulk charges in the 13.6V > 14.2V range.



What PWM controllers do is reject the power above the bulk voltage.
What MPPT controllers do is regulate and utilise it.

Why that's a lot like saying...



Well yes it is... but,
...it's more of a 36 cells operating at 83% duty thing rather than 30 are flat out and 6 are idle.

But you can use that power with MPPT...or you could get the right solar panel for your system.


Would that be the same in effect as this?



A 17% smaller panel that performs the same? Or a same sized panel that performs 17% better.
Yes, indeed, with PWM.

Unfortunately 30 cell, high quality solar panels are not a thing. 36 cell are the standard.

So were I to say a 100W standard 12v solar panel on a good PWM controller is setup to run at 83% peak performance. Mounted horizontally so derate another 30%... so it's a 60W panel on PWM, and a 70W panel on MPPT.

This is a very telling graph I pulled from a 90W solar panel.
Assuming MPPT can use everything under the curve so 100% rated.
Let's look at the MPPT is 30% better than PWM myth....



A PWM controller directly connects the battery to the solar panel. This makes the effective maximum power point of that system to be (battery voltage x solar current) fixed at around 14V in practical use.

That means only the shaded portion is available to PWM systems.
While the shaded and unshaded portion is available to MPPT sytems.

Let's interrogate that graph for more data..





Interesting findings.



How is MPPT 30% better when there's only 20% more power available from the system?

Cold weather uprating was it?



0.61V x 36Cell X 5A = 110W = 30% more better

If you ever saw 0°C and 1000Wm² solar irradiance at the same time and if the cell temperature wasn't generally 50°C hotter than atmosphere being that it lives in a glasshouse facing the sun...


What does MorningStar claim? 10% > 15%!

Here we have Normal Operating Conditions Vmp of a 60-Cell Solar panel ≥ 2x(12volt).

If this was 24V PWM system. MPPT would have a 3V advantage compared to it...

In a fortuitous turn of events I only blew up the easy to get to FETs

When I say easy to get to ...wielding a soldering iron from above a heatsink, around bodge jumper cables and capacitors in a teeny channel because the back of the board is an Epoxy slab.


Easy like not impossible..:P



I'm very glad these lived.





I've soldered the new FETs to the legs of the old. I cut them as long as I could. I've even managed not to solder any legs together (world of pain).



Seeing as the FET bus is in parallel and I didn't order enough reinforcements I ran the test at 75% complement and current limited it with a 12.5A charger boosting a fully charged battery.



It's cycling through charge stages and behaving like recovering..
I haven't load tested it or output current sensed.


She'll be right in no time...
"RS.. I need another bag of them FETs, 30 sticks of hot glue anna boxa band aids!"

I like it. The grunt and the charge tenacity.:cool:
She's sure aggressive & persistent.
Constant output, no more generation reduction from low alternator RPM in traffic, thanks to it's input load B2 regulation.


Sure expensive for a 120A 1200W Continuous DC - DC Charger.


Test Conditions:
5 year old flooded battery charged to specific gravity 1.275, watered.
C20 discharge applied for 10 hours.
100% rated capacity confirmed (temperature compensated).
B2B Charger wired on 35mm² from Alternator B+ to Service Battery Bus Bars with 35mm² feeder.
4mm² original equipment ground cable with onboard fuse installed.
Temp sensor installed.
Manually switched.


SOC at commencement of charge according to

Trimetric (trusted): 52.5%
Smartgauge (tuned low, tracking): 45%

Resting Voltage: 12.1V



B2B Inrush Surge: 92A




I can't tell who won.

The Sterling 1200W Continuous DC - DC Charger is €500, a used magnetic latching Alright SW80 contactor cost me €30.
My SW80 does E-Start, full rated load, 100% efficiency and can be fixed with either end of a hammer, also waterproof and fanless. :P


I like both.

It's extra redundancy.

High charge rate for windy motorways (alternator temperatures).
"Low" Charge rate for pootling through town (fan belt wear reducing)


This Sterling 1200W Continuous DC - DC Charger is this much better than a good cable job anna contactor.



Which is to say about 10 times better than standard and double better than my Split Charge Contactor in practical use.
It's most effective in the 70% - 90% range...it has filled under the curve very nicely in the happy battery zone.





Give me a welder, metal, a dedicated alternator and a pulley kit I can do you an even less favourable result with double the input potential, inherent redundancy and cooler alternator temps.


Or stick a 50Ω wirewound potentiometer beside yer dash ammeter on the D+ from the dash light above it.


0.5k€ for a double better charge powah source with short life capacitors in a hot sealed box?
Worth it?



I've derated mine to it's apparent nominal max operating current. It seems I mighta forgota put 40A back into the box when I reassembled it.
Dear Charles...maybe I left them on the table or is the rating 50% inflated or was it 67% efficiency? who nose? Maybe they'll come back? I'll be sure to let you know if they do!


I guess I didn't over-spec it at all. :rolleyes:

I can now replenish 90Ah of my 220Ah battery in 1.5 hours or 100 miles.
That used to take ~2.5 hours (~7x better than standard).


My Service Battery is getting a wee bit fond of it's evaporative coolant (H²O), arah about 100ml a month....the Sterling will probably raise that loss to ~125ml.

B2B Charge Profile: Flooded; 14.8V regulated...with temperature compensation 18mV per °C (fixed: conservative), what does naff all under 5°C because the output max voltage is 15.1V





Battery temperature at 60A sustained, C4 charge...nothing to get excited about.

"Soft Start" is a ~60 second activation delay.