PV array example 12V battery with 75V solar charger:
• Minimum number of cells in series: 36 (12V panel).
• Recommended number of cells for highest controller efficiency: 72 (2x 12V panel in series or 1x 24V panel).
• Maximum: 108 cells (3x 12V panel in series).
If 36 cells produce roughly 20V Vmp, and 72 cells produce roughly 40V Vmp, how is it possible to be more efficient to go from 40V to 14.5V for bulk charging, rather than from 20V to 14.5V?
From what I understand, buck step-down converters have higher efficiency when input and output voltages are closer together. And the MPPT charging circuitry should be very similar in design.
MPPTs work by switching the input at a specific (high) frequency.
This switching frequency allows high efficiency at higher input voltage.
This in contrast to PWM controllers that have highest efficiency when input voltage is just a tad above output voltage.
It’s just a (very) different way of working.
This is very abstract and doesn’t really answer my question. How?
I’m fairly certain this MPPT controller has a classical buck switching step-down converter inside of it, and those work more efficiently when the voltage differential is lower.
I have used many MPPT devices. But most of the cheap ones deliver around 90% - 94% of PV Input > Battery Output. The 6% - 10% of the power is wasted as heat during the conversion.
Victron says they deliver 98%+ efficiency, but turns out they too are stuck at 94% - 95% at the most (from the units which I’ve bought). Even though there sensors data says 98%, if you cross-check it, you will find out the true efficiency.
Many of the marketing materials show MPPTs are 98% efficent. But they do not say if its the MPP tracking efficency or the PV > Battery conversion efficency.
And even if they do, those stats are based lab tested at certain input voltages and at certain current limits and certain battery voltages, etc.
I understand that advertising is a best case scenario. What I don’t understand is why the manual would say something that based on all modern electrical theory should not be true.
Recommended number of cells for highest controller efficiency: 72
They are clearly saying that the controller itself, not the overall system efficiency, is higher when input voltage is around 40V, not when it’s 20V or 60V.
And embarrassing that nobody caught and fixed it for so long.