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Q & A On The MPPT Solar Charger

While we know that our revised MPPT Solar Charge Controller will be welcomed by many readers who had requested improvements, there are bound to be other questions, like "Can we now push the revised design to handle more solar panels?" The answers could be yes, no and maybe, as demonstrated by this selection of recent questions from readers. . .

By John Clarke

Q

I intend to build the MPPT regulator to try this technology out at one of my radio repeater sites.
    All my sites operate at 12V but it seems there could be a further efficiency gain by using an MPPT regulator with a 24V panel. The original article outlines some component changes to swap the whole system from 12V to 24V so it seems simple to leave the battery side components for a 12V battery but change the solar side components for 24V.

This leaves the inductor which may require a higher value of inductance to operate in 24V to 12V mode. Is this possible or is the inductor likely to saturate if used in this way?

A

Click for larger image

It is unlikely that higher efficiency would be obtained using a 24V solar panel with a 12V battery. The high efficiency of MPPT is achieved because the solar panels are operated at their maximum power point for charging and because the panels are at a higher voltage than the battery voltage.

A 24V panel would be running at about 36V when delivering its maximum power and this would need to be stepped down to between 12 and 14.4V when charging the battery. Compared to the typical step-down from 18V when operating at the maximum power point for a 12V panel, we would expect that the charger will have greater losses when operating with a 24V panel.

The charger would certainly work with a 24V panel and 12V battery although L1 will need to be larger at 20uH with 14 turns on the former. This core should still be suitable for use without any danger of saturation.

Overall, we would not recommend purchasing a 24V solar panel on the off-chance that it will deliver any better overall efficiency than would a 12V panel with the MPPT charger.

On the other hand, if you already happen to have a 24V panel then there is no reason why you cannot press it into service in the way you suggest.

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