Tech Q & A: Off-Grid System Design Part 2

Q: How do you balance the best practice of oversizing the PV array with the charge controller manufacturer’s max DC input recommendation?

A: Best practice is to not exceed the manufacturers maximum recommended PV power for a charge control. Some will have that limit very close to the rating of the charge control and some will allow much more PV power than the nominal rating. For example, the Morningstar TS-MPPT-600V is allowed to have double or more PV power compared to what would fully load the 60A output. This can be useful if it is used for a backup system. Then during early and late hours or cloudy days the production of the solar array will be strongly enhanced. Also keep an eye on temperature de-rates on the charge control, some start to de-rate at only 78°F.

Q: If the inverter controls the DoD, why do we need the charge controller?

A: Charge controllers regulate the energy created from the solar array into the batteries and diverts the energy when the batteries are fully charged.  Charge controllers monitor the batteries State of Charge (SoC) for proper charging but do not isolate or disconnect the load from the batteries at low Depths of Discharge (DoD).

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