on MPPT & the “live use” of house power

I’m am not an MPPT cheerleader;  I am a pragmatist and a cheapskate frugalista, in that order.

Lately  have been chafing at the objection that MPPT only provides benefit during the Bulk phase of charging.  It’s true that MPPT “boost” is more dramatic during Bulk, but we should be clear why this is so:

MPPT provides benefit whenever demanded current exceeds the PWM controller’s delivered current.1 Batteries in Bulk mode will gobble up all the current the controller can deliver;  MPPT shines.  No arguments there.

Therefore, it is said, MPPT does no good when battery charging current requirements taper off in Absorption and Float modes.  True enough, BUT… There can be additional current demands in those modes on top of what is required for charging.  **Any house power loads** will increase current demand.

Consider the following system:

  1. a PV panel that
1. generates 100W measured at 17v (Vmp)

2. generates 82.18W at 14.8v (Vabs)

3. generates 73.29W at 13.2v (Vfloat)

4. generates 51.52W at 12.1 (50% SOC)
  1. 2x 6v GC batteries bank (200Ah) that float C/100 @ Vfloat

  2. PWM controller

1. 2.0A (26.4W) used for float, leaving 46.99W for "live use" house loads
  1. MPPT controller
1. 90% efficient DC/DC

2. 2.0A (26.4W) used for float, leaving 63.6W for "live use" house loads

This results in a max of 35.34% increase in “live use” capacity during float.  Basically the same “boost percentage” manufacturer’s claim for MPPT.  So we see that MPPT has the same advantage over PWM in all stages of charging anytime the system wants more than the PWM can provide.

PWM and the discretionary 12v circuit

Earlier I frothed and ranted about a discretionary 12v circuit that would only be used when the batteries were being charged.  I failed to notice PWM controllers would be able to make use of this setup, albeit supplying a lower amount of current.

In summary

Rather than saying MPPT only works better during Bulk we could more accurately say “MPPT works better any time the PWM controller is maxxed out.”

  1. assuming PV voltage is higher than present charging voltage.