backchannel: mppt addendum

A continuation of this reply.

Consider the most common panel on this forum:  the renogy 100w mono

And the most common snowbird temp, 70F.At that temp the panel can produce 87w at a Vmp of 16.44.

Highest normal Vabs and Vfloat used to give PWM a fighting chance.  Only a minority of ‘dwellers actually Absorp/Float this high so MPPT advantage will be understated in the following examples:

early bulk 12.1v, sunrise PWM: 64.01W (5.29A * 12.1v) PPT: 82.620W (5.29A x 16.44v * .95) MPPT advantage == 29.07%, but we already knew it would be something like that.  Everybody agrees on this part.

mid bulk 12.7v PWM: 67.18W (5.29A * 12.7v) PPT: 82.62W (5.29A x 16.44v * .95) MPPT advantage == 22.98%.  MPPT advantage less dramatic as Vbatt comes up.

absorption 14.8v PWM: 78.29W (5.29A * 14.8v) PPT: 82.62W (5.29A x 16.44v * .95) MPPT advantage == 5.53%.  Getting close there!  By the mid-90s F PWM will break even with PPT in this setup.float

PWM: 73.00W (5.29A * 13.8v)

PPT: 82.62W (5.29A x 16.44v * .95)

MPPT advantage == 15.50%

followup questions

  1. what charging stage do your batteries spend the most time in?

  2. is your house system used for stuff (lights, charging phones, running a fan) or does it only float batteries?

  3. have you ever used so much power during the day that your controller kicked back into Bulk?

  4. do you want or need 5.53% to 29.07% more power from your panels?

  5. is that additional power worth the $$$ of MPPT to you?

It might be entirely reasonable, based on priorities and situation, to answer “No” to the last question.  But that doesn’t mean the advantage doesn’t exist.

Elsewhere in the thread

Float is the reason we tell people that you can not practically charge with a generator. You can not force the last 10% into a FLA bank with higher voltages.

No, you’re talking about Absorption.

My Lifelines are AGM and are suppose to be done once the acceptance rate drops to a certain point in absorb.

Now you’ve got it right.  After they are “done” the charger goes to float.   No charging occurs in float.

Timers lead to chronic undercharging unless monitored closely.

So set the timer for longer.

Mine is a Midnight Kid with a WhizBang Jr. It uses a shunt to measure end amps. The WhizBang Jr. and shunt adds the end amp function to get rid of the timer.

And all for just $439! ($359 + $52 + $28) for a 30A controller with end amps support.

That’s cool, but I set my $200 40A tracer for a 3hr absorb.