PWM charge controller output technical question

machineman

Not having put a PWM controller on a scope, I have question about what its really outputing. Which of these 2 scenerios is how a quality PWM outputs:
1. The PV voltage is passed thru and pulsed at a fixed frequency and varibale duty cycle. 17V in and 17V out pulsed and clamped to the battery voltage. Lower duty cycle reduces the current into the battery for the bulk, absorb, float stages. No Inductor capacitor smoothing.
2. The PV voltage is regultaed down to the bulk, absorb, float voltages and then PWM output with fixed frequency and variable duty cycle. Essentially a buck converter with specific voltage setpoints.
I have seen schemaitcs on various sites showing both methods. The first is easiest but is it OK to pulse the full panel Vmp into the battery?
Thanks

zoneblue

Firstly a PWM charge controller only pulses once absorb starts. During bulk, it just literally hooks the pv to the battery via some kind of blocking diode/fet to prevent reverse flow at night. During absorb or EQ stages, PWM reduces the duty cycle to maintain target charge setpoints. There is no smoothing, thats how its cheap and cheerful, and lightweight, not much to go wrong. A mmpt controller OTOH uses inductors and electrolytic capacitors as part of its buck dc converter topology.

machineman

OK, that about what I was thinking. Bulk DC=100%, Absorb DC<100%variable downward, Float=Low DC% to maintain float voltage. And MPPT chargers reduce voltage and boosts available current like a Buck circuit.

BB.

Yep... PWM is an "on/off" switch. MPPT are typically some form of Buck type down converter.

Note, MorningStar has an option on some of their PWM type charge controllers... "High" frequency switching can make for lots of audio noise. They have a "slow switching" mode that is quieter in the audio frequency range (I think it was for telecom systems originally).

-Bill

machineman

BB. wrote: »

Yep... PWM is an "on/off" switch. MPPT are typically some form of Buck type down converter.

Note, MorningStar has an option on some of their PWM type charge controllers... "High" frequency switching can make for lots of audio noise. They have a "slow switching" mode that is quieter in the audio frequency range (I think it was for telecom systems originally).

-Bill

Is there an optimal PWM frequency for lead-acid battery charging? I searched and found 50Hz up to 1KHz..

BB.

Not sure... The PWM controller turning on/off rapidly does somewhat "emulate" the actions of a desulfator.

In theory, higher frequency "holds" the battery at the correct voltage better (think of the battery as a "big capacitor"--Higher frequency makes the "charge/discharge" micro cycling range smaller--But it also makes audio and RF noise possibly higher.

At this point, I don't think I would care unless I had a "whine" in audio equipment and I wanted to kill that noise--Then I would try the "slow switch" option.

-Bill