Maine Sail, writing at marinehowto and various forums, is one of the best resources on the ‘net on matters related to batteries and their charging. He has real experience and performs valuable and well-designed experiments on our behalf.
I will take my reputation in my own hands and push back on one of his statements. I don’t think it’s wrong; I think it’s misleading because of how it is worded – the scope is narrow but the wording is overly broad.
Here we go
Because the battery essentially determines the current which can flow into it, when the controller reaches absorption voltage, MPPT gains over PWM are only possible during BULK charging. > > https://marinehowto.com/mppt-vs-pwm-solar-controllers/
In a charging-only scenario that is true.
In a real life scenario (which involves both battery charging and user loads) an MPPT controller will typically have an advantage over PWM any time heavy loads are applied By heavy I mean “at the limits of either controller’s capabilities under present conditions”. We could also say “PWM’s capabilities” since it will have the lower output of the two.
Let’s consider this system:
- Battle Born LFP pack, pseudo-Absorption at 14.2v and pseudo-float at 13.4v. Discharged to 80% SoC overnight, ~12.4v.
- the most popular panel, a 100w Renogy mono panel, Vmp 18.9v, Imp 5.29A.
- MPPT assumed to have 5% conversion losses, 10% panel derating and 2% wiring losses, all non-issues for PWM since it’s not using that voltage anyhow.
I include this only for completeness. PWM gets clobbered in early Bulk but we already know that.
Absorption and Float
In absorption (charging only) we have already stipulated there is no difference between PWM and MPPT. There are edge cases where there might be a difference but that’s not what we’re here to discuss. I’ll stipulate the same for Float.
Real life intrudes
Now let’s assume we are in Float later in the day and want to crank up a load. What can the controllers harvest?
- PWM = max 70.9w (13.4v x 5.29A)
- MPPT = max 83w (100w x .95 * .90 * .98), +17% MPPT advantage
So what, it’s only 12.1w? Two things:
- MPPT can make 17% more power in Float in this example. The same thing happens in Absorption but the difference is smaller because Vbatt is higher at that time (+10.6% mppt advantage).
- it is demonstratably not true that “MPPT gains over PWM are only possible during BULK charging”. They are possible in Absorption and Float, too, whenever relatively heavy user loads are added in.
Now if you only want to run a 40w load then either controller will do it. If you want to run a 75w load in float the PWM can’t do it and will draw from the battery. Which will decrease Vbatt. Which will decrease PWM output. Which will decrease Vbatt, and so on, and so on in a PWM death spiral.
I like PWM
This is your reminder that I like, own, and use PWM controllers. They are terrific value for money and can be tweaked to increase their performance. My intent here is to point out the quote above is true only in a limited context (charging with no other loads). When loads are considered the claim is no longer necessarily true.