In the spring of 2014, early May to be exact, I set up a series of tests to compare MPPT controllers to PWM controllers for charging a LiFePO4 battery bank. Some on the net professed that an MPPT controller simply won't work with LiFePO4 batteries because the voltages are higher and an MPPT's boost works off the differential between panel voltage and battery voltage. The reality is that while a LiFePO4 battery has a nominal voltage of around 13.2V, and stays there most of the time, it is also a bank that for a solar array is very hard to raise the voltage of. I knew this net theory to not to be the case because I use an MPPT controller on my own vessel.
MPPT In A Nut Shell:
An MPPT controller ustilizes excess panel voltage, in bulk charging mode, and turns it into usable charging current.
MPPT vs. PWM Closer Look:
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 in BULK mode. This is important!
Bulk charging simply means the battery terminal voltage has not yet risen to the limiting voltage or the controllers absorption voltage set point. Think of a solar controller as a bulk charger then a voltage limiter and it becomes much easier to picture what is actually happening.
In bulk charge the solar panel is putting out all it can based on the conditions presented to the panel. These conditions include solar irradiance, shade, panel angle/orientation and panel temperature. In bulk mode the battery terminal voltage is at its lowest has not yet risen to the controllers limiting voltage. In bulk mode the battery terminal voltage slowly rises to absorption or the limiting level. With an MPPT controller bulk charging is where the voltage differential between the panels voltage and the battery voltage are the widest. A larger difference between solar array voltage and battery terminal voltage means the MPPT controller can effect more boost by using the excess panel voltage and converting it into usable charging current.
In absorption charging the battery can not continue to take all the current the panel can provide so the voltage climbs to the limiting voltage where the controller now enters a PWM or pulse width modulation mode. In PWM (absorption) the controller only allows enough current from the panel so as to not over shoot the controllers voltage limit.
In absorption mode an MPPT controller is doing virtually the identical thing a PWM controller is, limiting the voltage to the battery to the preset level. The battery decides how much current it can take at that particular SOC and voltage thus no "boost" can take place. MPPT boost only occurs during bulk charging. There is no real quantifiable boost benefit of an MPPT controller once the batteries have attained the limiting voltage, only in bulk.
All 12V nominal solar panels are in the 17-19V range and lead acid batteries charge at 14.1V to 14.8V depending upon brand, type and chemistry. This means that extra panel voltage goes to waste with a PWM controller. With a PWM controller in bulk the panel is essentially direct connected to the battery. This means the panel voltage becomes identical to the battery terminal voltage, minus any wiring voltage drops of course, and the panel slowly increases battery voltage to the absorption level. Because of this direct connection the excess voltage is simply not used as efficiently as it could be. Once at absorption voltage the PWM controller limits the voltage to the absorption set point just an an MPPT controller does. There are no gains or boosts in bulk with PWM and the extra panel voltage simply goes to waste.
So how does this apply to LiFePO4 or large lead acid banks?
Because the LiFePO4 battery is a very stubborn source to move or increase the voltage on, the solar controller remains in bulk mode for much longer than it does with lead acid batteries. The 400Ah bank seen here only charges to a max of 13.8V but with the solar panels in this test it would not come up to 13.8V until the battery was at nearly 99.5% SOC. In contrast a typical AGM or flooded lead battery will come up to 14.4V significantly easier and significantly faster with the same current source. This higher absorption voltage 14.4V vs. 13.8V means less MPPT boost towards the tail end of bulk. Longer bulk times mean longer boost times for MPPT controllers. The MPPT controller on our boat is almost always in bulk mode with the LiFePO4 bank..