MPPT ?? - Page 1
Forum section: electronics / beginners

author: Phaedo
author_rank: Contributor
timestamp: December 29, 2023, 02:44:12 pm
content: Hello...I was watching a bunch of videos on MPPT and PWM controllers for Solar PV applications.I am quiet confused about most aspects.From what I figured out, the general topology seems to be Solar PV ---> MPPT controller or PWM controller ----> BatteryPWM controller seems to be straight forward enough (i guess...) . It would be a buck boost controller which will match the Solar PV voltage to the Battery voltage and not worry about the current drawn , with a maximum limiting value of the current. It will work in a CV output mode.MPPT is more confusing. From what i could figure out, it will track the PV input and optimize the output , so that the power (VxI) drawn at the input will be at a maximum. Since the output, i.e. the battery has to be charged to a fixed voltage , how would the MPPT controller logic work ? Would the output be in CV or CC mode while charging the battery.

author: globoy
author_rank: Frequent Contributor
timestamp: December 29, 2023, 04:19:15 pm
content: I won't speak for all MPPT topologies but in a pair of small 12V (35W and 50W) MPPT chargers I designed, I put a simple software controlled asynchronous buck converter between the panel and the battery. The controller monitors Vsolar, Isolar, Vbattery and Iload, and controls a PWM output that drives the switching MOSFET in the buck converter (it also monitors battery temperature for compensation purposes). It controls the PWM period in order to keep the panel near its maximum power point (maximum Vsolar x Isolar). A larger PWM value = more power transferred. Battery voltage is monitored for charge control. The battery voltage sets the buck output voltage and only changes as current is delivered to the battery or taken from the battery. You don't need fast buck control because the battery voltage never changes very quickly. In my system I estimate the current going into the battery based on the power into the buck converter adjusted by a buck efficiency lookup table minus the load current. The estimated battery current is also used by the charge algorithm. When the battery (+ load) doesn't require as much current as the panel can put out then the buck converter is allowed to drift to the appropriate power level by the firmware. There are conditions under which the MPPT algorithm can drift away from the actual MPPT point and find another local maxima. To handle this, on occasion the firmware will slew the PWM through the entire range in order to determine the current MPPT point to reset the MPPT algorithm.I suspect larger chargers have plenty of additional complexity but in my case this simple system has worked very well under a lot of conditions. MPPT delivers significant advantages over the older PWM-only charge controllers.

author: Phaedo
author_rank: Contributor
timestamp: December 30, 2023, 05:08:36 am
content: QuoteThis is the part which is confusing to me.Assuming the battery has fallen to 10V in the night, what would the output voltage of the buck converter when it begins to charge the battery again ?Which of these would be correct :A) The buck conv. duty cycle increase till it can push enough current (into the battery) to bring the battery voltage to 13.5V , and maintain this duty cycle ?B) The MPPT algorithm tracks the Solar input V and I till it finds the maximum power point. It then maintains this duty cycle , irrespective of the output voltageand current into the battery. I suppose if the maximum voltage or current into the battery is breached,then this would override the MPPT condition.QuoteThis is to save on the need of current sensing hardware on the battery side ?

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 30, 2023, 09:38:34 am
content: Quote from: Phaedo on December 30, 2023, 05:08:36 amB

author: Peabody
author_rank: Super Contributor
timestamp: December 30, 2023, 03:50:05 pm
content: Not being an EE, this has confused me too. If you only have one switching circuit and one inductor, you can control one side or the other, but not both at the same time. But I guess B has to be the answer. During the CC phase, you don't care about output voltage because the battery is a big current sink and the output voltage is going to be very close to the battery voltage in any case, and all you really need to do is make sure the charge current doesn't get too high. Not sure about the CV stage unless you just ignore the MPP altogether.

author: fourfathom
author_rank: Super Contributor
timestamp: December 30, 2023, 04:21:03 pm
content: A PWM controller does not use any sort of switching regulator. These are generally just a transistor/FET switch, or even a relay, that merely connects/disconnects the solar panel directly to the battery with a duty-cycle set by the battery voltage. There may be current-sensing that adjusts the duty-cycle as well. You can do this direct connection because a solar panel is essentially a voltage-limited current-source -- you can short out a panel with no ill effects. The simplest "PWM" controllers merely disconnect the panel when the battery voltage rises above a setpoint, and connects the panel when the battery voltage drops below this setpoint. The fancier ones attempt to implement some sort of bulk/float charging pattern.The PWM converters do waste available power when the battery voltage is not equal to the panel MPP Voltage, this wasted power being dissipated in the panel.

author: tooki
author_rank: Super Contributor
timestamp: December 30, 2023, 08:08:45 pm
content: Quote from: Phaedo on December 29, 2023, 02:44:12 pmYes, a lithium battery must (as the first stage of charging) be charged to a specific voltage. Emphasis that in this phase, the CC phase, you charge ittoa specific voltage, notata specific voltage. As I understand it, this is how it works: an MPPT charger will determine what input current level produces the highest power output, and then pumps that current into the battery (buck/boosting if necessary to keep it within a useful range). So it’s CC, but may be at a current lower than the battery is capable of charging at. Once the CC phase is over because the termination voltage has been reached, the charger will regulate (buck/boost) the output to that voltage, but limit the current so that the charging power is limited to the MPPT power value.E.g. suppose that at a given amount of sunlight, a panel’s output voltage is 4V with a 100mA load, 3V with 200mA, and 1.5V with 300mA. If you calculate the power, you get 400mW, 600mW, and 450mW, respectively. So to maximize power yield, you’d limit your current draw to 200mA. If your battery is already at 3V, then you have no choice but to boost the charge voltage, so you might boost it to 3.1V at 190mA, then 3.2V at 187mA and so on, until terminating at 143mA at 4.2V. Then you’d keep charging at 4.2V until the current dropped to whatever your termination current is.

author: Phaedo
author_rank: Contributor
timestamp: December 31, 2023, 05:51:44 am
content: Thanks for the replies !Just as an aside : There are a lot of these cheap DC DC modules available Aliexpress, Banggood etc. Is it possible to remove the presets for CV , CC and feed a voltage command from a DAC,PWM etc ? Then these could be used for MPPT purposes , I guess.Just an idea

author: Psi
author_rank: Super Contributor
timestamp: December 31, 2023, 10:14:12 am
content: Quote from: Phaedo on December 31, 2023, 05:51:44 amThere are also MPPT chargers on aliexpress too.Also some switchmode battery charge regulator ICs come with the MPPT feature built in. The LTC4162 comes to mind, there's variant's of that with MPPT.Actually I think they all support MPPT but some variants have it on by default so you don't need a MCU to enable it over SPI.

author: globoy
author_rank: Frequent Contributor
timestamp: December 31, 2023, 05:57:35 pm
content: QuoteThis. I implement a Bulk-Absorption-Float charge cycle. Essentially there are two subsystems running: The MPPT algorithm which, when enabled, tries to optimize power out of the panel and the charge control algorithm which tries to charge the battery. For example with your 10V the charger would enter Bulk charge mode and attempt to push as much current as the buck (running with MPPT) could supply to the battery until the voltage met the [temperature-compensated] Bulk voltage threshold at which point it would switch to Absorption charge mode. In this mode the MPPT algorithm operates slightly differently and will decrease the power transfer if necessary in order to maintain the battery voltage at the Bulk threshold. Then when the charge algorithm sees the charge current decrease to some percent it switches to Float mode. In all cases but Bulk the MPPT algorithm may be limited in order to limit the current going into the battery in order to ensure it doesn't charge past the current charge state voltage threshold (actually there is a maximum solar current limit in all cases to protect the hardware in case the user attached the charger to a large panel).QuoteYes. Current flow to the battery is bi-directional and I didn't want the additional expense of a bi-directional current sensor.