Hello,
I’m designing a battery charger for a CubeSat using the MP26124. I’m using the same circuit as the typical application in the schematic. The only difference is that my charge current is set to 0.5A.
It works great in normal operating conditions, i.e. when the input power is greater than is consumed by the battery charger.
However, if the input to the battery charger cannot supply the power required for VBAT @ 0.5A, the input voltage drops, the battery charger turns off, and turns back on when the input voltage is above the threshold. This cycle repeats indefinitely while not enough input power is supplied.
The input voltage is generated with a step-up converter which generates 20V from solar cells - which I am currently simulating with a bench power supply.
My question is if it would be possible to reduce the charge current in constant current mode, when the input power is less than the desired charge power.
I have thought of two ways of doing this:
- tune the voltage or current control cycles of the battery charger to reduce the charge current in constant current mode, so that the input voltage is maintained close to the dropout voltage of the charger.
- I could monitor the input voltage to the charger and toggle the ~EN signal if it falls below some threshold - could be considered a crude form of “PWM”
Any other suggestions for how to achieve a variable charge current would be very welcome, especially if they don’t require adding new components to the circuit 
Thank you,
José
The power converter can’t put out more power than the solar panels are giving you. It it tries to the solar panel voltage will collapse. What you want is a charger with a MPPT ( maximum power point tracking) system. This adds an additional control loop that seeks to maximize the power extracted from the solar cell.
A traditional charge has basically two limits Battery Current and Battery Voltage
A MPPT charger has three limits battery Current, battery voltage and solar panel power.
Hi jshannon, thank you for your answer. Of course I don’t want to put out more power than the solar cells are generating; I want to put in the same power that is being generated (minus conversion losses).
In other words, I would like to dynamically adapt the charge current flowing into the battery based on the amount of power that is available.
Indeed, this is basically a form of MPPT; which controls the voltage and current drawn from the solar panels by changing the load impedance. But to achieve that, I need to be able to “tell” the charger” to use a lower current limit in CC mode.
Sure, one simple way this is done is to add a control loop that backs off charging current if the panel voltage falls below a set point. If you look at solar panel VI curves you see there is a relatively constant voltage that is the point of peak power. There are chargers in the market with this function built in. Alternatively if you have a micro available you can roll your own. Measure Vpanel and V battery and the charge current then set the duty cycle of a PWM and increment or decrement a required. In conditions where the panel has sufficient power the PWM will be controlled by charge current or charge voltage in conditions where the panel can’t satisfy the charge loop then the PWM would be controlled by finding the PWM setting that results in maximum charge current. That duty cycle would be roughly Vbattery/Vmpp at an initial guess. Vmpp moves around with temperature.
Looking at the data sheet, hey why not. I would pull down on either the I comp or the Vcomp pin in the event that the panel voltage goes lower than the Vmpp. You are going to need a reference, and op amp and a diode so that the op amp only pulls down.