I have a BQ24650 circuit that is exhibiting some odd behavior with regard to the MPPSET voltage divider.
Originally I had a board populated with the circuit shown in the attached PDF. Note the DNP (Do Not Populate) designations were followed except for D26, R58, D25, R69 which were populated and R78, and R79 which were not populated (I wanted the LEDs during testing).
This circuit was working OK. I was monitoring input voltage & current from a 5V panel and output voltage and current to my 3.3V (3.6V charge voltage) LiFePO4 battery. However, I would get a lot of times throughout the day when the charging would stop (current flow went to 0). I figured this may be due to the slight drops in voltage when the sun would go behind clouds. I figured since the chips turn on voltage is 5V, the chip may be turning off. I figured I would try a 6V panel. Note that I had run the 5V configuration successfully for several days.
I put a 6V panel in and things seemed to be a little better, but since my MPPSET level was still 5V I was losing some efficiency. Therefore, I attempted to change the voltage divider values to set the MPPT to 6V. To do this I used the following equation.
MPPT = 1.2(1+R67/R64) --> Leaving R67 alone I calculated a standard resistor value of 95.3K for R64.
I replaced this resistor and put the system outside to test for a day. When I checked the data I found that there had been no charging all day.
I noticed that when I connected a supply to the input or my battery to the output the STAT2 LED would turn on indicating charge complete. This was not the case previously during my successful testing.
After spending a little time trying to fix things, I figured I must have accidentally shorted something and blown the circuit. I resolved myself to populating another circuit (Very time consuming). This time I made two so I could have both a 5V MPPT and 6V MPPT board.
Upon completion of the circuit I tested both. The 5V MPPT board originally worked perfectly fine when I tested it with my bench supply (Gave a 3.6V output and STAT1 LED would turn on if the battery was connected). The 6V MPPT board however, did not work (Gave only 1.8V originally at the output then 2.3V after some cleaning and reconnecting it. Also the output seems to vary based on the input. I was using about 6V input originally and got those values. Then I varied the input thinking that maybe 6V wasn't enough now that the MPPT was 6V, so I increased it to about 10V and noticed the output climb. However, it didn't get to 3.6V as it should).
I figured I must have done something wrong with this board when soldering it, so I decided to switch the working 5V MPPT board to be a 6V MPPT since I had already done testing at 5V MPPT and needed to do the 6V MPPT tests still. Therefore, I replaced the 121K R64 with a 95.3K. This resistor is relatively isolated from other components and I am very confident that I didn't alter any other parts of the circuity when I did the replacement. However, when I then connected this board to my supply the device failed (Originally the output was 3.6V for a second or two then started to rise. I disconnected power in hopes of not damaging anything, but it looks like I was too late. I then connected a battery and noticed that the STAT2 LED was on like it would be in the initial board).
I tried switching back to a 121K resistor at R64, but the circuit still would not work.
In summary I have now had 3 boards which I configured for 6V MPPT fail. Two of these boards were known to be working with a 5V MPPT (R64=121K) prior to switching to a 6V MPPT (R64=95.3K).
How is this possible? Doing some calculations, I would have needed an input voltage over 35V with that voltage divider (R64=95.3K) in order to go past the absolute maximum 7V on the MPPSET pin.
