Solar Charge Controller 6V@2A SLA

Power Tracking 1.2A Solar Panel Powered 3-Stage 6V Lead-Acid Fast/Float Charger.

Project Page:

Electrical Schematic: |500x375

Board Design: |500x455 |500x455

Long story short, I debugged and debugged... then finally got it to work (with some room for improvement).

point of interest: problem with ICM7555 one shot controlled by TLVH431 reference, 15016^0,ICM7555OneShotWithResetProblem.jpg 15016^0,ICM7555OneShot.jpg

I used 30AWG wire for this fly wire rats nest and liked it. 15016^0A(1OF2)_RWRK.jpg 15016^0A(2OF2)_RWRK.jpg

The list of changes that I need to spin into an updated board.^0A,Schematic_RDLN.pdf

The new schematic and board, linked from the top post, is for 15016^1 board. I have also added a switch (not on redline) that will allow attaching the battery in the dark without having it disconnect.

Got rev 1 boards and removed parts from original rev 0 board to place them on the new ^1 board (are my really that cheap, guess so). So far, testing looks good but I never checked the Ideal-Diode circuit, and now I know it is worthless above about 6V due to the emitter-base breakdown voltage, anyway removing Q2 and R5 got me going. This video helped me:

I have updates and some short videos of testing and debugging on circuit board page (see post #1), but now it is time to get a few PV array and batteries.
15016^1A Schematic RDLN.pdf

Rev^1 is power-constrained at sunrise when the battery was disconnected and thus caused supplied power to hiccup every time the MCU started, this is very bad and will cause SCR latch-up of a CMOS device. Also, the relay bounce can cause latch-up.

A few more updates* to Rev^2, but this is working well, now. At first it made sense to snap this in next to my project boards, but I do not want those large enclosures floating around in the yard, so while stressing about getting this solar buck charger to work (the way I wanted), the reality is it is going to be integrated into my controller board. With that said, I am so glad I decided to do this stand alone, it has been a challenge, but seeing it work is awesome.

I will try to sum this up, but there is a lot going on, so see the LT3652 datasheet* also. I did not quite go with the LT datasheet for the battery temperature compensation after finding some SLA data** from PowerStream and Panasonic. The room temperature float voltage is 6.83V, and it takes any DC power source above 14.5V and bellow 32V, but will pull any power it can from a constrained source at 14.3V. That means when the rain clouds show up 20 mA may be the max from PV (34mA after converted down to 6.8V) and the battery will have to make up the difference (these MCU's only work if there is a power glut). The power up sequence I ended up with prevents starting the load (Vout) until the battery has some reserves, although the value (6.5V) used is not really optimized. This allows a clean power up event (at 6.5V) even if the PV is providing a constrained (morning) power flow. And if the sun does not keep up with the load it will disconnect (at 6V). The buck converter would then immediately raise the PWR voltage (unless the sun is blocked) and connect the battery which starts to charge up. This slow connect and disconnect process is acceptable for an MCU or any CMOS device. An MCU can also track Vout and sleep or cause a disconnect at the time of its choice rather than wait for the 6V sensor to decide.

This is the everycircuit simulation* of the power sequence control, I know it is impossible to follow, and that is my complaint. This could really be a useful tool to show how things work, but since I can't add my own box of things that should be in a sub-circuit (TLVH431) and lock the layout paths it is almost, but not quite, worthless. It is still fun to play with. The device parasitics have also been a problem, e.g. the Zener between a BJT base and emitter and the body diode found in discrete MOSFET, which is a problem in all the SPICE simulators on my list (ICAP, LTSPICE, PSPICE).

Update to ^3, so it is the fourth attempt, each was a learning experience. This time I didn't learn anything which was odd, cause I swapped out two TLVH341 and a pile of supporting parts for a TPS3700 window comparator and added temperature tracking for the solar panel. The window comparator voltage thresholds are a little different for this revision, at about 5.8V the battery disconnects to prevent damage to the battery. When the sun is powering the buck converter enough to cause 5.9V at its output the battery will connect, but the MOSFET (Q5 in ^3) that connects really only blocks power from the battery (it has a body diode that will let charge in from the buck converter). Once the battery is charged to about 6.58V power is sent (Q1 in ^3) to the load (and latched in that state). Anyway, the power sequence management voltages have been refined. The temperature tracking of a solar panel is neat, it shifts the power point voltage as a function of a thermistor temperature (attached under the panel).

Update to ^4, which adds more protection (reverse battery). It seems nearly bulletproof now, except if the PV is connected to the load.