Compact PWM PV Charge Controller using 08M2

[Solar Mike]

Here is a basic design for a PWM type PV solar charge controller, using a Picaxe 08M2. Ideal for smaller 30W panels charging a 10AH or so lead acid battery.
I have a number of projects coming up that require small panels hooked up to various size 12 volt gel or Lifepo4 batteries, in stand alone field operations powering pumps, control valves etc. After playing with various designs for small MPPT controllers, MPPT is really over-kill for small setups, so am switching to simple PWM only.,
Sorry about smd components used here, everything I build now uses smd components.

Having many small dual logic level mosfets (30v @80amps 5.2mR) left over from a Lifepo4 BMS project, decided to use them in this design; the PV charger part is 2 back to back devices (single package) driven by a current to voltage isolated mosfet driver, these drivers originally designed for use in SSR's have on\off times approx 1mSec so the PWM frequency must be quite low, quite ok for a battery charger.

The high side load switch has two in parallel to bring the series resistance down, this switch gets turned off should the battery volts get too low.

A novel part of this circuit is the load switch is driven from the C.3 input only pin, by programmably setting the C.3 pull-up on, the output voltage divider feeding the schmitt trigger inverter gate is set high; and low when the pull-up is disabled.

Schematic below:










Cheers
Mike

 

[Solar Mike]

PCB 72x60mm:



Gerber files Attached.

I am waiting for the pcb's to come back, will then test it and write some software, post back here later.

Cheers
Mike

 

[papaof2]

Thank you. I have some reading and circuit following to do this afternoon ;-)

 

[Solar Mike]

PCB's are back, have made one up for testing, looks ok no errors, writing software for it now.

 

[papaof2]

I'd checked in yesterday to see if you had the boards back so my guess on production & shipping seems to have been close.

Like the cat who ate cheese and lay by the mousehole, I'm waiting with bated breath ;-)

 

[Solar Mike]

Finally have some software, attached as zip file. Code is well commented, so I won't elaborate further here.

Have given the code and the board a good test, all works as expected, no changes required to pcb layout.
For the 12v gel battery I was using and the 30watt PV panel, resistors R1 = 20k, R2 = 100k, R15 = 47k, R17 = 2k7.

The software has a calibration mode for setting the Battery and PV voltage sense pots.
Charge and load currents up to 5 amps, no additional heat sinking is required; if currents are higher from a larger panel then place a piece of silpad material under the pcb and bolt to an alloy plate.

Cheers
Mike

 

[Solar Mike]

Here is a picture of a the built up board used for initial testing.

 

[fernando_g]

Beautiful and well executed project.
I really like the fact that you have included the component's value in the silkscreen.
I quickly browsed your code. Will ask questions later if I have any doubts.

PD: it is amazing the capabilities of those SMT Mosfets.

 

[Solar Mike]

I have also created an alternative variant of this low power PWM controller using a PICAXE 14M2, it used shunt PWM regulation and will have lower standby power when not under charge, which maybe more important for smaller batteries; also has extra leds showing charge status.

Look over on The Backshed

Cheers
Mike