Why does my homemade charge controller switch rapidly between charge and dump?

[damosuz]

I have built the following circuit which uses the comparators inside a 555 timer to control the charge of a 12 V battery from a solar panel. The circuit is adjusted (via pots R1 and R2) so that when the battery voltage gets under 12 V, the relay connects the solar panel to the battery so it charges while when the battery voltage gets over 14 V, the relay connects the solar panel to a dummy resistor so it dumps energy.

When I try the circuit with the panel outputing 15 V and the battery at 10 V, the relay swithes very fast between charge and dump. Does anyone know why?

Thank you for your help.

 

[berkeman]

The 555 pinout looks a bit odd to me. Where is pin 5? Have you checked that pinout against the 555 datasheet?

 

[jim hardy]

check battery voltage with solar panel directly attached.
If you have a sulfated battery it'll be greater than your dump setpoint.

http://batteryuniversity.com/learn/article/sulfation_and_how_to_prevent_it

 

[damosuz]

I tried with a different battery and it seems to be working. The battery I was using must have been sulfated!

Thank you.

 

[jim hardy]

Great.

Berkeman's suggestion about pin 5 was right on. 555 wants a capacitor there to smooth its internal reference .

Some capacitance from node R1-R2-R8 to common would help noise immunity.

555 datasheet suggests robust capacitance across supply near the chip for it draws considerable current during switching. The cmos versions are less rambunctious.

Nice job!
There exist "smart" analog battery charger IC's made for this job, but for a one-off hobby application yours looks fine.

http://www.ti.com/lit/an/slua115/slua115.pdf