# charge controller question: disconnect pv with mppt from full battery

I'm creating a battery charging model with pv and a buck converter controlled via a P&O mppt algorithm. My charge controller currently disconnects the pv panel from the battery as the latter reaches a certain voltage level. As a result I get massive current oscillations on the input side of the buck and voltage oscillation on the output side. Also, when the panel is reconnected, there is a high current/voltage peak on the output side.

What the charge controller does is admittedly quite rudimentary: it just disconnects and connects the pv panel without altering the mppt algorithm. There are no dummy loads nor diodes preventing negative currents.

Now, I would like to ask what the charge controller should optimally do when the battery reaches its maximum charge. Should the mppt be switched off and the duty ratio of the buck be regulated to zero before disconnecting the panel? Should it be disconnected in the first place? Should I use a dummy load that connects to the buck when the battery is disconnected?

In order to mitigate the oscillations, I set the buck duty ratio D to zero when I disconnect the panel but the oscillations remain.

Clearly I'm just missing some basic knowledge on how a charge controller operates. I tried searching the forum and elsewhere on-line but didn't find what I was looking for. I'd greatly appreciate any insights.

Thanks!

• The question probably does NOT have a general answer - it is probably specific to your implementtion so you need to post much more detail including a circuit diagram and the IC(s) used etc. | Also, you describe what happens BUT is this a real-world design or a simulation. Somulations can behave in bizarre ways which do not reflect real world results - especially so under extreme boundary conditions - which is just what you have. ... – Russell McMahon Apr 13 '18 at 22:34
• ... | You say you get "massive current oscillations" on the input to the buck converter when duty cycle is set to 0%. How can this happen - the buck converter cannot draw "massive current" if effectively off OR if it does it's not REALLY off - where does the current go? ie Tell us enough to let us really understand your actual problem. – Russell McMahon Apr 13 '18 at 22:34