Solar help - combining mono and amorphous on single mppt

[maej]

I have a 30w ebay special mono panel mounted on my pushpit and connected to my domestic bank of 2 * 110aH bateries via a Tracer 10a mppt, also from ebay. System is 12v.

I also have a 15w amorphous panel that is not currently installed.

Can I simply connect the wires from the amorphous panel to the MPPT controller input terminals along with the mono panel, effectively adding it in parallel? Or is this stupid / pointless / dangerous for any reason?

the combined wattage is well below the MPPTs max rated current, and with one panel good in direct sunlight, and one in overcast conditions I'm hoping it'll give me a reasonable trickle charge every day.

 

[noelex]

You can connect the two panels up in parallel. You wont do any damage, but because the Vmp is likely to different the efficiency will suffer a little.
An alternative is to connect the 15w panel directly to the battery via a fuse. There is a slight risk of overcharging the battery. This is very unlikely in the UK, but it would be better to disconnect the 15w panel if you were leaving the boat with no load over the summer period, if you do connect things this way.
The later approach will give you a bit more power out of the panels.

 

[maej]

...but because the Vmp is likely to different the efficiency will suffer a little...

Thank you.

Yes, the mono says 18v, the amorphous is 17.5v so there is a difference.

Would fitting the amorphous panel through a cheap pwm charge controller work? I worry that with my scatty mind I'll forget to disconnect the panel during a heatwave.

 

[noelex]

Thank you.

Yes, the mono says 18v, the amorphous is 17.5v so there is a difference.

Would fitting the amorphous panel through a cheap pwm charge controller work? I worry that with my scatty mind I'll forget to disconnect the panel during a heatwave.

0.5v is normally just acceptable, but with panels of different type and in a different location there will be more of a difference.
Yes you could fit a cheap PWM regulator, but the cheap ones are often not very good (and its not worth spending a lot for a regulator that is only serving a small cheap panel) which leaves the efficiency close to using your existing regulator and probably not worth the cost and trouble.

This is a guess at the sort of difference I would expect with the different options.

Direct connection of the 15w panel is the most efficient.
15 w panel connected to its own cheap regulator -5 % on the total solar output.
15w panel connected to your existing MPPT regulator -10% on the total solar output

Keep in mind these numbers are a guess, but they quantify the sort of difference I am thinking about. If you want a foolproof easy system (nothing wrong with that) I would connect the 15w panel to your existing regulator and live with the loss.

 

[maej]

Thank you noelex for the great advice.

I think what I will do is connect it directly to the battery for winter, then as the days draw out in spring I'll connect it through the mppt, then direct to the battery again as the days draw in again in Autumn.

Thanks

 

[noelex]

Thank you noelex for the great advice.

I think what I will do is connect it directly to the battery for winter, then as the days draw out in spring I'll connect it through the mppt, then direct to the battery again as the days draw in again in Autumn.

Thanks

Sounds like a great solution if you can connect it both ways easily. It will have the side benefit that if the MPPT controller fails (and the cheaper MPPT controlers do have a high failure rate) the self discharge of the battery will still be replaced by the 15w panel over winter.