Adding a new higher powered solar panel in parallel?

[steve yates]

I currently have a 50w panel on my wee boat, and now it has a sprayhood Ihave space to add a 100w panel.
Q1, i can remember the specs of the 50w panel but iirc the current was in the region of 3.5a, the new panel has a rating of 5.69a by connecting them in parallel what are the realworld downsides with the different amp ratings and possibly v ratings too. ( no idea what the 50w was originally)
ie is it enough to not bother and just replace the 50 with the new 100? Or os it still worth the gain?

Q2. Just to add to the pot, the 50w is on the companionway hatch, whichis now under the sparayhood.
It still gets plenty light at the moment while sunis lower in the sky but I would imagine it is very often shaded in the summer when the sun is higher. Again, will this drag everything down enough that I would be better just ditching it?

 

[Stemar]

Am I right in thinking that a panel on the sprayhood would be partially shaded by the boom much of the time? If so, you'd get more power out of two 50w panels - one in full sun, giving its all and the shaded one not giving very much, than one partly shaded 100w job. That was the advice I got when fitting panels to the hatch garage on my previous boat, but it was a good few years ago, so it might be out of date.

 

[Sea Change]

As I understand it, when you place panels in parallel the one with the lowest voltage can drag down a higher voltage one. Ideally you want them to have the same Vmp for no losses. But unless the figures are hugely different you'll probably be better off connecting up the 50w than not.
I'ma bit hazy on this but that's what I recollect anyway.

 

[noelex]

If you have an MPPT controller, if the voltage (Vmp) of the two panels is similar (within half a volt), the results will be OK with parallel connection. If the panels will see different conditions, as you indicated, two controllers would be significantly better, but this is not essential.

If you have a PWM controller, providing they are both nominally "12V" panels, a single controller will still deliver optimum performance.

In either case check the controller or controllers have adequate current capacity.

 

[andsarkit]

The panels will have blocking diodes otherwise they would discharge the battery at night so one panel cannot drag the other one down. The highest voltage output one will provide all the current until the current draw drops voltage to the level of the second panel which will then contribute. I don't think matching panels is very important as one panel will probably be partially shaded and so the outputs will be different anyway.
Two MPPT controllers would be best but probably OTT for a small system. I don't know your system but if you have two batteries and a VSR or Cyrix you could put one panel on each battery with a cheap PWM controller on the small panel.

For some years I had a 40w panel feeding two 75Ah batteries without a controller and the batteries never got above 13.7V and were always fully charged for the weekend.(typical UK summer weather and a swinging mooring so variable orientation)

 

[noelex]

The panels will have blocking diodes otherwise they would discharge the battery at night so one panel cannot drag the other one down.

Blocking diodes are very rarely installed by solar panel manufacturers. The diodes seen are bypass diodes.

The discharge at night is blocked by the solar controller (nearly all solar controllers have this feature), but most solar panels will only have a very slight discharge at night even without a controller.