24V system - is 3 PV in series okay?

[force9BOAT]

Hello,

My question is a little bit related to the previous post from Chris R.  I currently have two Evergreen Solar 100W panels connected in series to charge a 24V battery bank.  I'm using a Xantrex C40 controller.

I would like to add more PV but wondering if I need to go up in increments of two PV panels, meaning another set of two panels in series added in parallel with the first set of two.  Or can I add one panel for a single set of three in series?  My concern is that three PV in series would push up the voltage too high without producing more current.  

So I guess my question is actually about the Xantrex C40 charge controller.  Will is covert the extra voltage to amps?

Rob

[Flux]

No the C40 won't do what you want. The MPPT controllers will, but they cost more.

With the C40 you will see no significant improvement with another panel in series. There will be a very marginal increase in current from current at peak power point to short circuit value, but to use the C40 you will need 4 panels with 2 parallel circuits.

Flux

[force9BOAT]

Thank you Fux,

That was what I was concerned about.  I'll buy two new panels.

Rob

[cabinfevor]

Flux,

I have a question regarding your reply about the C40 not working well with 3 panels in series. I may be wrong so I want to ask. I was under the inpresion that the C40 was capable of 125 volt input. Knowing that there is less current at higher voltage for the same amount of power I would have thougth that wiring panels in series up to the 125 volt limit would be better than wiring all of the panels parrallel. The higher voltage and lower current would require smaller wire ga from the array to the controller and less voltage drop because the voltage drop is directly related to current draw.

Am I missing something with how the C40 converts its incoming power (volts x amps) back to charge voltage? Am I losing charge current buy wiring 2 or 3 24volt panels to a C40 rather than wiring them in parralell?

I curretnly have two 24 volt 175 watt panels wired in series to a C40. I did this figuring I'd get an earlier start in the mornings by making over the charge voltage sooner and longer into the evening when the panels are not making enough voltage if they were wired parrallel. I generally see as much as 80 volts when I disconnect them from the controller so I was thinking that another in series would work better and still be in the 125 volt limit of the controller.

Do you campain that I'd get beter performance if I wired mine parrallel?

thanks

[ghurd]

"missing something with how the C40 converts its incoming power (volts x amps) back to charge voltage"

The C40 Does not do that, because it is not MPPT.  Amps into a C40 = amps out of a C40.

A MPPT is like the MX60 for example.

You are losing a lot of power. Very nearly half.

G-

[cabinfevor]

G

thanks for the heads up. I've obviously goofed up by wiring these series. I'll change them next time I'm there.