Help - compatibilty different solar panels
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I have a small home 12 v system I installed several years ago with 4 Solarex SX 50 panels. I'd like to about double the system. The Solarex SX 50 is no longer available. How can I tell if another panel is compatible with my existing panels? Thanks, Chuck
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If you are going to add panels in parallel, they should have, approximately, the same Vmp voltage. If you are going to add them in series, they should have the same Imp current.
If you are going to add the new panels in series/parallel with the first set, you should probably check the old set with a DVM and make sure that they are all still good (Voc voltage and Isc current, and, if you can, Imp when connected under load to the battery bank).
The Solarex data sheet (PDF).
If you will have to add another solar charger (first one is at maximum power--just connect the second controller, in parallel, to the battery bus in parallel with the first), then it does not matter what panels you use as long as they meet the needs of the new charge controller.
If one doesn't have an MPPT charge controller, is there still a reason to have Vmp about the same for parallel panels?
Vmp does not need to match when paralleling panels for a PWM controller--As long as the panel voltage is, roughly, the battery bank voltage + 2 volts controller drop + any wiring/fuse drops--you are fine.
If you are using lead acid batteries, typically 15-15.5 volts for equalizing a flooded cell lead acid battery means a 17.5 volt panel or greater...
You can parallel any panels that are at least ~17.5 volts and all will work fine with a PWM controller (assuming the controller's voltage and current limits are respected).
If a panel is less than 17.5 volt (say 15 volt)--it will still work, but only during the bulk charging of the battery. Once the battery voltage rises above ~13 volts (in this example) it will stop supplying current to charge the battery.
i have to point out here that typical pv vmps have reached 17.5v only in recent years with higher outputs coming from the modules through better technology. many pvs were around 17v or slightly higher and work just fine with pwm controllers so there is no rule of equal to or greater than 17.5v for feeding pwm controllers. many batteries also do not require 15.5v to charge as many are below 15v so even the 2v rule you're flaunting(which isn't written in stone as mandatory either as it could be as low in some cases as 1.5v) will be good for pv vmps of below 17v. maybe when adding blocking diodes you may wish to require 17.5v or greater vmp.
for the record bill i have used 2 us64s in parallel with blocking diodes through a sunsaver 10 into 3 paralleled wally world deep cycle batteries in the past and it worked just fine. i still have those diodes in place, although i should remove them as i don't have to worry about battery backfeed at night with an sb50.