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 > slightly different solar qx

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wanderingaimlessly

SOBOVA

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Posted: 04/07/21 04:18pm Link  |  Quote  |  Print  |  Notify Moderator

Am looking at solar on a small class A.
I currently have 3 group 27 batteries (rv/marine) for the house. (prior owners had just installed 2 new, and I had one from a trolling motor setup) they will be replaced in about 2 years with 4 GC-2 batteries.
I have run across a 325 watt panel , which is at approximately double the voltage you see on the typical RV panels.
Am I correct that a 325 watt panel at the following specs is approximately 4 to 6 times the charging capability as compared to a normal 100 watt panel from renogy or another small panel?

REC - TwinPeak2 Mono - 325W Solar Panel
Dimension (mm) 1675(L) x 997(W) x 38(D)
Net weight 18.5kg
Module efficiency 19.5%
Rated Maximum Power (Pmax) 325W
Current at Pmax (Imp) 9.56A
Voltage at Pmax (Vmp) 34.0V
Open-Circuit Voltage (Voc) 40.3V
Short-Circuit Current (Isc) 10.15A
Nominal Operating Cell Temp. (Tnoct) 44.6°C
Maximum system voltage 1000V DC
Maximum series fuse rating 25A
Model REC325TP2M

Manufacturer REC
I believe it should easily be enough for my planned usage, or, should I buy 2 and also due to the higher voltages, what should I get for a charge controller?

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Posted: 04/07/21 04:25pm Link  |  Quote  |  Print  |  Notify Moderator

100W vs 325W is 3.25 times the power.

You need a MPPT charger for the 325W panel which converts the panel amps and voltage to the battery amps and voltage. For a discharged battery 325W/14.8V = 22A (maximum) charging minus wiring and controller losses.


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Posted: 04/07/21 04:27pm Link  |  Quote  |  Print  |  Notify Moderator

Doesn’t the old saying “watts is watts” apply? And just under 10 amps max isn’t going to be enough for a four battery 6 volt bank IMO. It is a start however. Don’t understand why panel is rated at 10+ amps. My three 100 watt panels putout 16+ under ideal conditions.


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wanderingaimlessly

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Posted: 04/07/21 04:28pm Link  |  Quote  |  Print  |  Notify Moderator

I understand the 100 vs 325 part, but what effect does the voltage difference have?
Open of 40v vs 20 on typical 100 watt panels?
Is there a doubling there as well?

wanderingaimlessly

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Posted: 04/07/21 04:35pm Link  |  Quote  |  Print  |  Notify Moderator

renogy 100 watt as found here
renogy

versus rec 325
rec

along with the 3.25 times as many watts, what effect does the voltage change have?
My first thought was a doubling, but I'm no arcky sparky whiz.

Would 2 of these 325 watt panels be grossly overcharging a 4 battery setup that primarily runs 2 cpap machines and an absorbtion reefer?

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Posted: 04/07/21 04:42pm Link  |  Quote  |  Print  |  Notify Moderator

Voc is no load and is used to help verify that a panel is working OK.

Vmp is the voltage at maximum power as is Imp the amps.

So 34.0*9.56=325W. That voltage and amps is converted to the lower battery voltage at higher amps for charging.

100W panels are often used with a PWM controller which uses Isc of the panel which will be about 5A+.

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Posted: 04/07/21 04:45pm Link  |  Quote  |  Print  |  Notify Moderator

The 325 will need an MPPT controller to do "24-12"

max amps to the RV 12v system (loads first , battery gets what is left over) with each aimed at high sun at noon

100w with PWM gets Isc--mine get 6.3 amps

The 325w loses 10% from heat loss (note they give the operating temp)
So 325 - 32 = 293w. Now with good wiring to controller, say a 2% loss so now at 287w input to the controller. Controller loss doing 24-12, maybe 5%, so lose another 14w = 273 output watts.

Amps out is output watts/ battery voltage so take three voltages:

273/12.5 = 22 amps
273/13.5 = 20 amps
273/14.0 = 19.5 amps

So pick 22 amps and that is 22/6.3 = 3.5 tines the amps of the 100,

What controller for the one 325? A Tracer 20 amper would do, but IMO go for a 30 amper for margin.

If you put two of those 325s in series, you could get 44 amps or so--pick a 60 amper? A 40 amper would do with a little over-panelling, which is allowed.

Each 325 has 40 Voc, so two in series would be 80, so the Tracer would work with its 100v limit.

44 amps on 3x 80AH =240AHis an 18% charging rate--no sweat--you can go to25-30% if you want.


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wa8yxm

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Posted: 04/07/21 04:48pm Link  |  Quote  |  Print  |  Notify Moderator

The higher voltage. with an MMPT controller, means you can use smaller wires between the panel and the controller. Likely the same wires you'd use for the 100 watt will work.

The MMPT controller converts the higher voltage to more amprage. but Volts * Amps still applies so at most a 325 panel is 3.25 times a 100 panel

And do not expect either panel to max out


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Posted: 04/07/21 04:52pm Link  |  Quote  |  Print  |  Notify Moderator

wanderingaimlessly wrote:

Would 2 of these 325 watt panels be grossly overcharging a 4 battery setup that primarily runs 2 cpap machines and an absorbtion reefer?
You need to due a energy audit for your total usage including the CPAP.

This includes lights, TV, etc. If the refer is on propane then it uses very little power, but on AC with an inverter a lot more.

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Posted: 04/07/21 04:59pm Link  |  Quote  |  Print  |  Notify Moderator

BTW I generally use 66% output of a flat panel for 4+ hours. Some days more some less.

So 20A*66%*4 hours = 52Ah (amp hours) for charging vs whatever your usage is.

You may have some wiggle room on a marginal day with 4xGCs which will have about 440Ah or 220Ah at the 50% SOC (state of charge). ie Depends upon your usage.

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