Panel to Battery Voltage Calculations...

ywhic

So my panels that I am looking at are 130w 17.XX volts VMP and 7.39 Amp IMP maximum normal (not overload ratings).

I take 14% off for the efficiency numbers the mfg says..

comes to 14.62 vdc per panel and 6.36 amps..

I take off the 2% per panel for the wire to the combiner.. 14.32 vdc and 6.23 amps

I take off another 1% for the panel to pass thru my combiner box with 2' of 8AWG wire.. 14.18 vdc and 6.167 amps..

I take off another 1.53% for the panel to pass thru the 6AWG (& 5') of wire that goes from the solar controller to the battery bank.. 13.96 vdc and 6.07 amps..

If I multiply 13.96 * 6.07 amps I get 84.7372 watts at the battery (with 100% sun, panel angle, and such).. which equals to about 65% efficiency..

(Glad I am getting at least 6 of these.. LoL)

Does these numbers sound about right for each panel??

Cariboocoot

Re: Panel to Battery Voltage Calculations...

Nope.
You're cheating yourself out of a lot of power.
Panels are current sources and will try to maintain peak current while letting the Voltage do what it will. Wire resistance takes away Voltage, not current.
So what will happen is that in full sun the panels will try to provide full current at whatever Voltage the battery is at. If the battery doesn't "need" the current it won't "take" it (internal resistance increases as charging progresses). By the time you get to the 14+ Volts of Absorb stage it is highly unlikely the panels will need to supply full current at all (load demands notwithstanding).
As such, your maximum power potential here is more like 7.39 Amps * 12 Volts or 88 Watts or 68%. Plus it could be higher if the batteries demand full current still just at the Absorb set point: 14.4 Volts * 7.39 Amps or 106 Watts or 81%.

Remember that panels will try to output their maximum current from zero Volts to Vmp. The power loss is all in the Voltage drop below Vmp, and the lack of draw for full current. If you want to figure it the hard way, do the Voltage drop calculations every step of the way and see how many Volts you lose. Chances are it won't be as much as what the battery will pull the Voltage down to while charging.

ywhic

Re: Panel to Battery Voltage Calculations...

Follow-up ??

If I did a 24v panel setup (35-37v).. and using the Morningstar TS-45 PWM controller and a 24V battery bank.. will the lead coming from the charge controller be less than 32VDC going to that 24v battery bank.. I'm thinking yes.. but unsure..

The controller is set for Absorb at 14.6*2 = 29.2 and Equalize would be 15.3*2 = 30.6

Cariboocoot

Re: Panel to Battery Voltage Calculations...

ywhic wrote: »

Follow-up ??

If I did a 24v panel setup (35-37v).. and using the Morningstar TS-45 PWM controller and a 24V battery bank.. will the lead coming from the charge controller be less than 32VDC going to that 24v battery bank.. I'm thinking yes.. but unsure..

The controller is set for Absorb at 14.6*2 = 29.2 and Equalize would be 15.3*2 = 30.6

Again: PWM controller, so the battery will pull the panel Voltage down to its level. You will see this on both sides of the controller during Bulk. When charging is at the Absorb stage you can see charging Voltage on the output side (around 28.8 Volts normally) and higher Voltage on the panel side because the controller is switching the connection between panels and batteries on and off to maintain the Absorb set point. This switching happens rapidly so meters will not detect the momentary drop in Voltage of the panels when the controller is "on". Something like the phenomenon of persistence of vision.

This is why "true" 24 Volt panels have the Vmp in the 35 range: so that after losses to heat and wiring there's still enough to push the batteries up to the Equalize Voltage.

BB.

Re: Panel to Battery Voltage Calculations...

Technically, Voc is around 20-22 VDC for a "12 volt" solar array at STC... And certainly can go higher in very cold weather. So you are looking at >45 VDC Voc-cold for a "24 volt" solar array (35-37 volt Vmp-array-stc).

When you have a PWM charge controller, it is basically an "on/off" switch... So you will see Vmp-array-temperature when the PWM switch is "off" or "open". And you will see Vbatt+wiring+controller voltage drop when the switch is "on/closed".

Or, roughly 29 volts + couple of volts drop = ~31 volts when charging (assuming you are charging at 14.5 volts equivalent to a 12 volt bank).

-Bill

ywhic

Re: Panel to Battery Voltage Calculations...

BB. wrote: »

Technically, Voc is around 20-22 VDC for a "12 volt" solar array at STC... And certainly can go higher in very cold weather. So you are looking at >45 VDC Voc-cold for a "24 volt" solar array (35-37 volt Vmp-array-stc).

When you have a PWM charge controller, it is basically an "on/off" switch... So you will see Vmp-array-temperature when the PWM switch is "off" or "open". And you will see Vbatt+wiring+controller voltage drop when the switch is "on/closed".

Or, roughly 29 volts + couple of volts drop = ~31 volts when charging (assuming you are charging at 14.5 volts equivalent to a 12 volt bank).

-Bill

I'm just asking as I am using a 32VDC max 60AMP breaker on the way down to the battery bank from the controller and wanted to know if you folks think it will hande the switch if I decide to sway back to a 24v system down the line, from the current setup of going with a 12v system.. don't want to have to buy another breaker if I don't need to..

Cariboocoot

Re: Panel to Battery Voltage Calculations...

May I give an entirely non-committed response and say "probably"?

The 32 Volt rating on the fuse is the maximum Voltage it is designed to interrupt. On the output side of the controller it should only ever see this Voltage level during Equalization. Likewise the fuse is meant to blow at 60+ Amps, and high current would usually only be seen at lower Voltages. From that aspect "it should work". From an NEC aspect it's wrong.

But what's more important is that if you up the system Voltage you'll halve the current for the same Watts. So is 60 Amps then the right value for the 24 Volt system? If you're doubling the Wattage output, yes. If not, no.

BB.

Re: Panel to Battery Voltage Calculations...

In theory, the battery bank is the source of high current in the event of a short circuit--So a 32 volt circuit breaker on a 24 volt battery bank should be fine. And if there was "high current" to the 24 volt battery bank, you would have 32 volts - 24 volts or 8 volts across the breaker--So, that would be OK too.

HOWEVER--I am not you, and I do not know your application.

I am a big believer in risk management. If your wiring is well supported, in conduit, mounted on non flammable surfaces--That is different than nailing wire to plywood/2x4 studs on an unfinished wall.

So, a layered approach (non-flammable metal or sheet rock or cement board mounting surface) and a good quality fuse/breaker operated within specifications gives you several levels of "things that have to go wrong" before you have a "fire".

Also, if you have a wiring "wall"--When designing equipment, we had to have a metal tray or small mesh metal screen which could catch dripping flaming plastic before it hit the floor and started a fire (plywood floor, carpet, boxes on ground, etc.).

I have seen even "fire rated" plastics/insulation burn very nicely.

-Bill

Cariboocoot

Re: Panel to Battery Voltage Calculations...

BB. wrote: »

Also, if you have a wiring "wall"--When designing equipment, we had to have a metal tray or small mesh metal screen which could catch dripping flaming plastic before it hit the floor and started a fire (plywood floor, carpet, boxes on ground, etc.).

I have seen even "fire rated" plastics/insulation burn very nicely.

-Bill

Ooh! There's a lovely thought to keep you awake at night, eh?