Calculating panel load for pwm controller

Would I be pushing my luck to have four (4x) 100 watt panels in parallel on a the Victron BlueSolar PWM Pro Charge Controller 12/24V-30A.

This calculates out to 33.3 amps.

But I’m under the impression that the voltage both open circuit and under load is higher than 12. And that the panels rarely product 100 watts each.

If I adjust the collective panel output to 90% I get 360 watts. And if I adjust the voltage to 13 volts, the calculation changes to 27.7 amps.

I don’t know how to adjust for a cold day. We can get down to 10-20 F occassionally.

I’d rather not smoke the little controller and can be quite happy with the three 100w panels I have but wouldn’t mind getting the most out of it.



Photon Sorcerer
PWM controller has just a switch between battery and PV panel. Its input will either be close to battery voltage when switch is on or near Voc when switch is off. Controller electronics may pull a little bit of current from panel when switch is off so Voc may drop a little. It also has a method to prevent panel leakage shunt resistance from discharging battery during the nightime. This can be done using two back to back MOSFET's, a relay, or just a blocking diode to prevent the reverse leakage.

Worse case voltage drop across PWM controller would likely be with a blocking diode drop. Usually only used on cheap low current PWM controller.

Maximum current through PWM controller will be Isc of panels. This assumes panel Voc is greater than about 1.2 times battery voltage where panel will operate in the near constant illumination current range. In this range the current is near Isc of panels where Isc is the given illumination conditions current generated by panels. Isc of panel is not effected much by temperature.
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Solar Addict
This calculates out to 33.3 amps.
Not so with a PWM controller. This type of controller is a simple switch that connects the panels to the battery. Thus the battery only 'sees' the panel current, A typical 100 watt panel will have a maximum current of just over 5 amps.
So with 4 panels, 20 amps is all you can expect at best.
With a MPPT comptroller all the panel power is converted ( less losses) to battery power.

Although the panel OC voltage will vary with temperature this is not normally a consideration with PWM controllers

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This is one of those days I wish I were smarter (chuckle).

To test my understanding:

-The voltage change with low temp will not be an issue since the PWM is rated 12-24v.

-The amps per 100 watt panel in real life is a bit over 5. To be safe, 5.25. Therefore, I could safely put five typical 100w panels (5.7 would be max)

Thanks for the help!


Photon Sorcerer
Pretty much.

Just check the Isc rating on panel and multiply x4 panels. That will be the max current the PWM controller could see.

Panel rated wattage is P_mp.

A '12v' panel has 30 to 36 series connected cells with about 0.5 Vmp per cell at 25 degs C so panel V_mp at 25 degs C will be between 30 x 0.5v = 15v and 36 x 0.5v = 18v.

If 100 watt panel, 100w/18 Vmp = 5.55A I_mp to 100w/15 Vmp = 6.67A I_mp.

I_sc is about 1.05 times I_mp so your Isc should be between 5.8 amps and 7 amps depending on number of cells in panel. The highest current you can be is 4 x 7 amps = 28 amps maximum. More likely 4 x 5.8 amps = 23 amps max. Either way you are less than 30 amp rating of controller.


Solar Addict
To avoid getting too technical look at the back of the panel. Unless you have sourced an 'odd' panel it will have 36 cells. It will look like this,
solar panel specification.jpg
At a a loading allowing 19 volts the current would be 5.3 amps, at a load loading allowing 0 volts, 5.7 amps. Your 12v battery under charge may have a voltage of 14 volts so the current will be a little over 5.3 amps.
This is the maximum current under test conditions, in practice it will be less, depending on your location, panel angle, weather conditions and the time of day.

5 panels will generate, when wired in parallel, a maximum of 26 to 27 amps under ideal conditions with a PWM controller.



Solar Addict
Does having battle born (two) affect these voltage calculation?
However a PWM controller is not the ideal solar controller for lithium batteries.
Its probable the ease of used programming and the interface of the Victron Smart MPPT solar controllers would complement expensive lithium batteries. There is some concern that the PWM effect may cause accelerated aging.
Note also that with a MPPT controller you get up to 30% more energy into the batteries from a given set of solar panels.
is this the controller you have?
victron PWM controler.jpg

are the batteries to be used in parallel for a 12v system?

I will go have a look at the panels. I don’t have the controller yet but I did order the Victron BlueSolar PWM Pro Charge Controller 12/24V-30A from Battle Born (with their blessing). Sorry-I don’t have a pic.

I can cancel the order and go with the MPPT is there is an issue with taking care of the batteries.

I was trying to save a few bucks. But battery health is priority. The batteries arrive Friday.

Thanks all.
I think it’s ok internet etiquite to provide this. Email response from BB customer support:

Yes, that controller will work for your system. This does not offer the Bluetooth communication like some of the other Victron controllers, but it will work as a reliable charger for lithium batteries. You can wire the panels in parallel and still have a little room for growth if needed.


Solar Addict
Something missing from the replies so far and the possible reason BB oks the use of this CC.
The current through the CC is the load current when less than panel Isc and you have full control over this.

Seems BB has determined the current to their batteries is under the limit for that PWM.


Solar Addict
Victron BlueSolar PWM Pro Charge Controller 12/24V-30A.

I am surprised that this controller is recommended by Battleborn. As a stand alone the settings are not optimum for their batteries ( related to their optimum settings), and the Victron unit needs the remote panel or a computer link for user settings.
Battleborn specify the charge current of the 100Ah battery at 0.5C max, 100 amps for the application with two in parallel, so its possible with the proposed setup, 5 X 100 watt panels , to have a higher charge current.

The thoughts that using a PWM system is affecting battery life is not backed by any research I can find, however some MPPT controllers, especially the Victron Smart range, offer easy user setup of charge parameters and having the status of charge conditions accessible via the app is useful in battery management. This may allow longer service life from the batteries.

The added advantage of MPPT controllers is being able to operate with panels in series or high voltage panels and to deliver all the panel power to the batteries.

I’ve reached out to BB customer support to reconfirm this controller. I was thinking cheap and easy was the way to go. But upgrading my choice to the SmartSolar MPPT 100/30 might be a better choice.

And if I understand correctly, some overpaneling would not be a concern with the MPPT controller.


Solar Addict
can cancel the order and go with the MPPT is there is an issue with taking care of the batteries.

I was trying to save a few bucks. But battery health is priority. The batteries arrive Friday.
Setting aside optimal high-amp charging an mppt can perform, the issue you need to be aware of is the ‘equalize’ function of a pwm that usually is defaulted to charging lead acid batteries.
It seems doubtful that battleborne would not know what their batteries need.
On the website, BB indicates that they can ship their controllers configured for BB batteries. And I made this request in the order notes. I’ll keep this thread posted on the response.
Thanks Mike. Would the math be the same for MPPT And PWM as below?

5 panels will generate, when wired in parallel, a maximum of 26 to 27 amps under ideal conditions with a PWM controller.


Solar Addict
Not the same,
With PWM the current maximum is the sum of panel maximum current, 5 x say 5.3 =26.5
With MPPT the process within the controller takes in maximum panel power and converts to a voltage for the battery.
Assuming the charge voltage determined by the battery is 14 volts, ( it will vary a little depending on the state of charge of the battery) the maximum current will be power/voltage, (5 x 100)/ 14 = 35.7 amps. A MPPT charger can only deliver current at its rated maximum, so with a 100/30 you would be limited to 30 amps output.

Four panels would give 21 amps with a PWM and give 28 amps with a MPPT.

Note that these are maximum assuming ideal conditions, in practice the current will be lower.
The MPPT allowing over panelling, will over the day, give a greater yield. You may loose a little around noon but will have a greater yield early and late in the day.

A factor not discussed is that a MPPT controller needs (battery volts plus 5) to start in the morning. There may be a slight advantage is connecting panels in series within the maximum voltage limit of the controller. This sets a limit of four panels, (22 volts x 4 = 88 volts) in series for the 100/30.
Panels in parallel would be limited to 5 due to the input limit of 30 amps.

Thanks for the detailed response Mike. I’ll work on digesting all this.

On the +5 volts to start the system, if I go parallel, would there be an advantage to one east angled panel to kick off the system? My flat roof is 5 degrees west facing.