Series solar panels voltage too high?

[Flippervan]

Hi all, joined just now to seek help on an issue with my campervan’s solar setup.

We have 4 100W panels wired together in series, into our PMW controller and into our leisure batteries. They’ve only been installed for a couple of weeks but I have concerns they’re working to their best capacity.

Each panel is specced at working voltage: 18V, and working current: 5.56A.

With 4 panels wired in series, theoretically voltage from the panels could be up to 72v.

Our charge controller is a cheap PMW unit, with a max solar input voltage of <55v, and a max voltage battery end of <34v. It has a rated current of 30A.

I realise the charge controller is rated to a lower voltage than our solar panels, but as I assumed we’d never get full output from the panels we’d be okay with that. Did I make a stupid mistake here?

Thanks!

[SunEagle]

Hello Flippervan and welcome to Solar Panel Talk

I would not say it was stupid but possible a bad decision.

First off a PWM CC is less efficient then a MPPT type. So it makes sense to get as many input amps as possible. With your panels wired in series you will only get about 5.5amps that can charge your batteries. That turns your 400watts in about 70 watts (5.5amps x 12.5v = 69amps)

Also as you noted the input voltage is higher then what the CC can handle. That can destroy the CC.

My suggestion is to wire the 4 panels in parallel. That will get you a little over 20amps for charging. While that still reduces your system panel wattage by about 33% you are getting all you can out of it. Of course that requires some type of combiner box with fuses for each panels.

Another suggestion would be to replace that PWM with an MPPT type CC. That will get you almost the full panel wattage for charging.

Rule of thumb.

PWM type CC is Amps in = Amps out
MPPT type CC is Watts in = Watts out.

By the way what is the amp hour rating of your batteries? You need to get about 1/10th that rating in amps for proper charging.

[Flippervan]

Thanks SunEagle,

That's interesting. I knew PWM CC are less efficient but I thought they would input both amps and voltage the same.

Looks like we have to buy a combiner box and go with a parallel setup then. I think that would be the more affordable option than going for a MPPT CC.

Our two batteries are 130AH each, so what's that, 260AH / 10, we'd need at least 26amps from our solar panel setup? Or would the 20amps you mentioned be sufficient?

Thanks in advance!

[SunEagle]

Thanks SunEagle,

That's interesting. I knew PWM CC are less efficient but I thought they would input both amps and voltage the same.

Looks like we have to buy a combiner box and go with a parallel setup then. I think that would be the more affordable option than going for a MPPT CC.

Our two batteries are 130AH each, so what's that, 260AH / 10, we'd need at least 26amps from our solar panel setup? Or would the 20amps you mentioned be sufficient?

Thanks in advance!

FLA type batteries should get a charge rate between C/8 and C/12 where C = the amphour rating. So with a 260Ah and 20amp charge that comes to a C/13 which may be light but then you may get more than 20 amps depending on the panel orientation. They are rated 5.56amp each and theoretically that should total to 22.24amps but you will get less than that.

Again with a PWM type CC the amps you put into the CC is what you get out. Because the panel voltage is higher then the 12V battery you end up losing a part of the panel wattage.

So in most cases a panel that is rated 100watts will only provide about 66 watts ( 5.5a x 12v = 66w). That is a 34% reduction in wattage.

[bcroe]

You are lucky if the charge controller has not already blown up. The panel voltage will
reach its peak at least some of the time, causing destruction. Putting 4 panels in series
with that controller will not give you any more output than one or two panels. The best fix
is the MPPT controller with sufficient voltage capability, but 4 panels in parallel will come
close to the same performance. good luck, Bruce Roe

[Mike90250]

There are 2 reasons for the minimun amps.

1) having enough power to recharge the battery in 1 sunny day

2) being able to gas or bubble the batteries while charging, to stir the electrolyte up. Not so critical in a camper that moves, but required for batteries fixed in place in a battery bank.

Depending on your usage, if you are able to fully recharge every other day at least, you have enough solar. But if you cannot recharge the batteries fully in 48 hours, they start to sulfate and irreversibly loose capacity. If you crank up the engine and move every day (and you have a battery isolator connected to your engine alternator) a 2 hour drive will go a long ways to recharging .

https://en.wikipedia.org/wiki/Battery_isolator

You can get cheap ones or expensive ones http://www.balmar.net/

[Flippervan]

Thanks again all, have decided on an MPPT controller to fully maximise our panels input.

Collecting this today and fitting to avoid any potential damage to our current PWM controller. Luckily weather hasn't been great on the West coast anyway!

https://clearcutconversions.co.uk/pr...e-solar-panel/

And thanks for explaining that Mike90250, we do have a split charge relay so we're getting power in when we drive. They'll likely be days/weeks of little driving over the summer though so I think it would be best to not rely on that anyway.

Guess it's a case of buy cheap buy twice, glad I managed to spot the issue before going any further though!

[Asterix]

Thanks again all, have decided on an MPPT controller to fully maximise our panels input.

Collecting this today and fitting to avoid any potential damage to our current PWM controller. Luckily weather hasn't been great on the West coast anyway!

https://clearcutconversions.co.uk/pr...e-solar-panel/

And thanks for explaining that Mike90250, we do have a split charge relay so we're getting power in when we drive. They'll likely be days/weeks of little driving over the summer though so I think it would be best to not rely on that anyway.

Guess it's a case of buy cheap buy twice, glad I managed to spot the issue before going any further though!

I suggest maximising by putting 2 in series and putting the other 2 in parallel with them.

Here is the reasoning behind the suggestion. A Solar Charge Controller will not charge a battery when the panel Voltage drops below the minimum charge Voltage(circa 13.5V). With 2 panels in series even in lower light conditions, there is ample Voltage for the CC to work. Having the 2 other panels in parallel with them gives you a charging current of around 11A in bright light and a lower figure on those days and times when light is sparse. Ample current to give a decent charge.

An MPPT CC is of course a must have, just on efficiency grounds alone, but even that will cut out, when the Voltage is too low.

Maintaining a charging period for the longest time possible is of course paramount.

[Mike90250]

The rule of thumb for panels is
Series panels need to be close to the same AMPS. the lowest rated amp panel will limit all the series panels to that amps
if you have two 7 amps panels in series with a 5 amp panel, you can only get 5 amps. and the voltage will add up

Solar panels in Parallel, need to be close to the same Voltage, the lowest voltage panel will limit the DC voltage to it's own voltage.
If you have three 18V panels and two 38V panels, in parallel, you will only get 18V but all the amps will add up. ( and you have to insure the wiring can handle the amps)

[Asterix]

The rule of thumb for panels is
Series panels need to be close to the same AMPS. the lowest rated amp panel will limit all the series panels to that amps
if you have two 7 amps panels in series with a 5 amp panel, you can only get 5 amps. and the voltage will add up

Solar panels in Parallel, need to be close to the same Voltage, the lowest voltage panel will limit the DC voltage to it's own voltage.
If you have three 18V panels and two 38V panels, in parallel, you will only get 18V but all the amps will add up. ( and you have to insure the wiring can handle the amps)

Flippervan first post.

We have 4 100W panels wired together in series, into our PMW controller and into our leisure batteries. They’ve only been installed for a couple of weeks but I have concerns they’re working to their best capacity.

Each panel is specced at working voltage: 18V, and working current: 5.56A.

So all 18V panels, all with a current of 5.6A

36V @ 11.2A when put in series and parallel.