# adding a new panel

yeah yeah yeah I know this has been hashed over again and again but im still struggling with it. I understand the two basic rules,series-amps,parallel-volts.sooo if I have two strings of four 75 watt siemens 17.0 volt panels in series and paralleled at the combiner and I want to introduce a shell 110 watt 17.5 volt panel do I just add the new panel in parallel with the combined strings???
the eight 75 watt panels are rated at 17 volts and 4.4 amps and the 110 panel at 17.5 volts and 6.3 amps. so because the voltage is close I can parallel this new panel??? I feel like im speaking some foreign language I dont understand. I hope you get my drift! thanks mark

• Solar Expert Posts: 1,164 ✭✭
Re: adding a new panel

Parallel.

For series panels must be close to the same amps, for parallel have to be close to the same voltage.

That also means that if you have 4 of the 75 watt panels in series, then you must also have 4 of the 110 watt in series.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: adding a new panel

mark,
if you had them all paralleled i'd say go ahead and parallel the shell with them. you said something of series/parallel already with the siemens pvs making the pv system as a whole a 24v operation. unless you have another 110w shell pv to be seriesed (or one with a similar current rating as your present 110w shell pv) with the present shell pv that you have then you can't add this to your present 24v pv system. slight mismatches in voltages for paralleling are ok, but not large mismatches and adding a 12v pv to a 24v pv system does not work out.
• Posts: 6,290 admin
Re: adding a new panel

Yeah, sounds like you have 2 parallel strings of 2 panels in series, which would present ~ 34 volts to the controller. Thus as Niel states, you'd have to have a second Shell in series with the new one you just got, which would give ~ 35 volts, which would work OK if paralleled with what you already have.
Again, if none were in series (all parallel) you could just add that one panel to the existing group.
By the way, just to clarify after reading your first post, it's the amps that add, but not the volts when panels are paralleled, but the voltage (not amps) is added when panels are in series.
Cheers
Wayne
• Posts: 6,290 admin
Re: adding a new panel

thank you all for the input. what I didnt mention was although the eight siemens 75 watt panels are mounted on the roof (4 on each rack) but havent been wired yet. the plan that was reccomended here was to series each rack of four (48 volts) and parallel the strings at the combiner and then to the mx60. what happenend was I ended up with more batteries than I had planned (980 amphour agm's) so I needed another couple hundred watts of solar panels. I am looking for another 100 watt panel but for the time being should I just go with the original plan (2 48volt strings paralleled at the combiner) and hold off on the 110 volt panel until I can find another one???
like I said nothing has been wired yet so im really starting from scratch.
will I be able to keep my battery bank temporarily charged with my 600 watt (8 75watt panels).??? thanks guys mark
• Super Moderators, Administrators Posts: 33,439 admin
Re: adding a new panel

Mark,

You have a nice MPPT controller their (MX 60)--and a 48 volt battery bank... So, your mix of panels will be more complex.

First, the rated Vmp voltage for each string must be greater than 62 volts DC...

So, you cannot add a single 110 Watt @ 17.5 volt (not 110 volt--as you just miss-typed) to the string in parallel.

You could add it in series with the 75 watt panels and increase the voltage of the string (as long as you are below the controller's maximum voltage). However, if you add it in series, then it will be about 75 Watts worth of power (because of the less current supported by the other 75 Satt panels).

And, if you have multiple parallel strings--each of the strings need to have their Vmp voltages match (you cannot have one string with 68 Vmp, another string with 120 Vmp, and/or another string with 17.5 Vmp)... Either it will not give you optimum power output (wasted power), or it will not give any power at all (17.5 volt panel in a system that needs ~60 volts to operate correctly).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 6,290 admin
Re: adding a new panel

Oh rats. so if I understand correctly the only way this panel will work for me is if I had another one and seriesed each into a string giving me two 60volt strings paralleled at the combiner,although the 110 watt panels will operate as a 75watt panel.yes??
• Solar Expert Posts: 1,989 ✭✭✭
Re: adding a new panel

Based on 4.00 watt solar panels, your best bet might be a second controller

The MorningStar Sunsaver Mppt street price is \$190-200 could take care of the 110 watt panel ... might be your lowest cost and loss way to make use of the extra panel.
• Super Moderators, Administrators Posts: 33,439 admin
Re: adding a new panel

Mark,

Think of series / paralleling batteries together.

If they are all the same batteries (say 1.5 volt "D cell" @ 1 amp) then putting 10 batteries in series would give you 15 volts @ 1 amp.

Take two series strings as above, and add them together in parallel, you would have 15 volts @ 2 amps.

Now, say you have 1x 6 volt car battery and you want to add it into the two strings above... Well, you could take out 4 D cells and replace those with the 6 volt car battery... But overall, the one car battery will still be limited to 1 amp (instead of 100 amps)--because the other batteries in series cannot pass more than 1 amp--so the 6 volt 100 amp battery is wasted. And, you had to take out 4 D cells anyway--and you have no way to connect them that is useful with the 6 volt car battery (other than spreading them out by add 2 to each string--making the overall voltage 15+1.5+1.5=17volts--you can do this with an MPPT controller--but the 99 amps of the car battery is still wasted--you could have just used more "D cells" instead--cheaper and just as much overall power available).

And let me back up a little bit--What is your battery bank voltage? Is it 12 volt or 48 volt? I kind of made an assumption that you had a 48 volt battery bank because you were going for a 48 volt array--but with a MPPT charge controller, that is not a requirement (Array voltage only has to be higher than the battery bank voltage to charge properly--there is nothing wrong with a, for example, 96 volt solar array with a 12 volt battery bank--will work fine and efficiently).

That matters... Because the only way you can use a single 110 watt panel would be to parallel all of the panels together to power your 12 volt battery bank

Anything else (24 or 48 volt battery bank) would require some other combination that probably involves tossing some panels or buying more 110 watt panels (2x 110 Watt panels for a 24 volt bank or 4x 110 Watt panels for a 48 volt bank).

-Bill

PS: The new MorningStar controller mentioned by Solar Guppy can charge a 12 or 24 volt battery bank.. So, depending on your bank's voltage--it may be a great solution, or not work at all.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 6,290 admin
Re: adding a new panel

WOW thank you for excellent tutorial. what I have is a 12 volt battery bank. the only reason is because I already had a couple big ticket 12 volt items(inverter and a few other 12 volt loads). seems nothing is wired yet im not opposed to paralleling all the panels. this would mean I could add any panel as long as the voltage was the same.correct? would there be any drawbacks here other than having to use monster size cable? which is a pretty big drawback I guess. thanks mark
• Super Moderators, Administrators Posts: 33,439 admin
Re: adding a new panel

Mark,

As long as you are within the input voltage and ratings of the MX 60 controller, use series fuses to each panel, to protect each panel against internal shorts (if a panel gets shorted, the other 8-9 panels will feed current to the one shorted panel and could cause a fire). And use heavy enough cable to feed the current to the controller--you should be fine.

And as long as the panels are close in voltage rating--they will all work just fine together in parallel (0.5 volt difference should not mater much at all).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 1,989 ✭✭✭
Re: adding a new panel

Your better off with two 48V strings ( no combiner required for two strings parralleled ) and a second controller. A combiner for 9 panels cost the same if not more than the MorningStar Mppt unit and you can get by with 1/4 the wire size to boot, again, the added wire for an all parrallel configuration costs more than the Second Mppt unit

For you needs to add the 200 buck mppt unit is the lowest cost solution, but as BB has expalined there are many ways to resolve this configuration issue.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: adding a new panel

sorry for the misreading of how you were configuring your system. i agree with sg for you to keep your original plans at a 48v pv system. seeing as how you are downconverting to 12v for the 12v battery bank this makes the 2nd controller to handle the odd shell pv very appealing as it is already setup to deliver 12v and just needs a controller to regulate it. if you find that another mppt is too cost prohibitive then any quality pwm controller will do too.
get a battery temperature sensor too.