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Well, the answer is that it really depends on the following factors:

• What Voltage your battery system is.

• What output Voltage each solar panel produces.

• What the maximum “Series connection Voltage” is for your solar panels (see manufacturer’s specs).

• How many solar panels in the system (we assume they are all identical specification) and how many MPPT solar charge controllers that you want to use.

• Are there any obstructions on the roof such as air conditioners or a sailboat mast that could cause some shading?

• What are the sunlight conditions where your RV or boat will be most of the time? Are there trees or anything that could cause partial shading?

In most cases with smaller solar panels (200 Watts or less) I will suggest using at least two solar panels and wiring them in Series so that their combined output Voltage will be as high as possible so that in most situations, the MPPT solar charge controller will be able to engage. For example, most 100 to 110 watt solar panels only produce up to about 19.6 Volts under peak conditions. If you have a 12 Volt battery system it probably needs to charge around 14.4 Volts and as most MPPT solar charge controllers need to see at least + 5 Volts higher than the charge Voltage to initiate, a 19.6 Volt solar panel is not going to have enough direct sun on it to get the 19.6 Volts much of the time. If we wire the two solar panels in Series the combined maximum Voltage output will be 39.2 Volts and even under partially cloudy or lower light conditions they will still likely be above 20 Volts so they will actually be able to charge the battery under a much wider range of conditions. Be sure to check the maximum Series connection rating for your solar panels – The SunPower 100 Watt semi-flexible solar panels can only be Series wired up to 48 Volts but others like the Go Power 100 Watt semi-flexible solar panels can be Series wired up to 600 Volts (many more panels than two).

Unfortunately many RV Manufacturers will "cut corners" and/ or use low quality products (and materials in many cases - such as the use of non-tinned copper wiring and wiring of smaller gauges than we'd like to see) to either maximize their profits or else to help keep the final sticker price down. I have found that this is particularly common when it comes to factory equipped/ factory installed solar equipment. In almost all cases the factory uses the cheaper/ non-programmable/ lower efficiency PWM style solar charge controllers which may be OK for use with the factory installed AGM house batteries but we certainly cannot use them when we upgrade to Lithionics lithium-ion batteries - primarily because we cannot custom program the Bulk Charge and Float Charge Voltage settings to those figures needed for use with the Lithionics batteries. So when the RV comes to me with a PWM controller (Pulse Width Modulation) or even a lower quality MPPT solar charge controller (Multi Power Point Tracking) I always have to replace it with a far superior MPPT solar charge controller like those made by Victron Energy. I feel bad for my Clients because essentially they paid for this original low quality product that unfortunately we cannot use. Not only are the Victron MPPTs affordable and easy to custom program but they can capture up to 30% more solar power versus the simpler PWM type solar charge controllers.

In addition, I've found that most RV Manufacturers don't really understand the best/ most optimal way to wire up the actual solar panels. Typically I find that they are always wired in Series - which would not necessarily be a problem if the actual solar panels themselves had a base output Voltage of at least 25 Volts. However, most of the time the RV Manufacturer has installed the smaller 100 Watt to 200 Watt semi-flexible solar panels which may only be able to produce a maximum of 20 Volts DC under peak/ ideal sun conditions. As mentioned, the problem with this is that the MPPT solar charge controllers typically need to see at least 4 Volts or 5 Volts higher coming in from the solar panels themselves than the bulk charge setting that we program in to the MPPT for the MPPT to be able to start up and actually charge the batteries. So for the Lithionics 12 Volt batteries, we always set the MPPT's bulk charge Voltage at 14.4 Volts (sometimes we bump this up to a maximum of 14.59 Volts) so if the MPPT needs 5 Volts higher than 14.4 Volts, this works out to 19.4 Volts needed just to get the MPPT to turn on. If the maximum Voltage that the solar panel can produce is 20 Volts, this means that when there are only slightly less than absolutely ideal sun conditions (middle of Summer, midday sun with no clouds) chances are the solar panels will not produce a high enough Voltage to get the MPPT to turn on and will not perform any bulk charging. So this means that both early and late in the day when the Sun is lower in the sky, if there is a wee bit of shade on part of the panel, if there is a bit of dirt on the solar panels or if there is any cloud in the sky chances are that you will not get much if any solar charging when the solar panels are wired in Parallel.

So when we have a "clean slate" for everything - like the solar, we can plan out and execute the installation in a way that will perform at the absolute peak that its capable of. In the case of these 20 Volt solar panels, we will always recommend that if we have just 2 of them that we will wire them in Series so that in almost all daytime conditions, the combined Voltage of the two solar panels will almost always be above 20 or even 25 Volts - permitting the MPPT to go into the bulk charging mode even early/ late in the day, under cloud or part shading and even when it is raining!

Yes, on a May 2024 solar only upgrade RV project (a Leisure Travel Van/ Sprinter Chassis), the Client had AGM batteries and just two of the Go Power 100 Watt semi-flexible solar panels and a cheap PWM solar charge controller from Go Power as standard equipment. The RV factory had wired these in Parallel and the Client found that very rarely would he ever get any solar charging. After Pinnacle Innovations rewired them in Series, added a Victron Smart MPPT and added the proper DC circuit breakers, the Client found that this made a whole World of difference! In fact, while the Client was here picking up the RV mid-afternoon I showed him on the Victron Bluetooth App the new MPPT and much to his amazement his batteries were actually charging in bulk mode with almost 7 Amps going in. The amazing thing is that the Sun was nowhere to be seen - it was actually fully overcast AND RAINING at the time! Under peak direct sunlight the maximum Amps these two panels can produce at the bulk charge Voltage would be 13.88 Amps - so almost 7 Amps is pretty good while raining. I can guarantee that when the solar panels were wired up the original way that under these conditions there would be zero charging going on...

A week later he followed up with me and thanked me - stating that the solar is now able to fully recharge his two small batteries each day and he is no longer having to charge them everyday with shore power - which is what he was forced to do every day for the past 3 years. His Son (whom also has a very similar RV that I installed an MPPT on in 2021) was so impressed that he decided to call me and have me rewire his Go Power 100 Watt semi-flexible solar panels in Series (as well as Parallel as he has four panels - not two). At the time I initially installed his MPPT I had recommended that I also rewire his solar panels that were factory installed by Leisure Travel Vans strictly in Parallel but at the time he was not convinced it was worthwhile. After witnessing the huge improvements in his Dad's almost identical solar system, he called me up and came in to have me also wire his solar panels in Series. As he has 4 panels, what I do is wire two together in Series, wire the other two in Series and then Parallel connect the 2 sets of 2 wires coming from each set of 2 solar panels. This way we combat the effects of both shading of one side of the RV as well as lower sunlight conditions to offer the maximum capability for solar charging.

Now if your solar panels are capable of producing lets say 25 Volts to 48 Volts or higher (typically larger solar panels up to 500 Watts) and you have several panels and there are likely to be things that cause shading on parts of the roof (a sailboat mast, trees, air conditioner etc.) then chances are that I will recommend either wiring the solar panels in Parallel to one MPPT solar charge controller or if budget is not a concern, ideally wire the solar panels individually to their own MPPT solar charge controller. This way when one panel is shaded the other one can be charging.

Just remember that in order to capitalize the most on solar power we want the solar panel system’s output Voltage to be a minimum of lets say 6 to 10 Volts higher than that of your battery system – otherwise the MPPT solar charge controller will only activate during peak mid-day sunshine. By wiring the solar panels in Series we can get solar charging happening during many more conditions – including early/ late day and when clouds are present. However if shading is likely to be a problem then we may prefer to wire the panels in Parallel or Series-Parallel if you have several solar panels. In many cases, the use of multiple MPPTs can increase the amount of solar harvested – especially when shading or partial shading is present.

You can see a great selection of solar panels, DC Solar Circuit Breakers and Breaker Boxes that we offer on our webpage at: https://www.getlithium.com/Off_Grid_solar_panels_page.html and you can select a great, industry leading MPPT solar charge controller from Victron Energy on our webpage at: https://www.getlithium.com/30a_Victron_Energy_MPPT_solar_charge_controllers.html

If you’d like us to do a no pressure evaluation of your solar system we’d be happy to do so. We will look at everything that connects to the solar and battery system so that we are certain to offer you exactly what is needed. Feel free to visit our website at https://www.getlithium.com where you can find out more including the pricing of the Lithionics systems and you can Contact Us by phone or by email on our Contact Us page at: https://www.getlithium.com/12-Contact-Us.html