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Old 04-14-2019, 04:20 PM   #1
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Connecting solar panels to two power banks.

Having a crazy idea and wondering if anyone done it.
Had a renogy pwm 30A solar charge controller and was wondering if I could connect them to my existing solar system to charge a power bank. I draw what I'm thinking about and wondering if it's doable. I know there are MPPT charge controller which you can connect to two power banks but they are very pricey. Attached is the skate of what I'm trying to accomplish.
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Old 04-14-2019, 08:27 PM   #2
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I use solar systems rather extensively for my ham radio systems. I operate systems that use a Pulse Width Matching (PWM) controller completely independent of my systems that use Maximum Power Point Tracking controllers.
The reason being that PWM controllers have to be matched to the voltage of both the solar panel system and your battery. There will be no available excess for the MPPT system, or you run the risk of catastrophic failure of one or both of the controllers, and overcharging of the PWM balanced battery if extra solar capability is added.
MPPT controllers continuously adjust their input to bring in the maximum power possible from your solar array, and can also vary their output power to match the attached battery. This means that MPPT charge controllers are more efficient than PWM controllers, and more effectively utilize the full power of your solar panels to charge a battery system. They are used on systems with solar panels that have charging capacity in excess of the battery capacity, so they are much more efficient in less than optimal charging conditions.

What will happen in your proposed system is the voltage and current will flow to the balanced PWM system, and excess current gets sunk in the battery because it is close to the total solar capacity in the first place. Or in more straightforward terms the battery on the PWM system functions as a wet capacitor and normalizes total solar output.
As a consequence the MPPT unit will allow that battery to discharge, as full load will be absorbed by the PWM side. Use of an solar panel in excess of the PWM rating associated with the battery will result in failure of the controller and possible fire hazard.
In larger solar panel systems designed to power something like an entire home, panel and battery voltage aren’t typically the same. As a result, PWM controllers are more suited for small DIY solar systems with a couple of low voltage panels and a small battery, and the MPPT controllers are much more flexible and can handle excess solar capacity efficiently.
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Old 04-15-2019, 01:09 PM   #3
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Quote:
Originally Posted by Oregonpharme View Post
I use solar systems rather extensively for my ham radio systems. I operate systems that use a Pulse Width Matching (PWM) controller completely independent of my systems that use Maximum Power Point Tracking controllers.
The reason being that PWM controllers have to be matched to the voltage of both the solar panel system and your battery. There will be no available excess for the MPPT system, or you run the risk of catastrophic failure of one or both of the controllers, and overcharging of the PWM balanced battery if extra solar capability is added.
MPPT controllers continuously adjust their input to bring in the maximum power possible from your solar array, and can also vary their output power to match the attached battery. This means that MPPT charge controllers are more efficient than PWM controllers, and more effectively utilize the full power of your solar panels to charge a battery system. They are used on systems with solar panels that have charging capacity in excess of the battery capacity, so they are much more efficient in less than optimal charging conditions.

What will happen in your proposed system is the voltage and current will flow to the balanced PWM system, and excess current gets sunk in the battery because it is close to the total solar capacity in the first place. Or in more straightforward terms the battery on the PWM system functions as a wet capacitor and normalizes total solar output.
As a consequence the MPPT unit will allow that battery to discharge, as full load will be absorbed by the PWM side. Use of an solar panel in excess of the PWM rating associated with the battery will result in failure of the controller and possible fire hazard.
In larger solar panel systems designed to power something like an entire home, panel and battery voltage arenít typically the same. As a result, PWM controllers are more suited for small DIY solar systems with a couple of low voltage panels and a small battery, and the MPPT controllers are much more flexible and can handle excess solar capacity efficiently.
You sure make total sense. Had a feeling it will be an issue when the circuit is open between the 2 charge controllers. May be adding a DPDT toggle switch so can switch between two system.
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Old 04-18-2019, 05:36 PM   #4
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If you are trying to keep both the house and chassis batteries charged, it is only own, but check out the Morningstar Sunsaver duo. It will keep both charged and you can choose if it is 50/50 or 90/10. Love mine, just wish it was MPPT.

Their support is great too.
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Old 04-20-2019, 02:07 PM   #5
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Originally Posted by robburns76 View Post
If you are trying to keep both the house and chassis batteries charged, it is only own, but check out the Morningstar Sunsaver duo. It will keep both charged and you can choose if it is 50/50 or 90/10. Love mine, just wish it was MPPT.

Their support is great too.
I have a 40 amp mppt with my power bank just wanted to use the 30 amp wmp one I have to charge second power bank. I think I can do it just need the battery isolate swith before charge controllers this way when one is full I can switch to other power bank.
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Old 04-21-2019, 10:12 AM   #6
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I am not sure I understand your thinking regarding the battery isolation switch. If you continue to power your MPPT controller without a corresponding load on the MPPT unit, you will damage it. Also, some solar cells can be damaged if operated in full sun without a charge controller and corresponding load to regulate output. Which it appears you have taken into account by routing the power to the other battery bank.


If it were me, and I did not want to spring for the appropriate MPPT controller, I would use a Perko 8501DP battery selector and switch the MPPT output from bank A to bank B. Forget the PWM controller altogether. I used this approach on my pontoon boat to charge the trolling motor batteries, then switched to charge the house batteries from the same solar panels and a single MPPT controller. It worked great in that application.
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Old 04-21-2019, 03:49 PM   #7
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I am not sure I understand your thinking regarding the battery isolation switch. If you continue to power your MPPT controller without a corresponding load on the MPPT unit, you will damage it. Also, some solar cells can be damaged if operated in full sun without a charge controller and corresponding load to regulate output. Which it appears you have taken into account by routing the power to the other battery bank.


If it were me, and I did not want to spring for the appropriate MPPT controller, I would use a Perko 8501DP battery selector and switch the MPPT output from bank A to bank B. Forget the PWM controller altogether. I used this approach on my pontoon boat to charge the trolling motor batteries, then switched to charge the house batteries from the same solar panels and a single MPPT controller. It worked great in that application.
I did some reading and seems like not a good idea to do it. Already my system setup works perfect. Guess I'll keep the other controller as back up. Have a Epever 4210 40 amp mppt which working great.
Thanks for feedback. Really appreciate it.
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Old 04-23-2019, 01:20 AM   #8
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Thanks for the lesson. Since you guys sound really knowledgeable about solar systems on an RV, I was hoping you can address a few questions that have concerned me for awhile:

I have 4 Deep Cycle 6V Golf Cart batteries. They are 220AH each... wired in series to achieve a 12V System... and in parallel to get ~440AH (under 20A load) of battery storage.

I have 4-100W Renogy solar panels on the roof and a Renogy "Tracer" MPPT Solar controller that is 4 years old.

I only use my RV 3-4 months every summer, but during a phone call with Renogy tech support one day... they implied my MPPT Solar Controller could be "getting old."

Do MPPT Solar Controlers "get old"?

Is there a life expectancy for these things that most people would agree on?

Are there a safety concerns with MPPT or PWM Solar Controller when they older than 3 years?

Note: I also use an "Isolater" or "VSR" between my house battery bank and my engine battery bank. This primarily so the engine alternator charges the house batteries when I motor down the highway, but I suppose it works just as well when the RV is in storage and the Solar panel "fill-up" the house batteries and trickle charges the engine batteries too. At least I think this is the case when the Solar Controller puts out more than 13.6V. Is this right?
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Old 04-23-2019, 02:35 AM   #9
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I think more information about your MH and your actual setup would be needed before making any Un-educated guesses.
I say this because my MH charges both the chassis and house batteries from the alternator when driving and from shore power and from the solar panel.This is what Winnebago did when they decided to fix the problem of the Chassis battery going dead while on shore power for long times.
So as I stated more data about your actual system would help.
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Old 04-23-2019, 09:02 AM   #10
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Or make life better and easier for yourself and your batteries. If these are both for house use, combine the two banks into one large "battery" and there will be no need for an involved set up. This large battery will charge and perform better that two separate banks, simplify your system and ultimately make your batteries last longer. I have been doing this on marine applications for decades. Chuck
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Old 04-24-2019, 11:15 AM   #11
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The marine scenario doesn't work for rv. Chassis battery for rv is different type battery than deep cycle house batteries .
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Old 04-24-2019, 01:50 PM   #12
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The marine scenario doesn't work for rv. Chassis battery for rv is different type battery than deep cycle house batteries .

Sorry but this is just not correct, A battery is a battery, rv or marine. And many are using 6 volt golf cart batteries for house banks, identical to what we use often for marine applications. Been doing this for about 30 years. The only difference in marine applications is that the panels are exposed to more moisture and possibly salt air. You are correct that a chassis or "start" battery is probably different than a house bank, but please reread the original post for the proper context for my replies. Chuck
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