Single solar charge controller Two battery banks Two inverter/charger

solarking · May 2017

Hi guys,

My solar array is 2000 watts. I have two different electricity meters/electric connections on my ground floor and first floor..
I have a 48v mppt charge controller.

I intend to do this so please tell me if I am doing something wrong,

I will be connecting 2 different 48v battery banks to the same charge controller with separate dc breakers for 2 battery banks(mcb will be connected to the output of the charge controller so the output will be split into two theoretically speaking)

Planning to connect 2 48v inverter/charger to each battery banks.
inverter/charger no.1 will be connected to the mains of the electric connection of ground floor and the output goes to supply the ground floor.
inverter/charger no.2 will be connected to the mains of the electric connection of first floor and the output goes to supply the first floor

I would like to know if the above setup is fool proof. Will the above setup works even if I use single battery bank with 2 inverter/chargers.

My another query is , is it possible to use only one inverter/charger for both floors. My electrician told me the neutral on the ground floor is different from the first floor and it cannot be mixed and when used in bypass mode both meters will run and I will be charged twice.

Note: I don't want to combine the battery banks to single battery bank as the second battery bank is quite old.
(Inverter charger will be 2.8kw(3.5kva).

bill von novak · May 2017

solarking said:

I will be connecting 2 different 48v battery banks to the same charge controller with separate dc breakers for 2 battery banks(mcb will be connected to the output of the charge controller so the output will be split into two theoretically speaking)

OK. In that case you are effectively connecting the two banks together. The older bank will die rapidly due to lack of current.

I would like to know if the above setup is fool proof. Will the above setup works even if I use single battery bank with 2 inverter/chargers.

As long as you keep both breakers closed it should work. As mentioned the weaker bank will tend to not last long.

My 
another query is , is it possible to use only one inverter/charger for 
both floors. My electrician told me the neutral on the ground floor is 
different&nbsp; from the first floor and it cannot be mixed and when used in 
bypass mode both meters will run and I will be charged twice. 

You have to make sure the neutrals are common, so you may have to do some significant rewiring. (Of course you have to ensure correct neutral/ground bonding.)

Note: I don't want to combine the battery banks to single battery bank as the second battery bank is quite old.

You can't get around that; that is what you are (effectively) doing by connecting the charge controller to both.

BB. · May 2017

I assume you are in/around Madurai India?

Assuming you are using a TSW/PSW (true/pure sine wave) AC inverter--They generally have transformer isolated AC output--So there is no neutral/issue between the the separate inverters. An AC transfer switch should take care of running your AC circuits.

However, if you are using MSW AC inverters (modified square/sine wave)--The neutral issues can be a big problem. MSW inverters, typically, do not have transformer isolated outputs and they do not like neutral bonding--Especially if your battery bank is ground referenced (negative or positive earth+safety ground). Having a ground bonded neutral on the AC output and a ground bonded/referenced battery bank will (generally) result in a dead short through the MSW AC inverter.

You cannot (effectively/efficiently) charge two battery banks with just one Solar (or AC) battery charger. In theory, you could use diode isolation between the two banks, but the 0.2 to 1.0+ volt drop across diodes makes for problems quickly and efficiently charging the two battery banks. Plus the solar charge controller needs battery power to operate its electronics. A blocking diode will kill power to the solar charge controller.

I would like to backup a moment and figure out your AC loads (Watt*Hour/kWH per day, peak Watts, etc.). Knowing that information, we would then size the battery bank to run the battery bank. And then the solar array to charge the battery bank and support your loads.

Once we better understand your power and needs (emergency backup power, full time off grid, trying to save money on power costs, any backup genet, etc.).

-Bill

solarking · May 2017

Thankyou for your reply bill von novak & BB.

Please check the image to know more about my intended setup.

bill von novak · May 2017

solarking said:

Please check the image to know more about my intended setup.

OK thanks.

If you close both breakers at the same time you are effectively connecting the banks together. That can work, but as mentioned the weaker battery will tend to die sooner. (You may also see some unexpected - and large - currents under some conditions through those breakers.) Or you could flip them back and forth so only one is closed at a time. That can also work, but I would worry about 1) forgetting and letting one bank die or 2) never fully charging either bank.

You have drawn the diagram to make it look as if you have grid power wired to parts of each floor and inverter power wired to other parts of each floor. That can work as long as the systems do not interconnect. If you want to use the same wiring for both inverter and grid power you need a transfer switch - either one inside the inverter or an external one. If you have a transfer switch with the right number of poles you can switch everything over but you _cannot_ provide three phase power (or indeed anything other than a single phase at the inverter voltage.) If that's OK then that might work for you.

As BB mentioned make sure you are not using an MSW inverter that is swinging both neutral and hot.

I don't know where you are (other than not the US since you refer to "ground floor" and "first floor" as different floors) so I can't speak to the details of how/where you will have to connect ground and neutral, or whether neutral will have to be broken by the transfer switch.

solarking · May 2017

Thankyou for your inputs Bill,

I don't understand the logic behind unexpected/large current. Could you please explain.
That was a mistake in the diagram, no the output of the inverter will not be connected to a different side.
Both floors have heavy loads isolated so the inverter will be only used for lighter loads excluding ac,microwave, fridge etc
3 phase doesn't affect the inverter input/output in first floor coz there is a manual transfer rotatory switch for phase selection before the isolated inverter input.
As you mentioned I am using a inverter/charger so there is no requirement for additional transfer switch.

Both the inverters are pure sinewave .but still I could hear a slight humming noise from the inverters.

Is it possible to connect both the inverter/chargers to a same battery bank(if one of the battery bank is closed). Both have different source of AC input and as far as my understanding I think the DC side doesn't matter. Will there be a problem when both the inverter/charger starts charging the battery bank at the same time?

Estragon · May 2017

Multiple charging sources to a single battery bank shouldn't be a problem. During bulk, both chargers (and any available solar) will contribute current to charging. Each source may see slightly different voltages during absorb so some may contribute little or nothing to absorb. In winter, I often use a generator to supplement weak morning solar and turn off the generator at absorb to let solar finish absorb to the extent it can with available light.

jonr · May 2017

> Is it possible to connect both the inverter/chargers to a same battery bank

Generally speaking, connecting two power sources in parallel depends on the design - imagine that supply A produces a .1V higher voltage than supply B. And that B then tries to reduce/regulate this voltage down by acting as a current sink. Ie, they are fighting each other. Fortunately, most chargers don't act as current sinks.

But some do - for example (the only one I can think of), a motorcycle alternator typically has a shunt regulator and if some other charger were to produce a "too high" voltage, it would try to short it out.

Lumisol · May 2017

LOL don't use a motorcycle to charge your battery bank then.

Marc Kurth · May 2017

In my experience with RV/Marine/industrial systems fed with shore power, alternators - plus aux generators and solar working together:
Multiple charge controllers into a single battery bank, do work together. I work with Concorde AGM's so maximum charge rates are essentially nonexistent, but be aware of that possibility when using other batteries.

In general, the source with the highest voltage will cause the others to back off. Typically, it does not slow the charge rate appreciably because the system is in Absorb by the that time anyway. I have seen a charging system throw and error code when it thinks it should be in float, but the voltage is at the absorb level.

I have not personally seen a system that sinks in order to bring the voltage down when another other source is "outrunning" it. Note that I am certainly not saying it can't ever happen. I am only saying that I have not ever seen it in my narrow world, but this is what I do for a living.

Marc

BB. · May 2017

For flooded cell lead acid batteries... There are changes in efficincy as state of charge, temperature, etc. happen.

For example, below ~80% state of charge, a flooded cell lead acid battery is near 100% efficient (within reasonable charging limits). And as the battery is >~90% state of charge, more energy is converted into Hydrogen+Oxygen gasses and heat.

For than 80% state of charge, a flooded cell can probably take 20%+ more rate of charge (based on 20 Hour ampacity of battery bank). However, a 100% State of Charge Lead Acid battery with a 2.5% rate of charge can overheat and fail.

Somebody once said that in the Electric Vehicle world there are folks that hit FLA batteries with very high current when they are very low state of charge (I don't remember the numbers).

Note that AGM and GEL batteries are also Lead Acid Chemistry... And they do not like elevated charging voltages (and presumably high charging currents) then they are near full charge either. AGM and GEL batteries do not withstand gassing very well at all (venting, lost of electrolyte, Many GEL batteries will form permanent gas pockets in the Gelled Electrolyte and ruin their capacity too).

-Bill

solarking · May 2017

BB. said:

For flooded cell lead acid batteries... There are changes in efficincy as state of charge, temperature, etc. happen.

For example, below ~80% state of charge, a flooded cell lead acid battery is near 100% efficient (within reasonable charging limits). And as the battery is >~90% state of charge, more energy is converted into Hydrogen+Oxygen gasses and heat.

For than 80% state of charge, a flooded cell can probably take 20%+ more rate of charge (based on 20 Hour ampacity of battery bank). However, a 100% State of Charge Lead Acid battery with a 2.5% rate of charge can overheat and fail.

Somebody once said that in the Electric Vehicle world there are folks that hit FLA batteries with very high current when they are very low state of charge (I don't remember the numbers).

Note that AGM and GEL batteries are also Lead Acid Chemistry... And they do not like elevated charging voltages (and presumably high charging currents) then they are near full charge either. AGM and GEL batteries do not withstand gassing very well at all (venting, lost of electrolyte, Many GEL batteries will form permanent gas pockets in the Gelled Electrolyte and ruin their capacity too).

-Bill

Charging a battery bank with 2 solar charger controller and 2 inverter/chargers isn't the same? If a battery bank can be charged with 2 solar chargers then why can't we use 2 inverter/chargers for same battery bank?

BB. · May 2017

You can use multiple chargers just fine. The total charging current should probably not exceed 25% (20 Hour ampacity). And >13% rate of charge, it would be great to have a remote battery temperature sensor (many solar charge controllers will accept an RBTS... Many AC battery chargers will not).

Not setting the charging voltage over the recommendation of the battery mfg. is important... That is how a lead acid battery (any type) "throttles" the charging current.

If you hit a Lead Acid battery with too much charging current (and/or too high of charging voltage), they will heat up. And the hotter the battery bank, the lower the charging voltage that you should be hitting the battery bank with. If you do not have an RBTS, the bank voltage will fall (~ -5 millivolts per degree C per Cell) and you can achieve thermal runaway. (more charging current, hotter battery bank, lower charging voltage, higher charging current from charging source).

If you have a 20% charging current Solar Charge Controller and a 20% AC Battery charger (on genset)--Just do not run them at the same time, unless you are monitoring the battery bank (charging voltage, battery temperature, how much bubbling in electrolyte, specific gravity, etc.).

I am not saying you will end up with an explosion and fire--Just that there is an increased risk of something going wrong. I always try to state people off with conservative/reasonable recommendations.

-Bill

solarking · May 2017

Let me explain my current setup and what I am going to add additionally,

I have connected the solar array to a 48v mppt charge controller which is connected in series to 4 x 12v 150ah battery bank. The mppt has a power saving accessory which I am using it to automatically turn off/on the inverter/charger. When solar power is detected this addon device turns off(usually this happens around 6 -6.30AM) the inverter/charger and this device turns on the inverter/charger when battery bank reaches 47.2V(this happens around 5.30pm). Battery bank gets charged by the Inverter/charger and stops within an hour(@12A charging current) and will be on trickle charging.

To my above setup I am going to add an additional inverter/charger(input power provided to this inverter/charger is from a separate grid). I will be using a programmable timer to cut off the grid supply to the inverter/charger. I will be using the programmable timer since I cannot use the addon device for the second inverter which takes care of the automatic turning off/on of the first inverter/charger.

I think would be facing the following issue in my above setup,

Since the second inverter/charger depends on the programmable timer it may turn off/on at different interval than the first inverter, if this happens then the second inverter/charger starts charging the battery bank before the first inverter/charger starts charging the bank.
I am not sure what will happen in the above case.

mcgivor · May 2017

In theroy depending on the state of charge of the battery the second charger will either charge or go into float and should have no effect on the first, when the first one "wakes up " it should see the battery voltage and contribute as required, assuming there is no conflicts between the two chargers.
Just a theroy.......no gaurentee

Estragon · May 2017

We've had a run of cloudy days and extra occupants, so I'm currently charging the 48v bank with two inverter/chargers off a generator, plus the two solar charge controllers, so no problem with multiple sources. I will turn off the generator with voltage hits absorb.

Part of your description isn't making sense to me though. Unless there are significant loads running, 47.2 is on the low side, and depending on battery type would likely take more than an hour to charge. Also, ending at 12a seems high for a 150ah bank to end charging.

solarking · May 2017

Estragon said:

We've had a run of cloudy days and extra occupants, so I'm currently charging the 48v bank with two inverter/chargers off a generator, plus the two solar charge controllers, so no problem with multiple sources. I will turn off the generator with voltage hits absorb.

Part of your description isn't making sense to me though. Unless there are significant loads running, 47.2 is on the low side, and depending on battery type would likely take more than an hour to charge. Also, ending at 12a seems high for a 150ah bank to end charging.

Yes, 47.2 is on the low side. Just now realised it, I will have to check my battery voltage before the grid supply is switched on automatically which usually happens around at 5.30 PM. The inverter/chargers are built in with smart charging algorithm so it adjusts the amps as the battery reaches the absorption state 12a is the bulk charge amps. I am using Tall Tubular batteries which are comparable to trojan's life and ruggedness though trojan is not tubular.

bill von novak · May 2017

solarking said:

Thankyou for your inputs Bill,I don't understand the logic behind unexpected/large current. Could you please explain.

Sure. Let's use an example:

You have your two systems, call them A and B. You have a 10 amp charging source so you figure "I'll use a 20 amp breaker, that should be plenty."

System A's inverter sees a very large load; call it 100 amps. It initially draws 100 amps from its own battery. The battery voltage drops. It then starts drawing power from battery B since battery B is at a higher voltage now. This continues until you are drawing 70 amps from A and 30 amps from B. The breaker trips. This is puzzling to you - "but I am only charging at 10 amps!" But since there are no diodes or switches to prevent this, it keeps happening.

Is it possible to connect both the inverter/chargers to a same battery bank(if one of the battery bank is closed). Both have different source of AC input and as far as my understanding I think the DC side doesn't matter. Will there be a problem when both the inverter/charger starts charging the battery bank at the same time?

Yes, as long as they are isolated (most are.) Should be no problem. The one with the higher setpoint voltage will do most of the work.

solarking · May 2017

bill von novak said:

solarking said:

Thankyou for your inputs Bill,I don't understand the logic behind unexpected/large current. Could you please explain.

Sure. Let's use an example:

You have your two systems, call them A and B. You have a 10 amp charging source so you figure "I'll use a 20 amp breaker, that should be plenty."

System A's inverter sees a very large load; call it 100 amps. It initially draws 100 amps from its own battery. The battery voltage drops. It then starts drawing power from battery B since battery B is at a higher voltage now. This continues until you are drawing 70 amps from A and 30 amps from B. The breaker trips. This is puzzling to you - "but I am only charging at 10 amps!" But since there are no diodes or switches to prevent this, it keeps happening.

Is it possible to connect both the inverter/chargers to a same battery bank(if one of the battery bank is closed). Both have different source of AC input and as far as my understanding I think the DC side doesn't matter. Will there be a problem when both the inverter/charger starts charging the battery bank at the same time?

Yes, as long as they are isolated (most are.) Should be no problem. The one with the higher setpoint voltage will do most of the work.

Now it is clear thanks Bill.

What happens when you connect a mppt and a pwm charge controller to the same array but charging 2 different battery banks.

mcgivor · May 2017

An MPPT controller needs a higher voltage, roughly double battery voltage, to operate correctly, a PWM needs a voltage slightly higher than battery voltage, so the MPPT would not fiunction correctly, if the voltage were high enough for the MPPT, the PWM would run inefficiently wasting excess as heat and damaging the unit.

solarking · May 2017

my mppt max pv input is 100v so I have connected 2 x 24v 72 cell panels in series and 4 such strings are connected in parallel.
Typical 48v PWM controller input voltage max will be 90v , array will be producing lesser than 80v even during peak sun.
When connected to load and charging is on voltage will drop below 65v so the pwm charge controller will be quite efficient. By doing this can't I charge 2 different battery banks efficiently compared to connecting 2 battery banks to a single MPPT charge controller?

mcgivor · May 2017

If the rating of the controllers are about equal in terms of current capacity, it would probably be better to split the array rather than sharing. Two MPPT controllers cannot share a single array due to tracking conflicts, not exactly sure how the MPPT would deal with a PWM controller connected, but it likely would cause some issues with the MPPT more so than the PWM.
Good question perhaps someone has a more definitive answer but my gut feeling is there would be issues.

littleharbor2 · May 2017

You can't connect two controllers to the same array. Why would you want to do this? If because of too much panel for one CC Just split the array.

mcgivor · May 2017

After thinking for a while There would be a problem when the PWM actually begins modulating, during bulk charging it would basically be an open switch, so not changing much in terms of current/voltage of the array, but when it begins to modulate, the switching on and off would confuse the MPPT 's ability to track. So splitting the array would be the best choice, I take it this was a hypothetical question.

bill von novak · May 2017

solarking said:

Now it is clear thanks Bill.

What happens when you connect a mppt and a pwm charge controller to the same array but charging 2 different battery banks.

It will drive the MPPT charge controller nuts. The PWM charge controller will be in one of two states: open (no connection) and closed (connected to the battery.) When it is open the MPPT controller will work normally. When it is closed the MPPT controller will see the V/I curve of the battery, not the solar panel - so its choice of operating point will be all wrong. And if the PWM controller is opening and closing its switch rapidly, the MPPT will never be operating correctly, since generally they do not track very fast.

solarking · May 2017

bill von novak said:

solarking said:

Now it is clear thanks Bill.

What happens when you connect a mppt and a pwm charge controller to the same array but charging 2 different battery banks.

It will drive the MPPT charge controller nuts. The PWM charge controller will be in one of two states: open (no connection) and closed (connected to the battery.) When it is open the MPPT controller will work normally. When it is closed the MPPT controller will see the V/I curve of the battery, not the solar panel - so its choice of operating point will be all wrong. And if the PWM controller is opening and closing its switch rapidly, the MPPT will never be operating correctly, since generally they do not track very fast.

Thanks Bill

Single solar charge controller Two battery banks Two inverter/charger

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