Charge Controller 48 Volt PV to 12V Battery Advice Needed

System · February 2010

A classroom I help with normally sets up two Mitsubishi 150's panels in parallel, plugged into a Sunsei 12/24V CC25000 charge controller. That connects to a 12 volt typical U.S. lawn tractor battery to demonstrate charging. Sometimes hooking up a small 400Watt inverter to the battery with a small load on the inverter (FM radio) for the students.

Another class has a pair of old Isofoton PV modules IS-150/24 (VoC 43.2 volts, Isc 4.7 amps, Imax 4.35 amps, Vmax 34.6, per panel). They want to run them in parallel, connecting to a 48 volt charge controller (as they think they can't fit that much power in a 24 volt charge controller) and use it to power the same kind of lead acid basic 12 volt battery.

Is there an affordable charger similar to the Sunsei that will do this?

There second question, is they want to know if the same 48 volt setup above, would work equally well with a Verizon FIOS battery instead (12 Volts, 7Ah, slow duty cycle).

Any help would be appreciated. Yes I realize in advance they should just go with a larger voltage battery or a battery bank but that is not an option, long story. They are trying to minimize space and also minimize weight and number of batteries. And they already own the PV panels...

Help?

BB. · February 2010

Re: Charge Controller 48 Volt PV to 12V Battery Advice Needed

dragon1 wrote: »

A classroom I help with normally sets up two Mitsubishi 150's panels in parallel, plugged into a Sunsei 12/24V CC25000 charge controller. That connects to a 12 volt typical U.S. lawn tractor battery to demonstrate charging. Sometimes hooking up a small 400Watt inverter to the battery with a small load on the inverter (FM radio) for the students.

Just to be clear--you are talking about these panels:

Mitsubishi PV-MF 150EB3

Pmax @ STC 150 W
Pmax @ PTC 134 W
Vmp at Pmax 23 V
Imp at Pmax 6.52 A
Voc @ STC 29.8 V
Voltage change -82 mV/C

Another class has a pair of old Isofoton PV modules IS-150/24 (VoC 43.2 volts, Isc 4.7 amps, Imax 4.35 amps, Vmax 34.6, per panel). They want to run them in parallel, connecting to a 48 volt charge controller (as they think they can't fit that much power in a 24 volt charge controller) and use it to power the same kind of lead acid basic 12 volt battery.

We need to clarify here... There are 12/24/48/etc. charge controllers... And there ar also 5/7/15/30/45/60/80 amp charge controllers... And there are PWM and MPPT type charge controllers...

Looking at the amount of output power for the charge controllers first... Power=Voltage*Current ...

So, we need to know the battery bank voltage 12/24 and how much current the charge controller will output, and what the solar panel configuration/options will be.

It sounds like, in all cases, you are asking about charging a 12 volt battery bank. Correct?

If so, then we have to look at the solar panels... For an inexpensive PWM type charge controller, the Vmp (voltage maximum power) of the solar array needs to be, at least:

Vmp >~ Vbatt-charging + 2 volts for controller and wiring voltage drops

For charging a 12 volt lead acid battery--typically the battery charges around 14-15 volts. And the Vmp of "12 volt battery charging panels" is around 17.198 volts... That is the most efficient voltage point for the system (using PWM controllers).

Your solar panels have Vmp= 23-34.6 volts (depending on which ones we are talking about).... What happens is, the farther above the battery charging voltage you get, the less efficient the solar panels are at charging the battery.

and To show you an example. Lets take your Mitsubishi 150 watt panels. Vmp=23 V and Im= 6.52 A. Assume PWM controller (simply a switch "on" or "off" pulse controller. To configurations. One with the two panels in series and the other setup with the panels in parallel. Remember voltage adds in series and current adds in parallel connections:

For the series configuration, the Vmp = 2*23 = 46 volts; and Imp = 6.52. When connected to a 12 volt battery bank, the voltage at the panels will equal that of the battery bank:

P=Imp*Vbatt-charging= 6.52 amps * 14.4 vbatt = 94 watts charging 12 volt battery

Now, for two panels in parallel, the Vpanel will still be around 14-15 volts of the charging battery, but the current will be 2xImp because of parallel connection:

P=Imp*Vbatt-charging= 2*6.52 amps * 14.4 vbatt = 188 watts charging 12 volt battery

If you had a pair of 150 watt solar panels which were designed for charging a 12 volt battery bank with Vmp=17.5 volts, then:

150 watt / 17.5 volts = 8.57 amps (Vmp=17.5v)
P=Imp*Vbatt-charging= 2*8.57 amps * 14.4 vbatt = 247 watts charging 12 volt battery

The other panels (Isofoton) have their Vmp even higher (34.6 volts)--so using a PWM controller to charge a 12 volt battery will be very usage of power (again, losing over 1/2 the panel wattage).

Your options are to

Connect Isofoton in parallel through cheap PWM controller and charge 12 volt battery bank (over 1/2 power lost)
Connect Isofoton in parallel and charge 24 volt battery bank (a good fit--not much power wasted).
Purchase a quality MPPT (Maximum Power Point Tracking) solar charge controller for $250-$400+ to use 95% of the Isofoton solar panels to charge either a 12 volt or 24 volt battery bank.

MPPT charge controllers are actually Power Converters. The take High Voltage/Low Current solar panel energy and down convert it to Low Voltage/High Current use by the battery (around 95% efficient)... MPPT is the "AC transformer) in the DC world.

You can read about PWM and MPPT solar charge controllers at these to FAQ's:

All About Charge Controllers
Read this page about power tracking controllers

Assuming you choose the correct Solar Charge Controller (based on input voltage and current, plus the battery bank voltage)--either type will charge fine. For your needs, the MPPT will give you 2x more power than a PWM on the Isofoton panels charging a 12 volt bank.

An MPPT controller will give you something like 30% more power vs a PWM controller when charging a battery bank with the Mitsubishi solar panels.

Is there an affordable charger similar to the Sunsei that will do this?

I assume a Sunsei is an inexpensive PWM type solar charge controller... It has the limitations of all PWM controllers vs solar panel Vmp/Vbatt mismatches.

You have to look at the charge controllers Voc (voltage open circuit) maximum rating against Voc of the solar panel... Some 12 volt controllers do no have very high Voc.

Note, the 12 volt vs 24 volt vs 48 volt charger rating refer to the programmed charging voltage of the battery--not the input voltage rating. If you have a 12 volt battery, you will always need a "12 volt output" capable charge controller (of any type). Note, some controllers can be programmed to charge 12 or 24 or 48 battery banks (switch settings or programmable charging voltage set points).

There second question, is they want to know if the same 48 volt setup above, would work equally well with a Verizon FIOS battery instead (12 Volts, 7Ah, slow duty cycle).

If I understand your question--a 48 volt charge controller is for charging a 48 volt battery bank. Would not be useful for charging a 12 volt battery/bank.

Any help would be appreciated. Yes I realize in advance they should just go with a larger voltage battery or a battery bank but that is not an option, long story. They are trying to minimize space and also minimize weight and number of batteries. And they already own the PV panels...

And this is going to be the question you need to answer... How much power do you want to get out of these solar panels into a battery bank...

If this is only done for demonstrations... Running a radio for 20 minutes a few times a year--then you don't really care if you are getting 80 watts or 280 watts out of the solar panels (cheap PWM vs expensive MPPT solar charge controllers).

If, however, you need to get some known amount of power out of the solar panels--then you are going to spend some serious money (for a class room project) to get the power these panels are capable of (assuming they are still functional and have not be damaged).

I am sorry for all of the verbiage--You are approaching the problem from the side and had some assumptions that probably where not correct (if I understood your post/questions correctly).

Please feel free to tell us what your exact needs are (battery voltage, battery AmpHour capacity, and how much power you need (amps, watts). And we can give you a more concise answer.

-Bill

System2 · February 2010

Re: Charge Controller 48 Volt PV to 12V Battery Advice Needed

Fantatsic response, thank you for the detailed answer.

To clarify a few points I made and answer questions you raised:

It is only for very brief classroom demonstrations, to briefly charge a single 12 volt battery. Probably all of once or twice a month. The idea is to give them a quick hands on seeing that amperage is additive in parallel PV and not drive the voltage so high that we can't use a readily available charge controller.

I realize the Isofoton's are not ideal for this, but the class already has the Isofoton panels, a 12 volt battery and then a 12 volt 400 watt inverter that runs off the charged 12 volt battery. And they have a small FM radio/mini boom box they plug into the inverter.

So we were hoping to buy a cheap 48 volt charge controller (preferably $100 or less) and whatever the best type is for this configuration in the price range, that would work with a pair of the Isofoton's in parallel with the existing batttery.

We thought we'd need a 48 volt controller, since the Isofoton's voltage seemed way above 24 even though Isofoton considers it a 24 volt panel in their literature. The Sunsei controller I mentioned could handle up to 50 volts but only with a 24 volt battery.

I did not realize that the charge controller's rating specifically referred to the battery voltage. Now I feel a little stupid for not realizing that. The Sunsei they have is a 12/24 and it had been explained to me that it referred to the panel voltage, which seemed odd...

So I guess I need a 12 volt charge controller that can handle the voltage of the Isofoton's in parallel and their amperage, now that I understand it better.

The battery they'd prefer to use is the one they already have, a spare Verizon FIOS battery (12 Volts, 7Ah, slow duty cycle). The lawn tractor style battery they have for the other class is 12 volts, I don't remember the amperage. I could get another one of those if I had to, but they liked the FIOS battery for compactness, weight and size.

A cheap PWM controller sounds like the best solution for cost, even though it is the least preferable solution as you pointed out if this was anything other than a demonstration trying to use existing equipment.

If there is a particular make & model of controller you think might fit the bill or manufacturer I should look at, I'd appreciate a recommendation. They need this for next Saturday.

Thanks again!

BB. · February 2010

Re: Charge Controller 48 Volt PV to 12V Battery Advice Needed

To be honest--you probably do not even need a charge controller... Just a fuse and polarized plug of some sort (solar panels when plugged in backwards--very bad news for the panel).

Just forget the whole idea of PWM/Charge controller and wire it to a used car battery. 4-6 amps from solar panels into a 60-80 Amp Hour batter is not going to hurt anything for a few hours of demonstration.

If you want to protect the inverter (and battery) put an inline switch with the battery and watch the voltage with a meter... When it gets near 15 volts, turn off the charging current.

When you are done, disconnect the loads and panels--and put everything away. Old lead acid batteries should probably be charged one a month for a day with a 1 amp AC maintenance or trickle charger to keep them from dying (but if is a used/recycled car battery--you probably don't even care--see if you can borrow an old battery or two from the local parts store).

Some notes--a car battery is pretty dangerous. You have high currents (100's to 1,000 amps or more) into a dead short. Filled with sulfuric acid. And if charging, will give off hydrogen gas which can be lit off with sparks from a switch. So keep any switches/clips away from the top of the batteries.

The 7 Amp*Hour battery is really too small for use with these panels... You are charging it with almost 5-10x as much current as normally would be done with your current solar panels. Short runs at charging will not hurt them--but heavy charging will overheat them and really cause a lot of hydrogen/oxygen gas generation.

Obviously, a small 7 AH battery will only provide enough power to run a radio and not much else.

-Bill

Charge Controller 48 Volt PV to 12V Battery Advice Needed

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