Charging Batteries

adas · March 2008

Aloha, help me understand this...
I have my large 24v battery and my panels put out about 30vmax to 28v around 3pm. Is this not enough to charge the batteries? Anyway, I was just starting my panel setup and did not use a charge controller, just manually watching the charging , but I am installing 2 MX60's tomorrow.

Here is my question....
I have been told to series 2 panels and get 55-60 or so volts and put this into the controller. Why not just put one panel through the controller...not enuf voltage?? In my mind it seems a waste to bump ot up to 60v when only 30v is needed. I also was told that the controller will not only trim your voltage from say 60v to the needed 30v and thereby double the amperage, so nothing is lost by series ' ing the 2 panels and in fact I will be getting out more than double the amperage. comments on this?
thanks
frank

mike95490 · March 2008

Re: Charging Batteries

Basicly, you were told right.

Can you tell us more about your setup ? Panels, model #, qunty and your battery bank, brand, # of cells

You must be blessed with a very large PV array, to need to MX60's

adas · March 2008

Re: Charging Batteries

mike90045 wrote: »

Basicly, you were told right.

Can you tell us more about your setup ? Panels, model #, qunty and your battery bank, brand, # of cells

You must be blessed with a very large PV array, to need to MX60's

Aloha, I have

40 x 70watt BP panels
3 x 24v electric forklift batteries
2 x MX 60
1 x 2400 24v tripp light inverter.

I am off grid and just setting up this system for my office. Right now I can run my security system, night lights in the compound, flourescent lights in the office, computer, small fridge and toaster oven. I have 24 of the panels directly hooked up to the batteries as of yesterday and for the last 3 days batteries are staying even and not more and not less charged. Tomorrow I will hook up the last 16 panels and series 2 panels together with the 2 MX 60's so I do not have to watch over the charge.
So series ing two panels to 60v into the MX60's is the way to go?
thanks
frank

mike95490 · March 2008

Re: Charging Batteries

Suppose the MX60's can output 60A @ 28V each

then their max input is 3600 watts ea

What is the voltage/amps of your individual 70w panel ?

niel · March 2008

Re: Charging Batteries

adas,
correct me if i'm wrong here, but if i remember right the bp70 is a 12v pv. it will probably have a vmp around 17v with an voc around 21 or 22v with an imp of about 4.1amp. to get output at 24v 2 pvs must be in series, but if the high pv temperatures are keeping the voltage too low then 3 or 4 pvs could be seriesed and downconverted to output at 24v. with 40 pvs to work with that does not work out evenly when using 3 in a string so 4 per string you'd go with.

the mx60 normally can have 48amps input in conjunction with the 60amps output rating to allow for nec rules, but there is a modification (approved?) for the mx60 to allow its output to be 70amps and a max input of 56amps. for a 48v pv input that would mean a max of 28amps inputted to the mx60 from the pvs as the current is about doubled in the downconversion to 24v. this increase in current capacity does increase the power handling ability of the mx60 as in a 48v system with the unmodified 60amp mx60 at 48amps would mean 48vx48a=2304w and for the modified mx60 with 56amps now inputable would mean 48vx56a=2688w.

adas · March 2008

Re: Charging Batteries

niel wrote: »

adas,
correct me if i'm wrong here, but if i remember right the bp70 is a 12v pv. it will probably have a vmp around 17v with an voc around 21 or 22v with an imp of about 4.1amp. to get output at 24v 2 pvs must be in series, but if the high pv temperatures are keeping the voltage too low then 3 or 4 pvs could be seriesed and downconverted to output at 24v. with 40 pvs to work with that does not work out evenly when using 3 in a string so 4 per string you'd go with.

the mx60 normally can have 48amps input in conjunction with the 60amps output rating to allow for nec rules, but there is a modification (approved?) for the mx60 to allow its output to be 70amps and a max input of 56amps. for a 48v pv input that would mean a max of 28amps inputted to the mx60 from the pvs as the current is about doubled in the downconversion to 24v. this increase in current capacity does increase the power handling ability of the mx60 as in a 48v system with the unmodified 60amp mx60 at 48amps would mean 48vx48a=2304w and for the modified mx60 with 56amps now inputable would mean 48vx56a=2688w.

Aloha, here are the specs for the panels. They are 24v and are Government overstock (whatever that is) and made in 2002.

Electrical characteristics: Maximum Power(Pmax)- 70 watts.
Open circuit Voc(V)- 44 volts.
Short circuit Isc(A)- 2.99.
Operating Voltage Vmp(V)- 29.80.
Current at Pmax or Imp(A)- 2.35.
Maximum System Voltage(V)- 600.
Module voltage(V)- 24.
Minimum Series Fuse(A) - 5.
Minimum Bypass Diode(A) - 6.
Maximum number of Modules in Parallel - 1.
Maximum Number of Modules in series - 11.

So for 40 panels the amperage at 29.80 v is 94 amp, needing 2 mx60's

frank

mike95490 · March 2008

Re: Charging Batteries

Operating Voltage Vmp(V)- 29.80

Ah Ha ! That's the critical voltage, that the MPPT circuit will try to track (+- a volt or three)

and with Current at Pmax or Imp(A)- 2.35 , 40 in parallel are too much for 1 controller.

If you place 2 panels in series, bumping the voltage up to ~88V, then you will see benefit from the lower total system current, and save some on wire costs & losses. Locate the controller near the batteries, and you will have short runs of the high current wires. That's what my plan would be.

Mike

adas · March 2008

Re: Charging Batteries

mike90045 wrote: »

Ah Ha ! That's the critical voltage, that the MPPT circuit will try to track (+- a volt or three)

and with Current at Pmax or Imp(A)- 2.35 , 40 in parallel are too much for 1 controller.

If you place 2 panels in series, bumping the voltage up to ~88V,

Mike

Aloha, Is this a typo to mean 3 panels for ~88v or 2 panels for ~58V?

Today when I came into work, I found the batteries were not really overcharged as I expected, but I discovered an employee using the electricity to grind rust for over 4 hours solid with a 4 1/2" grinder. The PV panel system was holding 27.2 volts with this load. Shutting off the panels the batteries tested still at 25.8v. Pretty fair since only 24 of the 40 panels are hooked up. Tomorrow we series the panels, install the rest of the panels and install the 2 MX60's. Making improvements by the day.....

frank

mike95490 · March 2008

Re: Charging Batteries

adas wrote: »

Aloha, Is this a typo to mean 3 panels for ~88v or 2 panels for ~58V?

frank

Not a typo

" Open circuit Voc(V)- 44 volts."

As you load lessens (batteries charge up, and don't need full power) your controller will not be drawing full power from the panels, and with less load, your panels slowly rise to the Voc . 2 in series will be about 88V.

Even a little bit of load will pull this down.

This is why MPPT controllers work, they prevent the battery bank from pulling the panel voltage below the Max Power Voltage "Operating Voltage Vmp(V)- 29.80. " which changes a bit over the course of a day.

Maybe someone here has a chart/graph of what the typical curve/slope is ?

crewzer · March 2008

Re: Charging Batteries

Adas,

Here’s what you need to know to configure your system.

Your “24 V” batteries will require a bulk-stage charge voltage of between 27.8 V and 31 V at 77 F, depending on the type and manufacturer. Using the MX60 controller, the array’s operating voltage must exceed that range by a couple of volts.

An array’s operating voltage will rarely approach its Vmp spec, which is based on very cold ambient conditions. Something in the 80% to 90% range is typical, and very hot temperatures can reduce Vmp even further to ~75% of STC Vmp. Applying this information to your BP modules, their individual bulk stage operating Vmp will typically be in ~23.8 V to 26.8 V range. This range is too low to meet the bulk stage charge requirements for your 24 V battery bank.

Wiring two modules in series will create a string with an operating voltage of ~47.6 V to 53.6V. This is high enough for the MX60 to charge your 24 V battery bank. Wiring 20 of these modules as a 2 (in series) x 10 (in parallel) configuration will create a 1,400 W STC array with a mid-day bulk-stage operational rating of ~50 V x 23.5 A Imp. Assuming 97% efficiency, the MX60 will convert this input to a charging output of ~29 V x ~39 A.

The MX60’s official PV array limit for 24 V battery applications is 1,600 W STC. Hooking a 1,400 W STC array up to each of the two MX60’s should work fine.

HTH,
Jim / crewzer

niel · March 2008

Re: Charging Batteries

thanks for correcting me on the specs for the pv as it is a rare thing to have pvs in that wattage range at a higher than 12v rating.
yes, i agree with these guys (me getting back to this thread a bit late) and that the vmp and voc specs you gave does shed much light onto this. 2 in series will work and that would yield 20 such series strings that you will have need of 2 mx60s to handle it. do not wire all 20 strings together, but have 10 strings together for mx60 #1 and the other 10 strings for mx60 #2.

Charging Batteries

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