Off Grid System, connect in Series or Parallel?

[howl78]

Hi everyone,

First time poster, long time lurker.....

I know that this question has been answered before, but it hasn't been too specific to my hardware. I have a cabin that's off-grid and we're installing the solar system this summer. Here's what I currently have:

5 x 245watt LDK Modules = 1225watts at 80% = 980watts.

MNE250STM-L -240 E-Panel white steel E-Panel
MS4024-AE 4000 watt inverter/charger
ME-RC50 remote display
ME-BMK-NS
1 MNEPV2 breaker for BMK
Classic 250 charge controller
2 MNEDC panel mount breakers (for the Classic)
250 amp Battery Main Breaker
DC-GFP - built into the Classic
Arc Fault Detector - built into the Classic
3 MNSPD (Surge Protection Devices)
General labor for assembly, test and crating
1 MidNite BTS, 1 Magnum BTS
Automatic Generator Start with ARC 50 remote upgrade

Also have 4 deep cycle 12V batteries, which will soon become 6.

My biggest question is: Do I connect my modules in series or in parallel?

Thanks in advance for your help!

[inetdog]

Hi everyone,

First time poster, long time lurker.....

I know that this question has been answered before, but it hasn't been too specific to my hardware. I have a cabin that's off-grid and we're installing the solar system this summer. Here's what I currently have:

5 x 245watt LDK Modules = 1225watts at 80% = 980watts.

MNE250STM-L -240 E-Panel white steel E-Panel
MS4024-AE 4000 watt inverter/charger
ME-RC50 remote display
ME-BMK-NS
1 MNEPV2 breaker for BMK
Classic 250 charge controller
2 MNEDC panel mount breakers (for the Classic)
250 amp Battery Main Breaker
DC-GFP - built into the Classic
Arc Fault Detector - built into the Classic
3 MNSPD (Surge Protection Devices)
General labor for assembly, test and crating
1 MidNite BTS, 1 Magnum BTS
Automatic Generator Start with ARC 50 remote upgrade

Also have 4 deep cycle 12V batteries, which will soon become 6.

My biggest question is: Do I connect my modules in series or in parallel?

Thanks in advance for your help!

Since you have an MPPT charge controller (the Classic 250), you want to connect your panels in as long a series string as possible without producing too high a Voc value for your charge controller. If the 245W LDK panels are the 245W STC 245D-20(S), the Voc at standard temperature is ~38 volts. Five of them in series would produce 190 volts. Even when you correct for the lowest likely winter temperature that is well within the allowed voltage range of the 250.

[howl78]

Since you have an MPPT charge controller (the Classic 250), you want to connect your panels in as long a series string as possible without producing too high a Voc value for your charge controller. If the 245W LDK panels are the 245W STC 245D-20(S), the Voc at standard temperature is ~38 volts. Five of them in series would produce 190 volts. Even when you correct for the lowest likely winter temperature that is well within the allowed voltage range of the 250.

Awesome! Thanks so much for the quick response. Much appreciated!

[Sunking]

5 x 245watt LDK Modules = 1225watts at 80% = 980watts.

Meaningless numbers. You have 1225 watt solar panel array. Main point is 5 is a prime number which means you only have 2 options all in Parallel or all in series. No question about it.

MS4024-AE 4000 watt inverter/charger

Way over sized inverter. Inverter should be approx the same wattage as solar panel array assuming the batteries can handle the current. You are looking at a 1000 to 1500 watt inverter max.

Classic 250 charge controller

Good CC will allow you to wire your panels in series for a 24 volt battery system.

Also have 4 deep cycle 12V batteries, which will soon become 6.

Huge mistake you will deeply regret. Use only a single string of batteries of appropriate Amp Hour Capacity. It is clear you are going with 24 volt battery. If you need 24 volt at 600 AH then use 4 volt 600 AH batteries. Otherwise kiss your money good bye.

FWIW you have not specified the battery capacity. With a 4000 watt inverter @ 24 volts minimum capacity is 1600 AH. Are those 12 volt batteries 600 AH? I can answer that for you. No where close to 600 AH. Doom and Gloom are in your future.

With your panels dictate what you can do at 24 volts.

Panel wattage = 1250 watts
Battery Voltage Capacity = 24 volts @ 400 AH to 600 AH
MPPT Charge Controller @ 24 volt = 50 amps
Max Inverter = 1500 watts.

[paulcheung]

Hi I don't mean to hyjack the thread, I just want to know is there any problems for a larger than needed inverter other than the initial high cost? The reason I want to know is in my country most of the people including me want to buy a larger inverter so we can expand when is financially possible.

Thanks

[SunEagle]

Hi I don't mean to hyjack the thread, I just want to know is there any problems for a larger than needed inverter other than the initial high cost? The reason I want to know is in my country most of the people including me want to buy a larger inverter so we can expand when is financially possible.

Thanks

The problem with an inverter larger than your battery system is that it can easily over discharge your batteries to the point of hurting them. People have used large inverters but they spend the time to track the usage and depth of discharge and stop before it goes below 30% of the battery capacity.

If you are going to purchase an inverter for a battery system it is worth spending the extra bucks on a Pure sine wave inverter instead of a modified sine wave.

[Sunking]

Hi I don't mean to hyjack the thread, I just want to know is there any problems for a larger than needed inverter other than the initial high cost? The reason I want to know is in my country most of the people including me want to buy a larger inverter so we can expand when is financially possible.

Paul the issue the inverter over taxing the batteries. A lot depends on battery type but I know you have Rolls. With that in mind your batteries can supply about a C/6 discharge current assuming they are fully charged up before voltage sag exceeds .3 volts. Above C/6 and the voltage starts to sag even more. It is a function of the battery internal resistance. As the battery discharges the internal resistance goes up, and the battery voltage decays which compounds the problem. Now that same C/6 curren tis sagging .5 volts in a battery SOC of 12 volts.

So what happens with the voltage sag of the battery, and the voltage drop on the wiring add up. So in effect you can have a fully charged up battery but with a heavy load the input voltage at the inverter can drop to cut-off voltage and trip the inverter off line when in reality there is nothing really wrong except the design and expectations.

As a rule of thumb and why it is so easy for many of us to spot the problem instantly, is the maximum size inverter is Battery Voltage x Battery AH / 8 = Max Inverter Wattage.

So what if you have a 12 volt 200 AH battery. Max Inverter = 12 volts x 200 AH/8 = 300 watts.

Sure you can put a 1000 watt inverter on the above example. As long as you do not load it up to much past 300 watts life is good. If the battery is fully charged up you can even draw the full 1000 watts, but it will trip off from under voltage pretty quick. And if the cables are not sized properly even faster and maybe some moke and fire on the cables and/or connectors.

[paulcheung]

Sunking,
Thank you for your reply, I have written outback about the controller and they requested some information from me and I sent them and still don't get back answer yet.

Can you tell me what is C/6 discharge current? I have no idea about it, I am new to these things. I have more questions about my system, I will start a new thread as I hi jack this thread enough already.

Thank you again.

[Naptown]

C/6 is the battery capacity in amp hours divided by 6
Charge rates are usually referred in the same way

For example a 100 amp hour battery the discharge rate is c/6 ( Which is a tad high for most batteries although not for yours)
100/6= 16.66 Amps

[paulcheung]

Thank you

[howl78]

Huge mistake you will deeply regret. Use only a single string of batteries of appropriate Amp Hour Capacity. It is clear you are going with 24 volt battery. If you need 24 volt at 600 AH then use 4 volt 600 AH batteries. Otherwise kiss your money good bye.

FWIW you have not specified the battery capacity. With a 4000 watt inverter @ 24 volts minimum capacity is 1600 AH. Are those 12 volt batteries 600 AH? I can answer that for you. No where close to 600 AH. Doom and Gloom are in your future.

With your panels dictate what you can do at 24 volts.

Panel wattage = 1250 watts
Battery Voltage Capacity = 24 volts @ 400 AH to 600 AH
MPPT Charge Controller @ 24 volt = 50 amps
Max Inverter = 1500 watts.

Ok, so all of that being said, the batteries are just what I have in place now hooked up to a generator. These won't be the ones used with the system. Since you seem to be keen on battery sizing, what do you recommend I use for batteries? Should I be using 4 volt 600 AH batteries? And how many?

[thastinger]

howl, just think of the battery as pounds of energy storage capacity. A 24V 400Ah battery bank is the same as a 48V 200Ah battery bank. Just a hypothetical here but let's say you're system design called for that size battery and say you could get the Trojan T-105RE batteries locally at a good price. You would need 8 of the batteries regardless but configuring them for 48V would be your best option because you could do that with one series string of batteries instead of having to parallel two strings in the 24V config.