Advice on system upgrade...12 to 24 volts....HELP PLEASE!!!

msgieser · July 2014

Thanks in advance for the help! Doing a system upgrade and need advice.

OK Coot, you have helped me before in setting up my present system, so Coot or anyone, what do ya think?

My present system (12 volt) = 3, 300 watt panels or 900 watts, feeding into a Midnight (3 panel) combiner (panels to combiner = 4-8 feet). Combiner to a Morningstar TriStar TS-MPPT-60 charge controller (1 foot wired with #2 welding cable). Charge controller to 6, Trojan T105RE, 6 volt batteries, (wired for 12 volts with #0000 welding cable interconnects), 6 feet with #2 welding cable. Batteries to a Xantrex 1800 watt pure sine wave inverter ( 4 feet wired with #0000 welding cable. Plus a Tri-Metric monitor, shunts, fuses, disconnects, etc in the mix.

Now, I have 4 new 250 watt panels to add. I realize if I keep this at 12 volts (in addition to adding another combiner box) I will need to add a second TriStar TS-MPPT-60 charge controller. I'm going up in inverter size to 3,000 to 5,000 watts (pure sine wave). Wire runs and sizes will be as they are presently. I will also add 2-4 or ?????? more batteries (same T105RE's....existing batteries are fairly new and never abused, so Trojan told me I should have no problems adding additional)

Is now the time to rewire the battery bank to 24 volts? I realize I could then get by without the additional charge controller, just one 8 panel combiner box. Since I'm going to a bigger inverter anyway....does 12 vs 24 volt offer advantages beyond the charge controller addition and inverter change issue? My wire runs are, and will remain fairly short, with good size wire. Are there additional advantages in 12 vs 24 volts..... and other thoughts and/or concerns you may have that I should consider now that I'm upgrading to a larger system? Thanks again for your past and present/future help....much appreciated!!!!!

mike95490 · July 2014

Re: Advice on system upgrade...12 to 24 volts....HELP PLEASE!!!

if your current 1800w inverter is straining and you think you need to go larger, you really need to jump up to 24v or think if you are going to grow again, go up to 48V.

2,000w at 12V = is only about 180 amps. That's going to be some cables, and fuse to manage that. half that amps at 24V (90A).

any planned usage of more then 2000w at 12V is just crazy talk.

vtmaps · July 2014

Re: Advice on system upgrade...12 to 24 volts....HELP PLEASE!!!

msgieser wrote: »

Charge controller to 6, Trojan T105RE, 6 volt batteries
<snip>
I will also add 2-4 or ?????? more batteries

Your signature says you have a 450 ah battery bank. At 12 volts that would be four batteries, not six.

If you reconfigure the batteries to 24 volts, that will be four batteries per string. If you have four batteries now, you can add four more. If you have six batteries now, you only want to add two more.

msgieser wrote: »

My present system (12 volt) = 3, 300 watt panels or 900 watts
<snip>
Now, I have 4 new 250 watt panels to add.
<snip>
I realize I could then get by without the additional charge controller, just one 8 panel combiner box.

Don't be so sure... first of all what is the Vmp of your present panels, and what is the Vmp of your new panels. If they don't match, you should not put them in parallel.

Next issue is the Vmp, even if they do match, the Vmp may be too low for a 24 volt system. If it is too low, you need to put two panels in series to make a string, but you would then need an even number of panels. Also, if you put them in series, they need to have the same Imp.

I need more details on the panels to give any advice on configuration possibilities.

--vtMaps

msgieser · July 2014

Re: Advice on system upgrade...12 to 24 volts....HELP PLEASE!!!

Ok...here are those numbers:

existing panels = Vmp 36.72 V
new panels = Vmp 30.70 V

existing panels = Imp 7.73 A
new panels = Imp 8.15 A

I do have six batteries. would add 2 or 6 more if I go to 24V. Thanks.

Cariboocoot · July 2014

Re: Advice on system upgrade...12 to 24 volts....HELP PLEASE!!!

msgieser wrote: »

Ok...here are those numbers:

existing panels = Vmp 36.72 V
new panels = Vmp 30.70 V

existing panels = Imp 7.73 A
new panels = Imp 8.15 A

I do have six batteries. would add 2 or 6 more if I go to 24V. Thanks.

That's just the problem vtMaps was referring to: Vmp's are too far apart for maximum efficiency if the panels are in parallel, and the Vmp of the new panels is too low to be used "single" on a 24 Volt system.

So you're up against using a second controller for the new panels (two in series) or finding different panels which more closely match the specs of the ones you've got.

Over-all going up to 24 Volts is a good idea for your increased power needs.

vtmaps · July 2014

Re: Advice on system upgrade...12 to 24 volts....HELP PLEASE!!!

Cariboocoot wrote: »

So you're up against using a second controller for the new panels (two in series) or finding different panels which more closely match the specs of the ones you've got.

I can think of a way to use one controller with your three old panels and with three of your new panels:

Put one old and one new panels in series to make a string. The Imp is not exactly the same, but close enough to work. Cariboocoot's suggestion to add another controller is probably your best option if you've already purchased the new panels and want to use all of them.

--vtMaps

Cariboocoot · July 2014

Re: Advice on system upgrade...12 to 24 volts....HELP PLEASE!!!

vtmaps wrote: »

I can think of a way to use one controller with your three old panels and with three of your new panels:

Put one old and one new panels in series to make a string. The Imp is not exactly the same, but close enough to work. Cariboocoot's suggestion to add another controller is probably your best option if you've already purchased the new panels and want to use all of them.

--vtMaps

Yes, that will work as long as you're willing to accept that the string Imp will be the lower of the two different panels.

Quite often it's not a matter of "this won't function at all" but rather how well it will function. For example the low Vmp paralleled to the higher will work, but when the battery Voltage rises and the array comes closer to the higher Vmp the lower value panels drop off in current as they are brought above their Vmp. This is not an easily predictable thing because the charge controller is actually going to pick the Imp*Vmp and it is not dependent on panel specs. As such the loss may never happen or it may be low enough to be inconsequential.

When dealing with dissimilar panels you do not get 'X' Watts plus 'Y' Watts equals 'X+Y' Watts unless they are on separate controllers. What you get is 'Z%X+Y' Watts and that percent may be good enough for the job at hand.

msgieser · July 2014

Re: Advice on system upgrade...12 to 24 volts....HELP PLEASE!!!

OK..I have the new panels so it's a go. I have no problem in adding a second MS-TS-MPPT 60 controller. So, I just wire the new panels into a second combiner box, combiner outputs into the second controller, and out of the second controller through cables to another connection on the batteries????? Basicially just duplicate the connections for the first panels? PLUS...decide if I'm going to rewire the present (adding 2 or 6 new) batteries to 24 volts. Then add a 24 volt inverter. Thanks

Cariboocoot · July 2014

Re: Advice on system upgrade...12 to 24 volts....HELP PLEASE!!!

msgieser wrote: »

OK..I have the new panels so it's a go. I have no problem in adding a second MS-TS-MPPT 60 controller. So, I just wire the new panels into a second combiner box, combiner outputs into the second controller, and out of the second controller through cables to another connection on the batteries????? Basicially just duplicate the connections for the first panels? PLUS...decide if I'm going to rewire the present (adding 2 or 6 new) batteries to 24 volts. Then add a 24 volt inverter. Thanks

That's pretty much it. With fuses/breakers in the appropriate places.

As for adding batteries ... try to keep the charging capacity in balance with the storage capacity; there's quite a difference between adding two batteries to give you two parallel strings @ 24 Volts and adding six batteries to give you three parallel strings @ 24 Volts. 50% difference in fact.

zoneblue · July 2014

Re: Advice on system upgrade...12 to 24 volts....HELP PLEASE!!!

I personally would first try the panels one of each into a single controller. You might lose a bit from the imp mistmatch, but youll save it back in controller idle losses. You most definately need 24 if not 48V for a 5kW inverter.

Advice on system upgrade...12 to 24 volts....HELP PLEASE!!!

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