Recomended MPPT input voltage for a 48 volt system

littleharbor2

Typical recommended MPPT nominal voltage input for a given system is double the battery bank voltage. I am contemplating switching my system over to 48 volts from 24 volts. I have 72 cell panels series wired 3s x 3p, Midnite classic 200. I am curious as to how the approx. 130 voc. , 105 vmp.  will work into a 48 volt bank, this being 1 1/2 times the nominal voltage, not double. At the present time this 3s x 3p configuration is charging my 24 volt bank and being 3 times nominal it creates more heat than I am comfortable with, especially in the warm Baja climate.

vtmaps

littleharbor2 said:

Typical recommended MPPT nominal voltage input for a given system is double the battery bank voltage.

I disagree.  The ideal string Vmp for an MPPT controller is about 20-30 volts above battery voltage.   With 60 cell panels, you need 1, 2, or 3 in series for 12, 24 or 48 volts respectively.  That works out to approx double the battery voltage. 

With 72 cell panels in a 48 volt system, if you put them 2 in series the voltage is barely high enough in hot weather and you may have to put the Classic in 'legacy' mode.  Three in series is perfect for a 48 volt system and is a good reason to have a classic 200 rather than a classic 150.

--vtMaps

littleharbor2

Thanks for your thoughts here. This is the first time I have heard the 20-30 volt differential but it makes sense to me. Less DC-DC conversion with it's inherent ineffiencies.  My classic 200 will be much happier.

Dave Angelini

When you use HV controllers or string inverters you are only concerned with hi enough starting voltage and not exceeding the 600 or 1000v spec when cold. Much easier when you need alot of power.
 I still have that 4024 and 4548 in the shed? Are you in Mexico now? I am asking for a friend.

littleharbor2

No Dave. I'm home on Catalina Island. The system is shut down except for a 2 module and small controller setup for mantainence charge. Cant bear the thought of running the Classic throughout the summer, especially when it's sitting idle all summer.  I also have another NEVER USED SW 4024 and a lightly used SW 4048. The 4048 is what I am going to change over to in the fall.

Vic

Agree with vtmaps   ...

With 72 Cell PVs,  on a 48 volt system,  you really have little choice.   Strings of three PVs on a 48 V system is sufficiently high to allow EQing almost any Flooded battery with hot PVs and a relatively cool battery.

MidNite Solar has added a Footnote  to their String Sizer Results,  regarding minimum Input voltage for MPPT CCs,  like the Classic:

"With MPPT controllers the higher the input voltage the less efficient they are. This is not a large value but it will add up to a little more heat in the controller and a point or two less in efficiency. BUT you also have to be careful not to have the input voltage to low. Most all MPPT controllers will want to see a minimum of 130% of the actual high battery voltage. So if we have a 48v battery and it has an Equalize voltage if 62.3 volts than we would multiply that by 130% and we would need a minimum of 81 volts on the input on the hottest day of the year in order to have enough headroom for the MPPT to work".

So with 72 Cell PVs,  and a 48 V Flooded battery,  strings of three PVs is the minimum,  and with 60 Cell PVs strings of three is even better,  as it easily meets minimum Vin required at a lower Vin to the CC.

On the 48 V systems here,  one runs strings of three 72 cell PVs  (106 STC Vmp strings),  and another runs strings of three 60 cell PVs (92 STC Vmp strings),   on Classic CCs.   Both systems could run lower Vin to the CCs,  but we need to run what the available PVs.

Vic

Dave Angelini

littleharbor2 said:

No Dave. I'm home on Catalina Island. The system is shut down except for a 2 module and small controller setup for mantainence charge. Cant bear the thought of running the Classic throughout the summer, especially when it's sitting idle all summer.  I also have another NEVER USED SW 4024 and a lightly used SW 4048. The 4048 is what I am going to change over to in the fall.

Dang, I have a part that needs to come back from Puerto Escondidio and it could there could have been a nice bonus in there for you.
Looks like you have more SW's than you need. Getting any tuna off Catalina?

littleharbor2

Sorry I couldn't be of any help.

Can you really have too many SW's


They are pulling in nice Bluefin daily on the backside if the Island

Dave Angelini

Nice ! You are living large down there!
 No Prob on the Baja express.  I think Downwind Marine got it done on the HF a few hours ago.

SouthernBajaGuy

littleharbor2

I joined this forum because I had a similar question regarding input voltage. I too live in Baja, a little bit north of Todos Santos.  I'm bringing a couple items down while building my house, but I don't yet need as much as I'm planning on bringing (5000 48v watt hybrid inverter and LiFePO4 48v 100ah battery).  It comes in a kit but I didn't want to buy the panels in the states and risk being harassed or charged at the border for them so I'm planning on buying them down there (Any leads appreciated; the place near me is expensive and very few options) so I wanted to make sure I had the minimum voltage required which isn't specified anywhere; it just has the operating range voltage.  But the kit comes with 6 x 200 watt panels which are each 12 volt, so that puts it at 72 volts.  I don't yet know the voltage on the panels either, so that's my issue. The store has 550 and 340 watt panels but aren't getting back to me about manufacturer or model. I already have 2 x 200 watt 12v panels so if I had to add those on top of the panels I buy, that would be ok too.  So I guess worst case scenario is I'd just have to buy more panels for the MPPT to function. Is that correct?  If the panels are 40v and I buy 2, it should be more than what the kit came with.  I see the date on this as 2016, so I hope you guys still use this.

BB.

For optimum results, you want the minimum Vmp-array to be (ideally) around 1.3x the battery charging voltage...

For a typical LiFePO4 a 12 volt battery may take around 14.2 to 14.6 volts charging... Then the array minimum MPPT voltage rating should be:

• 4 series LiFePO4 batteries * 14.2 volts nominal charging voltage * 1.3 MPPT voltage factor = 73.84 Vmp-array (STD)

This allows for the array/panels getting hot in summer sun and Vmp-array falling as panel temperatures rise. MPPT controllers can only "drop" Solar Array voltage to Vbatt charging voltage. This will give you the minimum number of Vmp panels in series.

There is another thing to watch out for... As panels get cold, the Voc (voltage open circuit) voltage rises... The minimum temperatures in Winter for Todos Santos are around 12C, so you will not see too much Voc voltage rise in winter... So you have to look at the Vpanel-max input rating for your MPPT controller to figure out the maximum Voc-array for the maximum number of panels in series.

-Bill

littleharbor2

 Look up the specs of the inverter. If you already have it, it may be on the side of the case. Most 48 volt, hybrid inverters have quite high MPPT headroom, although I just came across one with a 140 Voc. max. That may be your case, If you haven't bought it yet, do yourself a favor and buy a high voc. model. They make wiring up your system quite easy with a single series string. BTW with a 48 volt system especially, Buy larger high wattage panels. They are considerably cheaper.
 I recently bought 4, 550 watt panels here in San Felipe for $1,000.