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.