48v bank to 54v bank

[DonutRoller]

Thinking of adding 1 extra battery  in series per bank to help capacity, inverter can handle voltage, midnight classic250. 2 banks of 8 Costco batterys. Can I simply ramp up the voltage,etc settings? Is this a good idea or am I missing something?  Thanks John

[Vic]

Hi D R,  Welcome to the Forum.

Increasing your system voltage by 6 V nominal is a large step.

Your batteries almost certainly are Flooded Lead Acids (FLAs).  You might have some issues with cold batteries,  requiring a higher Absorption voltage,  and it is possible that you might not be able to do an effective EQ,  especially on cold batteries,  as the Inverter might not be able to accommodate the high battery voltage.

There could be other effects that might be undesirable.

What is the brand and model number of your inverter?

Thanks,   Vic

[DonutRoller]

The inverters an outback fx3048t 42-68 vdc input range, batterys are flooded 6 v golf cart batterys.

[Vic]

Hi again D R,

Thanks for the added information.

IMO,  your proposal to use 27 Cells on a 48 V system is NOT a good idea,  just generally.

What is the suggested Absorption voltage  and EQ voltage?  Based on guesses,  it appears that you would not be able to do an EQ on your proposed battery bank,  even when it is at 25 C (77 F),  without exceeding this MAXIMUM battery voltage to the inverter.  Most probably,  the inverter would shut down,  due to high voltage,  if an EQ was attempted.

AND,  sometimes,  inverters do interesting things when the battery voltage is high,  like clipping the top of each sine wave on the inverter output,  and so on.

You MIGHT see about 8% increase in Capacity,  if one additional 6 V battery per string  was added,  but this Capacity increase is quite small.

Assuming that the suggested Vabs might be about 2.45 Vpc X 27 cells = about 66 V,  without any Temperature Compensation,  and it appears that any real EQ voltage on batteries that are at 25 degrees C would not be possible.

IMO,  there are reasons that running 9 ea 6 V batteries on a typical 48 V system is not at all common.

Without specifics on the charge/EQ requirements for your batteries,  seems to me that 9 batteries per string seems like a bad idea,    IMO    ....

FWIW,   Vic

[DonutRoller]

Thank you for helping me, I guess a 3rd bank is in my future.

[Vic]

Ok D R,

Well,  as you may know,  parallel batteries often can cause problems.  Personally would not run more than a single string in an off-grid system.  IMO,  two strings is marginal,  and three strings is trouble.

This is because parallel battery strings usually do not share currents well,  and the current sharing worsens as the battery bank ages.

Trying to find batteries of sufficient Capacity to meet your needs in a single string is often the best approach.  This CAN mean that the battery bank is a bit more expensive than one made of a number of parallel strings of smaller C batteries.

In addition,  adding batteries to a bank at a later time can make things even worse,  as  older and new batteries will diverge farther from each other.

If you must use parallel strings,  be certain to  wire them correctly,  for a least bad configuration,  and also  get a Clamp on DC Ammeter to monitor how well the strings are sharing current.

If one pays careful attention to string balance (and take remedial actions when necessary),  and monitor the SG of EACH cell monthly (or more often),  one might be able to manage multiple battery strings,  but it takes quite a lot of effort,  And,  as batteries age,  it takes yet more time.

FWIW,  just my opinions,  do not mean to rain excessively on your parade.    Vic