2 x 50% BATTERY

 

Assume we have a110VDC installation of 2 x 50% batteries, previously sized at 82 x LBE600P (600Ah) per 50% battery. Both batteries are connected by a normally open DC bus-tie A. Should the need to arise to close the bus-tie, what would be the magnitude of the circulating current between a potentially fully charged 50% battery and a fully discharged 50% battery:

 

 

On the basis that the battery complies to DEP standards, i.e. 110VDC +10%/-15% without dropping diodes, the end cell voltage after a full discharge will be 1.14V per cell.  On closing the bus-tie A there will be a potential difference between the 2 battery halves of (82 x 1.2V) – (82 – 1.14V) = 4.92V.

Each LBE600P cell has an internal resistance of 0.3mOhm per cell when fully charged.  On discharging this will increase significantly so a fully discharged battery will have a significantly higher internal resistance.  For the purposes of this example we’ll assume the worst case which is the lowest resistance.  So the complete DC circuit comprises 82 cells x 2 batteries x 0.3 mOhm = 0.0492 Ohm

For a short period immediately after the bus-tie is closed a current will circulate from the fully charged half battery to the fully discharged half battery of 4.92V / 0.0492 Ohm = 100A.  This will very quickly diminish as the battery voltages become more equal.