Another Battery Box and some new Cells

As with all things related to this conversion there is a change.  The battery system is moving from just A123’s to a group of A123’s and some CALB CA60FI’s, the rear battery box is actually the perfect size for the CALB’s and will house 74 of the 60Ah cells along with the EMW 10kW charger.  This means there is a new battery box that will be in the front of the car to hold the A123 cells that will remain.  The 3P groups have been reconfigured to 4P so that all of the groups exceed 60Ah, the lowest CALB cell will determine the pack capacity.  As 4P groups that will never be fully charged I am hoping they will be trouble free.  In an attempt to go easy on the CALB cells, the battery current will be turned down to 500A for normal driving, (meaning just over 8C peaks) and possibly as high as 750A in a “sport” setting (12.5C peaks) based on Jack R’s 12C testing of the 40Ah version of this cell.

The battery box is fairly compact at 24″ x 8″ x 12 1/4″ and will hold either 20 groups of 4P A123’s or 19 of the CALB 60Ah cells if I choose to swap out the A123’s in the future.  This box will fit between the frame rails in front of the motor but behind the AC condenser.  Due to the placement of the hood latch and pop up headlight mechanism this battery box will need to be removed or at least lowered to access the cells.  It will be raised up from the bottom and bolted in place.  This box will be insulated and heated the same as the rear box.  Due to the lack of access, a Cell Log 8 Breakout module will be installed for easy measurement of cell voltages.  The cell log 8’s won’t be on all the time and will only come on by pressing a momentary push button that turns them on and allows the voltages to be read.

As usual I start with a simple rendering, this also lets me generate a cut list for the steel that I need.

Front battery box

Then the actual frame with a layer of primer/paint.  This time around it was much easier, I learned a lot building the first box.

front battery box

Then on to the aluminium skin, it’s riveted on just like the last box, but that process also went much smoother.

The box, ready for some rubberized undercoating to protect and seal it.

Test fit with some A123’s and CALB’s to confirm I got the sizing right.  There is 1/2″ foam insulation as well as a resistive heater in the bottom of the box.

The other notable change is the switch to bottom balancing.  Given the hybrid pack of A123’s and CALB’s I would prefer to use the bottom portion of the A123’s capacity and limit how much they are charged.  Each of the cell groups measures 76-78Ah but they will never be charged over 61Ah (estimate of the lowest CALB 60Ah cell).  Since the pack will seldom be discharged near 100% the A123’s will effectively be used only in the middle 60% or so of their capacity.  Along with the lower battery current settings and the move to 4P from 3P the A123’s should have an easy life.  This does change the normal charging scheme slightly, since the A123’s will never reach 3.5v a new calculation is required for safe CV voltage.  During charging the A123’s are around 3.35v as they pass 60Ah so 20×3.35 + 74×3.5 = 326V or 3.468V per cell.  I will likely charge to 3.45v/cell as a safe starting point and see if the charger terminates as expected when the CALB’s are nearly full, I may need to raise/lower the final voltage accordingly.

 

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