Tips for prismatic cells

A user guide.

Terminals

Nut height

Connection to a cell must be such that the current flows directly from the cell's terminal to the bus bar or cable terminal. You must not rely on the mounting bolt to carry the current.

Unfortunately, in some cells (namely Shandong HiPower) that can be hard to achieve:

• The large nut securing the terminal to the case is installed with a locking compound
• That locking compound creates an insulating layer
• The large nut may be taller than the cell's terminal
• The connecting bus bar (or the terminal on the connecting cable) rests on the large nut
• Since the nut is isolated, current flows through the mounting bolt instead
• The bolt is usually made of a coated steel which has a relatively high resistance
• This results in a high temperature rise

The solution is to ensure that the large nut does not extend past the cell's terminal surface. If you cannot screw in the nut further, you may need to use a planer to file down the top surface of the large nut.

Prismatic cell terminal close-up.

(Thanks to Brian Hall of Thunder Struck EV for the tip.)

Aluminum to copper

Before connecting to prismatic cells with copper bus bars (dissimilar metals), burnish the aluminum terminals slightly to remove the coat of aluminum oxide, and cover with Noalox anti-oxidant compound, which you can get from, among others, Grainger.

(Thanks to Bull Dube' of KillaCycle for the tip.)

Side clamping

Li-ion cells expand as their State Of Charge reaches or passes 100 %.

The pressure is not due to the act of charging, nor to heating; it's the state of being fully charged that results in high pressure.

The expansion is due to mechanical expansion of the active materials, and to gases released when the cells is full, which are reabsorbed during discharge.)

A cell may be damaged if this expansion is not contained (due to layer separation).

• On one end, cylindrical cell inherently contain that expansion by virtue of their shape
• On the other end, pouch cells do not contain that expansion at all, as their walls are soft
• Prismatic cells are somewhere in between, as they do have a case, but it is not strong enough to fully contain the expansion

At what point do the cells expand, and what pressure do they exert on their walls?

It depends on the cell's design; some manufacturer's cells expand noticeably at 100 % SOC, and some only if they are grossly overcharged.

We tested a CALB cell, and found no change in the pressure exerted on its walls, even when overcharged to 4.2 V (their maximum voltage is 100 %).

Prismatic cell pressure test fixture

Therefore, we feel that, when properly protected by a BMS, these CALB cells do not need a constraint, at least not at room temperature.

(This test was performed at room temperature. We don't know what would happen if these cells were both hot and completely full.)

We do not know how cells from other manufacturers behave; unless you have tested your cells, and discovered that they do not expand at 100 % SOC, you must constrain them.

Also, if you are daring enough to use Li-ion cells without a BMS that shuts down charging when any cell is full, then constraining a cell might (might) help it survive the abuse of overcharging.

The expansion of a stack of prismatic cells is contained by:

• Placing hard plates on either side of the stack
• Squeezing the stack of cells with the plates as far as possible (use threaded rods and screw in the nuts as far as they go)
• Clamping the lot by keeping the threaded rods in place or with metal packing straps

Prismatic cells clamped with metal plates and straps
(Left: Honda S2000 Electric Vehicle Conversion. Right: Casa Del Gato)

(Thanks to Richard Hatfield of Lighting Electric for the tip.)