Making your own lithium battery

Voltage and touchability

The system voltage is usually determined by the connected device. Nevertheless, it may be good to take a look at what consequences this has for machine safety and the touchability of live parts. If the contacts are not allowed to be touchable, this may require more expensive connectors and shielding.

Charging and discharging currents

A cell has a certain internal resistance. As a result, it generates heat which is proportional to the square of the current.

Heat = I² x R

To keep the cell stable, a maximum current for both discharging and charging is indicated in the cell’s datasheet. Usually the maximum allowable charge current is considerably lower than the discharge maximum.
With P parallel connected cells, the current is divided over the cells, so the total current may be P times higher.

I_{Pack MAX} = P x I_{Cell MAX}

BMS parameter calculations

The BMS must of course be suitable for the number of cells in series (S). It must also be able to handle the maximum continuous current.
Many BMSses do not mention the short-circuit current. The short-circuit current is calculated from:

I_SHORT = P x V_CELL / R_CELL

For example, for a pack with 10 cells in parallel, each with an internal resistance of 45 mOhm:

I_SHORT =10 x 4.2 / 0.045 = 933 Amp.

There are also (high power) cells with an internal resistance of approx. 10 mOhm. Then the current becomes:

I_SHORT =10 x 4.2 / 0.010 = 4200 Amp.

Heat generation of the cells

One of the trickiest things about a battery is getting rid of the heat. Proper removal ensures that the battery will function up to the desired temperature.
The important limits come from the cells. For most cells the following applies:

• Max. temperature during discharge: 60 grC.
• Max. temperature when charging: 45 grC.

If the working temperature of the battery is to be 40 grC, the ‘headroom’ is only 5 grC during charging and 20 grC during operation.

Heat generation in the BMS

A component of the BMS for lithium batteries is an electronic overload and short-circuit protection. The switching elements have a resistance, which generates heat which is proportional to the square of the current.
This heat must also be dissipated.

Heat removal

When charging and discharging currents are low, heat development will also be low and little attention needs to be paid to heat. With high currents, however, this is very necessary. There are several methods to remove heat. From low to high cost:

• Air cooling
• Heat conduction to the housing
• Liquid cooling

When your application needs to operate in sub-zero temperatures, heating may also be required.