Battery behavior
The battery voltage is the element to identify the system charge level. The smartboxes are produced with two types of 12V and 24V batteries made up of 4 cells and 8 cells respectively.
The discharge of the battery does not have a linear but a curvilinear trend. Here is the graph of the typical single cell discharge curve:
The voltage of the entire battery is always calculated as the sum of the individual cells. For example a 12V battery with 3.2V cells will have a voltage of 3.2 * 4 = 12.8v.
Keep in mind that a cell is considered to be completely discharged at 2.6V, so the battery 2.6 * 4 = 10.4
Below are the general specifications of the individual cells and their batteries.
| Description | Nominal voltage | Voltage 100% | Voltage 0% |
|---|---|---|---|
| Cell | 3,2 | 3,4 | 2,6 |
| Batt 12 V | 12,8 | 13,60 | 10,4 |
| Batt 24 V | 25,6 | 27,20 | 20,8 |
Attention, the BMS to protect the cell battery has a discharge cut-off threshold which disconnects the battery and prevents it from malfunctioning. It is important to always size the systems well to avoid system shutdown phenomena as much as possible.
## How the various cells behave
Each cell can have a slightly different charge level from the others. The BMS, in addition to having protection functions, adopts strategies to realign the state of charge when possible.
The BMS by its nature stops the charge when one of the cells reaches its maximum allowed and stops the discharge when one of the cells drops below the minimum allowed. This second condition can give the illusion that the BMS has disconnected earlier than necessary because the displayed voltage is always the total of all the cells.
An example with 8 cells:
| Cell | Voltage |
|---|---|
| 01 | 3,2 |
| 02 | 3.21 |
| 03 | 3.25 |
| 04 | 3.25 |
| 04 | 3.25 |
| 06 | 3.23 |
| 07 | 3.26 |
| 08 | 3.23 |
The total value is: 25.88v Even though the theoretical minimum should be 25.60 the BMS will break off at 25.88 because cell 01 has reached its minimum.
Important! The nominal value of a cell battery is always to be considered the minimum voltage below which it is best not to go. this behavior is actually true with all batteries, even Pb ones.
Temperature considerations
The worst enemy of lithium cells is certainly low temperature. The cells, as indicated in the datasheet, should not undergo charging processes at temperatures below 0°C. From our tests it is however possible to carry out charging cycles at temperatures below zero with a significant reduction in overall performance.
However, it should be considered that a SmartBox rarely reaches such internal temperatures because the electronics always generate heat which always keeps the cells at a higher temperature even in extreme environments.
However, below are the specifications of the cells used:
| Description | Suggested |
|---|---|
| Suggested SOC usage window | 10%-90% |
| Charging Temperature | 0°C-45°C |
| Discharging Temperature | -20°C-55°C |