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Lithium Battery Manual
3. System design
When designing a system with a lithium battery, a basic understanding of how the battery interacts with the BMS and how the
BMS interacts with loads and chargers is required.
3.1. The battery cell alarm signal
The battery communicates with the BMS via its BMS cables. If the system contains multiple batteries, all batteries are connected
in series via the BMS cables and then the first and last battery is connected to the BMS.
The battery monitors its cells and it will send an alarm signal to the BMS in case of a:
• Low cell voltage alarm signal
• Low cell voltage pre-alarm signal
• High cell voltage signal
• Low temperature signal
• High temperature signal
The BMS will act by turning loads and/or chargers off as soon it receives an alarm signal from one of the cells in the battery.
The alarm communication process between the battery and the BMS is depicted in images below.
BMS
The BMS receives an alarm signal from a battery cell
BMS
The BMS receives an alarm signal from a cell in a multiple battery setup
3.2. The pre-alarm signal
The purpose of the pre-alarm is to warn that the BMS is about to turn off the loads due to cell under voltage. For example; you
would want an early warning that loads are going to be turned off while manoeuvring your boat, or if lights are turned off when it is
dark. We recommend connecting the pre-alarm to a clearly visible or audible alarm device.
Switching behaviour
In case of an imminent under voltage shutdown, the pre-alarm output of the BMS will switch on. In case the voltage continues to
decrease, the loads are switched off (load disconnect) and at the same time the pre-alarm output will switch off again. In case the
voltage rises again (operator has enabled a charger or has reduced the load) the pre-alarm output will switch off, once the lowest
cell voltage has risen above 3.2V.
The BTV ensures a minimum delay of 30 seconds between enabling the pre-alarm and the load-disconnect. This delay is to allow
the user a minimal amount of time to prevent the shutdown.
3.3. The BMS
Besides making above pre-alarm signal available, the main purpose of the BMS is to control the chargers and the loads. There
are two ways that it can do that:
1. By sending a signal to the charger or load.
Page 5
System design
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