The LiFePO4 battery: preventing cell under-voltage, over-voltage, and over-temperature The first line of protection is cell balancing. All Victron LiFePO4 batteries have integrated cell balancing.
The second line of protection consists of:
- shut down of the load in case of an imminent cell under voltage, and
- shut down or reduction of the charging current in case of an imminent cell over-voltage, high temperature
(>50°C) or low temperature (<0°C).
The VE.Bus BMS is the core of the second protection line.
However, not all loads or chargers can be controlled directly by the VE.Bus BMS.
In order to shut down such loads or chargers several VE.Bus BMS controllable Cyrix switches are available.
Cyrix-Li-load The Cyrix-Li-load will disengage when its control input becomes free-floating.
If the battery voltage recovers after disconnecting (which will happen when no other loads are connected to the
battery), the output of the BMS will become high and the Cyrix will reengage after 30 seconds. After 3 attempts
to reengage, the Cyrix will remain disengaged until the battery voltage has increased to more than 13V (resp. 26V or
52V) during at least 30 seconds (which is a sign that the battery is being recharged).
Cyrix-Li-Charge The Cyrix-Li-Charge will connect a battery charger with 3 seconds delay:
- if the Charge Disconnect output of the VE.Bus BMS is high, and
- if it senses 13,7V (resp. 27,4V or 54,8V) or more on its battery charger connection terminal, and
- if it senses 2V or more on its battery terminal (the Cyrix will remain open, if not connect to the battery).
The Cyrix-Li-Charge will disengage immediately whenever its control output becomes free-floating, signaling cell
overvoltage or cell over temperature.
In general, a cell over voltage alarm will reset shortly after charging has been stopped. The Cyrix will then
reconnect the charger. After 2 attempts to reengage with 3 seconds delay, the delay increases to 10 minutes.
Whenever the battery voltage is less than 13,5V (resp. 27V or 54V), the Cyrix will disengage with a delay of 1 hour.
Note 1 : In the case of zero discharge current, or small discharge current, the Cyrix will not disengage shortly after
the charger has been switched off and/or disconnected because battery voltage will remain higher than 13,5V.
Note 2 : If, after the Cyrix has disengaged, the output of the battery charger immediately increases to 13,7V or
more, the Cyrix will reengage, with 3 seconds delay.
Cyrix-Li-ct The functionality of the Cyrix-Li-ct is analogous to the Cyrix-ct.
The Cyrix-Li-ct will parallel connect a lead-acid starter battery and a LiFePO4 battery:
- If the Charge Disconnect output of the VE.Bus BMS is high, and
- If it senses 13,4V (resp. 26,8V) or more on one of its power terminals.
The Cyrix will disengage immediately:
- When its control output becomes free-floating, signaling cell over voltage or cell over temperature,
and/or
- When the battery voltage drops below 13,2V.
Start assist function: a short positive pulse will close the relay for 30 seconds (see figure on page 2).
A built-in transient voltage suppressor will limit the voltage spike that may occur when the Cyrix suddenly
disengages due to cell overvoltage or over temperature.
