- The relay in question may not be enabled in the profile settings. By default, all of the relay outputs on the BMS are disabled. This is done as a safety measure to prevent unintended charging / discharging in case the wrong relay output is used by the installer by mistake. To be used, the relay must be enabled on the “Relay Settings” section of the battery profile and these changes must then be uploaded to the BMS.
- The relay may have already latched off or the current limits may prohibit the relay from activating. The discharge enable relay, charge enable relay and charger safety relay will all latch in an off state once the BMS determines that the pack cannot accept any more charge (for charge enable and charger safety) or provide any more discharge (for discharge enable). These are linked to the charge current limit (CCL) and discharge current limit (DCL) and there are many factors that contribute to these limits (battery voltages, pack temperature, resistances, SOC, etc).
NOTE: The output status of all the relays can be viewed by going to the “Live Text Data” screen on the BMS utility and selecting “Advanced Parameters” from the drop-down menu at the bottom of the screen. Additionally, by selecting “Limits Parameters” the charge completion status can be viewed (if the charge completion status is active then this means the charger safety output turned off because the pack is full).
Resolving the issue:
Step 1. Check for conditions that cause the relays to latch in the OFF state.
The following conditions will cause the relays to latch in the OFF state:
- Charger safety relay will latch OFF when the charge current limit (CCL) hits 0A (indicating no more charge is allowed).
- Charge enable relay will latch OFF when the charge current limit (CCL) hits 0A (indicating no more charge is allowed).
- Discharge enable relay will latch OFF when the discharge current limit (DCL) hits 0A (indicating no more discharge is allowed).
NOTE: Even though the respective current limit may be reading a positive number while diagnosing the problem, the relays will latch OFF once the limit hits 0A during operation (even briefly). A positive current limit does not necessarily imply that the relay should be engaged.
The following conditions allow the relays to RE-ENGAGE under certain circumstances:
- The charger safety relay output can re-engage after a set period of time by adjusting the “Charge Re-enable Timer” parameter on the “Cell Settings” section of the battery profile. This means that the charger safety relay will turn back on after X number of minutes have passed since the charger safety relay turned off.
- The charge enable relay can re-engage either when the charge current limit (CCL) rises to a certain level or the state of charge (SOC) drops below a certain percent. These settings are available on the “Relay Settings” section of the battery profile settings.
- The discharge enable relay can re-engage either when the discharge current limit (DCL) rises to a certain value (eg: the pack voltage recovers, the pack warms up, the SOC goes up, etc) or when the state of charge (SOC) rises above a certain percent. These settings are available on the “Relay Settings” section of the battery profile settings.
- All relay outputs can re-engage once the BMS is power cycled (that is, either restarted or when the BMS loses both Charge Power and Ready Power and goes into low power sleep). The most common example of this would be plugging an electric vehicle into the grid, having the charger safety relay turn off once charge is complete and then unplugging the vehicle (BMS goes to sleep). Once the vehicle is turned on or plugged back in to the grid, the BMS will allow the relays to turn back on again.
Step 2. If the BMS has not been power cycled since the relay was enabled in the profile settings, try restarting the BMS.
The relays are latched off by default, if the relay is being enabled for the first time it is advised to restart the BMS to activate the relays. Methods to restart the BMS:
- The easiest way to reset the BMS is via the BMS utility. This can be accomplished by going to “File” -> “Restart BMS”.
- The BMS can be power cycled by removing both Charge Power and Ready Power (on the Main I/O harness) allowing the BMS to go into low power sleep. Re-applying power to Ready Power or Charge Power brings the BMS back out of sleep and the relay statuses are reset.
- The entire Main I/O harness connector can be unplugged from the BMS and re-inserted to cause the BMS to restart.
Step 3. Check to ensure that discharge is not prohibited in the current operating mode.
The BMS allows for discharge to be prohibited unless the Ready Power signal is energized (signifying that the application is ready to draw power). It also allows for discharge to be prohibited if both Ready Power and Charge Power are energized at the same time (in an electric vehicle, this may indicate that the car is still plugged in to the grid while the driver is attempting to drive the car away). If these options are enabled, verify that the appropriate conditions have been satisfied before proceeding. These settings are available on the
“Relays” section of the battery profile settings.
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