Charger Safety Relay

This relay is used by the BMS to control a battery charger (or other charge source in a defined charging period). If enabled, this relay output on the BMS will be driven low to allow charging until the BMS determines the charger should shut off, at which point it will float high. This relay latches off by default, but can re­engage based on certain conditions if programmed to do so.

For CANBUS­enabled chargers, the status of this relay is transmitted digitally in addition to controlling the physical relay output. For this reason, this relay is strongly recommended to be enabled for most applications, even when the physical relay is not used.

Important Note: Disabling this relay alters the behavior of the BMS unit. If this relay is enabled, the Charge Current Limit will latch at 0A once the relay output turns off to prevent any further charging. This prevents any charging from occurring since the battery pack is full. There are several conditions listed on this settings page for re­enabling the output. Once the output is re- enabled, then the Charge Current Limit will go above 0A. The CCL will not latch at 0A if the relay is not enabled.

**Click here for a functional flowchart of how the relay works.

Settings:

?Enable the charger safety relay output?

Check this box in order to use this relay. If this check box is not checked, the BMS will not use this output. The behavior of the BMS will be altered if this is not enabled. Please refer to the Operational Manual for more information.

Trip relay off only when charge current limit hits 0 amps

Trip relay off if charge current limit hits 0 amps OR pack current exceeds charge current limit by XX %

The charger safety relay will always turn off in the event that the charge current limit drops to 0 amps (which always happens any time any cell reaches the maximum voltage, but also happens for other criteria ? see Charge Current Limits in the operational manual for more details). Selecting the second option will additionally cause this relay output to turn off in the event that the BMS measures actual current flowing in the battery in excess of the charge current limit at the time. The measured current must exceed the current limit by the programmed percentage in order to trip the relay off.

Important note: If the relay trips off due to high current, the relay will remain latched off until CHARGE power is removed from the BMS. The options below for allowing the charger safety relay to turn back on will not cause the relay to re­engage if it trips due to current. Note that while all relays have the option of using this feature, the programmed percentage is shared between all relay outputs.

Max time [minutes] that relay can stay on XX This setting allows the user to specify a maximum total amount of time that the relay can be on. The timer is reset when CHARGE power is first applied to the BMS, and time begins accruing as soon as the relay turns on for the first time. Note that the relay may not always turn on at the same time CHARGE power is applied to the BMS, as could happen if the battery is too cold to charge (in which case, the timer starts accruing when the charger safety relay turns on to allow charging). Once the timeout has expired, the charger safety output will latch off and will not turn on again until CHARGE power is removed from the BMS and re­applied, or the BMS is reset from the BMS software utility.

Turn relay output back on every XX minutes

After the charger has turned off due to a cell reaching the maximum voltage, this option will turn the charger back on after the specified amount of minutes. The charger will then turn off again when any cell reaches the maximum voltage again. One of two options may be specified ? only while cell balancing is still active will cause this happen while balancing is active and then latch off the relay. even after cell balancing has finished will cause the charger safety relay to turn back on every specified number of minutes indefinitely (unless the max time for the charger safety relay to be on is specified and exceeded, in which case the charger safety output will remain off).

The minimum amount of time that can be specified is 15 minutes.

This feature is not recommended for general use and is primarily used only for two scenarios:

1. Increased effectiveness for iron phosphate cells (for new battery packs) ? Due to the shape of the charge curve for iron phosphate cells, balancing can be more effective for brand new packs if the voltage is periodically brought up to the maximum cell voltage. This is typically only useful when the BMS is performing the initial balance on a battery pack. While it may be useful to cycle back on every 20 ? 30 minutes for new iron phosphate battery packs, it is not generally recommended to leave this feature engaged for regular use. It is also not recommended for chemistries other than iron phosphate.

2. While we do not recommend the use of always on DC:DC converters from the traction battery in an electric vehicle, if one is used, it may be necessary to periodically top off the battery. This feature or the feature below can be used to periodically top off the battery.

Important note: While cycling the battery charger during the initial balance of a battery pack may be useful, it is not recommended for production environments and should never be allowed occur rapidly. Cycling the battery charger on and off adds stress both to the battery pack and the battery charger and may decrease the lifespan of both. In applications where it is necessary, care should be taken in calculating the frequency that the charger is cycled back on.

Re­enable relay when SOC is below XX percent AND at least XX minutes have elapsed

This output is used for turning the charger safety relay back on if the battery falls below a minimum state of charge (unless the max time for the charger safety relay to be on is specified and exceeded, in which case the charger safety output will remain off). This is useful for energy storage applications where periodic topping­off of the battery may be necessary and when it is also necessary to top off after a discharge.

Important note: Cycling the battery charger on and off adds stress both to the battery pack and the battery charger, may decrease the lifespan of both, and should be limited to where necessary. In applications where it is necessary, care should be taken in calculating the frequency that the charger is cycled back on.