Low Battery Cut-Off with FNDC

[raysun]

Lithium batteries are a challenge to integrate with FX class inverters due to the mismatch between Low Battery Cut-Off (LBCO) voltage required by many batteries and the LBCO voltage range settable in the Battery Protection parameters of the FX class inverters.

The FLEXnet DC battery monitor can be used in this case to track the LBCO voltage, and shut down the inverter output in the event the battery voltage is at or below LBCO.

A system with FX/FXR inverter, FLEXnet DC monitor, Mate, and Hub is required. Also a length of light gauge two conductor wire to connect the FNDC Aux relay to the Inverter / On-Off terminals on the FX.

Wire pair will connect to the Inverter On/Off terminals. The other ends will attach to the Aux terminals on the FNDC. There is no polarity to the wiring.

The FNDC Aux will be programmed for LBCO by setting the (Voltage) Low parameter, (Voltage) High parameter, and Low Enable Delay.

Programming for a 48V nominal battery that requires a 50V Low Battery Cut-Out voltage, 52V Low Battery Cut-In voltage, and LBCO Delay of 10 minutes. Relay Mode = Auto

No other parameters are set.

NOTE: The FNDC Aux is a latching relay. In order for it to be enabled (contacts closed), the battery voltage must first be greater than or equal to (Voltage) High.

[SandyP]

Lithium batteries are a challenge to integrate with FX class inverters due to the mismatch between Low Battery Cut-Off (LBCO) voltage required by many batteries and the LBCO voltage range settable in the Battery Protection parameters of the FX class inverters.

The FLEXnet DC battery monitor can be used in this case to track the LBCO voltage, and shut down the inverter output in the event the battery voltage is at or below LBCO.

A system with FX/FXR inverter, FLEXnet DC monitor, Mate, and Hub is required. Also a length of light gauge two conductor wire to connect the FNDC Aux relay to the Inverter / On-Off terminals on the FX.

Thanks for the write up on LBCO.

Do you feel that the Victron BMV's relay function (Low Battery Voltage and/or SoC) would also be an option for those with FX inverters but no Hub or FNDC?

[raysun]

Sure, the BMV series can provide LBCO Aux switching from a "stand alone" battery monitor.

[SandyP]

Hmmm, looks like I will need to upgrade to the BMV-7## series as the earlier models have the relay normally open whilst the 7## series allow you to change that to normally closed.

[raysun]

Hmmm, looks like I will need to upgrade to the BMV-7## series as the earlier models have the relay normally open whilst the 7## series allow you to change that to normally closed.

The 600 series have both Low Voltage and High Voltage sensing logic, do they not? If the relay is NO, then perhaps the Relay High Voltage parameter can be set to LBCI voltage, and the Clear Relay High Voltage parameter set to the LBCO voltage.

FWIW, the FNDC relay is operating like that. It's NO, so the LBCI voltage must be reached first to energize the relay.

[SandyP]

Hmmm, looks like I will need to upgrade to the BMV-7## series as the earlier models have the relay normally open whilst the 7## series allow you to change that to normally closed.

The 600 series have both Low Voltage and High Voltage sensing logic, do they not? If the relay is NO, then perhaps the Relay High Voltage parameter can be set to LBCI voltage, and the Clear Relay High Voltage parameter set to the LBCO voltage.

FWIW, the FNDC relay is operating like that. It's NO, so the LBCI voltage must be reached first to energize the relay.

Yep, it has both high and low settings and you are correct that, based on the manual, you could "trick" it by setting high as low and visa versa assuming they do not have any "hidden" logic that checks the relative values of each.

Rl: Relay low voltage. When the battery voltage falls below this value for more than
10 seconds the alarm relay will be activated.
Rlc: Clear relay low voltage. When the battery voltage rises above this value, the
relay will be de-activated. This value needs to be greater than or equal to Rl.
Rh: Relay high voltage. When the battery voltage rises above this value for more
than 10 seconds the relay will be activated.
Rhc: Clear relay high voltage. When the battery voltage falls below this value, the
relay will be de-activated. This value needs to be less than or equal to Rh.

[raysun]

It should be fairly easy to test on the next charge cycle.
Set Rh = Absorb Voltage - 0.2V
Set Rhc = Float Voltage + 0.2V

For testing, Ah and Ahc parameters could be set to the values above to have a really annoying audible (and visual) indication the programming is working.

Better for real life, set Al and Alc so the alarm is enabled during the LBCO event.