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I was reading in the electrical systems of the a320 FCOM and in one of the notes it stated that the batteries have an automatic cutoff logic which prevents them from getting completely discharged when on the ground. (Parking)

But how does this logic exactly work? As the batteries are permanently connected to the hot bus in which they will continue to discharge

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    $\begingroup$ Not an expert but some kinds of batteries cannot be completely discharged without affecting its life cycle and can even require a replacement instead of a inexpensive recharge. Of course in flight use is critical and thus must be allowed to be drained to the last bit $\endgroup$ – jean Aug 9 '18 at 20:28
  • $\begingroup$ @Jai, are you looking for a description of the logic (parameters etc.), or a description of how this logic actually (mechanically/electronically) prevents battery drainage? $\endgroup$ – Jpe61 Oct 6 '19 at 14:12
  • $\begingroup$ @Jpe61 I'm looking for a description of how the logic prevents battery drainage specifically when the aircraft is parked on ground. $\endgroup$ – Jai Oct 6 '19 at 15:55
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    $\begingroup$ @Jai selectstriker2 has in his answer described the conditions required for cut-off, and the method used (automatic contactors), how specific an answer do you want? $\endgroup$ – Jpe61 Oct 6 '19 at 16:09
  • $\begingroup$ @Jpe61 the same answer is also available in the FCOM. I kindly request you to read the description I have put. How does the system prevent complete discharge of the batteries when the aircraft has been parked on the ground. Since number one the battery contactors are now open when on the ground preventing charge and secondly the batteries are permanently connected to the hot bus which will cause them to discharge. So how does this system prevent complete discharge? That's my question $\endgroup$ – Jai Oct 7 '19 at 6:27
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According to this page: http://theflyingengineer.com/projects/airbus-tech/electrical/

A Battery Automatic cut-off logic prevents the batteries from discharging completely when the aircraft is on the groung [sic] (Parking).

Automatic battery contactors open when :

  • The a/c is on the gorund [sic].
  • The BAT P/b s/w’s are at AUTO
  • The main power supply (EXT PWR+GEN) is cut off
  • Battery Voltage is low.

Nothing should automatically disconnect the batteries from the corresponding HOT BUS, so if something is left on the HOT BUS it would continue to drain the battery.

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The battery automatic cut off only works when the batteries are left on AUTO. When the battery voltage is low, the battery contactors automatically open, which cuts off power from battery bus and the DC essential bus which preserves the battery charge. It is there for a situation where someone forgets to turn off the batteries during an aircraft securing.

enter image description here Battery contactors.

However, the hot buses remain powered. So, the logic does not entirely prevent the batteries from discharging. The two voltmeters for the batteries on the overhead panel are actually connected to the hot bus and they keep showing the battery voltage even with the batteries off. And this can slowly discharge the batteries. That is why, it is asked in the FCOM to check if the batteries have a minimum Voltage of 25.5 V, if the aircraft was parked unpowered for more than 6 hours. And if the Voltage is below the limit you need to connect the external and charge them for 20 minutes.

When the aircraft has to be parked for a long period of time, the batteries have to be physically disconnected from the aircraft. According to our engineers, they take the batteries off, if the aircraft has to be parked for more than two days.

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