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IRS DC FAIL

Started by emerydc8, Sun, 30 Dec 2018 23:20

emerydc8

Right. With a loss of all generators the service bus is not powered, so the battery chargers won't be charging.

John H Watson

QuoteIsn't the 767 text above contradictory if it says "each charger can recharge a completely discharged battery" -- and then further below it mentions the 4v limit? I would have thought "completely discharged" means 0v.

Boeing does this all the time. It's like caging gyros... In one part of the manual it tells you to release the knob quickly. In another part it says do it smoothly (almost contradictory). Boeing clarified it for me... release the knob quickly, but don't let it slam into the case.

A battery can obviously discharge to zero (as there are devices on the hot battery bus which don't use relays). They can be recharged from dead flat in the workshop. They aren't thrown away.


Hardy Heinlin

Regarding the 4v limit on the 767 battery charging system again:

Does anybody disagree if I say that this 4v limit is also applied to 747 batteries?


Regards,

|-|ardy

John H Watson

It's also mentioned in the 744 books... but I'm not sure why the "when first turned on" is added.

QuoteThe battery charger does not turn on unless the battery
voltage is above 4 volts when power is first applied to the
battery charger.

Also in the Maintenance Manual:

Quote(7) A discharged battery (20 volts) should completely recharge within 75 minutes. If the charge
command continues beyond this maximum charge time and approaches 100 minutes, the
charger will switch into the constant voltage charge mode (27.75 volts).
(8) If the battery sensing connector is disconnected, a sensing loop in the charger will be opened
causing the charger to shut down.
(9) The battery charger will not start unless the battery voltage is above 4 volts when power is first
applied to the battery charger.

emerydc8

If it takes 75 minutes to charge from 20v to 24v, then presuming the charging is linear (a major presumption), it would take 6.25 hours to go from 4v to 24v. Does that sound right?

Hardy Heinlin

#25
Quote from: John H Watson on Wed, 16 Jan 2019 08:28
I'm not sure why the "when first turned on" is added.

Perhaps because when the total discharge happens in flight, you want to keep the charger powering the bus and ignore the battery costs; and when it happens at the gate you have the chance to replace that battery.


Cheers,

|-|ardy

John H Watson

Quotethen presuming the charging is linear (a major presumption)

I don't have a graph for battery charging below 25 volts. The graph above 25 volts is quite bizarre (definitely non-linear). Initially, constant current is used to charge the battery (this results in a non-linear voltage).

QuotePerhaps because when the total discharge happens in flight, you want to keep the charger powering the bus and ignore the battery costs;

Confusing. I can't see, from the schematics, how the charger knows the battery volts (as distinct from the voltage on the hot battery bus). If the volts on the hot battery/battery busses are decreasing all the time down to below 4 volts, then recharging is pointless and perhaps dangerous. There's either a major short in the battery or on the battery busses. Note: If the battery gets too hot, an overtemp switch in the battery will activate a relay to stop Ground Service Bus power reaching the charger (This is assuming the hot battery bus has enough power to activate the coil of the relay). I assume "first turned on" refers to power first reaching the charger via this relay. If overtemp activates the relay (and there is sufficient power to activate it), shutting off power to the charger, then, if the battery cools down but is still below 4 volts, the charger will not reactivate.
The battery also has a temperature sensor which talks to the battery charger and will stop the charger operating via charger internal logic.

emerydc8

QuoteI don't have a graph for battery charging below 25 volts. The graph above 25 volts is quite bizarre (definitely non-linear). Initially, constant current is used to charge the battery (this results in a non-linear voltage).

Thanks, John.
Quote
On 744's, I seem to recall that it was difficult to get power on the ship with a significantly discharged battery.

Were you referring to a situation where you had a weak/dead battery and with ground power available you tried to close the external power relay to power the AC tie bus? Maybe EXT 1/2 have a minimum battery voltage to activate a relay too.


Hardy Heinlin

When the battery provides 25v and the charger is disconnected, the EICAS indicates 25v.

When that same battery (25v) is being charged by 28v, the EICAS in PSX still indicates 25v. Shouldn't it indicate 28v then? How can the voltmeter sense the battery's theoretical output voltage (25v) while the charger sets 28v at the battery poles?


|-|ardy

John H Watson

Quote from: emerydc8Were you referring to a situation where you had a weak/dead battery and with ground power available you tried to close the external power relay to power the AC tie bus?

Yes, as I recall (vaguely).

Quote from: emerydc8Maybe EXT 1/2 have a minimum battery voltage to activate a relay too.

Sorry, my memory is a little fuzzy of the individual circumstances. In theory, Ext Power #1 supplies power to Bus Control Unit #1 directly (not through relays) and the BCU looks at external power quality. When you push EXT PWR #1 button, it commands BCU #1 to close the #1 external power relay (using dc power derived from the direct AC external power source) if the Ext Power source is good. So, in theory, there should be no Ext #1 relay problems due to a flat battery.

I don't know if power going to BCU#2 has any bearing on what happened. Many moons ago, we used to only plug in one Ext power source (until we found that Ext 1 power leads were overheating). Also, many moons ago, the aircraft were wired differently and BCU#2 had less options regarding power sources. If Ext 2 wasn't plugged in, BCU#2 may have relied on battery power (to close things like the #2 Ext Power relay and the SSB). PSX models the later circuits, so you may not see what I saw. Now I'm starting to wonder if we did get half the ship powered, but not the other half (due to BC#2 and SSB problems).

Quote from: HardyWhen that same battery (25v) is being charged by 28v, the EICAS in PSX still indicates 25v. Shouldn't it indicate 28v then?

I believe so.
If you recall, the charger has different modes. If the battery voltage drops to a certain value*, the charger is switched to a constant current charging mode and as a result, the battery voltage starts to rise. It deliberately overcharges the battery, then switches to a constant voltage charging cycle (which is 27.75 volts). As far as I know, this is what you should see on the EICAS, but the graph shows battery overcharging voltages up to (just over) 31 volts. I can't say I noticed this in real life. Of course, it may have happened (I just wasn't watching).

*I recall there are other factors which initiate the constant current charging mode. Hardy... see the charging profile in Elec Book 2, page 255.

(EDIT: The manuals do indeed state that the voltage indications show charger voltages. They say you have to disable the charger to show actual battery voltage).

emerydc8

QuoteMany moons ago, we used to only plug in one Ext power source (until we found that Ext 1 power leads were overheating). Also, many moons ago, the aircraft were wired differently and BCU#2 had less options regarding power sources. If Ext 2 wasn't plugged in, BCU#2 may have relied on battery power (to close things like the #2 Ext Power relay and the SSB).

Thanks for the info, John. On the 744F, we never select EXT#2 since the cargo loading system would lose its power. It's just plugged in and "available." Of course, we don't have the huge galleys and other loads in back to worry about overloading anything.

Hardy Heinlin

#31
The 4 v limit also prevents a flip-flop in the respective battery charger disable relay.

If you look at page 210 of the Aerowinx manual, at the top left of the diagram, MAIN B/C DISABLE RELAY:

When the MAIN B/C voltage is normal and the main battery voltage is zero, and the Standby Power selector is set to BAT, the relay will depower the B/C. And the empty battery is unable to keep the relay energized. So the relay relaxes and repowers the B/C which re-opens the relay etc.

If the B/C stops operating when the battery is below 4 v, the relay cannot open and the flip-flop is stopped.

But it doesn't matter anyway; in PSX you need years to completely discharge the battery by flight deck actions. When all avionics stop using battery power because the voltage is too low, only some lights will continue to discharge the battery.

There is a malfunction called "Battery bus failure" for either battery. This activates a "battery cell open circuit" like in the big sims. This sets the battery voltage to zero instantly.

I just implemented the 4 v limit. Strangely, the flip-flop was already prevented before I implemented the 4 v limit :-)


|-|ardy

emerydc8

Thanks, Hardy. So the trigger is below 4v. If it sees 4v the chargers will still charge?

Jeroen Hoppenbrouwers

3.999V vs 4.000V I bet.

Hardy Heinlin

#34
My charger deactivates when below 4.0000 v.

Now I know why the flip-flop was already prevented before I added the 4 v limit: My charger disable relay can be energized by any voltage greater than zero. It's the only relay that has this magic. I think I did it intentionally. I don't know how the real system would behave; the disable relay may already relax at 7 v or whatever, and at that point the flip-flop would start, theoretically, in the real system.*

Anyway, it's nearly impossible to get in such a low voltage range, even with time acceleration set to factor 64. In that range the only consumer of battery power is the relay coil itself (280ma) -- and the BCU if not powered otherwise.


|-|


* Theory: There may be another hidden system component that can detect whether a battery is installed at all, or whether the battery has an open circuit. If the charger is the only power source to the hot battery bus, you want that power -- and you get that power if you don't set the standby power selector to BAT. But if it's set to BAT on the real deck, you may hear a continuous rattling noise -- or not, if the system can detect it.

John H Watson

Quote* Theory: There may be another hidden system component that can detect whether a battery is installed at all, or whether the battery has an open circuit.

I don't know about the open circuit, but the charger knows if there is a battery physically installed. Installing the battery completes an "interlock" circuit (i.e. a loop of wire running from the charger to the battery and back again)

JRBarrett

#36
I can't speak to the battery charging system or batteries in a 747-400 specifically, but in general, a NiCad aircraft battery can be fully discharged to zero volts without harm. In fact, doing so is actually a regular part of NiCad battery maintenance, but a full discharge and recharge is normally done on a test bench where individual cell voltages can be monitored during the recharge cycle.

By contrast, fully discharging a lead acid battery will quickly cause irreversible  damage to the cells in the battery. If you leave your headlights on all night in your automobile, you can probably get the vehicle started with jumper cables, but even after the battery is fully recharged by the alternator, it will never again be as good as it was before. Its overall lifespan will have been shortened considerably.

Aircraft NiCad batteries come in a variety of capacities - 40 amp/hours seems typical. If a 40 AH battery has been fully discharged, it must be recharged using constant current at 40 amps for a minimum of one hour to restore basic capacity. When done on the bench, it is typically overcharged an additional 4 hours at 1/10 C (which is 4 amps for a 40AH battery). At the end of the topping charge in constant current mode, the charger voltage will be up around 32.5 volts. On a test bench, this would be the point where the electrolyte levels in the individual cells would be adjusted/topped off with distilled water.

In an aircraft which has a dedicated battery charger, it would be essential that no load be placed on the battery for at least the first hour of recharge. The aircraft charger will typically use constant current until the combined cell voltage reaches 28.5 volts, at which time the charge will float in constant voltage mode.

Though the aircraft charger can "bring the battery back" from a full discharge, it is not really designed for that purpose. If an accidental full discharge happens at a location where fresh spares are available, I'm sure maintenance would want to install a replacement battery rather than trying to use the aircraft charger to restore the dead unit.