This is a 767 question, but I think it should also apply to the 744 as well. I was in KSEA this morning and discovered, upon showing up at the plane, that someone -- probably one of the ground personnel -- turned the battery switch on and left the airplane overnight.
So the battery was down to 4v. Of course, maintenance suggested that because the AC and DC busses were powered via the ground power unit, it wouldn't be long before the battery was charged up. I wasn't optimistic at all. It's true that with external AC power connected, the battery bus is powered by the left main DC bus and the hot battery bus is powered by the ground service bus via the battery charger (similar to the 744 IIRC); but the battery itself is still dead and I'm not entirely convinced that a battery that has dropped down that low could even be charged. And even if it could be charged, I would have some concerns about overheating issues.
We don't have the luxury of seeing the battery voltages on the lower EICAS like you can on the 744. I guess that's the evolution you can see from the 767 to the 744. As pilots, we have no minimum voltages for dispatch because we can't even check them. It is strictly a maintenance function to display the data in the picture below. Even then, I think it can only be displayed on the ground. So, I was sitting there hoping there would be something that prevented us from getting too far into the pre-flight setup with this very weak battery, because I really didn't want to take this thing into the air like this.
Sure enough, within a minute of turning the IRUs to NAV, I saw the DC FAIL on all three units. That ended the discussion and we wound up replacing the battery. But it got me wondering -- What voltage is the IRU looking for in order to satisfy its internal DC test? Also, I'm not positive, but I think there is a minimum battery voltage required just to close the battery bus relay. Anyone have any info on this? I'm thinking the 744 is very similar. Thanks.
Jon
(http://www.hoppie.nl/forum/var/dead-main-battery.jpg)
Jon,
I recall reading that one of the conditions for battery charger shutdown to occur is sensed battery voltage of 4 VDC or less, which happened in your case.
I would guess that a good test to determine wether dispatch would be "safe" when maintenance gets pushy (without specific guidance regarding minimum battery voltage) would be to place standby power selector to BAT position. If you loose standby equipment, hold the hotel van, please. EICAS maintenance pages would be a much better option (although we are not allowed to touch them, unless it is to attempt to clear STATUS messages before dispatch).
Recently, during "peak season" with a mechanic onboard, we had an issue that triggered a STATUS message (can't recall what it was). We involved him in the discussion, after which he quickly displayed the related maintenance page in lower EICAS (in flight). Up until then, I also thought they could only be displayed on the ground.
When I get a chance, I will see if I can find the minimum voltage required to satisfy IRU ON DC test.
Regards,
Mariano
From a 744 AMM:
"The right and left IRUs remain powered from the airplane battery until the voltage drops below 18 volts...".
Regarding minimum battery voltage for satisfactory IRU ON DC test, I'm guessing the number is eighteen (although probably higher than that).
I hope resident engineers can chime in.
Regards,
Mariano
QuoteSo the battery was down to 4v. Of course, maintenance suggested that because the AC and DC busses were powered via the ground power unit, it wouldn't be long before the battery was charged up. I wasn't optimistic at all.
We would have changed the battery if it was down to 4V. A significantly depleted 767 battery can take up to 75 minutes to recharge (which would probably cause a flight delay).
Fortunately, I was at main base, so there was always a plentiful supply of fresh batteries in the workshops.
Don't worry about the battery overheating. The charger knows how hot the battery is, and won't charge the battery if it is too hot. As Mariano says, the chargers have logic in them which prohibits the recharging of a battery if it's below 4volts.
On 744's, I seem to recall that it was difficult to get power on the ship with a significantly discharged battery. I don't know if this was main bus power only (It may have been ok down to a fairly low value with Ground Service Bus power applied. The GSB allows the batteries to recharge).
QuoteRegarding minimum battery voltage for satisfactory IRU ON DC test, I'm guessing the number is eighteen (although probably higher than that).
My training manuals do say 18Vdc is required for an IRU on DC Test pass.
18V DC is also the limit in PSX.
QuoteFrom a 744 AMM:
"The right and left IRUs remain powered from the airplane battery until the voltage drops below 18 volts...".
Thanks, Mariano. I should have thought to go to the 744 manual to find it. Our 767 manuals don't list a voltage.
QuoteWe would have changed the battery if it was down to 4V. A significantly depleted 767 battery can take up to 75 minutes to recharge (which would probably cause a flight delay).
The mechanic let the battery charge for about 20 minutes while he was negotiating with Delta over a new battery. It didn't budge from 4v during that time period. When you say 75 minutes to recharge, is that using a special charger or is that with the aircraft charger? Maybe Mariano is right about a certain minimum voltage to even start the charging process and 4v was just too low.
Quote18V DC is also the limit in PSX.
Thanks, Hardy. I should have known this bridge had already been crossed by PSX, probably long ago. In addition to the IRS DC FAIL, I noticed that the fuel spar valve lights were illuminated and there was an APU fuel valve EICAS. The fire squibs wouldn't test either. All these items are on the hot battery bus. I would have thought that if there was DC power going to the hot battery bus from the charger that these items would still operate. I wonder if it takes 18v to operate them too. Obviously the IRUs are looking at the battery output, not the voltage supplied to the hot battery bus from the charger.
In any case, thanks to all of you for the input.
Jon
I believe that the hot battery bus, by definition of hot, shares its voltage with all peers, both providers (the battery and the charger) and consumers (IRS, fuel valves, and the rest that sits on the hot bus). Hence, all peers must be able to cope with all voltages between zero and 32 Volts. Probably all equipment on the hot bus is able to determine for itself whether the bus provides sufficient voltage to operate, whether it comes from the charger or the battery. 18V then seems a reasonable number for all of them. Possibly the valves may go lower?
Likewise, a hot bus should not allow a charger to charge the battery while NOT powering any other equipment on the bus. True?
Hoppie
Quote
Likewise, a hot bus should not allow a charger to charge the battery while NOT powering any other equipment on the bus. True?
You would think so, but how is it that 28 volts is on the hot battery bus via the charger yet the spar and APU fuel valves that are powered by that bus are still not working?
Quotebut how is it that 28 volts is on the hot battery bus via the charger yet the spar and APU fuel valves that are powered by that bus are still not working?
The battery charger cannot selectively choose not to charge the battery (because the battery is below 4V) but then provide power to the Hot Battery Bus. Everything is in parallel with no relays between them.
The 4Vdc provided by the battery is probably not enough to activate the motorised spar valve. Note that the SPAR VALVE light on the 767 (above the fuel cutoff switches) uses power from the Master Dim and Test System. This will not be powered by the hot battery bus.
Also, should the charger bluntly put 28V on the bus, it would probably overcharge the battery (sitting at 4V) to destruction in seconds. To limit the charge to, what, 50A or something (?), the only thing the charger can do is to limit its output voltage. Therefore the hot battery bus will see, 5V, 7V or something and most loads will still not engage.
When the battery charges, the voltage required to maintain the appropriate charging current will rise, and the charger will steer that. At a certain moment, loads will engage, and the charger will need to provide more and more current and voltage, until it matches the battery and the system stabilizes at about 28V.
Even if the charger would be a steady 32V output voltage and the current limiter would be a simple (but heavy) resistor, the hot battery bus would still see that increasing voltage over time.
Hoppie
If the books say the charger can charge from 4volts, then I'm sure the charge profile in the charger can handle the battery charging without destroying the battery. 38 amps is used to charge the battery.
QuoteAs the battery charges, the output voltage varies with the
battery state of charge and load resistance. Initially it is about 28
volts dc, and the current remains at 38 amperes. As the battery state
of charge increases, the output voltage rises until the inflection
voltage is reached.
The inflection voltage varies with battery temperature (higher
temperature gives a lower inflection point) but is about 31 volts dc at
normal ambient temperatures. At the inflection voltage the battery
voltage increases until the appropriate overcharge is completed.
The amount of overcharge is a fixed percentage (12%) of the time it
takes for the battery to reach the inflection voltage.
After the overcharge period is complete the battery charger switches
to the constant potential mode where it charges the battery at a
constant voltage of 27.75 volts dc limited to 38 amperes.
NOTE: The battery charger will revert to the constant current
mode if a power interruption occurs or the battery voltage
falls below 23 volts.
Quote
If the books say the charger can charge from 4volts, then I'm sure the charge profile in the charger can handle the battery charging without destroying the battery.
Did you find something in the book referring to a minimum voltage for charging the battery?
Quotethe books say the charger can charge from 4volts
The 4 volts is referring to the battery voltage. As your battery didn't charge, perhaps the
displayed 4 volts you saw may not have been a real 4 volts, but a rounded value, say, 3.7 volts (I recall that Hardy had a few theories on rounded voltages). The charger may have been using a
non-rounded value, so refused to charge the battery.
Just perusing the AMM on this and I see that the chargers will shut down with a sensed battery voltage of less than 4v.
Does anyone know the minimum battery voltage to close the battery bus relay (744 or 767)?
(http://www.hoppie.nl/forum/var/pages-from-24___126.jpg)
Quote from: emerydc8Just perusing the AMM on this and I see that the chargers will shut down with a sensed battery voltage of less than 4v.
Sorry, I thought that was already understood. That was what I was implying when I said...
Quote from: John H WatsonIf the books say the charger can charge from 4volts, then I'm sure the charge profile in the charger can handle the battery charging without destroying the battery.
Quote from: emerydc8Does anyone know the minimum battery voltage to close the battery bus relay (744 or 767)?
I'm not sure why this is important in your situation. As long as the battery is actually 4 volts or above, the battery charger will be operating and supplying 30 or more volts to the hot battery busses and the battery relays (assuming you haven't got the Standby Power switch in BAT).
The battery relays (specifically) are not intelligent. They are simply mechanical devices with coils and springs and things. They will close at a certain current depending on age/condition, circuit resistance, etc.
If you are attempting to power the busses with only a half-charged battery, as soon as the relays activate, the equipment will be attached to the battery and the current drain from the battery may be so great, the relays might de-energise again. There's no point pushing the battery switch until you can assure that all the equipment currently connected to the battery will function properly.
Am I right if I think the 4v limit is also applied to 747 batteries? In PSX the charging can work from 0v on.
Isn'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.
I believe "completely discharged" means "zero from a rechargeability point of view." Operationally ok, just empty, like an empty fuel tank. Fuel tanks also cannot be emptied by regular pumping below a certain level. Battery drain shuts off at 4V. The rest is residual (there is no electron scavenger :-) ).
A battery below 4V isn't discharged -- it is broken and needs to be repaired. A charger cannot do this. Like a leaking fuel tank.
Hoppie
Understood.
I just took the term "charge" as synonymous with "voltage". No charge, no voltage.
QuoteI'm not sure why this is important in your situation. As long as the battery is actually 4 volts or above, the battery charger will be operating and supplying 30 or more volts to the hot battery busses and the battery relays (assuming you haven't got the Standby Power switch in BAT).
I was thinking more along the line of what would happen if we took it in the air with say 10v and lost the generators. Would the battery relay stay closed with only 10v? Of course, it's purely theoretical because if you need 18v to align the IRUs then we wouldn't be taking it anyway. The reason I ask is that I got into a discussion with our fleet manager about this. The DC-8 used to have a minimum battery voltage to close the battery relay and power the battery bus, although it's been so long I've forgotten what it was. Probably anyone that did know has long since passed away.
Quote from: emerydc8 on Thu, 10 Jan 2019 01:28
Would the battery relay stay closed with only 10v?
You mean 10v from the battery and 0v from a failed charger?
Right. With a loss of all generators the service bus is not powered, so the battery chargers won't be charging.
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.
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
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.
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?
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
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.
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.
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
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).
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.
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
Thanks, Hardy. So the trigger is below 4v. If it sees 4v the chargers will still charge?
3.999V vs 4.000V I bet.
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.
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)
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.