Battery Charger=tru?

[LEM]

Hy,
Can't we say that the battery charger is a Transformer Rectifier Unit?
Turning 115/400 AC into 28 DC, that's the job of a TRU....

[mono]

Most dedicated battery chargers are much more than a TRU. Nicad batteries require for optimum charging a constant current form of charge (except on the airbus of course-they have to be different)

Therefore a battery charger output voltage is carefully varied such that it will supply a constant charging current to the battery.

Further refinement and research has lead to the development of the reflex charger. With this type of charger the battery receives pulses of current (shown to further increase battery life and reduce the chances of thermal runaway). Many of the more modern a/c types now have this type of charger fitted.

Under certain circumstances however, a battery charger can convert to TRU mode, when it will produce a regulated 28 volt dc output. Some triple autoland a/c force the charger into TRU mode in order to provide a 3rd independant DC power source for autoland capability.

[LEM]

Very informative as usual, mono.

My second question was going to be about "The limited pulse type battery charger converts 115V AC to 28V DC. (B733), but you seem to have already answered that.

Third question (eventually I'll send you a box of italian wine!):

"With the ground cart plugged in,, the ground service bus powered, the Battery Switch ON, and the DC Meters Selector in the BAT position, the voltage will read 28 volts, indicating the hot battery bus is being powered from the battery charger." (B733)

I seem to understand this is the only way to check if the battery charger is working (or not?):confused: , and why does the ground cart have to be plugged in?


Thanks a lot
LEM

[mono]

Looks like ASFKAP beat me to it.

In a word LEM yes the external power has to be in. It is one of 3 preconditions for the charger to be in TRU (constant potential) mode.

These are as follows.

(1) Auxiliary relay to bat bus relay R327 energizes due to loss of T-R 3 output.
(2) Standby power relay manual R328 energizes when STANDBY POWER switch is set to BAT.
(3) Ground refueling power select relay R10 energizes when ac external power is connected to the external power receptacle.

The charger will also be in TRU mode when battery voltage is at or above the charger threashold value.

When the charger is in charger mode the battery potential may be 2-3 volts above 28Vdc It may also be below 28Vdc but steadily rising (especially after an APU start)

If the charger is not working expect to see 2-3 volts (or more) below 28Vdc

Another quick check for correct charger operation would be to move the stby power switch to BAT for several seconds and then to auto. In the BAT position the meters should show 28Vdc, when auto is selected the voltage should rise to 31/32Vdc then slowly fall back to 28V.

I think that's about right but it's a long time since I worked the 737.

Cheers.

[LEM]

Thanks Gentlemen for your kind replies.

I must admit I'm having difficulties getting the whole picture 100% clear.

To avoid asking too many questions, I'll try in another way:

Please correct me if wrong:


1)"Another and more basic question: What is the best way to get a good reading of the actual battery "quality" (voltage?), prefferably without depowering APU or ground pwr, if possible? "


Select STBY PWR to BAT position. The Battery voltage reading is the real one.


2) When starting the APU, as soon as the starter cuts out, I can check the battery charger is functioning correctly by observing a temporary voltage rising in the BAT position, not only by checking the ammeter reversing from "-" to "+".

Or is it sufficient under this scenario to check the ammeter going "+" to say "THE BATTERY CHARGER IS WORKING"?

[mono]

If I read Capt Haddock's question correctly, the only way to check BATTERY voltage is with the meters in the bat position and with the Batt charger cb pulled or with an airplane powered only by the battery. With the AC busses powered the battery charger feeds the hot battery bus and may mask a poor battery. On power up prior to starting the APU or connecting ground power the battery voltage should be checked for at least 24Vdc.

Note the GPU only needs to be supplying power and connected to the a/c. Ext power does not have to be selected on. As long as the GPU is live and connected to the a/c R10 will be energised and the batt charger will be in TRU mode. Therefore selecting ext power off will NOT give true battery voltage.

regarding the second question, this depends on the scenario LEM

If no other power is available, after starter cutout the battery voltage will slowly recover but he ammeter will still show a -ve current flow. It will not start charging until the APU geny comes on line.

If another power source is connected to the busses then as soon as the starter cuts out the charger will start to recharge the battery and current will show +ve.

I would always recommend belt and braces, so check both the ammeter and the voltmeter.

[80/20]

The constant voltage trickle-charge function maintains full battery charge and prevents internal gas development. The limited pulse-type charger normally powers the hot battery and switched hot battery bus, and maintains the battery at full charge by converting 115V AC to 28V DC. Reading BAT BUS voltage or even selecting STBY PWR to BAT position will give a reading influenced by the charger and is not a good indication of real battery voltage or “quality”.

During the APU start cycle, the battery charger is automatically disconnected by the APU starter battery charger cutout relay to prevent charger overheat. This is the best time to get a good voltage reading from the battery since it will show how the battery handles the load without any assistance. The 737-300/400/500 maintenance manual will only permit one APU start attempt if voltage drops below 22 (if the APU starts at all). This is to prevent a poor battery from being dispatched.

I take my BATT voltage reading during APU startup. I check the charger by watching how it should send series of short pulses to the positive side of the DC AMPS METER after APU start and when the battery is fully charged.