I have been reading parts of the FAA docket at:
Accident Investigations - NTSB - National Transportation Safety Board.
Old stuff, but interesting.
The battery Boeing wanted.
Page 15 of the above docket refers to a Boeing Specification Document” (SCD) which is used to describe an individual airplane component.
The SCD referred to states that “The [APU] battery shall be capable of supplying 18kW constant power for 45 seconds to the Start Power Unit (SPU) with a minimum battery terminal voltage of 20V for three consecutive APU start attempts with 60 seconds rest between each start attempt”.
18kW at 32Vdc needs 562A, and at 20 Vdc needs 900A.
So Boeing was expecting the APU battery to supply between 562A and 900A for 45 seconds at a time.
In contrast,
The battery Yuasa supplied.
The 787 batteries are made up of several Yuasa LPV65 batteries in series. The same current passes through all the batteries.
These batteries are 75Ah types with a Yuasa specified maximum discharge rate of 5C, or 5x75 = 375A.
So Yuasa was not expecting to provide more than 375A.
How much current does an APU on a 787 actually take to start?
The FAA noted this disparity (562A or more called for by Boeing vs. 375A maximum available from the Yuasa batteries).
They then noted that the APU on the incident aircraft actually used less power than the Boeing SCD called for, and that there were sudden changes in power consumption during the start sequence, but that testing was done with “resistive banks”.
The FAA docket says that a start on the APU on the incident aircraft took 35 seconds and actually used power as follows:
Initially about 9.6kW (32V at 300A)
Then 8.7kW
Then 10kW for 10 seconds
Then 4kW
10.3kW for about 20 seconds
Then zero for 47 seconds
Then charging at 46A.
If the voltage available is 31V or less, then the maximum APU start power of 10.3kW requires at least 332A and probably more.
It would be interesting to know what factor limits the Yuasa battery current to 375A?
And is this limit independent of temperature, and of the duration of the current?
BTW, is there any information about how many of the 150 replaced batteries were APU batteries and how many were main batteries?