From Pub User: A bit of Googling would suggest that a beast of the sealed lead-acid variety could be made but would weigh a little over double.
That sounds like a trivial cost, but I'm not sure any other technology could produce the peak current required of the 787 battery when starting the APU. Any experts care to comment?
Wouldn't describe myself an expert, but I do have experience with Lead acid batteries in the renewable energy field. A little over double is fanciful due to the way apparent battery capacity varies with the rate of discharge.
If you are judging the weight based on a 75Ah lead acid battery, then that battery 'capacity' is obtained by finding that the battery can withstand a 3.75A load for a 20 hour period before the battery voltage drops to a point where the battery is considered fully discharged. If you upped the discharge rate to say 7.5 amps the battery would probably not give a 10 hours of discharge as a linear regression would suggest (75ah/ 7.5A = 10 hours), instead it would probably give 9.5 hours for a capacity of 71.25Ah. If you increase the discharge to say 20 amps, instead of the battery lasting the linearly theoretical 3.75 hours it would probably last 3 hours giving a battery capacity of 60Ah. And it gets worse from there - a 150a discharge current would probably give only 15 minutes before being flat - so 37.5Ah. This is known as the Peukert Effect.
Someone on here mentioned peak loads of 700a when starting the APU - to cope with that load you'd probably need a battery in the 400-500ah range - that much lead will end up weighing well over 140kgs (battery alone - not counting interconnects, racking etc. The end effect of having to redesign the racking etc and the additional space required would mean the impact with lead acid would be several hundreds of Kg's more. Lead acid because of this quirk in its chemistry (Peukert Effect) is not well suited to high current short duration/low weight applications.