The following table is intended to give a side by side reference of all the various lithium polymer batteries that I know to work reasonably well in an X400. Obviously the biggest problem here is that different motors will have different demands from the packs, so take careful note of the 'C' rating depending on your motors expected amp draw.

This isn't meant to be a recommendation of what pack to use but more a reference for comparison of size weight, capacity and discharge ratings. I'll add to the table as I get positive test reports.

For an explanaton of the lab reports contained on this page from Mark Hopkins
please click here ---> Understanding the Lab Reports

Mark Hopkins has also written an article all about Lithium cell safety.

All lab tests are conducted by Mark Hopkins, British and World Duration Record holder for Radio Controlled Electric Helicopters (Rechargeable Cells). The tests are performed under laboratory conditions in a consistent environment using constant current dischargers.
Manufacturer Capacity Type/Voltage Weight 'C' rating Size (mm) Review Lab Test
PolyQuest XP 2100 3S / 11.1 179g 20C 127x40x21 Report
Thunder Power (pro-lite) 2000 4S / 14.8 155g 10-12C 50x65x25 Report
Thunder Power (pro-lite) 2000 3S / 11.1 120g 10-12C 50x65x20 Report
Kokam FMA (gen4) 2000 3S / 11.1 173g 15C 80x45x26 28/02/2005 Report
Polyquest (slim) 2200 3S / 11.1 161g 12C-16C 130x47x18
Thunder Power (gen2) 2100 3S / 11.1 145g 10C-12C 98x34x20
Tanic 2220 3S / 11.1 145g 10C-12C 105x35x21 24/02/2005 Report
FlightPower 2500 3S / 11.1 162g 10.5C-14C 94x46x20
Apogee 2170 3S / 11.1 144g 10.5C-14C 90x40x15
Thunder Power (gen2) 1900 3S / 11.1 130g 10C-12C 69x45x20
Polyquest (slim) 1800 3S / 11.1 126g 12-16C 100x36x22.5
Polyquest (slim) 1800 4S / 14.8 161g 12-16C 100x36x29.5
FlightPower 1600 3S / 11.1 110g 10.5C-14C 74x43x20
E-Tec 1700 3S / 11.1 125g 10C 65x43x16 E-Tec Report
Jet Power 2200 3S / 11.1 156g 8C 65x40x26
Kokam 1500 3S / 11.1 100g 8C 74x37x17
Thunder Power (gen2) 1320 3S / 11.1 91g 12C-16C 65x34x20
Thunder Power
(gen2)
1320 4S / 14.8v 114g 10C-12C 34x65x26 TP Report