Originally Posted by Crunchmeister
J battery won't fit the E90. The J battery is a 9.6V made with 2/3A mini cells and designed for the TM P90. Echo 1 have made a small adjustment to the internals of the battery compartment allowing the use of a side-by-side 9.6V mini battery, like the one that comes with it.
However, those aren't typically available. I would recommend going to www.cheapbatterypacks.com
and build yourself a custom 9.6v 1500 mAh pack using Elite cells. It will cost about the same as a cheapo battery back would, but it's made with the best cells on the market and quality wiring and workmanship.
My apologies, I was under the impression the Echo1 was a clone (body wise) of the TM.
Another solution would be to dabble with Li-Po batteries, a 7.4v is smaller than an 8.4mini and will have the same power as an 8.4v large.
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.