ko esprit II vantage replacement batteries

[Big G]

I replaced the batteries in my tranny with some £8 from HK GP 3000mah nimhs a few months ago. since my tranny is the older of the several vantages it doesn't have the tabs to just pop AA's in so I soldered them together into 4's (obviously).

I only seem to get 2 nights racing out of a charge then they go flat really quickly. When charged they read 11.0v, but once the alarm goes off at 9.0v they drop voltage very quickly.

Any suggestions for good batteries to replace mine or indeed proper packs?

maybe 2x 3cell 11.1v heli packs would be better wired in parallel...?

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