Dude if your NIMH packs are 10 years old there shot trust me no wonder why you have no power or punch with your tanks a 10 year old NIMH is beyond shot and its life was well over..
if your packs are 7.2v 6 cell and when they charge if they go over 10 volts when they peak charge there done for. A 2S ipo will be far much better! and they charge at high rates and can be charged many times faster than NIMH packs if your lipo has a high C rating. normal lipo is x1C 3000 = 3amp charge but you can easy go 2 amp and with high C much more so they charge much faster than NIMH[TABLE="width: 100%"]
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[TD="bgcolor: #f0f0f0"]SPECIFICATIONS[/TD]
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Capacity: 3300mAh
Rated Voltage: 7.4V
Max Charging Current: 6.6A
Max Continuous Discharge Current: 30C (99A)
Burst Discharging Current: 55C (181.5A)
Watt Hours: 25Wh

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battery specs from above 3300 lipo with charger see you can charge at higher rate so it will charge more than 2 times faster than a 3300nimh pack, See above this 3300 lipo can be drained continuously at 99Amps your old NIMH packs probably cant handle 10 amps!