Hi There
I have used a 7.4V 1800 mAh Stick battery in this one. it slips in right but half of the front side of battery will hang out.I strapped it tight. some where during crawling on some angles when front suspension is pressed all the way up, the battery hits the rocks. which is not something we want but it happens on just those certain angles. I have not used an 8.4v n probably won't recommend using one. Using a stacked up battery pack *(the one that comes along) is highly recommended as it keeps the vehicle balance right as well.
Cheers n if you have any more questions
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