First up, a quick disclaimer: Building and battling fighting robots is dangerous and should not be attempted without great care! But hopefully you knew that anyway.
Lithium Polymer batteries are where it’s at now, and while they’re relatively tough…. they’re not combat robot tough…. far from it.
Lithium Polymer Batteries have a very, very peculiar discharge cycle. Essentially, they start at about 4.2V per cell, discharge to about 3.8V and stay there for quite a long time. Then, suddenly (and usually without much warning), they fall off a cliff. We’ve found this out the hard way. Thankfully, our drive controllers cut the power if it drops below 3.4V – but we got slightly worried when one of the batteries was not being correctly being identified by the charger as a 6S pack.
Simply put: We’ve already got pretty close to killing a battery.
As to the why: We ran a much, much longer test session than was originally planned on the morning of 11th December and over-discharged one of the packs. Thankfully, with some very gentle charging, we believe we’ve rescued it for now – however, it will be noticeably “weaker” as a pack from now on and much more likely to give us grief – lower capacity, failing to charge correctly, and, most worryingly, potentially catching fire. Thankfully, that hasn’t happened yet – and we hope it never does – but we obviously have to be very aware of it from now on….
At the slightest hint of an “inflated” cell in a LiPo pack – that’s pretty much it – curtains for that pack. Given what the Wars puts them through – it’s a miracle that we don’t have a greater number of LiPo fires, especially with the large number of spinning weapons in the modern Wars. LiPo packs unfortunately don’t take well to having holes shoved in them so axes and spinning blades are our main concerns there.
So… if you’re running LiPo battery packs: Check them regularly for signs of damage, use a LiPo battery alarm to alert you before you over-discharge them during testing, and most importantly stay safe!