So I bought a used 12s6p battery that looks great but had like 1000 miles on it. It runs great when it’s above 30%, gives me like 30 miles on it but then when it hits 30% it suddenly discharges super quick, I’ve been forced to push twice now because of it. Before I open it I figure I’d ask, could this be because of broken parallel cells? Why would it have normal range above a certain point then drop? I’m also using a step down buck converter to power some led lights if that matters.
30% doesn’t really mean much. Is that just what the meter says? Get a multimeter and read the voltage of the pack. If you’re running a vesc, you should have sensible soft and hard cutoff voltage values set that should give you a good idea of when your battery is nearing empty and it’s true useable capacity.
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I have it to a battery guage, so at around 40V, that’s when the soft cutoff for my vesc is too, the hard cutoff is at 36V. 50.4V - 36V is what I programmed the 100-0% for my battery meter. But the voltage just drops like a rock once it gets to 40V. Like I can feel it sagging then it just starts getting slower and slower and starts crawling after like 5 minutes.
For the record I’m also running pneumatics 50psi, 62-20T and get around ~25-30 miles in range. So it seems about right I think but I still don’t get why the voltage just drops.
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It’s also charging super quick, like 0.1V every 15seconds with a 5A charger
The more it charges the longer it takes to charge 0.1V too, at 38V it was charging 0.1V every 15s now at 41.5V it’s charging 0.1V every 40s
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take a video of the odd seeming behaviour
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I’m not sure how I’d take a video of it. I don’t have a vesc bluetooth module installed either so I have no way to show how my battery discharges
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Who did you buy it from? Was it super cheap?
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What cells are in that guy? 25-30mi on penumatics is pretty good range.
Liion voltage doesn’t change linearly when charging or discharging, and your charger doesn’t charge linearly, either. It chargest the fastest when the battery is the emptiest, and tapers off near the end.
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That’s what I was thinking too but I haven’t seen it be this extreme before, though I’ve only had a 10s2p before this. At around 41.5V it seems to be charging as normal.
They’re Sanyo 2070c cells
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Got it from francisco lol so yes I got it cheap but I trust his stuff 100%, I was thinking it’s either acting funny because of temperature or battery chemistry or something, I’ve never used a big custom built battery before, always prebuilt branded packs.
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Charging quickly at lower voltage is normal. Think of moving in reverse up the discharge curve. From 3.0 to 3.1 is very little Ah change. From 3.5 to 3.6 is much more. Assuming the cell drops off at 3.0v.
It could be the Behavior is cutoff set too High (I’m guessing it’s this). To be sure I would suggest finding out the cell type. It will tell you how low a cell can be discharged. 25r drops.like a rock at after 3v.
30q pulls like a champ through to 2.5. Each cell type is slightly different.
For reference: a badly sagging cell
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What Mr Lamp said.
You’re def getting good range out of that pack though. You can adjust your cutoff values so that you have a bit more warning before the thing dies on you though.
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Honestly I’m feeling really stupid right now. I was panicking about maybe having damaged the battery I completely forgot about the battery discharge curves. I researched them for like an entire week back when I was thinking of making them too…
I’m using Sanyo 2070c cells for the record, and I’m pretty sure the cut off voltage is at 40V
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Yea, I guess I was freaking out over nothing lol, this was my 3rd build but my first really pricey one so I’m noticing every little thing and paying attention to every tiny detail. The soft cutoff is at 40V but I’m going to change that to 42V and hard cutoff to ~38V just so I don’t damage it like an idiot.
Thanks for the help and the knowledge. I always enjoy learning from this hobby even if every lesson starts from anxiety
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Yeah I mean, batteries are a use.item. they won’t blow up from setting cutoff to 36v. I personally think a hard at 36 with soft at 38 is fine for most cells. Many cells are safe to go to lower than that. My qb26800 are factory rated down to only 3v.
You might want to find out what the manufacturer spec is for low voltage. Then set hard to that (times series cell count), and soft to something like 2v above that. Usually a good bet. Safe but you’re getting everything you paid for 
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It’s a very solid pack so I’m glad I’m just an idiot and I didn’t mess anything up during my build 
Thanks again
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it might also be that a P group is off balance and lower than the rest.
Depending on the BMS/VESC settings, it might trigger cutoff early/abruptly due to this one group being lower than the rest
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I feel like his range would be worse in that case
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It’s also possible, I’ll probably open it to check sometime soon since I have a module I need to install
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