Unfortunately my caliper is made of stainless steel and I made the mistake of measuring a cell once, never again 
I tried to keep the jaw away from the positive terminal but they still made contact
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One idea: you can insulate the tips with something like thermal shrink tubing, then either calibrate for the (minimum) padding, or measure the padding with the caliper itself and subtract it from your measurements.
Yeah, I have a couple of other chargers that will take the USB chargeable 21700.
Or just cover the plus terminal with a piece of tape and subtract the thickness… but something like “Scotch” clear tape won’t make that much difference anyway. Probably not enough to impact a go/no go for fitment anyway.
I measured around 71.15 mm
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Thanks for that!
The largest battery I have is a Sofirn 21700 5000mAh flat-top (came with my SC28); I don’t have calipers but from the data I can get from Sofirn and from @HKJ’s review it should measure between 70.6 and 71.1mm (@HKJ measured 71.0, Sofirn says 70.9+/-0.3) and it’s a VERY tight fit with barely a small fraction of a millimeter to spare:
(picture of the MC3000 bay’s rear terminal with this battery on it).
And let me tell you, it was a real PITA getting this battery in there, took me a minute of pulling the rear terminal as far open as as it would go with a finger from my left hand, and then using my right hand to try and wiggle the battery in there.
SkyRC should have made the bays longer, even 5mm would have been a big help, but a full centimeter would have been even better. I hope they revisit that in a future edition of this otherwise awesome charger.
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Yep, the MC3000 is borderline unusable for 21700s. It’s so freaking hard to get an 21700 in or out and will cost you the occasional fingernail tip. The device was obviously not designed with 21700s in mind.
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cost me a few wrappers as well 
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I wonder how popular the 21700 was back when the machine was designed (what, 12+ years ago). Much less something like anything with a USB charging port or protection circuit. But now, it is what it is. The thing has cell length limitations. It would take some significant redesign to fix the problem. I am not optimistic that it will ever happen. Fortunately all of the 21700s that I have tried fit OK. this includes the Molicells, High drain mystery “Lishen” cells (all 4 different types) Wurkkos and Sofirn 21700s, 50E and 50G, etc. Though yes, I would like more clearance.
Keep in mind mine is the model delivered before the changed the “bumps” on the contacts. That probably makes some difference.
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For sure, I was trying out a recently made MC3000 with Molicel P45Bs and they barely fit, there is maybe 1-2mm of clearance left. It was super annoying to get them in/out and the cells jammed with the slightest tilt.
Really? Wouldn’t it be just a matter of a longer box and terminals/springs?
You would need to redesign the entire PCB on the inside
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Good point, as the terminals/springs are indeed attached to the PCB. But a “major redesign”? Woudn’t it be just a matter of elogating the area between the terminals to accomodate the larger distance?
Yeah but that means designing and testing a new PCB and housing and then updating the entire production chain and it seems they’re too stingy to do that, unfortunately.
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I think that changing the length of the sliders, the rails they slide on, the springs, the board, and the dimensions of the case (new molding equipment) would be considered to be a major redesign. Then what to do with the existing models? Close out sales? Price increases on the new one (the biggest ding the MC3000 gets is pricing).
So even if you won’t give me major significant, how about non-trivial ??
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The practicalities of ROI. Profit margins, sales volume, etc. It is quite hard to be altruistic when the main goal of all companies it to make money at an acceptable margin to stay in business. I tend to think that the MC3000 is subsidized by the other SkyRC products.
Actually I am very pleasantly surprised that they still produce the machine this far from release. There are many, many chargers have come and gone.
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OK, points granted – I was thinking more like a customer than a manufacturer. As they say, when we try and walk a mile in their shoes even mentally, things look pretty different…
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They do say that 21700 is one of the sizes the MC3000 is compatible with in their advertising (then as now). Maybe only flattops? Maybe as suggested elsewhere in this thread, 21700 were not as common when the MC3000 was developed as they are now.
And like Mandrake50, I am pleasantly surprised that the MC3000 is still produced. And has Firmware updates - even if we are not sure what said updates do!
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Sadly the Mac side has not seen firmware updates in a few years, it is stuck at 1.15 (2020) but Windows has 1.18 (2023). When I inquired about it, they didn’t indicate if / when the Mac firmware updater would catch up to the Windows one.
Hopefully I don’t derail anything, but has anyone had charging stop with a “connection break” error even when all is clean and good?
When charging 4 new Molicel P28a simultaneously at 3 amps, I came back to it with 2 having completed, but the two rightmost cells weren’t charging and had a “connection break” error.
I did restart the charge and they finished, so the slots do still work.
Is this a quirk of the charger?