This reminds me I didn't make the post I intended about battery cells!
So far it has been a complete failure: I wanted to do a comparative review of a few cells, so I bought some models to test... and the review is not going to happen I'm afraid!
First I found these really cheap Chinese IMR's: purple packaging with what looks like a flame logo, 2500mAh IMR Li-Mn rated 35A, and they were so much cheaper than the other high discharge cells that it would have been really cool if they worked in the Zion.... but... they are button top!
So first tip: don't buy button top cells, only flat top ones will fit the Zion! I kinda managed to squeeze them in once but it put too much pressure on the springs and they were not fully inserted (couldn't close the Zion's back door) They fit in somehow in the X-Max V2 / Storm pen-style vape but you can't screw the bottom fully due to the button top (but at least they are usable... I have real doubts about the 35A dischage though, for that price it's too good to be true)
Then I had 4 KeepPower 3400mAh protected cells in my power bank. I thought I could test them (they are very good japanese cells and circuits in a chinese enclosure) but when I contacted the factory they told me they are only rated 5A continuous and 8A peak... So again, this is not going to happen!
Second tip: the Zion requires high discharge cells, a minimum of 10A continuous is needed but RBT recommends to get at least 20A continuous. Do not use cells rated under 10A or you risk to damage them or worse.
Protected cells tend to have lower discharge rates but there are a few that could work. They should be safe to use in the Zion. An other alternative would be to use IMR cells, that are even safer (but not completely fool-proof either) At this point I don't recommend using non-IMR non-protected cells unless you are pretty sure that they can safely discharge below 3V and that they have the right discharge C rate.
This leads me to the cells I've been using with Ashton so far: VapPower IMR 2500mAh rated 35A continuous and 50A peak. They work really good. Now there is some controversy about their real C rate, as it appears they are repacked LG cells (and LG say they are only 20A continuous), but after some research it appears they can really handle the 35A and LG was very conservative when they released their specs.
Why not buy directly LG or Samsung cells you might ask? Well you could, but as I said earlier there are so many counterfeit cells on the market it's not easy to be sure you are getting genuine ones. At least with these VapPower (and for now) you can be relatively sure they sourced the real deal for you (but they will surely get faked at some point too...)
We've been measuring our cell voltages by hand with
@pakalolo after every session and the discharge with the way we use the Zion is pretty consistent. With a pair of fully charged cells I can get 3x 15-20 min sessions, so somewhere between 45 min and 1h.
I'd like to test with higher capacity cells but I'm not sure the extra capacity could be used in practice. These VapPower can already go as low as 2.5V/cell and the Zion will cutoff way before that (I managed to get them to 3.4V but they should never go below 3V because the device will stop before that point)
Often higher capacity cells achieve more mAh by allowing a lower discharge voltage and/or a higher full charge voltage, this is why I'm not sure we would really get any gain... but I could be wrong and I invite you to experiment and post your findings! But stay safe!