I recommend manual batteries with KR808D-1. I find thinner juices work better in the cartos (I use 20% or less VG juices, particularly light colored ones that leave little or no flavoring sediment to clog the carto). For decent battery life, I recommend the standard aka 78mm over the shorty/65mm batteries. The standard makes a 120mm long ecig w/ the carto. All decent minis are thicker than a cigarette (around 9.2mm) but look slimmest in dark colors.
These are real 3.7v ecigs and with the Smokeless Image/Bloogplanet/Halocigs cartos or especially the Vapor4Life WOW cartos can deliver a pretty sharp hit with some flavors and nicotine. For example, some 24mg vapers cannot vape some flavors of vapor4Life WOW cartos is 24mg. Some of those flavors have a sharp-edged hit. I think some of Vapor4Life's flavors are overly flavored, but some I like a lot.
Re blank cartos - difficult to impossible to clean and dry and have work as well as when new. Some people succeed. Many don't bother trying. Only put juices you know you like in cartos. On a manual KR8 you can use a DSE901 standard resistance atomizer and drip tip to test juices (atomizer may arrive with nasty tasting primer on it so * blow out, drip 4 drop son atty, let soak in, vape mostly off without inhaling, repeat from * until it tastes OK)
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How I fill Vapor4Life KR8 cartos and Boge 510 cartos
Pop out the soft cap with an unbent paperclip in the mouth hole, very careful not to catch stuffing or the wire loop.
Shake and open the eliquid
Hold a bit of tissue on the threaded end hole, tilt the carto, and add 18 drops per ml of capacity into the stuffing (Boges and most KR8 cartos are about 1ml).
Wait 15 seconds or so then see how many more drops the carto stuffing will soak in easily.
Clean up the threaded end, tilt open end down into tissue, puff into threaded end to clear air channel (Boges and the KR8 cartos I use have a defined center air channel and I want to see light through that)
Sop up any loose juice above the stuffing
Cap and let it settle - I lay it on different sides for 15-20 minutes to make sure every bit of stuffing gets damp so I won't singe the carto. if you use thicker juice than me (I use 20% Vg or less) then you may need to wait much longer.
While vaping, if the vapor gets a bit light or a bit dry or the carto is getting hotter than normal, I take the carto off the battery and add eliquid. I sometimes use a plastic or delrin drip tip as a mouthpiece on Boge and KR8 cartos (not to drip into since the juice would head for the central air channel)