So... Skree... umm where bouts you from? Just wanna make sure I'm nowhere near you!
I'd never trust stacked 18650s!
Sheffield matey. I see your point, there is a lot more material to go up in the event of a cell failure, but 18650s are less likely to fail than 18350s simply because being bigger they can handle more load. In the case of a stacked mod, this is even more true as the vamo's PWM is stepping battery voltage down, and thus is drawing less amps from the battery (at 7.4v) than it would be at 3.7v single battery to create the same wattage at the coil
I did electronics to a pretty high level, and have used all types of battery chemistry widely throughout many hobbies i have. I can and will go into stacked batteries a little here but please don't think i'm ranting, or being a git, i'm not.. i just know what i'm doing and hearing some things provokes fairly blunt reactions or essays of scientific overkill from me. I was going to save this for a guide, probably will copy and paste it to a guide later
It is an extemely common misnomer to call one 18650 a battery, it is not a battery, it is a cell. If you join it in series (stacking) with another identical cell, that is a battery. All modern cell types are designed to be put in series with another identical cell. Without exception. Car batteries are made this way, a collection of cells in series. There is no increased risk from putting cells in series regardless of cell chemistry, provided the resultant battery is used correctly.
The issue with putting lithium based cells in series is no two cells are *absolutely* identical. Slight variances in the chemical slurries in the cells, slight differences in internal resistance etc mean that two cells, even in series, will discharge at slightly different rates. This is to say, when my vamo cuts off, one cell might be at 3.303v and the other might be at 3.297. If putting those cells on a charger that doesn't really monitor charge individually, they may then come off the charger at slightly different voltages again. This is why some people who 'stack' recommend swapping the cells position (one in before the other) every charge, to sort of balance that out. Won't make a great deal of difference, that imo but whatever floats their boat
If you don't have some way of balancing out the difference between cells, what happens is one cell will gradually become really out of balance with the other. This causes problems when charging (one cell does not get fully charged) and discharging (that cell will become overdischarged). This is not so much of a problem with a decent intelligent but otherwise fairly standard charger as such chargers cut off by battery voltage. You'd probably get away with it as long as your charger has individual charging channels.. but a good rc charger gives you very closely matched voltages (
)The difference in voltages (state of charge) can also cause massive current flow between the two cells on being connected together in series and in circuit. this is why indivdual cell charging voltage monitoring is important
A proper charger, designed to charge cells in series, such as those sold in the radio controlled model industry, usually has a cell balancing function. That is, at the end of charging, the charger equalises the voltage (State of charge) of the cells in the battery. They will both come off at the same voltage, say 4.201v for a total battery voltage of 8.402v. Radio controlled model batteries use NOT safe chem lipo cells, often up to 6 cells in series, with absolutely no protection at all, not even a metal casing on the cell.
If the cells are balanced every charge, it avoids entirely the problems of cells charging at different rates and the cells are then safe to use 'stacked'. This is of course, on two cells that have never been charged any other way, never used on their own, always have been a pair from their very first use and were bought together.
Of course, this does not affect the problem of cell failure, all cells can fail, but if you're using safe chem LiMN batteries (and i do, unprotected ones at that. Vamo V2 appears to have a hotspring but i am getting myself a 2 cent fuse to use to protect me against vamo failure), the most that can happen is a vent of gases
If you use 'stacked' mods, google 'lipo balance wiring' and have a look at RC balance chargers such as the turnigy accucel 6 (what i use)
I merely added lipo balance wiring to a 2x18650 battery box and fitted 4mm banana plugs to it so it connected to my charger, in effect my cells are always connected in series and kept perfectly balanced to each other. As always, this is a case of what is right for you.. i really know what i am doing, so can monitor my cells correctly, have the right gear, etc. I do not recommend stacking cells to anyone who does not know exactly what they are doing, because done incorrectly, it can be dangerous.
edit: i don't have a camera worth a damn, but when i have one, or use of one, i'll take a pic of the vamo in its current ludicrously large state. I'll also try and take a few pics of my charging setup.
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