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E-cigarette battery upgrade

Status: Proposal
January 16, 2014
E-cigarette battery upgrade The cheapest E-cigarette available in my area is the Vapourlites, apparently the tiny lithium battery that fits inside the "normal cigarette size" steel tube is only rated 180mAh, with a current draw just over 1A it doesn't last long - and takes between 2 - 3h to recharge. With this state of affairs, it proved neccessary to have 3 on charge while using a fourth - an expensive proposition. Initial experiments with 3x AA Ni-Cd cells showed promise, but this arrangement is only just over 3x the original battery at typically 600mAh. Ni-Mh cells were also tried, the current crop come out between 3 - 4 times the capacity of Ni-Cd, typical ratings being around 2300mAh - unfortunately the Ni-Mh cells have significantly greater internal resistance, so a significant proportion of the energy is wasted heating up the cells - in practice the Ni-Mh cells didn't go any further between recharges than the Ni-Cd ones. On a recent visit to a local DIY store, permission was obtained to rummage the recycling box for scrap electronic bulbs (hopefully LED) - there was only a blown halogen capsule/reflector assembly in the bulb department, but among the jumble of assorted batteries was a lithium laptop pack, this was hurriedly transported back to the bench and pried open. Some of the cells were in a sufficient state of charge to operate the E-cigarette and were taken to be in fit state to proceed with building a battery modification. Initial experiments with charging these cells from the original Vapourlite charging equipment proved workability - a less than fully charged cell lasted all day. Unfortunately the Vaporlites charger does not supply enough current to charge the cells in a reasonable time, one of the prototypes was left charging a full 24h and still didn't reach full charge. The charge cut off chip is hidden inside a very small capsule that also contains the pressure sensor that detects when air is being drawn through the device. This part cannot be modified and cannot handle any more current (it gets hot). Obviously lithium cells need to be treated with a healthy respect - malfunction of the end of charge cut off chip would likely have unfortunate consequences. These cells need more current and the original charge control chip can't do it - so another way had to be found, the simplest approach is a shunt regulator, but it has to be set pretty precisely! The TL431 "programmable zener" seems made for the job. Its maximum current is rated at 100mA, so the current limiting resistor was calculated for this current on the basis; 5V - 4.2V = 0.8V, divide that by the 100mA and you get 8R - nearest prefered value is 8R2 and an actual current limit (at 4.2V) of 0.0976A. To make the battery mod, it will be neccessary to peel open the steel tube of an old battery assembly to recover the threaded insert that the cartomiser (tip) screws into. This threaded insert is the only way for current to get in or out of the battery assembly as it also screws into the charger. As the battery no longer has an outer steel tube, it becomes impractical to use the pressure sensor and the charge management chip in it is replaced by this modification. On the new assembly the pressure sensor is replaced by a manually operated microswitch - this does however leave the problem of how to close the circuit while charging, a low forward drop Shottky-barrier diode across the switch takes care of this - but you have to take into account this volt drop when setting the shunt regulator to the exact correct voltage. The calibration procedure is as follows: With no battery adjust the trimpot so the shunt regulator is cut off, connect a partially discharged battery and measure the forward drop of the Shottky diode, finally disconnect the battery and adjust the shunt regulator for 4.2V + the forward drop measured on the diode. The TL431 has a much sharper knee than regular zener diodes, so cut off of charge current is pretty quick when the terminal voltage gets to what is set. A note on the mechanical construction: The problem of mounting the insert that the cartomiser (tip) screws into, onto the cell, is solved with a short stub of hot-melt glue stick, the thin ones sold in cheap DIY stores aren't really up to it but you can get thicker glue sticks from reputable retailers, simply melt the end of the stub (jet flame lighter or modellers pencil blowtorch) and press the insert (with wires already soldered on) into the melted glue before it cools. This procedure is repeated to attach the stub to the lithium cell, or iff you use some kind of battery holder that works too. About 1/2" to 3/4" stub is fairly bendyand is resilient enough to be carried in a jacket pocket, and even if it does come undone - its easily repaired.
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