lipo discharger with ~3.75V cutoff

[pldaniels]

Hello everyone,

Been working on a new project, when I should really be working on my buck converter and a lot of other things.

Anyhow, this is a discharger (great, more complicated than a light bulb or a 10W resistor), however it's a discharger designed to bring lipos down to 3.7~3.8V for the purpose of storage and/or shipping.

Keeping lipos full charged for extended periods is not good for them according to reports starting to appear more frequently.

At first there doesn't seem to be a lot of use for this circuit to people, however I've personally found that many times I've charged up packs for the day of flying only to get there and the weather is sour or the plane crashes within the first 30 seconds :o

So, anyhow, to the device, basically it's just 3 separate dischargers which use a 3.3V zener coupled with PNP and NPN 500mA generic transistors and some feedback to produce a circuit that has a fairly sharp turnoff at approximately 3.75V and enough hysterisis to prevent the circuit from wildly oscillating due to cell bounceback.

I'm still tweaking the final values as it's a tradeoff between avoiding the soft zener knee vs low quicent current during the off period.

The discharge rate is about 250~350mA through a 3W 15R resistor. I'll actually change those to radial ended 5W wirewound resistors later.

http://www.pldaniels.com/flying/albums/PLD-LS-01/mpic00013.jpg

http://www.pldaniels.com/flying/albums/PLD-LS-01/mpic00017.jpg

[pldaniels]

Here's a typical discharge graph of one of the cells.

Quicent current is ~2mA at the moment, I can tighten this up to be about 1mA but at the same time it's a bit annoying because it once more makes the zener behavior unpredictable.

Some people have asked me why I didn't use a microcontroller. Well, to be honest I did consider using some Atmel attiny13 AVR's, they're lovely little uC but tends to push the costs up. I aimed to make this as low cost as possible and using discrete parts.

http://www.pldaniels.com/flying/images/PLD-LS-02-discharge-03.png

[pldaniels]

Made some more adjustments and changed the discharge/load resistor from a 15R 3W to a 10R 5W ceramic.

Discharge is now stopping at 3.68V (loaded) and floating back to 3.70V, very nice and clean.

The production design will end up having only a single 4 pin JST-XH connector on the end and people using a 3 pin (2S lipo) can use an adapter lead or cut out the center tongue. I looked at putting two plugs on but it would mean making physical jumpers across the board and that's never pleasant. Also, going this way with one plug I can edge mount it which allows for much easier package sealing.

Paul.
http://www.pldaniels.com/flying/albums/PLD-LS-01/mpic00020.jpg

[pldaniels]

I forgot to say, I'd need jumpers with two connectors if I was putting them side-by side, as apposed to front-back like they are in the previous photo.

Additionally, from a safety point of view, it's better to only have one plug and a down-converter lead to 2S as it prevents people trying to plug two packs in at once.

Paul.

[Dan Baldwin]

How 'bout a schematic, Paul.

Dan

[pldaniels]

Here's the schem.

Values are moderately critical, especially the 330R|150R divider setup that feeds the zener as this is a balance between a low off quicent current vs zener regulation current.

The LED needs to be a red or green type with 1.8~2.0V Vfwd.

Watch out for the thermal stability of this circuit as getting the zener too hot will make things go out of wack badly.

Paul.

[pldaniels]

Just a note - depending on the LED your cutoff will vary. Red LED's will give you about 3.70V where's green will push that up by 0.6V or so.

Be aware that because we're operating the zener in the knee the exact cutoff will vary somewhat depending on the brand/make of the zener involved. You can adjust this by tweaking with R1.

Next thing I'm trying to determine is how to make a bootstrap option for when the cell voltage is above the cutoff but below the hysteresis level of the circuit - it, say you only used the pack enough to draw it down to 3.85V which could still be in the hysteresis area of the circuit and so it won't activate the discharge cycle.

Paul.

[pldaniels]

Well, here's what she looks like in pre-production.


http://www.pldaniels.com/flying/albums/PLD-LS-01/spic00104.jpg

http://www.pldaniels.com/flying/albums/PLD-LS-01/spic00105.jpg

http://www.pldaniels.com/flying/albums/PLD-LS-01/spic00106.jpg

Paul.