Low Battery Voltage/Battery Saver - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: October 07, 2014, 04:20:48 pm
content: Hi,I need to TOTALLY cut power to a circuit when the level of the battery powering it drops below a certain level. Most circuits have constant drain on the power source due to ICs, reference voltage dividers, regulators etc.. The problem with this when a circuit is powered from a rechargeable lithium battery pack is, if the device is left unattended for a long period, the battery can drain to a level where it can no longer be recharged (well it can be recovered but it's a pita). So I need to completely cut all drain to the battery when it reaches a certain level. The theory for this is pretty simple, a p-fet with the gate held to the battery +ve with a large resistor and pulled to ground by an npn bjt. The base of the npn has a resistor (1K) to ground and the anode of a zener diode whose cathode is connected to the +ve of the battery . Thus when the battery gets too low, there's no longer any base voltage, so the transistor turns off causing the fet to turn off as well, in that state there's virtually no drain (there's a little but it'll be tiny) on the battery. But it has problems, near the threshold, there's going to be some ohmic behavior in the npn and p-fet and maybe some oscillation as the power is cut and the battery recovers a little, it'll switch back on again. I can't put any hysteresis on the threshold because doing so would require a constant drain on the battery, so I'm a bit stuck as how to implement it.Many thanks for any advice.MArc.

author: Zepnat
author_rank: Regular Contributor
timestamp: October 07, 2014, 08:54:32 pm
content: Try these, 38p, maybe mod them into circuit.http://www.fasttech.com/products/1030/10003003/1225100

author: LightYear
author_rank: Newbie
timestamp: October 08, 2014, 12:08:05 am
content: Maybe try adding these three components:Resistor between Zener anode and gate, say 470R.Capacitor in parallel with resistor tying NPN gate to ground, say 100uF.Resistor between NPN emitter and ground, say 10R.In simulation I get a big hysteresis effect, and <90uA in the off state. May take some tuning though - the RC gives a psuedo-hysteresis effect via a time delay and the emitter resistor contributes to an actual hysteresis effect. You might also try replacing the NPN with a Darlington pair, allowing you to lower the base current even further.

author: Rick Law
author_rank: Super Contributor
timestamp: October 08, 2014, 05:36:46 am
content: Quote from: Zepnat on October 07, 2014, 08:54:32 pmI second this. I have two sets (8 total) of them. While I have not used them extensively yet (used 1 from each set for about a month), I tested them well when I first got them and all 8 performed to spec. My two sets are one pair of 2Amp ones, and three pairs of 4Amp ones.They stop charging when they should, they stop discharging when they should, and the resume (after cut off) when they should.The soldering do look second rate, but they work better than they look. Some lumping at the IC pins and the bolbs do not look as well form as I like. One reason may be on the controller IC, two IC pins are shorted by a lump. They are suppose to be since the two pins goes to the same thick trace on the PCB, but I don't like looking at a clump of solder bridging two pins.I tested the cut-off/resume voltages carefully. The 2Amp pair was very consistent with each other. The six 4amp ones were equally consistent with each other. Lack of consistency would be a sign of poor quality in my book. I could not test their cut off current as accurately as I tested the voltages as the DMM is too slow to catch the exact current at cut-off. Their consistency increased my confidence in them significantly.Since I tested them longer than I used them, I don't know how well they hold up long term after just about a month. For now, I am very pleased by their performance.Rick

author: Unknown
author_rank: Unknown
timestamp: October 08, 2014, 09:27:46 am
content: @Zepnat & RickThose are interesting little things, very sparse with components so their operation must be fundamental. They are a little low discharge rate for my application though, 4A per cell I may get away with though if I were using Li-Ion cells. Unfortunately I'm using Li-Po packs which have all the cells integrated, but it could be possible to replace components on those modules to change their characteristics to what I need, would take a little reverse engineering though, but that's always good fun, and they aren't very expensive either. I do need some e-cig stuff from FT so I'll pop a few of those on the order list.@LightYearNice one, I was wondering if a largeish cap could be used to hold the hysteresis somehow but I couldn't quite figure out how to implement it. 90uA is pretty good. That gives over 40 days per 100mAh left in the battery. If I cut the power at say 3.0V per cell, that leaves plenty of charge, it'd take years to fully deplete.EDIT: Playing with the simulator and your suggestions I got this workingThere's a good second delay between the threshold being reached and the transition between on and off, which will deal with the oscillation, when havily loaded it'll hiccup every second or so which is fine. On this circuit the threshold is about 9.6V which is 3.2V per cell, which is almost smack on what I was aiming at. The drain at 9.5V is about 500uA but when the battery falls to 9 volts there's barely any drain at all, <10uA. So that seems to be perfect. Then again simulation and real life are two very different things, so time to crack out a prototype board and variable power supply to test the bugger. Thanks for those tips