Designing low voltage battery protection circuit - good reference designs? - Page 1
Forum section: electronics / projects

author: IanB
author_rank: Super Contributor
timestamp: October 16, 2016, 10:29:28 pm
content: Application: I have a fluorescent lantern powered by 6 D cells. I can put NiMH cells in it to avoid issues with storage, leakage and replacement of alkaline cells. However, upon testing it the lamp will run continuously with a supply voltage from 9 V down to below 3 V. If I accidentally left it running with NiMH cells it would likely damage the batteries by reversing the polarity of one or more of them.Therefore, I wondered about inserting a low voltage cut-off between the batteries and the lamp. This would disconnect the batteries once the voltage got below (say) 6.6 V and not reconnect unless the voltage was above 7.5 V (say).Design requirements: The circuit would be upstream of the power switch so it should have a very low standby current (microamps). The voltage thresholds should be adjustable (by resistor dividers?). I envisage it could be constructed from SMD components on a small circuit board that fits in the battery compartment between the battery terminals and the spring clips. There is plenty of room for this. The operating current of the lamp is approx. 0.6 A, so I think a switching capability up to 1 A should be sufficient.Any thoughts on what would be a state-of-the-art IC for this job? I'm guessing this must be a common application for Li-ion batteries and so there must be standard chips for the purpose. Is there a single chip solution? I'm not sure what keywords to search for.

author: Ian.M
author_rank: Super Contributor
timestamp: October 16, 2016, 10:54:50 pm
content: The problem with state of the art protection ICs is they will mostly be optimised for LiION or LiPO cells and a fixed 3.00V or 6.00V cutoff maybe with a small (+/- 0.1V) trim range wont be any use to you.However, take a look at theLTC2934that can monitor two supplies with 10Meg dividers with a total current consumption under 2uA. You'l lneed a micropower LDO to keep its Vdd under 5.5V and a power MOSFET to do the switching, but even so you should be able to keep the idle consumption well under 5uA.I'd want at least a 3A switching capability as the lamp almost certainly pulls a lot more current at startup.Also most NiMH cells don't hold charge well when stored. If that's a significant requirement you'd do better to design a 2S LiPO pack for it, which with leading brand low self-discharge cells could be good for a year or so.

author: moffy
author_rank: Super Contributor
timestamp: October 16, 2016, 11:00:20 pm
content: The LT1120 is a 5v micropower regulator(40uA) with a REF, Comparator and a shutdown pin. It basically has all the functionality needed for your purposes as well as being low current.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 16, 2016, 11:16:39 pm
content: Would a simple lm10 based circuit do what you want?

author: Ian.M
author_rank: Super Contributor
timestamp: October 16, 2016, 11:23:43 pm
content: Only if he can tolerate 1/2mA quiescent drain.There's precious little capacity between 1.1V and 1.0V nominal end of discharge and next to none below 1.0V.Typical self-discharge for a D cell NiMH is already gulping 0.5% C per DAY so it probably doesn't make any sense whatsoever to add a low drain cutoff before the power switch. A level detector driving an obnoxiously loud Pizeo beeper, *AFTER* the power switch might be a better choice or if its before the power switch it needs to sense if the switch is on and deliver full trumps of doom mode or if its off chirp plaintively with a very low duty cycle to remind you the b----r needs charging ASAP.IMHO that's a better solution anyway as combined with a two input voltage detector like I suggested, you can give the user warning the light is about to fail before the critical shutoff point and let the user choose whether another minute or two's light in an emergency is worth damaging the batteries.

author: IanB
author_rank: Super Contributor
timestamp: October 16, 2016, 11:55:30 pm
content: OK, thanks for the ideas on the LTC2934 or LT1120. It seems like "micro-power" and "battery management" are good keywords.As to the battery supply, six low self-discharge NiMH cells provide 8000 mAh at 7.2 V and are the correct fit for the battery compartment. I can check and cycle the batteries once a year and they will be ready if needed.I admit I didn't even think of a Li-ion conversion, but I would be dealing with geometry issues to fit equivalent capacity into the same space.

author: IanB
author_rank: Super Contributor
timestamp: October 16, 2016, 11:59:18 pm
content: Quote from: AcHmed99 on October 16, 2016, 11:54:42 pmLooks nice also. Looks like that and a MOSFET would be the basis of a solution?

author: Ian.M
author_rank: Super Contributor
timestamp: October 17, 2016, 12:46:39 am
content: Yes the LTC1442, a MOSFET, a pizeo beeper (with a series diode, powered from after the switch) and a three resistor divider would be close to ideal. You could get the low battery warning and cutoff control all in one chip. Datasheet saved for future projects.

author: IanB
author_rank: Super Contributor
timestamp: October 17, 2016, 01:38:10 am
content: I will have to figure out how to do the circuit logic (off at 6.6 V, on at 7.5 V). The maximum hysteresis supported by the LTC1440 is 100 mV, so I think I need to use two comparators, one for on and one for off. But I don't know how to combine them to provide the required state logic. If I try to construct a Schmitt trigger using an external feedback resistor network around a comparator I think I would need a regulated supply voltage, not the variable voltage from the battery (which points me back to the LT1120).(I should mention I have never designed anything like this before, so it is very much a learning exercise, maybe an excuse to try something out and do a bit of prototyping.)

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 17, 2016, 02:04:16 am
content: Quote from: Ian.M on October 16, 2016, 11:23:43 pmGood point. Perhaps it could trigger a mechanically latching relay and turn off (at a level that could power the relay). I don't know how much space the OP has to use.

author: tautech
author_rank: Super Contributor
timestamp: October 17, 2016, 02:27:18 am
content: Quote from: IanB on October 16, 2016, 11:55:30 pmExactly.When I needed those attributes I ended up using ICL7665

author: Ian.M
author_rank: Super Contributor
timestamp: October 17, 2016, 02:52:42 am
content: You don't need a regulated supply voltage as the variation of hysteresis with supply voltage can be compensated for as the supply voltage at the switching points is known.I think this LTSPICE simulation comes pretty close, ON at 7.46V and off at 6.64V with a quiescent current under 3uA. In real life it should be possible to hold it under 10uA. It doesn't use any unobtanium resistor values either. Even if you scale all the comparator resistors by a factor of 1/10 to make them easier and cheaper to get, and reduce the board surface leakage problem, the total quiescent current is under 5uA (hint: plotlimit(-10u,Ix(U1:V+)+I(R1),10u))

author: IanB
author_rank: Super Contributor
timestamp: October 17, 2016, 06:37:47 am
content: Quote from: Ian.M on October 17, 2016, 02:52:42 amI see. Thanks for the circuit idea, I'll experiment with it a bit.

author: Ian.M
author_rank: Super Contributor
timestamp: October 17, 2016, 06:50:24 am
content: Note the series combo for R1 - that doesn't make much sense for 1% resistors, so if you need it *really* close to those setpoints, and assuming you scale the resistors down by a factor of 10, it will need a 100K trimmer between R1 and R2, wiper to -in. You can tweak the hysterisis the same way with a 220K trimmer between R3 and R4 but will need to drop R3 to 82K to get enough trim range. It also require expert tongue angle control.

author: IanB
author_rank: Super Contributor
timestamp: October 17, 2016, 07:20:24 am
content: The exact set points aren't that critical, they were just in the ballpark. The key parameters are to cut off the battery when the voltage reaches 1.1 V per cell to minimize the danger of reversing one of them, and since the resting voltage of a drained cell will rebound to about 1.2 V, to make sure the battery voltage is above 1.2 V per cell before reconnecting.Other considerations might be to include a little bit of a time delay to avoid the circuit cutting out during a power on surge, and maybe to ensure the circuit powers up in the on state if the voltage is between the upper and lower limits. Perhaps a capacitor across R2 would provide the necessary filtering against power on voltage dips?I also notice the IRF7210 is discontinued, with newer devices like the IRF9395M seeming to be the recommended replacement. Older, "simple" packaging gets replaced with modern "fancy" packaging. It must be fun keeping up with discontinued parts like this if designing products in an industrial setting.

author: BravoV
author_rank: Super Contributor
timestamp: October 17, 2016, 07:35:46 am
content: Not sure if this fits your requirement, an interesting circuit that I saved and archived long-long time ago from other forum, it was designed by a forum member called Mr.Al, he is also a member here, though not very active.Here you go :The description is there, but you can read the details discussion -> ->HERE, beware, its quite an old thread, 12 years ago.

author: IanB
author_rank: Super Contributor
timestamp: October 17, 2016, 08:27:35 am
content: Quote from: BravoV on October 17, 2016, 07:35:46 amThanks, I know that forum.But, gosh, that's a complicated circuit! It uses so many parts. One of its drawbacks is that it requires connection to every cell in the battery. That could be possible in an integrated battery pack, but it would be hard to arrange in an open battery compartment.

author: BravoV
author_rank: Super Contributor
timestamp: October 17, 2016, 08:30:46 am
content: Quote from: IanB on October 17, 2016, 08:27:35 amYeah, agree, but the sure thing is, it protects every single of your NiMh cell effectively as you want.

author: Ian.M
author_rank: Super Contributor
timestamp: October 17, 2016, 09:00:18 am
content: That gets FUGLY real quick. To get the parts count per cell right down, maybe a PIC12 MCU for each group of three cells running directly off them, that spends most of its time asleep to minimise current consumption, monitoring the two intermediate taps using ADC channels and signalling the next PIC up via Zeners to level-shift its momentary cutoff and enable output signals. So that's two ADC or comparator inputs, two outputs upwards and two inputs with pullups from the PIC below. The bottom group of three obviously doesn't need the inputs from below, and the top one would also do the latching logic and drive the high side MOSFET. Its spare I/O pin could be connected to a button to provide manual switch control. Each cell would need a Schottky diode across it so that in the event of deep discharge due to storage, any attempt to turn the MOSFET on couldn't cause cell reversal at a voltage high enough to damage the PIC(s) You'd also need mechanical keying in the battery holder to prevent physically reversed cells making contact. I make that one component per cell if you really Muntz it by leaving out the Schottkys, and 2.33 per cell if you include 0.1uF decoupling for each PIC.Re: choice of IRF7210 - LTSPICE's MOSFET selector is extremely clunky with no options to filter by continuous drain current, dissipation or gate threshold. The latter is a particular problem that has even caught out Dave before now in one of his videos. I could have simply fiddled with the parameters of the ideal MOSFET model, but I try to make the effort to use physically realisable components. Anyway, MOSFET substitution is not exactly rocket science if your favourite distributor has good enough parametric search so the fact it was obsolete and rated for far more current than you need didn't strike me as relevant - its a bit like dropping in a 2N3055 bipolar model - if your circuit can drive it it will be able to drive most other power NPNs.

author: IanB
author_rank: Super Contributor
timestamp: October 17, 2016, 06:02:11 pm
content: Quote from: Ian.M on October 17, 2016, 09:00:18 amOK, no problem. I wasn't sure whether you were recommending a particular part or whether you had just dropped in a placeholder part, so I looked it up to find out its specs. And that caused me to start musing over how long particular parts remain in production before they get replaced by the "next new thing".

author: lowimpedance
author_rank: Super Contributor
timestamp: October 18, 2016, 02:37:30 am
content: This simple circuit will do the disconnect at least. As for reconnect well some additional circuitry to replace the switch will be needed.