3.7v LiPo undervoltage protection - Page 1
Forum section: electronics / projects

author: lk
author_rank: Regular Contributor
timestamp: March 20, 2013, 09:02:13 am
content: Greetings,Im getting a few of the 3.7 LiPo batteries from Hobbyking that steve mentions in one of his videoshttp://www.eevblog.com/2012/12/03/eevblog-393-lipo-battery-discharge-testing/. Im planning to use them in some home made garden lights.But then I realiesed that there are is undervoltage protection on them, “No problem” I think, and start designing my own, and that went quite well if I have to say it myself, that is until I realized that I will not be able to charge the battery with this .So my problem is I can’t really wrap my head around how, I set this up so that this circuit so that it can be connected between my battery and my load, I guess every modern mobile phone battery contains this setup.I suspect that I will need atleast one mosfet more to make it work, but I cant really seem to make it work in my head ?U4 is a regulator (MCP1703) that I use as a reference of 1.8 volts, I use this because it has a quiescent current of only 2 uA , and the comparator U1 (MCP6541) has a quiescent current of around 600 nA.Battery is on the left and the load on the right in the schmatic, for some reason the connecotors did not go to the pdfSo my big question is how do I get this thing to work both to feed the load and being able to charge the battery. Anyone that can nudge me in the right direction.I only need to charge the battery with 500 mA, 2A tops from the load.-lk

author: moemoe
author_rank: Regular Contributor
timestamp: March 20, 2013, 11:26:25 am
content: Why not use a single chip for this, the STM810, seehttp://www.st.com/web/catalog/sense_power/FM1946/SC665/PF78092Quote

author: ConKbot
author_rank: Super Contributor
timestamp: March 20, 2013, 12:00:08 pm
content: If you dont want to roll your own,http://www.batteryspace.com/pcmforli-coli-mn-nibatterypacks.aspxHowever relying on it as a low voltage cutoff will result in a short cell life, so something else to cut it off at~3.4V/cell will result in a longer cell life and a runtime thats not much shorter as theres hardly any energy between 3.4 and 2.4v/cell in a li-ion pack.

author: lk
author_rank: Regular Contributor
timestamp: March 20, 2013, 12:09:28 pm
content: Hi Moemoe,thanks for the reply, you want me to use the voltage detector instead for the comparator? I would still be left with my main problem that i need to somehow be able the charge the battery, which is not possible in my current setup.and i cannot find any of the STM809, STM810, STM811, STM812, that supports a cutoff voltage closer to 3.5 volts that i would like, so i have some time to run on if the sun should not be out, and i don't want to kill the battery fully.And Greetings to ConKbot aswell,The cutoff voltage in my circuit is set at around 3.5 volt with a little bit of hysteresis so that i guess is mostly okas the draw is around 5uA with the current the voltage divider takes. But to answer your question i would very much like to roll my own low voltage cutoff as its a great learning experience-lk

author: cyr
author_rank: Frequent Contributor
timestamp: March 20, 2013, 12:47:15 pm
content: Why is it not possible to charge? Just connect the charger to the battery, the low-voltage cutoff is only needed for the load right?

author: lk
author_rank: Regular Contributor
timestamp: March 20, 2013, 12:59:15 pm
content: Hi Cyr,imagine the battery and the other parts(pir, charging circuitry, leds, control) are enclosed in a lamp outside, that needs to work automatically. I could have put this in the charging circuity, and i could worked around the problem. But that ship has sailed.so i need to protect the battery independently.-lk

author: bfritz
author_rank: Regular Contributor
timestamp: March 22, 2013, 06:42:47 pm
content: The companies that have the lion's share of the market for Li cell technology protection IC's are Mitsumi and Seiko. Why not choose one of their parts?http://www.mitsumi.co.jp/latest/Catalog/ic/battery/index_e.htmlhttp://www.sii-ic.com/en/param_chrt.jsp?subcatID=5I've seen some of these available through major distributors, so they are available. Just need to choose parts that fit your cell count, and desired protection levels.

author: CarlG
author_rank: Regular Contributor
timestamp: March 22, 2013, 08:31:15 pm
content: Hi there, I must ask the same question as cyr: Why is it not possible to charge?Quote from: lk on March 20, 2013, 09:02:13 amWhat exactly is preventing it? Regardless of whether you connect the charger at the source or the drain of the MOSFET switch, the battery will be charged. What other options do you have?The only problem I see is the risk of exploding LiPo batteries...how have you planned to cut the charging?And, of course, the possibility of oscillating comparator function, but that may be overcome to some extent with hysterisis trimming. And perhaps some logic gates.

author: lk
author_rank: Regular Contributor
timestamp: March 23, 2013, 10:02:30 pm
content: Hi CarlG,I dont know what was going wrong with my simulations earlier, but i saw 1 volt drops, when the current was going from drain to source on the pchannel i tried with, in my simulation, i just tried again , and now there is only a drop of 10mV or something there abouts, so i guess i might have seen a problem where there was none.Im handleing the charging of the battery with a MCP73831, so that should be alright.-lk

author: CarlG
author_rank: Regular Contributor
timestamp: March 23, 2013, 10:40:56 pm
content: Hi, fine that you got it right. Maybe you didn't run the simulation long enough?Initially the substrate diode of the MOSFET conducts the current. When the battery voltage reaches the trip threshold of the batlow circuit it enables the switch again.I am pretty sure that you will have trouble with getting that circuit to work. A latch that turns off the switch until a proper charging cycle is performed may make it better. But still, it's a challenge to design a charging device/circuit that doesn't make the battery blow up under any conditions.