I think I would start looking at one of the ICs available that are used to = power an item from an external power supply, but when the supply is removed= will run the unit from an internal battery. There are various ones availab= le, but the killer may be that these are also designed to charge the intern= al battery (but you may be able to make it think the internal battery is al= ways fully charged). e.g. this app note http://cds.linear.com/docs/en/design-note/dn1008f.pdf Refer to Fig 2, if your external battery supplies the 'adapter' input, and = you make the resistive divider suitable values, will it cut out at an appro= priate voltage and switch to the internal battery? Got to there from http://www.linear.com/products/usb_power_manager_(powerpa= th,_battery_charger) I believe Maxim have similar devices. > -----Original Message----- > From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf > Of Chris McSweeny > Sent: 02 December 2016 09:14 > To: Microcontroller discussion list - Public. > Subject: Re: [EE] LiIon protection circuit - custom higher cutoff voltage >=20 > Thanks - I think the issue is that the sense point is used both for low v= oltage > discharge protection and for overcharge protection. If I shift the voltag= e for > one I'm also going to shift the voltage for the other, so if I raise the = discharge > cutoff voltage I'm also going to raise the overcharge cutoff voltage and > effectively disable it (well the cell would go bang before it kicked in!) >=20 > Having slept on it, it looks like the solution is to have an additional c= utoff > circuit as discussed before, using a TLV431 and probably a P channel Mosf= et > to simplify the circuit (current draw is so low - I think about 30mA aver= age, > though I'm seeing a peak of about 80mA - that the advantage of lower Rds = of > an N channel seems irrelevant). I'm sure that would satisfy the requireme= nt - > the only question is how fine to cut the cutoff voltage where we trade of= f > losing run time by cutting off too early and having it fail to cutoff. >=20 > Chris >=20 > On Fri, Dec 2, 2016 at 2:52 AM, RussellMc wrote: >=20 > > You may be able to reduce the voltage that the cutout sensor sees. > > A series Schottky diode in the sense line - Vf is very low at low > > current, but even that may be too large a drop. > > > > A resistor divider would probably work. > > > > The main issue is probably whether the sense interface point is used > > bidirectionally for some other function. > > > > > > Russell > > -- > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .