1-cell li-ion battery under-voltage protection - Page 1
Forum section: electronics / projects

author: morfeus
author_rank: Newbie
timestamp: December 10, 2015, 11:15:41 pm
content: Hi,On a device I designed, I had only possibility to power it using external DC adapter. I would like to add backup battery with automatic switch-over so I have used MCP73837 for that. There is a good reference design for just what I needed. The problem I run into is how to add battery under-voltage protection. I was thinking to use TL431 Precision Programmable Reference for that. Putting all together, the final schematic is as in attachment. Will added under-voltage protection work as expected? Thanks.Kind regards,Gabrijel

author: Ian.M
author_rank: Super Contributor
timestamp: December 11, 2015, 12:23:20 am
content: No. Your MOSFET body diode is the wrong way round, the TL431 will have at least 1V drop when its on, more likely closer to 2.5V, which will be marginal for gate drive to the MOSFET, and there's no path to take its gate low to turn it on. Also the R2,R3 divider chain will be drawing 40-50uA from the cell all the time taking it ever further into non-recoverable discharge if you store the device uncharged - potentially fatal to the cell if it doesn't have a large reserve capacity.Basically your design is totally FUBARed.

author: morfeus
author_rank: Newbie
timestamp: December 11, 2015, 07:31:40 am
content: Hi Ian.M,regarding wrong mosfet orientation, I was thinking the same but then I found this:http://www.microchip.com/forums/m739544.aspxand all described sure makes sense! Body-diode would let uncontrolled current flow to battery damaging it!If anyone has some starting point for me how to make battery under-voltage protection, I would be very grateful.Kind regards,Gabrijel.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 11, 2015, 07:42:01 am
content: Either use a dedicated chip, or a small microcontroller. Utilize sleep modes in the microcontroller and google around how to measure its own input voltage without constantly drawing current. I use ATTiny25/45/85 in a similar case and it's very suitable for the task. Some requirements for the micro:- Runs from 2.0V- Using sleep modes, consumption less than, say, 5 µA- Integrated voltage reference- Integrated ADC- A way to measure its own voltage is a plus. An alternative is to use a resistor divider network with one of the legs connected to an IO pin so that it can be switched on only during measurement.- Integrated temperature sensor is a plus.Then you can rather easily implement:- Low-voltage cutoff- High-voltage safety cutoff against faulty charger- (With integrated temperature sensor:) Low-temperature charge cutoff (prevents charging below 0 deg C or so)If you have a battery-using product that already has a micro, then the best way is to integrate everything there, as the main application usually has means to shut itself down in code, without an extra power switch mosfet.If you have low-current discharge, keep the under voltage limit high enough. The datasheet values are for the specific discharge current and define the 0% SoC that way. If your current is lower and you just use the same voltage limit, you go slightly below 0%, which may or may not be a problem.If you discharge at low rates only, a 3.2V cut is usually good.

author: Ian.M
author_rank: Super Contributor
timestamp: December 11, 2015, 09:05:04 am
content: Ah, yes, you've got a path to the load from the external power in via D3. If you are going to do that, then you *CANNOT* use the same MOSFET for under-voltage cutout. The Microcchip AN1149 circuit uses the MOSFET to isolate the battery from the load when external power is appled and relies on the external power voltage being greater than the battery voltage so D1 (your D3) is forward biassed and the body diode reverse biassed. Remove the external supply, and even if you try to turn off the MOSFET by raising its gate to its drain voltage, it will still conduct and power the load via its body diode.

author: amyk
author_rank: Super Contributor
timestamp: December 11, 2015, 10:43:54 am
content: A DW01 and a dual MOSFET like a 8205A seems quite popular. You can find plenty of prebuilt PCBs using them, just search for "lithium ion battery protection PCB".

author: morfeus
author_rank: Newbie
timestamp: December 11, 2015, 05:31:23 pm
content: Hi,amyk, thanks! Iwill do as you have proposed so the final circuit will be as in attachment (if anyone ever needs something similar, I am sure that will be happy to find it here)