Undervoltage protection for Lithium-Ion batteries - Page 1
Forum section: electronics / projects

author: SenSeS
author_rank: Contributor
timestamp: January 12, 2016, 02:43:44 pm
content: I've recently included Lithium-Ion batteries in one of my projects and for the protection I have been relying on a battery pack with an internal protection circuit uptil now. Since I want to be able to use Li-Polymer pouches which typically don't come with this internal protection I want to include on-PCB protection. I was looking for a chip that would provide both overvoltage and undervoltage protection but when browsing Farnell for components most of them only have overvoltage protection. Is there a reason for this? Why should you want only overvoltage protection? How dangerous is it not to include undervoltage protection and how is this typically dealt with in consumer electronics? Thanks in advance!

author: dmwahl
author_rank: Contributor
timestamp: January 12, 2016, 03:14:45 pm
content: There's not necessarily a "typical" solution used across the board, but generally over-voltage protection is handled by the charging circuit. Under-voltage protection can be handled by small voltage monitor such as the NCP300/301 line of parts in combination with some shutoff circuitry to disconnect the load. Where a hard shut-down is not ideal, such as a phone or computer, more complex logic is required to keep track of charge level as well as voltage to prevent over-discharge.There's no one answer to your question, but if you can give some more information on what you're wanting to do it would help. Over-charging is generally more dangerous than over-discharge as the amount of stored energy is much greater in the charged state, but either state should be avoided.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 12, 2016, 06:24:38 pm
content: It's not dangerous to overdischarge the cells, but it's dangerous to charge them after it has happened.So in overdischarge situation, cells are only destroyed, but not in a dangerous way, because:Proper chargermustdeny charging (and report "broken battery") when the starting voltage is too low. I.e., a charger needs low-voltage limit too!If the charger doesn't check the voltage, it's not a li-ion charger, but a bomb.This makes the undervoltage burden on the load side a bit easier; you don't need to design the low-voltage cutoff extra robustly; only robustly enough that destroyed batteries won't be economical or nuisance problem. It often happens in laptop packs that they stop working, because the power management is poorly designed; but it doesn't become a safety concern, because the battery management refuses to charge those damaged packs!In any case, the answer to your question is: it depends! Usually, many designs have some kind of CPU or microcontroller there anyway, and it can check the cell voltage and disable any loads and go in low-power sleep mode when the battery is drained.My rules of thumb for charging:Below 2.0V - best to discard for safetyBetween 2.0V - 3.0V - run a conditioning charge cycle - i.e., charge very slowly (at C/100 max!) until at 3.0VOver 3.0V - allow normal charge.My rules of thumb for discharging:3.2V = cutoff for very slow discharge, i.e., practically no current peaks needed.2.5V to 2.8V = cutoff if you need higher currents; the cell voltage sags during those peaks, and it's OK.2.0V = some power tool cells may go this low during very high currents. Check the datasheets.If you follow these limits, you always have a cell that is above about 3.1-3.2V when you start the next charge cycle. Just make sure your monitoring circuit doesn't draw excess quiescent current. The self-discharge in the cell itself will be very small, but as there is practically no charge left, even a small external load can drive the cell below the allowed limits.

author: sleemanj
author_rank: Super Contributor
timestamp: January 12, 2016, 09:18:12 pm
content: Dw01 protection ic with fs8205a dual-fet is the common pairing for protection in chinese products.

author: aargee
author_rank: Frequent Contributor
timestamp: January 14, 2016, 05:34:51 am
content: I'm using these little boards for converting NiCd drill packs to run from LiPo batteries.The circuit works quite well at cutting off at under voltage.http://www.siliconchip.com.au/Issue/2013/September/LifeSaver+For+Lithium+%2526+SLA+Batteries