Charging random 3.7 V Li-ion batteries (repurposing old batteries) - Page 1
Forum section: electronics / beginners

author: Razor512
author_rank: Regular Contributor
timestamp: December 17, 2012, 12:40:33 am
content: I have a few old Li-ion 3.7V batteries from various smartphones and cameras and would like to re-purpose them as I have found them to work quite well in LED flashlightsOnly thing I am worried about is relying on the low voltage protection of the battery, or find a way to have the flashlight not function once the battery hits a certain threshold.The leveling circuit in the flashlight will keep the voltage at 3.6V even if the battery voltage goes down to 1.6V (using 2 AA batteries)I am able to get better battery life from the Li-in battery but I have no idea of how much damage I am doing to it by relying on it's internal protection. Does anyone have experience with re purposing old batteries for use in flashlightsAnd if it is safe to rely on the internal protection, I have a more expensive 10,000 mAh 3.7V battery that I would like to use

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 17, 2012, 01:05:36 am
content: it largely depends on the cut off on the batteries board,some drop down to 2.6-2.7v which is far far too low, where as others go to 3-3.3 which is more acceptable.

author: Razor512
author_rank: Regular Contributor
timestamp: December 17, 2012, 01:49:04 am
content: I will run it down until it cuts off. (if the cutoff happens in the lower 2.7V range, is it likely to kill the battery with just a few cycles or will it still last a reasonably long time (hoping to have the battery last around the same number of cycles as the average Ni-MH battery.My ultimate goal is to eventually upgrade the flashlight to a 1000 lumen Cree LED (probably 4 of them) by using thermal epoxy to attach it to an old socket 462 heatsink (to keep the LED cool, then also buy something to better regulate the power and handle the higher power LED (don't need it to be very small since I plan on using it as a work light (if things go as planned, I will have the equivalent of a $100 flashlight, for around $20-25)

author: amyk
author_rank: Super Contributor
timestamp: December 17, 2012, 09:25:30 am
content: It depends on the particular cell's chemistry; for 18650s, somespecify a cutoff of 3V, some2.75V, andsome 2.5V. So 3V should be safe for all types, but you might not get full capacity out of it.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 17, 2012, 10:53:37 am
content: Quote from: amyk on December 17, 2012, 09:25:30 amThe amount of charge left beyond the 3v mark is almost negligible, if you are using mobile phone batteries, the phone will usually cut off around 3.2v - 3.15v, but the battery will be able to supply power until it is at around 2.7v before it cuts off, this is probably to extend the cycle life, and min operating voltage of components or something.

author: T4P
author_rank: Super Contributor
timestamp: December 17, 2012, 12:32:02 pm
content: You need 4 3.3A constant current drivers then and 4 XML-U2s then.Note: SST-50s and SST-90s are more efficient (at much higher currents) but it's a order of magnitude more expensive