Protection Circuit for Li Type batteries

(Including Li-On, Li-Poly and LiFE)

Ive had Li-On batteries Ive salvaged from a laptop sitting around for a few months and Ive finally found a place to use them. I have a safe Charger in place Based on one of Microchips Charging IC's. I can vary the charging rate to be anywhere from 0 to 500mA. I have a larger IC (That I havent used) thats able to be used to charge up to 1 Amp (Same Family, just bigger ).

So, for the application I want to put it into (Im basically replacing NiCad batteries from a old drill to Lithium Ones, They fit quite well and have enough space for a PCB or Protection foam). The whole reasoning behind this project is to reuse and re-purpose something (ie laptop batteries)

So bottom line, I know I need protection circuitry to prevent the batteries from Over Discharging, and Ive seen such protection PCB's online. Now For all intensive purposes, I would like to make my own (however, If this is unfeasible to do, I'll just buy the PCB for the protection circuitry). Overcharging isnt much of a problem Nor is Consuming alot of current (they are rated for 4000mAH) since the battery charger takes care of the overcharging problem. So the problem to tackle is Over discharging. From My understanding, the point where they get cut off is around 2.5V.

Can this be done with a simple Comparator circuit, a mosfet and a voltage reference?. Im thinking Once Vbat Reaches 2.5V, The mosfet will turn Off and Cut Power to the main Circuit so you cant use the drill anymore.

2nd Question: Can I used 2 Chargers In series to charge 2 Batteries in Series? Ie, Wire up One Charger with one battery, and do the same with the 2nd Battery but in series. I do plan on using the batteries in Series (this drill was only 6V or So-Its really old but it worked fine until the batteries died). See attached Diagram. Im only doing this so the cells get balanced when they are charged and so I dont have to get another charging IC.

Here is an example of an Li protection IC: I would not ttry to duplicate it in discrete circuitry, but it's theoretically possible.

https://www.electro-tech-online.com/custompdfs/2011/12/S8200A_E.pdf

You may want to check and see if the Li batteries have a low enough internal impedance to feed a drill's high peak currents. Ni-CD and NI-MH have ultra low ESR, Li batteries are higher when comparing similar size.

Unfortunately I couldnt find any model number on the batteries. Their wrapped in Cardboard. Unwrapping it reveals nothing. Only thing I can say is that the pack had 8 batteries (14.8v @ 3.2AH total). Im assuming each battery is 3200 mAH. So I really cant check their internal Impedance. Is there a fool proof way of Calculating it? Ive thought about using a Simple Series Resistance Circuit, and using Ohms Law to Calculate internal Impedance. There was a way Ive found to do this, By measuring the voltage before applying a load, and then after. Since Current is the same in a series circuit, I could then find out the current and then calculate internal impedance. In theory this method should work.

It seems easier to just buy the protection circuity as its only $3 but I do like have the openess of being able to design my own protection circuitry using a IC

Li-Ion battery cells from a laptop will be for low power, not the very high power for a tool. Also, they will not be new so their capacity will be lower than their rating. They might catch on fire when the tool draws its very high current.

Years ago I used the Li-Ion cells from a laptop in an electric RC model airplane to replace the old Ni-Mh battery. They worked but got extremely hot. The current was 3A to 5A.

Hmm Safety is always my concern with These Lithium Batteries (with the exception of LiFePO4). Maybe Im better off Buying new Batteries (My actual original plan was to use LiFe Batteries) and putting these batteries to use for a LED flashlight or something.

A question out of Safety: My Charger has a Pre-Condition Mode that slowly brings up the cell voltage if its below 3V at a rate of 10mA. Some of the cells I have measure Less than One volt, Can they be safely charged? Or Should I Just Recycle them since the cell voltage is too low ?

A rechargeable lithium cell that has been discharged below 3.0V is ruined. Its capacity is drastically reduced. The protection circuit should have disconnected its load before its voltage got too low.
If you carefully charge it at a low current until its voltage is above about 3.2V then it can be normally charged at a higher current and it can be used if the reduced capacity is good enough.

And you are correct. My measurements Prove that (not that I didnt believe you ). The Cells that were 0 volts didnt take a charge too well. They sat at 0.4v and didn't budge. I didnt want to temp fate so I pulled them after a few moments. The other cells took a charge readily, the ones that were below 1v (they were around 0.4v) accepted a charge and still hold it. I decided to test one of the cells that almost had a full charge on it, It read a little under 4v. I connected it to a 4.5 Ohm resistor and took measurements every 6 minutes for a hr. My tests Conclude Their capacity is around 750mAH, a far cry from the original Capacity, unless Capacity Adds in series, which I dont think it does. These cells are also Old, above the Life span for Li-On Batteries.

So looks like I have a choice to make. Take them to a scrap yard (or recycling center) Or Put their low capacity to use. As hard as they are to let go, I think Im going to have to recycle them. Im hoping the local scrap yard takes them, it would be a pity to let something Like Lithium Go to waste.