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Conver Dewalt ni-mh charger to charge li-ion batteries.

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delambo

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Having once again killed the cells in my Dewalt drill I have decided to fit the battery pack with li-ion cells this time rather than the ni-mh that I have used in the past. Naturally I will need to modify the charger in some way to stop it from over charging. Has anyone done this before? Any suggestions as to how I might go about it? Any help gratefully received.

Thanks.
 
before proceeding, I'd like say, my Ni-Mh battery pack died suddenly so I opened it up, expecting a sludgy mess or something. Instead the batteries are connected using very thin metal slivers, which are rusty and broke off, and open circuits all over. I just reconnected the batteries by soldering wires on each terminals and that did the trick.

If it's not the case with yours, I'd like to offer my experience. I made 3 DIY battery packs using used Nokia Li-ion cells and they work fine as long as you isolate the connection during use or charging. The cells themselves will not overcharge unless you leave it for the whole day, I suppose.

Here's some pics:

**broken link removed**

**broken link removed**

As you can see charging is done individually for each battery.
 
Hello,

Charging Li-ion batteries is much different than charging NiMH batteries so you need a different charger. You might be able to build one from your old NiMH charger using the wall wart that probably came with it. The circuit will be much different though, and you can not use the NiMH circuit for the Li-ion cells.

The basic idea for an Li-ion charger is to build a voltage regulator that is current limited. The voltage is set to around 4.15v (max 4.20v) and the current limit is set according to what kind of cells you buy as the manufacturer specifies this value.

If you want to build one you should first check out your wall wart and see what voltage it puts out. You'll need to know how many cells you are going to put in series too.
You can buy chargers already made on the web though in case you dont want to actually build one. Building one requires some experience constructing electronic projects like soldering and knowledge of how parts should be connected.
 
I am an automotive electronics engineer by trade (repairing ECU's, radios etc etc). I appreciate the problems with ni-mh battery packs, and have replaced the cells inside on several occasions I could just do the same again but I now want lighter, better quality li-ion technology which does not suffer memory effect and will not drain when not in use.

Dewalt supply a charger which will charge ni-mh, ni-cd and li-ion battery packs but it is £150stg and I can buy a whole drill with 2 li-ion packs and a charger for that. There is nothing wrong with my charger nor with my drill so I dont want to go down the route of a new set. Dewaly will modify an old ni-mh / ni-cd charger for free if you upgrade to their li-ion batteries but again the cost of buying the new packs is more than buying the whole set which is really waste full. If Dewalt can modify a charger then so can I but I just want to know what they do. If they chuck the whole board in the bin and fit a new one in the case then that is what I will do and will design my own circuit but I don't believe they would do this.

I am quite happy with connecting the cells in the pack etc and I appreciate that my existing charger will not work which was the purpose of this thread. I already have a supply of 2.4ah 3.7 volt cells to use which I want to connect and charge as a pack not individually.

Thanks for the advise.
 
Charging Li-Ion is simple, as MrAl said you need a current-limited voltage regulator, with a precise voltage reference with low temperature drift. Don't overcharge ! Using a switching charger will avoid a clunky heatsink. You could even hack a laptop supply brick to supply the power.

If you want to maximize the life of your battery pack, you will definitely need an overdischarge protection. Using the pack till the drill stops working will overdischarge the cells and quickly render them useless. You could use a MOSFET to switch off the load, or a warning buzzer, etc.

You might also want to consider a balancing charger. If a cell is weaker than the others, it will only get worse with time, and this cell will bring down your pack.

You could simply place the charger into the battery pack (which makes it a lot easier to access individual cell voltages, and perhaps place a temperature sensor too, just to be safe).

Or place the current-limited voltage source in the "charger", and the cell balancing/protection in the pack.

There is a battery charge/management PMIC category on DigiKey, you'll find the chips you need there.
 
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Hi again,


Oh good so i guess you have a good idea what you are doing then. I wanted to make sure because Li-ion batteries have been known to explode and ignite and this happens mostly when they are charging.

As peufeu pointed out, a good voltage reference is a must too. This is because (as you may know) the voltage tolerance near the end of charge is very tight, meaning you should set your top output to 4.15v or so, but not to exceed 4.20v. These cells are unforgiving when it comes to over voltage so this is first priority.
Second priority is limiting the current, which for your cells would probably be 1 amp.
All this means that you charge using a current regulator first, limited to 1 amp, then after the voltage gets up near top end like 4.15v the charger must cut back the current to maintain the voltage at or near that level.
One simple solution would go like this:
Build a current regulator that measures the current and limits it to 1 amp, paralleled with a voltage regulator that starts to regulate at 4.15 which means the current drops to less than 1 amp after the voltage regulator takes over.

There are simpler ways too however, and the simplest way i know of is to use an LM317 with an input series resistor to limit current. The LM317 voltage set resistors are selected to provide 4.15v output (a simple voltage regulator which requires 2 resistors) and a third power resistor on the input to limit current. It's quite simple.


If you intend to charge more than one at a time such as in series, then you need to monitor the voltage on each cell individually and either cut back when one cell reaches 4.15v or use a charge balancing circuit that allows one cell to be charged while the other one sits there at 4.15v (as it is done more or less). It's important to monitor each cells voltage in the pack because if one cell goes above 4.20v it will get ruined very quickly or worse cause a fire.

Also as peufeu pointed out, there are chips that are made for this although i find rolling my own circuit to be more interesting and much more flexible. It's up to you though. You can then go with a switcher or the simpler linear.

Oh yeah there are other precautions too, such as not allowing the voltage to drop below 2.5v on any single cell.
 
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I heard that DeWalt uses a different kind of Li-Ion battery cell (A123) that is fully charged at a voltage less than 4.15V.
 
Thanks for the help so far. I do not know how manufacturers charge the cells independently though since for instance in my Dewalt battery pack there are only 2 terminals which will allow flow of electrons both directions (to charge and discharge). The charger omits around 14.4 volts to charge a 14.4 volt pack. So my plan was to connect the cells in parallel pairs, then to connect 4 pair in series to get to around the 14.4 volts that the NI-MH pack is now.

Also I am interested in the laptop charger theory. As I understand it (and I stand to be corrected) is that the charger is nothing but a transformer and the intelligent part of the charging circuit is integral to the laptop, so if I connected a 14.4 volt laptop charger to my drill battery it would continue to give 14.4 volts at the rated current for ever if it were permanently connected.

I am interested in giving this circuit a try and modofying the values of the components for 4 cells as per this chart, and maybe add some thermal cutout, then put the over discharge protection in the battery pack.

I have also contemplated trying to get hold of a 14.4 volt laptop battery and removing the cells and circuitry from inside and placing them inside my battery box, I could then simply use a transformer inside my existing charger, or I could put a female round connector on the outside of the pack to connect to a transformer.

Any thoughts on the above?
 
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Well, if you have the complete system you can split it in two, and the interface is only two wires :

Part 1 : current-limited voltage source, outputs a very precise 4.15V/cell (LiIon) or other voltage for other chemistries like LiPo, A123 etc : inside the power brick
Part 2 : protection (overcurrent, overcharge, overdischarge) and balancing : inside the battery

> the charger is nothing but a transformer and the intelligent part of the charging circuit is integral to the laptop

Yes, the power brick is a simple switchmode power supply, whose purpose is :
- saving weight and space in the laptop (always advertise the weight without charger !...)
- saving lots of certification fees (only the brick has to be certified for mains voltage safety, not the entire system)

Usually laptop batteries have a connector with lots of pins so I guess the balancer must be in the laptop itself. Here is a pack I hacked :

LiIon Helmet Light | Peufeu's Electronic Stuff

I have a stand-alone charger for it, which has a little connector which uses all the pins, so it must have a cell balancer too.
 
Sometimes a "Charge Balancer" is used. This is a circuit that may be part of the pack that monitors the voltage of each cell and does not allow any one cell voltage to exceed 4.20v.

The way some chargers do it is they switch the cell out when it is charged, and short circuit it's position in the string. You can most likely find more info about this on the web.
 
OK,

I have found an interesting thread on charge balancers here

I suppose I could try to build this circuit in to my battery pack and put the circuit from here in to my charger beefed up to take 4 cells using the information here.

I know the circuit for the balancer is for li-po cells but I am sure it would work with li-ion just the same?
 
If you do not charge a Lithium battery correctly then it will probably catch on fire. You do not extinguish a very hot Lithium fire with water.
 
OK,

I have found an interesting thread on charge balancers here

I suppose I could try to build this circuit in to my battery pack and put the circuit from here in to my charger beefed up to take 4 cells using the information here.

I know the circuit for the balancer is for li-po cells but I am sure it would work with li-ion just the same?

Hello again,


Yes, if the circuit works (and it looks like it does) then it will work for Li-ion.
You'll probably have to use 1 percent resistors for the 10k and 15k however if you use a 1k pot as they have in that schematic or else you may not be able to adjust to the correct cutout voltage point, like 4.15v or so. The adjustment sensitivity is about 40 ohms per 0.01 volt, which is 4000 ohms per volt, and 40 ohms is about 12 degrees on a 1k pot, but only 2.4 degrees on a 5k pot so it probably wouldnt be a good idea to go to a 5k pot instead of the 1k.
 
You do not extinguish a very hot Lithium fire with water.

As a professional firefighter I am well aware that the only extinguishing media for lithium based fire is Copper powder or graphite powder (sadly we don't carry it on the pumps though) however that is why I said earlier I would add some thermal protection - fire requires heat oxygen and fuel, by removing the heat source before it gets too high there will be no fire.

Hello again,


Yes, if the circuit works (and it looks like it does) then it will work for Li-ion.
You'll probably have to use 1 percent resistors for the 10k and 15k however if you use a 1k pot as they have in that schematic or else you may not be able to adjust to the correct cutout voltage point, like 4.15v or so. The adjustment sensitivity is about 40 ohms per 0.01 volt, which is 4000 ohms per volt, and 40 ohms is about 12 degrees on a 1k pot, but only 2.4 degrees on a 5k pot so it probably wouldnt be a good idea to go to a 5k pot instead of the 1k.

Thanks,

I will run some tests and keep you all updated as to my findings.
 
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Hello again,


Oh great that would be nice as i like to use Li-ion cells too and thought about doing my drill over with Li-ion too. When i bought my drill it came with two (yes 2) battery packs and after about 6 months both (yes both) packs were completely dead, unable to hold a charge anymore. I've since been using sealed lead acid but that's big and heavy and i have to use a cord to the drill from the battery. The only thing i didnt like about Li-ion is the cells i have are probably only rated for about 10 amps max draw, while the drill takes about 35 amps at some times when screwing in those drywall screws. I dont think the Li-ion cells would like that.
What kind and size Li-ion cells are you planning to use with this?
 
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