I want to charge an 18-V lithium ion battery pack from a cordless tool that was a freebie. The battery pack seems to have three terminals: Plus, Minus, and something that I interpret as being for a temperature sensor.
Assuming I don't need a particularly quick charge, is there a safe and simple way to charge this battery? Can it just be a power supply in series with a resistor, or does it need to be more elaborate?
Buying a charger for this battery would be more than I want to spend.
>I want to charge an 18-V lithium ion battery pack from a cordless tool > that was a freebie. The battery pack seems to have three terminals: > Plus, Minus, and something that I interpret as being for a temperature > sensor.
> Assuming I don't need a particularly quick charge, is there a safe and > simple way to charge this battery? Can it just be a power supply in > series with a resistor, or does it need to be more elaborate?
> Buying a charger for this battery would be more than I want to spend.
> Thanks in advance!
That battery chemistry does not tolerate charging abuse very well. I would recommend against what you are purposing.
>I want to charge an 18-V lithium ion battery pack from a cordless tool > that was a freebie. The battery pack seems to have three terminals: > Plus, Minus, and something that I interpret as being for a temperature > sensor.
> Assuming I don't need a particularly quick charge, is there a safe and > simple way to charge this battery? Can it just be a power supply in > series with a resistor, or does it need to be more elaborate?
> Buying a charger for this battery would be more than I want to spend.
> Thanks in advance!
The problem is that if you put too much current through a lithium battery, you can ignite it. I've never seen one go, but they apparently make a spectacular fire. I've heard that they have been restricted on cargo flights for this reason (ie, a short causes one to go, which causes the rest of the crate to go).
The other problem with LiION batteries is that they need to be held at a constant voltage for a period of time after the 'constant current' phase to get their full charge. Once they reach the max voltage, the current they pull will gradually taper off until the charger shuts off. It isn't recommended to put one of these on a 'float charge', like a lead acid battery.
If you have or can get a 24V regulated source*, I can show you a circuit that will safely charge the battery, using 4 transistors and a few resistors (all available from Radio Shack). It should be easy to build, but you'll need to solder it together and adjust it, meaning you'll also need a voltmeter. You'll also need an external alarm to tell you to turn it off, so you don't cook the battery too long.
> >I want to charge an 18-V lithium ion battery pack from a cordless tool > > that was a freebie. The battery pack seems to have three terminals: > > Plus, Minus, and something that I interpret as being for a temperature > > sensor.
> > Assuming I don't need a particularly quick charge, is there a safe and > > simple way to charge this battery? Can it just be a power supply in > > series with a resistor, or does it need to be more elaborate?
> > Buying a charger for this battery would be more than I want to spend.
> > Thanks in advance!
> The problem is that if you put too much current through a lithium battery, > you can ignite it. I've never seen one go, but they apparently make a > spectacular fire. I've heard that they have been restricted on cargo flights > for this reason (ie, a short causes one to go, which causes the rest of the > crate to go).
> The other problem with LiION batteries is that they need to be held at a > constant voltage for a period of time after the 'constant current' phase to > get their full charge. Once they reach the max voltage, the current they > pull will gradually taper off until the charger shuts off. It isn't > recommended to put one of these on a 'float charge', like a lead acid > battery.
> If you have or can get a 24V regulated source*, I can show you a circuit > that will safely charge the battery, using 4 transistors and a few resistors > (all available from Radio Shack). It should be easy to build, but you'll > need to solder it together and adjust it, meaning you'll also need a > voltmeter. You'll also need an external alarm to tell you to turn it off, so > you don't cook the battery too long.
Yes, I would greatly enjoy seeing such a circuit. I may still wimp out because of the safety issues, but at least I will have an idea of how a charger should work. Also I actually have a timer that I plug my soldering iron into, so I don't forget to turn it off.
>> >I want to charge an 18-V lithium ion battery pack from a cordless tool >> > that was a freebie. The battery pack seems to have three terminals: >> > Plus, Minus, and something that I interpret as being for a temperature >> > sensor.
>> > Assuming I don't need a particularly quick charge, is there a safe and >> > simple way to charge this battery? Can it just be a power supply in >> > series with a resistor, or does it need to be more elaborate?
>> > Buying a charger for this battery would be more than I want to spend.
>> > Thanks in advance!
>> The problem is that if you put too much current through a lithium >> battery, >> you can ignite it. I've never seen one go, but they apparently make a >> spectacular fire. I've heard that they have been restricted on cargo >> flights >> for this reason (ie, a short causes one to go, which causes the rest of >> the >> crate to go).
>> The other problem with LiION batteries is that they need to be held at a >> constant voltage for a period of time after the 'constant current' phase >> to >> get their full charge. Once they reach the max voltage, the current they >> pull will gradually taper off until the charger shuts off. It isn't >> recommended to put one of these on a 'float charge', like a lead acid >> battery.
>> If you have or can get a 24V regulated source*, I can show you a circuit >> that will safely charge the battery, using 4 transistors and a few >> resistors >> (all available from Radio Shack). It should be easy to build, but you'll >> need to solder it together and adjust it, meaning you'll also need a >> voltmeter. You'll also need an external alarm to tell you to turn it off, >> so >> you don't cook the battery too long.
> Yes, I would greatly enjoy seeing such a circuit. I may still wimp out > because of the safety issues, but at least I will have an idea of how > a charger should work. Also I actually have a timer that I plug my > soldering iron into, so I don't forget to turn it off.
> Thanks!
View with courier or other fixed width font.
Here is what you need:
1 10k trimmer. A cheap one is fine, the setting is not too exact. 1 1k resistor 1 10k resistor 1 1.8k resistor 1 1 ohm 2W resistor (one of the big square ones) 2 2N3906 or pretty much any TO-92 PNP transistor 1 2N4401 NPN (or any TO-92 NPN transistor) 1 IRF1405 N channel MOSFET (or equivalent, see below) 1 heat sink for the MOSFET (TO-220 variety) 1 mounting screw and insulator for mosfet and heat sink 1 regulated 24V@1A power supply
The mosfet and NPN transistor form a fixed current source. They limit the current through the battery to a value near 700mA. The two PNP transistors compare the output voltage with the voltage on the wiper of the trimmer. When the output voltage on the - terminal gets down to the voltage on the wiper, the current source is turned off. As the battery keeps sucking current, this differential pair will keep the voltage constant.
You adjust it by putting a 10k resistor across the output (instead of the battery). Then, use the trimmer to set the - of the output (using your multimeter) so that it is 18V below the + output.
You could probably get away with a 20V Power supply with these parts. With an IRF1405, you might be able to skip the heatsink. However, you probably won't be able to get one of those at radio shack. Get a couple of TO-220 case N channel mosfets and a heat sink (along with some heat sink paste) and a mounting kit. Handle the mosfets carefully, because you can blow out the gate pretty easily with a static shock.
Regarding NPN vs PNP (in case you don't know)
For NPN, the collector is the more positive one, and current flows from collector to emitter.
For PNP, it is the other way around, and the emitter is the more positive one, and current flows from emitter to collector.
> >> >I want to charge an 18-V lithium ion battery pack from a cordless tool > >> > that was a freebie. The battery pack seems to have three terminals: > >> > Plus, Minus, and something that I interpret as being for a temperature > >> > sensor.
> >> > Assuming I don't need a particularly quick charge, is there a safe and > >> > simple way to charge this battery? Can it just be a power supply in > >> > series with a resistor, or does it need to be more elaborate?
> >> > Buying a charger for this battery would be more than I want to spend.
> >> > Thanks in advance!
> >> The problem is that if you put too much current through a lithium > >> battery, > >> you can ignite it. I've never seen one go, but they apparently make a > >> spectacular fire. I've heard that they have been restricted on cargo > >> flights > >> for this reason (ie, a short causes one to go, which causes the rest of > >> the > >> crate to go).
> >> The other problem with LiION batteries is that they need to be held at a > >> constant voltage for a period of time after the 'constant current' phase > >> to > >> get their full charge. Once they reach the max voltage, the current they > >> pull will gradually taper off until the charger shuts off. It isn't > >> recommended to put one of these on a 'float charge', like a lead acid > >> battery.
> >> If you have or can get a 24V regulated source*, I can show you a circuit > >> that will safely charge the battery, using 4 transistors and a few > >> resistors > >> (all available from Radio Shack). It should be easy to build, but you'll > >> need to solder it together and adjust it, meaning you'll also need a > >> voltmeter. You'll also need an external alarm to tell you to turn it off, > >> so > >> you don't cook the battery too long.
> > Yes, I would greatly enjoy seeing such a circuit. I may still wimp out > > because of the safety issues, but at least I will have an idea of how > > a charger should work. Also I actually have a timer that I plug my > > soldering iron into, so I don't forget to turn it off.
> > Thanks!
> View with courier or other fixed width font.
> Here is what you need:
> 1 10k trimmer. A cheap one is fine, the setting is not too exact. > 1 1k resistor > 1 10k resistor > 1 1.8k resistor > 1 1 ohm 2W resistor (one of the big square ones) > 2 2N3906 or pretty much any TO-92 PNP transistor > 1 2N4401 NPN (or any TO-92 NPN transistor) > 1 IRF1405 N channel MOSFET (or equivalent, see below) > 1 heat sink for the MOSFET (TO-220 variety) > 1 mounting screw and insulator for mosfet and heat sink > 1 regulated 24V@1A power supply
> The mosfet and NPN transistor form a fixed current source. They limit the > current through the battery to a value near 700mA. The two PNP transistors > compare the output voltage with the voltage on the wiper of the trimmer. > When the output voltage on the - terminal gets down to the voltage on the > wiper, the current source is turned off. As the battery keeps sucking > current, this differential pair will keep the voltage constant.
> You adjust it by putting a 10k resistor across the output (instead of the > battery). Then, use the trimmer to set the - of the output (using your > multimeter) so that it is 18V below the + output.
> You could probably get away with a 20V Power supply with these parts. With > an IRF1405, you might be able to skip the heatsink. However, you probably > won't be able to get one of those at radio shack. Get a couple of TO-220 > case N channel mosfets and a heat sink (along with some heat sink paste) and > a mounting kit. Handle the mosfets carefully, because you can blow out the > gate pretty easily with a static shock.
> Regarding NPN vs PNP (in case you don't know)
> For NPN, the collector is the more positive one, and current flows from > collector to emitter.
> For PNP, it is the other way around, and the emitter is the more positive > one, and current flows from emitter to collector.
Thanks for the circuit! I entered it into LTSpice today, and simulated it by letting it charge a capacitor. It's a lot simpler than the mess of op amps I was thinking of.
Firstly, the third terminal probably isn't a temperature sensor, but a voltage sensing terminal to measure the battery terminal voltage at its terminals.
Secondly, although I've designed several Li-ion chargers, what you're proposing is so dangerous I am not going to offer any advice on how to charge it. I would advise, though, having a bucket of dry sand on hand to put out the fire. Water, foam and CO2 won't work, you need to block O2 and water getting to the fire. -- Nemo
On Jan 17, 2:42 pm, Nemo <P...@nospam.nospam.nospam.nospam.co.uk> wrote:
> Firstly, the third terminal probably isn't a temperature sensor, but a > voltage sensing terminal to measure the battery terminal voltage at its > terminals.
> Secondly, although I've designed several Li-ion chargers, what you're > proposing is so dangerous I am not going to offer any advice on how to > charge it. I would advise, though, having a bucket of dry sand on hand > to put out the fire. Water, foam and CO2 won't work, you need to block > O2 and water getting to the fire. > -- > Nemo
Thanks for the advice. I gathered that I did not really want to mess with these batteries. And then I looked up the price of this power tool. I will put it out on Craigs List.
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