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Author Topic: Protected LIthium Ion Charging Circuit  (Read 3709 times)
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I bought 5 3.7v protected lithium ion batteries off dx, and put them in a series configuration. I have a AC to 19v DC converter which maxes out at about 4.5v before it will blow. The question here is: do the batteries give their own resistance and control the charging current to a stable 1-2Amps? Or if I have to place a resistor in the circuit after the battery that will only allow a max of 1Amp.

The batteries are protected and shut off at 4.2v and 2.75v, so there is no need for complex charging circuits. I just need to know if the battery will limit the current intake manually through its own resistance, or if it will take as much as it can before either the DC converter or the cells explode.
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I bought 5 3.7v protected lithium ion batteries off dx, and put them in a series configuration. I have a AC to 19v DC converter which maxes out at about 4.5v before it will blow. The question here is: do the batteries give their own resistance and control the charging current to a stable 1-2Amps? Or if I have to place a resistor in the circuit after the battery that will only allow a max of 1Amp.

The batteries are protected and shut off at 4.2v and 2.75v, so there is no need for complex charging circuits. I just need to know if the battery will limit the current intake manually through its own resistance, or if it will take as much as it can before either the DC converter or the cells explode.

Lipos always need fairly complex charging circuits (compared to other rechargeable types) even those with protection circuits.  This is an area you should read up on extensively before tinkering, since LoPo's respond explosively to mistakes.

A good starting tutorial; http://learn.adafruit.com/li-ion-and-lipoly-batteries/overview
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I bought 5 3.7v protected lithium ion batteries off dx, and put them in a series configuration. I have a AC to 19v DC converter which maxes out at about 4.5v before it will blow. The question here is: do the batteries give their own resistance and control the charging current to a stable 1-2Amps? Or if I have to place a resistor in the circuit after the battery that will only allow a max of 1Amp.

Neither. Lithium Ion chargers need to have some 'smarts' to properly charge such cells. Such charging circuitry usually checks for the cells terminal voltage to decide to enter the charge process or not, then it supplies a controlled constant current rate of about 1C rate while monitoring the gradual rising terminal voltage. When the terminal voltage approaches  around 3.9-4vdc then it switches to a controlled constant voltage charge of 4.2vdc. So while your 'self-protected' cells may indeed limit possible damage to the cells, charging in a simplistic manner as you have described may shorten the total cell life. Your kind of on your own if you wish to deviate from the manufactures recommended charging method for your cells.

Lefty


The batteries are protected and shut off at 4.2v and 2.75v, so there is no need for complex charging circuits. I just need to know if the battery will limit the current intake manually through its own resistance, or if it will take as much as it can before either the DC converter or the cells explode.
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Also, assembling larger packs like you have done presents additional issues;

from a later page of the tutorial I linked to above;

"So the next question is whether you can connect batteries in series to get higher voltages? This is also discouraged because the battery wont be able to be charged in a balanced manner. You should purchase a lithium ion pack that is preassembled. "
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Mate , in my opinion you shouldnt consider playing with lipos unless you know what you are doing, search youtube for what can happen to an overcharged lipo,

the protection that you mention is normally there to stop overdischarging the cell which damages them a lot,

If you have wired up a 5s pack, then you need to balance them properly each charge or you will end up with a nasty result,
making your own packs is dangerous, you need to ohm match the packs, so unless your 5 cells were identical internal resistance then you will find that they discharge unevenly, the result of this will be harsh,

lipos, when they get below a certain limit, normally about 3.4v SUDDENLY lose capacity at an alarming rate, if only one cell of your 5s is dodgy then it will discharge rapidly, reducing the overall voltage of the pack and possibly causing a shutdown of whatever you are using, and of course if it doenst shut down then you will be damaging that cell. 

I paid about 15 euros for a hobbyking charger, it can do most battery types, and it balances lipos, its not perfect but it works,
i recommend buying a lipo charger and not messing around, also add a balancer cable to your pack,
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Mate , in my opinion you shouldnt consider playing with lipos unless you know what you are doing, search youtube for what can happen to an overcharged lipo,

While I don't disagree with this piece of advice, it presents a chicken and the egg scenario.  While a person can (and should) read up on the technology and such, you really don't gain experience without hands on experimentation and the resulting mistakes.  So in my opinion, using reasonable caution, experiment as desired, but read a little more first.
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I know the danger of lions, the charging experiment will be done in the driveway... in case it turns out like july 4th. From the information that I've been seeing, the battery needs a current limiter, and for that I bought a 20w 20ohm resistor. The dc converter output current gives 19v giving me a bit under 1C.

The configuration im going with is charger+->1A fuse->lions->resistor->charger-
The lions are still a 5 pack of 3.7v lion cells of dx. Each cell has over/under discharge protection, so at worst case I'll just have an uneven charging, but it wont differ by that much, since while charging lions they exponentially resist current while finishing their charge cycle, each battery will be near at least 4v.

I'll post the results once the experiment is complete. My biggest worry is destroying 20 bucks worth of lions, since getting more would take another month or so. I also have all the supplies to create an arduino voltmeter, so i'll be using that and some mosfets to turn the arduino into a smart charger along with the other requirements of this project.
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The problem is not that the failure will occur every time you charge it in that way, but that it will eventually fail in a spectacular way.  Lipos can not use an over voltage for charging like you propose.  Each cell must be charged with a maximum of 4.2v and when charged in series like you propose you simply can't guarantee that.  Series wired packs will usually have additional connections which allow for balanced charging which take into account the differing interbal resistances and hence the different charging voltages that would otherwise appear across each cell...

Unless your goal is simply to see what happens when you incorrectly charge a lipo (the fire), you are making a major mistake proceeding.  What you are doing will destroy those lipos you spent money for and their only value will be as an overpriced piece of fireworks...

Again, I strongly urge you to do some reading on the subject and learn something before you try to 'experiment'...
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A bit more detail on how the individual cells are protected from over / under charging would help.
Charging Lipos in series needs cell balancing circuitry and its unclear from your description whether this exists
or not.
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If you discharge them in series the first pack to reach low cutout voltage will disable the entire string. That's not a terrible situation, and under most circumstances it will only occur when all the packs are low anyway. If one of the packs is going bad however it will result in reduced usable capacity for the full string, but this is a failure condition for that individual pack and having or not having the protection circuit on that pack does not change the fact that a failure occurred. It's worth noting that if the protection did not exist that further discharge of that individual cell would probably leave it permanently dead.

If you charge them in series, current limited to the capability of any individual pack, the first pack to reach high cutout voltage will disable the entire string leaving the other batteries partially charged. You should reconnect them in parallel and charge them at 4.2V, and again you still have to make sure you charge the parallel pack at a current lower than any single pack can handle. There is no risk in charging them in parallel other than being frustrated by potentially tripping an individual cell's current rate limit and being forced to disconnect and reconnect that pack. Ideally you would charge the packs individually using the current and voltage limited method described by retrolefty.

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