greatest danger using li-ion cells occurs during re-charging

[cy]

unfortunately inherent dangers of using li-ion cells are not always apparent. cpf'ers are on bleeding edge of technology, using consumer grade loose li-ion cells.

greatest danger using li-ion cells occurs during re-charging, not during use. most lights are constructed well enough to prevent dead shorts. use bare li-ion in series with extreme caution. protected li-ion cells are always preferred.

some li-ion cells are indeed protected for high & low discharge. overcharge protection is where it's needed the most. Pila's contain overcharge protection. (According to AW all of his protected cells contain overcharge protection.)

if you are not going to invest in a hobby grade charger. safest li-ion cells to use is with overcharge protection.

new Pila chargers will terminate charge at 4.2v. most low end chargers will trickle charge after light turns green. a trickle charge will eventually overcharge.

li-ion cells have a narrow charge widow of 4.10v-4.20v, without a natural termination point. And will accept charge long as you feed current to cell. until thermal runaway is reached... rapid disassembly of cell (explosion) if other cells are close by (secondary explosions) possibly burning down your home.

it's not possible to charge li-ion in series and stay balanced without individual leads going to each cell. This requires charger with multiple channels or balancer module. safest way to charge li-ion cells is in singles! multiple li-ion cells can be charged in parallel. make sure cells are within .5v of each other to prevent excessive currents as cells start to balance each other. charge rate should be set to 1c of lowest rated cell in stack. http://candlepowerforums.com/vb/showthread.php?t=161299

personally I would not use low end chargers. Invest in a good hobby charger. I use schulze and triton. lots of other choices. it may be worth reading balance cradle thread. http://candlepowerforums.com/vb/showthread.php?t=158121&highlight=explosion

it's a relatively new trend to use multiple li-ion cells to drive hotwires. li-ion cells when discharged in series at high rates will become unbalanced. there's evidence dentrites will form during slow charge after cell reaches 4.2v and during high discharge exceeding mfg recommendations. these sharp formations could cause an internal dead short.

if you charge those same series of unbalanced cells. charger will charge up to correct voltage/termination point for number of cells. but internally some cells will be undercharged, some will be overcharged. if unbalance condition becomes bad enough you will reach thermal runaway (explosion).

C & D sized li-ion cells have been available in limited numbers with little guidance of dangers during re-charging. accidents with C/D sized li-ion cells will not be minor events. (AW's C li-ion contain protection PCB)

The RC community has been using and recharging lithium polymer packs for sometime. there's been a number of explosions documented. mostly during recharging. http://www.rcgroups.com/batteries-and-chargers-129/

So far we have only one documented instance of a li-ion cell exploding on cpf. this was caused by operator error, selecting an incorrect setting on the charger. http://candlepowerforums.com/vb/showthread.php?t=152013&highlight=explosion
charger involved in this explosion caused by user error is http://candlepowerforums.com/vb/showthread.php?t=117653&highlight=norm
fortunately no one was injured and house didn't burn down. but results could have been much worst.

it's extremely hard to cause a NMH battery to explode. VS it's relatively easy to cause a li-ion cell to explode by overcharging.

until A123 type cells become available in sizes cpf'ers can use. it's my desire to raise awareness of inherent dangers of li-ion use and avoid series of explosions and fires RC community has experienced.

apologies for length of post. I've tried my best to avoid technical mumbo jumbo....for more details http://candlepowerforums.com/vb/showthread.php?t=106242&highlight=explosion

If any information needs to be updated or corrected. please let me know and I will get it done. due to evolving nature of state of li-ion cells. this post is subject to revision.

if I had to make a few recommendations for using li-ion cells:

DISCLAIMER/CAUTION: I am not a battery expert nor do I pretend to be one. Please verify this information for yourself. Use this at your own risk. Not responsible for anything. Information for consumer use of loose li-ion cells is constantly evolving.

1. recognize greatest danger of using li-ion cells occurs during re-charging.
2. invest in a hobby grade charger and avoid use of low end chargers.
3. use li-ion cells with internal protection circuits (overcharge, over-discharge). especially when used in series.
4. don't charge in series without balancer leads going to each cell. w/independent channel for each cell.
5. safest way to use/charge li-ion cells is in singles.
6. don't use li-ion cells in series without protection circuits.
7. beware of dead short dangers. li-ion cells discharge at high rates
8. never charge li-ion cells unattended
9. charge li-ion cells in a protected area, so if fire should occur. your home will not burn down.
10. discharge larger li-ion cells to 3.4v before shipping. this removes almost all stored energy in cell.

 

[Josey]

Thanks for the information, Cy. I've been charging my AW Li-Ion C cells at 2 amps, which is the highest my Tenergy balanced charger can go. But that's the initial charging rate. The last three hours or so, the charge rate drops down slowly from 1 amp to 0.4 amp, when the signal to stop charging beeps. Since this charge rate is well below 1C, how much of an issue is it?

 

[cy]

josey, that's normal... a good hobby li-ion charger will ramp up current (.1C) slowly until a set voltage is reached, then ramp to max current selected by user or charger. then at a set voltage charger will step down current to a low level until final voltage or 4.1 or 4.2v is reached. at that point charge will terminate.

you can fast charge at 1C up to aprox. 90% of total capacity of cell, but then it will take longer to reach final termination point. or you can charge at .5C up to aprox. 90% slower, but then it's shorter to reach final termination point.

Thanks for the information, Cy. I've been charging my AW Li-Ion C cells at 2 amps, which is the highest my Tenergy balanced charger can go. But that's the initial charging rate. The last three hours or so, the charge rate drops down slowly from 1 amp to 0.4 amp, when the signal to stop charging beeps. Since this charge rate is well below 1C, how much of an issue is it?

 

[Led_Blind]

Thanks for the information, Cy. I've been charging my AW Li-Ion C cells at 2 amps, which is the highest my Tenergy balanced charger can go. But that's the initial charging rate. The last three hours or so, the charge rate drops down slowly from 1 amp to 0.4 amp, when the signal to stop charging beeps. Since this charge rate is well below 1C, how much of an issue is it?

it is all about overcharging. If after the charge cycle has stopped 0.4a is still delivered to the cell then there is a definite chance of overcharging. Dont worry about the 2a, those C cells are almost double that in capacity.

I have blown a lipo, it puffed up like a ballon on a normal charge cycle. Got to full charge fine, flew the heli fine but after that it was recycled

 

[cy]

cpf'ers are on bleeding edge of technology, using consumer grade loose li-ion cells.

until recently loose li-ion cells have not been available direct to consumers. most li-ion mfg still refuse to sell large loose li-ion cells to consumer.

JSB was directly responsible for this trend by financing the original protected R123 li-ion cells. JSB suffered huge financial losses due to this. We owe a debt of gratitude to JSB and should try to support him by purchasing products from JSburleys.

For the record Siverfox, Newbie and JS are the real experts of battery technologies. I've posted this information separate from first post due to length. there's so much information to cover... it's incredibly hard to be concise.

with C size li-ion cells available and used in series for hotwire applications. As these newbie users of li-ion cells attempt to re-charge these C li-ion cells in series without proper precautions.

the safety issue has become acute!

direct sellers of larger sized li-ion cells are almost all from far east. which for all practical purposes sellers from far east have zero liabilities from accidents. If say someone's house was to burn down from a defective product. it would extremely difficult, if not impossible to hold those folks accountable.

 

[balazer]

As far as I've read, the DSD charger is one inexpensive lithium ion charger that will not overcharge your cells.

If you use such a charger, are you really at any more risk than if you use any other lithium ion devices, like a cell phone or laptop? Those devices, too, have occasionally been shown to explode or start fires.

 

[brightnorm]

Cy,


This is important information that should be seen by every CPF member. IMO it should be stickied, and not just in this forum.

Brightnorm

 

[cy]

low end li-ion chargers are notoriously fickle on actual point ending charge. I've taken 5 separate nano chargers and ended up with 5 slightly different termination points. 4.12v to 4.3v

further all of the low end charger shown in pic didn't truly terminate, but continued to trickle charge after green light came on. New pila charger are among the few exceptions.

a safe way to use low end chargers is recognize you cannot leave cell charging in cradle for long periods. charge attended and remove cell after light turns green. then quickly measure final charge a few cycles to determine actual voltage.

if you want to experiment how quickly trickle charge builds up. simply leave cell in cradle for a set number of hours, then retest for voltage. don't allow cell to go above 4.20v

correct operating range for most li-ion cells is 4.10 - 4.20v range. look to your mfg for exact spec's. best choice if you charger gives one, is to charge to 4.1v termination. only charge to 4.2v when you need max capacity.

low end chargers typically charge at aprox. 500 milliamps or less. note as li-ion cells sized go up, amount of current needed to charge at most mfg is .5C to 1C. smaller chargers simply cannot supply enough current to meet these needs.

cell phone and laptop are usually UL approved devices and contain a host of protections built in. laptop battery packs comes with protection and most monitor cells in series of three. explosions are rare but do happen. as you know loads of recalls involving laptop batteries.

part of the safety issue with using loose li-ion cells is directly related to the perception that it's safe to plug in a li-ion charge and forget about it.

some li-ion cells and chargers sold contain little to no protection. for instance the explosion reported on cpf was caused by user error. charger in question contain no protection if user selected wrong voltage setting. http://candlepowerforums.com/vb/showthread.php?t=152013&highlight=explosion

so if you selected two cell setting and proceeded charge one cell. this would result in serious overcharging and likely explosion. there will be no warning... charger will simply overcharge cell(s). http://candlepowerforums.com/vb/showthread.php?t=117653&highlight=norm

you can do the same thing with hobby chargers too. it much harder for warnings will be issued several times by triton and schulze wrong chemistry is selected or wrong number of cell is selected etc.

but fooling any charger can be done. you can cause a li-ion cell to blow up by chargering it at say NMH settings.

note: you cannot safeguard against stupidity! It's no problems to safely use li-ion cells. once you realize the consequences of overcharging loose li-ion cells.

As far as I've read, the DSD charger is one inexpensive lithium ion charger that will not overcharge your cells.

If you use such a charger, are you really at any more risk than if you use any other lithium ion devices, like a cell phone or laptop? Those devices, too, have occasionally been shown to explode or start fires.

 

[SilverFox]

Hello Cy,

The actual maximum voltage for Li-Ion cells is more precisely stated as 4.200 volts, plus or minus 0.05 volts.

I think it is better to target 4.200 volts and understand that there is a little room for error, than to target 4.25 volts and forget that the $10 volt meter being used may not be totally accurate.

I hope you can understand this subtle difference.

Tom

 

[cy]

Tom, thanks for the correction and edited accordingly to 4.1v - 4.2v

the 4.15v to 4.25v range come from white papers, depending upon mfg.

totally understand why it's much better to state 4.2v with a .5v room for error.

 

[MrAl]

Hi there

I build all my chargers with a built in difference circuit that constantly monitors
the voltage and cuts back current more and more as the voltage gets closer
and closer to 4.2 volts. For example, at 4.100 volts the cell might get a full
1 amp of current (max for that cell) but at 4.190 it may only get 250ma and
at 4.199 volts it may only get 5ma. If the cell could ever really reach exactly
4.200000 volts it would get a theoretical zero amps of charge, effectively
cutting it out altogether. If there is any little self discharge which brings the
cell down to 4.199999 volts for example then perhaps 1ua of current would
then begin to flow, but this isnt really strictly speaking a trickle charge because
it's not constant...it's directly related to the cells voltage and its internal
resistance. A true trickle charge could eventually cause an overcharge condition,
but be aware that some manufacturers *may* call a charge that is indeed related
to cell voltage (as with all my chargers) a trickle charge, when really it isnt.
To find out for sure i guess you would have to measure your charge current
when the cell is right up at 4.200 volts. If the charge current is extremely low
then it's ok...if it's still 50ma then it isnt ok to leave on charge.

 

[SilverFox]

Hello Al,

I am afraid your set up is what is considered as trickle charging.

When the cells are healthy, this should not present any problems, however as the cells age they will be subjected to a trickle charge for an extended period of time.

First, let's take a look at a typical specification sheet for ah Li-Ion cell. Here is one from Panasonic. Notice that in the charging reference they refer to a 50 mA cut off for these cells.

The range is from 0.2C to 0.5C for the cut off current.

This means that when the current falls below the cut off value, the charge shuts off. The charger remains off until the voltage drops to around 4.05 volts. Then it starts up again, and once again completely stops when the current drops below the cut off current.

Back to the aged cell. The cell is considered "dead" when its capacity drops below 80% of its initial capacity. The reason the capacity drops is because the internal resistance of the cell goes up. This increase in internal resistance limits the maximum voltage of the cell. This means that it spends more time holding the constant 4.2 volt as the current tapers off. If your current never shuts off, you can effectively trickle charge it for quite a while as the cell struggles to stay at 4.2 volts.

As soon as you remove the cell from the charger, the voltage will drop on a cell with increased internal resistance. Your cell may have been charged to 4.2 volts, but when you take it off of the charger it may drop down to 4.12 volts, or something like that.

The lithium metal plating occurs when the cell is at its highest potential and has a very slight charge continuing to go into it. When you review a typical charging curve, you should see about 1/3 of the time spent in the Constant Voltage phase, 1/3 of the time the current will be tapering off rapidly, and the final 1/3 of the time is a very slight taper off in current until the current shut off value is reached.

Aged cells respond to charging differently. They will have a reduced Constant Voltage phase, a similar taper phase, and a greatly extended "trickle" charge phase. This is also what happens during charging at higher charge rates.

The extended amount of time spent during the last phase of charging is not healthy for the cell because it promotes the plating of lithium metal out of the electrolyte solution. This can also become a safety issue.

It just so happens that when cells are at 80% of their initial capacity, the third phase takes around 2 - 3 times longer to complete. This results in more time spent in this potentially "dangerous" zone.

Some of the hobby chargers have a fixed cut off of 100 mA. When the charge rate drops below 100 mA, the charge stops. Others use 0.05C of the charging current you select. You end up with slightly less charge, but avoid any issues of lithium plating. The other plus is that the charge time is reduced.

I would suggest that you add a charging current cut off to your charger.

Tom

 

[65535]

Standard CC/CV charging in parallel banks IMHO is the best way to charge, you can set the desired voltage you want, lower voltage increases cell life expentancy. Also the charge terminates itself once the current trickles to nearly nothing the cell is charged, works great on LA batteries, and lithiums, of all types, a Good CC/CV transformer, coupled with both a current and voltage measuring device of high accuracy make one of the best chargers out there.

 

[mdocod]

this is a great and informative thread. Thankyou!

 

[jumpstat]

So what recommended for a highend li-on charger that have all that is required to safely charge and take care of batteries? For your info I use AW's protected cells and Tekin charger with direct charge mode at 250-350mA CC, overcharge handled by the battery circuit.

 

[SilverFox]

Hello Jumpstat,

You need to start using a charger designed for Li-Ion cells.

Using a charger designed for NiCd cells and relying on the cells protection circuit to stop the charge is not recommended. A NiCd charger is capable of blowing the protection circuit on your cells.

The NiCd charger charges at a constant current and the voltage ends up where ever it does. The charge is terminated on a drop in the peak voltage.

A charger designed for Li-Ion cells uses a Constant Current to raise the voltage to 4.200 volts, then clamps the voltage at 4.200 volts while the current drops off. The charge is terminated when the current drops below around 50 mA, or 0.05 times the charge current. This is called Constant Current/ Constant Voltage charging. It is similar to what is used for lead acid batteries.

Pila has a very good charger that follows the CC/CV algorithm and it shuts the current off at the end of the charge. It is called the Pila IBC charger. Do not confuse the old style Pila chargers with the IBC one, the old ones were designed for the old style Pila cells only. The IBC Pila charger will charge all cells, protected and unprotected, safely.

Tom

 

[LuxLuthor]

It is obvious that there are still an unknown number of people who have not yet heard about the dangers with Li-Ion charging. Once you hear about it and understand, then you can take the proper steps to ensure safety.

There are some that believe any of these threads talking about the explosions and fires that can happen relatively easily with Li-Ion charger is like Big Brother trying to shove something down everyone's throat. In reality, it is a thoughtful campaign to simply make people realize that charging Li-Ion cells is not like charging NiCad or NiMH.

Thanks for all the information in this thread.

 

[petrev]

What Lux said - Info everyone should know

Thanks cy

Newish Thread

Day to Day - DN/Tenergy4P, FMA-Cellpro/HD6S and Voltcraft Balanced Sys. Charging

 

[MrAl]

Hello Tom,

Hello Al,
I am afraid your set up is what is considered as trickle charging.

What is there to be afraid of
As i said in my previous post, some people call it trickle charging
and some dont. You or anyone else can call it that if you like, and
that's fine.
I dont call it that because i dont call a charge algorithm that can
eventually reach near zero current trickle charging, although i
dont think i ever let it get that far in actual use because i
always take it off charge before long anyway. Proabably the longest
i have ever left a cell on charge like this was 24 hours.

I would suggest that you add a charging current cut off to your charger.
Tom

Well, in one of my chargers i built in a circuit to detect end of
charge at some low level like 50ma, but with my other charger it's
a matter of taking a circuit that works fine the way it is and
adding to it more parts and more board space. I would quickly do
this if i ever found it to be critical to the operation of the
unit, but for years now it has worked just the way it is and it
doesnt seem to bother the cells that i have been using.

 

[SilverFox]

Hello Al,

I understand that you are pleased with your battery and charger performance, however the question still remains...

Why do the Li-Ion battery manufacturers specify a low current cut off at the end of the charge?

Tom