Charging batteries in series?

Is it safe to charge unprotected lithium ion and NiMh cells in a battery holder that puts the batteries in series? I use a Maha C777plus-II charger. Will this setup charge all cells evenly or is there a danger of overcharging some of the cells?

Hi there,

My charger does two NiMH in series, but i prefer chargers
that do single cells as they charge more evenly.
Also, if you have a device that needs an odd number of
cells (like 3 cells) you cant charge that odd one without
having another dead cell to charge. It's a pain.

Take care,
Al

My triton will charge all cells in series(protected or not) except sla's.
Chris

[ QUOTE ]
MrAl said:

Also, if you have a device that needs an odd number of
cells (like 3 cells) you cant charge that odd one without
having another dead cell to charge. It's a pain.

Take care,
Al

[/ QUOTE ]

I have several holders that go into lights made by fivemega and others which make it easy to hold an odd number of batteries. I am just not sure if there could be a problem with uneven charging especially with unprotected Li ions.

li ions on the inside of a protected pack are in series all the time.If your cells are used and charged together all the time a good charger will equalize them as part of the chargeing process (topping off)AFAIK
Chris

Hello Grubbster,

There is no problem charging NiMh cells in series. It works best if they are all discharged to the same level before charging. To insure cell balance, it is usually recommended to do a 0.1C charge for 14-16 hours every 5-10 cycles of use.

Li-Ion cells can not tolerate overcharging. Because of this, you can not balance a series battery pack by topping off or trickle charging. There is no problem charging Li-Ion cells in series as long as they are at the same discharge level. When they get out of balance, you have to charge each cell individually to bring them back in balance.

Li-Ion cells are considered to be balanced when the resting voltage of each [edit] fully charged [end edit] cell is within 0.02 [edit] this goes as high as 0.05 volts [end edit] volts of each other.

Tom

edited for clairity.

Thanks for the info guys. /ubbthreads/images/graemlins/happy14.gif

Interesting topic... Since the ICE like the Triton has the ability to charge 1 to 4 Li-Ion's. Sounds like the key is that the cells are in similar discharge states. I have noticed that when my RCR123's are removed from my flashlights after use, both cells 9 times out of 10 will have identical voltage readings. Now if there is a way that I can discharge the two sets of two cells using the ICE I could then safely charge the 4 cells in series, right?

Why would trickle charging in series equalize anything? I think that should be in parallel to be equalized. Can someone confirm that? That's what I do when charging batts for my Mag85. I normally put three AA's each in three seperate 4AA-D parallel adapters (nine batteries total) and put them on my Vanson charger overnight.

Found at
http://www.xintechs.com/tips/batt/

Charging two batteries in serial is OK, this is the case of 99% of chargers. But, charging in parallel is a very bad thing, because battery's internal resistance is not constant and varies between cells. If you charge two batteries in parallel, one might be still undercharged while the other is already overcharged because their resistance and other characters are different even if they are made in the same group by the same manufacturer. One battery gets more currents than other and reaches full state earlier. If charging stops as soon as one is full, the other is under charged. This result in short battery life between charges, but does not harm the long-term battery life as overcharging is avoided. Overcharging generates lots of heat and dry out batteries

This site also has good info on batteries
http://www.batteryuniversity.com/partone-24.htm
Chris /ubbthreads/images/graemlins/grin.gif

Hello Dave,

I have edited my previous post to clear things up a bit.

You can take your pair of Li-Ion cells from your light and charge them up in series. At the end of the charge, let them rest for 15-30 minutes and measure the voltage of each cell. They are in balance if the difference is less than 0.05 volts.

If you find the cells are out of balance, run them for a while in your light and then charge each cell individually. This will balance them back up.

Tom

Hello Sinjz,

Let's take an example of charging a weak NiMh cell and a strong one.

When you put the cells in parallel, the strong cell will reach full charge first and signal the charger to shut off. This results in a properly charged strong cell and an undercharged weak cell.

If you put both cells in series, the charger is looking for 2X the single cell voltage and will continue to charge both cells until that voltage is reached. This results in a slightly overcharged strong cell and a fully charged weak cell.

Nickle chemistry cells can take a small amount of overcharge and turn it into heat, unfortunately Lithium chemistry can not do that. With Lithium cells, the strong cells becomes overcharged. This results in damage to the strong cell, and in extreme cases, "rapidly venting with flame."

I am not aware of any battery pack manufacturers that endorse parallel charging of NiMh cells.

Tom

Hello Chris,

[ QUOTE ]
Christoph said:
Found at
http://www.xintechs.com/tips/batt/

Charging two batteries in serial is OK, this is the case of 99% of chargers. But, charging in parallel is a very bad thing, because battery's internal resistance is not constant and varies between cells. If you charge two batteries in parallel, one might be still undercharged while the other is already overcharged because their resistance and other characters are different even if they are made in the same group by the same manufacturer. One battery gets more currents than other and reaches full state earlier. If charging stops as soon as one is full, the other is under charged. This result in short battery life between charges, but does not harm the long-term battery life as overcharging is avoided. Overcharging generates lots of heat and dry out batteries

[/ QUOTE ]

I believe it is important to understand that this quote is refering to NiMh cells. We have been discussing both Li-Ion and NiMh cells here, and I just want to make sure that people understand the reference.

Tom

[ QUOTE ]
SilverFox said:

When you put the cells in parallel, the strong cell will reach full charge first and signal the charger to shut off. This results in a properly charged strong cell and an undercharged weak cell.

[/ QUOTE ]

I don't think so. The error is in assuming the stronger cell appears stronger to the circuit. When you put the cells in parallel, the stronger cell discharges into the weaker cell, in the proper orientation. The cells become equalized, with the weaker cell receiving the charge of both the charger and the stronger cell. The stronger cell can't signal the charger to stop, since the charger sees only the capacity of the parallel set. Any excess charge received beyond the capacity of one, is discharged into the other. Only when both can take no more, is there any indication that one or the other is full.

I have Ansmann, AccuManager, Vanson, C.Crane, La Crosse, Energizers, Ray o Vacs, Sony, and various imported chargers. If I had to give them all up but one, I'd keep the parallel charger, the C.Crane; even though I know the blasted spring would eventually pop on the contacts. I've had to buy three so far because of it, but until someone else makes as decent a charger, without such a faulty tension spring, I'll keep buying them. The only chargers I call quality, on a personal level, are those that charge individually or parallel.

Series charging can't and won't give individual attention to the cells. Individual charging of course does evaluate each cell. In parallel it's not necessary because the charger treats the whole set as one cell; the individual cells take care of one another. In series the individual cells try to destroy one another.

this is so complex an issue. that its hard to explain all the way around, so i will avoid trying to agree with all that was said above, even though most of it is acurate, its dependant on the charger type, rate, etc etc. meaning it could be 100% wrong in different situations.

everything is right in the context it is used, everything becomes horribly wrong if its used out of the context it was intended. which ALWAYS happens when people like me dont know what the heck you guys are talking about /ubbthreads/images/graemlins/smile.gif (newbies)

EX: overcharge ni-?? PACK to balance the cells at 4A , BAD news.
(am i being anally critical, when most chargers out there only work one way? mabey)

and that is the problem , dependant.

if you are charging to a specific voltage, then parellel charging is PERFECT, if your just tossing a specific ammount of current in then stopping, then it sucks.
see dependant.
if you keep a specific voltage on a set of parellel cells of any type on till each of them reaches that voltage point, the current is balanced out till each it topped off the same.
this works EXCELLENT for li-ions.
AND you completly balance out the top charge for all of the cells, what could be BETTER? nothing, but one expert says its a disaster, because of the METHOD of charge.
so now read over that again, what METHOD did i say this would be effective for? Huge difference here in how its done.

rebalancing a Series pack of Ni-??? is done by slow overcharge, OR completly seperating the cells as individuals and cycleing them seperatly.
you can NOT properly rebalance the cells in a ni-??? pack by blasting them, and stopping when some arbitrary high voltage is reached, when some arbatrary current is input. there will always be some cell in the series pack that will not be at full charge.
rebalance is hugely important, because it reduces reverse charge
what we have here is assumptions made based on the charging TYPE, or based on one or anothers manufactures method for charging.

it becomes nessisary to think more LOGICALLY, so you can analize WHAT is occuring on the METHOD your using.

1) in series each does get the same current, but does that mean that each will charge the same . . . NO. but very close if they were all discharged the same.

2) in parellel they recieve current based on the state of charge (voltage differential) a lower voltage (charged) one is likely to get more current passing through it. but depending on how the charge is stopped, the low one might still be low when your done.

3) every cell in every battery pack is different in some small way, total capacity, it charged voltage, energy to storage abilility, even discharge rate.

4) cell imbalance sucks it is what does (series) PACKS in more than anything

5) any type of overcharge to ni-??? cells should only be done SLOWLY , so if you use this method to rebalance the few that have not reached the top, dont blast it then.

6) seperate PACKS to singles or parellel sets every once in a blue moon to rebalance or cycle rebalance them.

7) li-ion SINGLE cell protection will cut off series charging to the other cell, so you can not get full charge on single cell protection without single cell charge SEE #6

8) li-ion PACK protection curcuits VARY, some of them the batteries act like single units (see7) and some Properly top off the lesser batteries in the pack before cutting the entire set off. cant guess which packs DIE

i hope this explains WHY there is 101 ways that its right and wrong to do things, because there is also 101 curcuits, and chargers , batteries, and ways they get applied, that make any outright single answer on HOW, neer impossible.

for simplicity, it might be best to picture it like you have one cell that is dead, and one cell that is fully charged. and you want them both to be fully charged.
its NEVER that bad, but if the method your charging can cover FOR that, then its easier to think about what your trying to do, in a very small way.

9) oops, most important. you can NOT charge UNPROTECTED li-ions in series, one always goes over. and if it goes over its DEAD. that last little bit of voltage at the top of the charge is to critical.

EX: even if you had a stop voltage of way LOW 8V one could be at 3.6 and the other at 4.4 (near ruined). or one could be at 4.2 and the other 3.8 and one is never fully charged.
continue doing this over and over, and the imbalance gets bigger and bigger.

stop voltages of 4.2x2 or 8.4v would be SURE that one was at 4.4 (nearing demise) and the other is barely reaching 3.9.

well you can, and if you watch what is happening to each seperate unit in the seires pack, you can do it without killing anything. but you still want to prefer charging each cell all the way up, so they act similar in discharge.

10) if stuff is out of balance, because of a few low cells, and the low cells never get full charge, and the high cells get overcharged, then it can get further out of balance over time.
IMO this is where SLOW overcharge of ni-?? PACKS with lots of series batteries seems to make more sence WHEN you cannot seperate the cells.
and probably why that method is used with stupid charges, for normal people, with many cell packs.

Hello Empath,

I was thinking about my post and realized that there would probably be some confusion. I could have used better terms than strong and weak.

The argument you present sounds good on the surface, however it overlooks the fact that it takes potential to perform the charging process. I do not believe hooking two cells in parallel will balance them, unless you leave them hooked up for an extended period of time.

As a matter of fact, Red Scholefield did some testing on this. He took a fully charged battery and hooked it up to a fully discharged battery and observed less than a 10% transfer of capacity in 24 hours.

The problem in charging cells is that they do not have the same capacities even when checked directly from the manufacturer. This imbalance is compounded by environmental conditions, storage times, usage, and charging methods.

VidPro has made the observation that the various arguments are dependent on charging rates. Let's take a closer look at this.

If you have two cells of differing capacity and charge them in parallel, the cell with lesser capacity will fill up first and go into an overcharged state. If you are charging at a fast rate (2C), the drop in voltage from the first cell reaching full charge will signal the charger to shut down, leaving the other cell undercharged.

If you are charging at a moderate rate (0.5C), once again the lesser capacity cell will fill up first, but the signal may not be strong enough to signal the end of charge. If this is the case, the lower capacity cell will "cook" until the greater capacity cell catches up. This would have the appearance of balancing the cells, but in appears that in actual use, the cells age faster than they would if charged in series. I do not have any "hands on" experience with this and am only reporting what others have discovered.

If you are charging at a slow rate (0.1C timed charge), the lower capacity cell will fully charge and the greater capacity cell will end up undercharged.

All of the series chargers that I am aware of use algorithms to take into account the differences in cell capacity while charging. They lower the charge rate towards the end of the charge so the lower capacity cells that are going into overcharge are treated gently while still allowing the higher capacity cells to attain full charge.

If I had to give up all but one of my chargers, I would stick with the Schulze isl-6. I am constantly amazed at its ability to charge all types of chemistries rapidly and gently.

Tom

Okay, I'm really confused now. I think it was SilverFox that initially told me that my method would work properly. Now he seems to be saying I'm killing my batteries. I'm confused. /ubbthreads/images/graemlins/confused.gif

Can someone please specify for me if MY setup works or not? I'm using a Vanson SpeedyBox (BC-1HU). I'm putting nimh AA's in three of the four slot of a Aventrade 4AA-D PARALLEL adapter. I do that with three adapters for nine AA's total. I then place these three D-sized adapters in the Vanson and let it charge overnight. According to the sheet that came with the charger, it is charging at 700 mA/h. It uses the Delta-V cut-off detection method AND a cut-off timer set at 11.5 hours for 'D'cells. I sometimes do a discharge with this setup as well.

Am I screwing up all my batteries??? /ubbthreads/images/graemlins/thinking.gif

edit: BTW it trickle charges at 1/10 constant pulse current, which would mean 70 mA/H. Lastly, assuming I'm not frying my batteries, should I be keeping them in the same sets of three or should I mix and match them?

sinz, do you use a dummy cell for forth cell in your adaptors? I guess that is a moot question, because you need all slots filled to complete circuit in adaptors. You are charging cells in parallel in individual bays in your Vanson at a very slow rate, 700 ma's, for cells probably 2000 mAh each, or more, or 6000+ mAh's total. Sounds like cells would equalize easily, over time.

Bill