# Li-Ion Parallel Charging



## SilverFox (Apr 13, 2007)

When you hook Li-Ion cells in parallel, they will equalize depending on their voltage differential. There has been some concern that the initial surge pulse of current may be a problem. Because of this concern, it is usually recommended to keep the voltage of the cells you are paralleling within 0.5 volts of each other.

To study this surge current pulse, I took a 26500 3200 mAh Li-Ion cell and over discharged it to a resting voltage of 2.097 volts. I then took a Sony US18650V 1600 mAh cell, which is rated to 10C, and fully charged it to 4.193 volts.

The 26500 cell has a 1kH impedance of 0.085 ohms and the 18650 has 0.025 ohms. A short piece of 12 gage stranded wire was used to complete the connection. It appears that the wire will add around 0.0008 ohms of resistance.

A Craftsman 82062 DC Amp Clamp Meter was used to measure the current. Voltage was monitored with a Fluke DMM.

I hooked the cells up in parallel, and observed a maximum of 6.5 amps for under a half of a second. The current quickly dropped to 3 amps over the next 2 seconds, then leveled out at 2.4 amps. It was still dropping at this point, but was not dropping as fast as it initially had been. I was watching the current, but when it leveled out at 2.4 amps, the voltage was around 3.104 volts. As the voltage rose, the current tapered off.

I disconnected the battery and the 18650 cell was at 4.012 volts, and the 26500 cell was at 3.121 volts. This gives me around a 1 volt differential. Hooking things back up in parallel again saw the current peak out at just over 3 amps and quickly settle down to around 2.4 amps and was steadily dropping.

With a 2 volt difference between the cells, I measured a surge current pulse of around 6.5 amps. When the difference was 1 volt, the current dropped to a little over 3 amps.

A similar test was run by others, and they observed that with a 1.4 volt difference, the surge pulse was 5 amps which quickly settled down to around 2.5 amps in roughly 10 seconds.

I measured that 93 mAh of capacity had been added to the empty cell. The duration of the test was about 2 minutes, so that makes the average equalization current about 2.79 amps.

It would be very unusual to have two individual cells with this much of a voltage spread, but if you were charging parallel packs, it is possible. Keep in mind that Li-Ion cells are OK with up to 10C pulses when they are empty, but they should not be charged at over 1C. In the example above, my 3200 mAh cell was subjected to a pulse of 6.5 amps, or roughly 2C, then the transfer rate dropped to around 1C and continued to drop from there. 

*Although I can see no problems with this, it is still recommended to make sure that you have less than 0.5 volts difference when hooking up in parallel.*

Now, lets look at some theoretical numbers. Ohms law says E = IR, or I = E/R. The difference in voltage was 2 volts and the total resistance should have been 0.1108 ohms. This predicts that our initial surge current pulse should have been around 18 amps.

Either my meter is incapable of measuring the fast transient surge pulse, or there are other things involved that we are not taking into consideration. If I have another 0.1969 ohms of resistance in the system, the numbers balance. It is quite feasible that the total internal resistance under load is greater than the individual 1kH impedance measurements. On top of that, the voltage differential is quickly diminishing. If the resistance is correct, that would mean that the actual voltage difference was only 0.72 volts.

If we look at the worst case with a 0.5 volt difference, using the same set up, we end up with a theoretical initial surge current pulse of about 4.5 amps, however the actual measured value would be closer to 1.6 amps.

So, what does all this mean?

If you charge Li-Ion cells in series, there can be a problem if your cells become unbalanced. However, if you charge in series and utilized a balancer, the problem goes away.

If you don’t have a balancer, you can charge your cells individually or in parallel. Individual charging has no special precautions, however when parallel charging, keep an eye on the voltages of the cells you are paralleling together to charge and it is best if the voltage difference is less than 0.5 volts. If you have a greater imbalance, start charging the lowest voltage cell first, then as it comes up to the value of the next cell, or cells, add them in. Parallel charging is very safe, but you need to be aware of the initial balancing currents involved when you first hook your cells up in parallel.

If you use a charging cradle, or jig, you can even parallel charge different capacity cells at the same time. You have to be aware of the initial surge pulse and the voltage differential, and also you need to make sure the charge current does not exceed 1C for the lowest capacity cell. This means that you could hook up 2 R-CR123 850 mAh cells, along with a 650 mAh 14500 cell, and throw in a 2200 mAh 18650 in your 4 slot charging cradle. Make sure that all of the voltages are within 0.5 volts, parallel all the cells, then hook up your charger. With those cells your charger would see 1 cell of 4550 mAh of capacity. Since 1C for your smallest cell (14500) is 650 mA, you would set your charger to charge at that rate. Assuming all of your cells were empty to start with, you would come back in around 4 hours and all of your cells would be fully charged, and equally balanced. If you group your cells with similar capacities, you can cut the charging time down to around 2 hours or less.

When parallel charging, set you charger cell count to 1 cell and adjust the charging current to a 1C rate for the lowest capacity cell.

Tom


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## VidPro (Apr 13, 2007)

a person might worry about it more if the added was inclusive of the charging TOO. (putting the batteries on a FAST parellel charger)

you had a VERY wide range there, and yet it handles it pretty good.
especially seeing that the battery that is getting hit hard and fast, is very low, as opposed to being already charged. (meaning over 80% with 3-6amps would be improper)

and with the D sized li-ions or them c sized they are now using, they should be carefull with even a 1C rate (over time) any burst pulse would be no biggee, but if they insist on charging in 15 mintues, they are gonna get surprises 

if they start using a 3amp charge on the c-cells , sombody is going to have an unnessisary issue, that would be solved by just waiting another flipping hour. (watts the rush

so where is the Chart, oh great chart master  then i dont have to read so much .


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## SilverFox (Apr 13, 2007)

Hello VidPro,

Sorry, no charts on this one... 

Tom


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## Josey (Apr 13, 2007)

Tom, I just ran into another problem. I have a parallel charger, but one of the Li-Ion cells I bought has no voltage. It shows 76,000 ohms resistance, while the other cells have resistance, if any, too high to measure. What happens if someone puts a shorted cell into a parallel charging rack, which they might not know when it's brand new? I've noticed other people finding cells like this.


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## SilverFox (Apr 13, 2007)

Hello Josey,

If you measure the voltages before you parallel your cells, you should be able to catch that.

0 volts is really dead. I have tried to totally drain cells and they still come back to over 1 volt, even with I discharge them to 0.1 volts at a 0.01 amp rate.

You should always check the voltage of any cells you get. This goes for NiMh as well as Li-Ion. If the Li-Ion cells are below around 3.5 volts, they may have problems. 

Tom


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## Josey (Apr 13, 2007)

Thanks, Tom

What you're saying, I gather, is that anyone who uses multiple Li-Ions should have a voltmeter.

I worry about slightly sick cells that are in the early stages of failing. Do we have to worry, for example, about two cells fully charged to 4.2 volts, but then fall to different resting voltages because one is not as healthy as the other.


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## Supernam (Apr 14, 2007)

How about just buy 2 separate single cell chargers? =P


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## cy (Apr 14, 2007)

Tom, thanks for this much needed info!!


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## 65535 (Apr 14, 2007)

Parallel charging is great for applications that use multiple cells. Glad you are spreading this around silverfox nice work.


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## LEDite (Apr 14, 2007)

Tom;

The cylindrical cells have a PTC device which heats up & limits the current levels. 

I think that is the reason you did not see higher currents in your parallel testing.

I am surprised at the 10C rating on the Sony 18650 cell. Even so the other cell's PTC probably limited the current.

Larry Cobb


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## AndyTiedye (Apr 14, 2007)

The only "0 volt" lithium ions I have seen are protected cells where the protection has triggered. Also some new protected cells are this way.
The protection should reset when you charge them.


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## SilverFox (Apr 14, 2007)

Hello Josey,

After parallel charging, the cells will come to a resting voltage. In multi cell applications, I match cells based on their resting voltages. The cells that have aged and have developed higher internal resistance will fall back to a lower open circuit resting voltage, and you can group them together for multi cell use.

However, if you are parallel charging, or individually charging, you are balancing the charge each time. You could use an old cell with a new cell and end up with cells that are out of balance at the end of the discharge, but this is not a problem if you re balance them during the next charge cycle. I think your performance may suffer a little, but that is about it.

Tom


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## SilverFox (Apr 14, 2007)

Hello Larry,

I think it takes more than 10 seconds to heat the cell up to a point where the PTC kicks in. While I am not totally ruling that out, the PTC trips that I have seen tend to abruptly stop the flow of current. I did not observe that during this test. However, I have not done a lot of pulse testing, so this is new territory for me.

In addition, prior to the surge current pulse test, I had run the numbers and tested each of the cells at very high current rates to make sure there would be no interference from the PTC. The cells performed reasonably well at a continuous load of 15 amps for 1 minute during these tests. 

The Sony "V" cells are used in power tools and are designed for high power applications. I believe the more common size is the 26650 which is rated at 20C or 50 amps.

Tom


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## SilverFox (Apr 14, 2007)

Hello Supernam,

Separate chargers work very well, however if your light uses 9 cells, you can charge all 9 cells in parallel in around 2 hours with 1 hobby charger and a parallel set up. To do the same thing would require 5 dual slot chargers. 

Now that I stop to think about it, that doesn't seem like a totally unreasonable way to go. That way you don't have to worry about imbalance at all and just charge each cell individually.

Tom


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## cy (Apr 14, 2007)

Tom, you have PM


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## goixiz (Dec 16, 2013)

I have a set of battery rated 4.2V 4400mAh for my light. Its an external battery for a small headlamp. I noticed that in order to charge my battery with the charger provided (output 4.2V) takes at least 8 hours and that seems a lot. The battery pack has 2x 18650 (setup in parallel mode) I have no idea what batteries they are and the pack is usuallly in my back pocket when in use.

Should i be concern about this battery pack of exploding ? (Ive been reading about 18650 exploding lately)

I would like to make my own battery packs with 3 or 4 18650 batteries in parallel - Where do i get the supplies to make such batteries ?

TIA


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## SilverFox (Dec 16, 2013)

Hello Goixiz,

Welcome to CPF.

It sounds like your charger has limited current. If you charged your battery pack at 4 amps, it would be charged in a couple of hours. When you parallel cells you need to double the charging current or put up with extra long charging times.

To check the general condition of your cells, charge them, let them rest for an hour, then measure the voltage. If your cells are close to 4.2 volts there is no reason to worry.

To make a battery pack you need a spot welder and some cells. You will probably need to practice welding before you get that down to the point where it is reliable. Care should be taken when welding cells because they can react adversely to excess high temperature and you may experience "rapid venting with flame."

Tom


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## cy (Dec 19, 2013)

blast from the past for an old thread ... Silver hope things are going well ..


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## SilverFox (Dec 22, 2013)

Hello Cy,

Excellent would be an understatement...  

Tom


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