Connecting cellphone/mp3 lithium ion batteries in parallel

[akikhia]

so i recently purchased an extended battery for my phone. i realized that the back cover for the extended battery left about 40% of the extra size it added to the phone empty. knowing that laptops at times have slate batteries i was wondering whether i can connect different sized batteries in parallel with the batteries in the phone to fill the gapps with some extra juice

my understanding is that these consumer batteries have great safety systems and parallel connecting them is not an issue if they are the same battery with same capacity

if it is possible is anything extra needed to regulate the batteries? would it be fine to use a solder on the terminals due to the heat it may apply to the battery? if it is possible where would be a good place to get these smaller lithium batteries if i dont find the right sizes?

 

[Sceadwian]

i was wondering whether i can connect different sized batteries in parallel with the batteries in the phone to fill the gapps with some extra juice

Absolutely not. Plain and simple.

The only way to do this would be to add the exact same cell in parallel and they would both have to be brand new to start with, it would also need a charge balance controller which is different from the basic protection IC's you see on single cell packs.

 

[Diver300]

I wouldn't agree with Sceadwian here.

If you want to parallel batteries they must be the same chemistry. That is not negotiable. You can parallel Li-Ion batteries very well, because the voltage changes with the state of charge, so cells in parallel will charge and discharge at the same time and they will stay balanced.

You must get the voltages the same before you connect them in parallel. If one is discharged, its voltage will be 3.5 V or so, while a full charged one is around 4.2 V. If you connect a discharged one to a charged on you will get a big current from one to the other, which is bad. However if you get them to the same state of charge they will be at the same voltage, so you can then connect them. Once connected they will stay balanced.

It really doesn't matter if you parallel different capacities. The maximum and minimum voltages that the charge / discharge circuitry subject the batteries to should be what all the cells can stand. The max and min voltages don't vary much between different Li-Ion batteries.

Putting Li-Ion cells in series is a lot more difficult to do properly. You would then need balancing circuitry. Unless the cells are identical, you won't get the full capacity from the larger capacity ones, and you then rely on the balancing circuitry to keep on cell from damaging the others.

 

[Sceadwian]

It really doesn't matter if you parallel different capacities. The maximum and minimum voltages that the charge / discharge circuitry subject the batteries to should be what all the cells can stand. The max and min voltages don't vary much between different Li-Ion batteries.

I'm sorry this is a direct contradiction of what I stated. You should under no circumstance ever use cells of differing capacity in either series or parallel, EVER. Cell imballance will ultimatly cause pack failure through the difference in cell resistance, it is not possible to safely charge lithium packs of different capacity in parallel or series, EVER.

Assumption of different capacity cells self balancing is faulty as the packs can only balance with an identical cell resistance which can only occur when the cells are the same capacity for a given charge state.

 

[akikhia]

I'm sorry this is a direct contradiction of what I stated. You should under no circumstance ever use cells of differing capacity in either series or parallel, EVER. Cell imballance will ultimatly cause pack failure through the difference in cell resistance, it is not possible to safely charge lithium packs of different capacity in parallel or series, EVER.

i tested this last night. with the safety chips in cell phone batteries, if one has a lower capacity than the other it would fill up first then completely cut off current flow and the other finishes off its charge.

Assumption of different capacity cells self balancing is faulty as the packs can only balance with an identical cell resistance which can only occur when the cells are the same capacity for a given charge state.

i am not sure why cell resistance is important here, packs dont need indentical cell resistance to balance out they just need to meet at the same voltage so why is it an issue for these cells to be connected in parallel and constantly balancing each other out? in fact it means that the current load drawn by the device is distributed among the batteries so less stress and potential to heat overall.

 

[Diver300]

I don't see that there is a problem with different capacity cells in parallel. It is quite common to have a lithium battery in parallel with a tantalum capacitor or two and probably a few dozen ceramic capacitors. I realise that the capacity of the lithium battery is several orders of magnitude larger than that of the capacitors, but they are all charge storage devices, and they are paralleled, and there is no problem.

Any control circuit for Li-Ion batteries will limit the charging current. With batteries in parallel, the current will be distributed between the batteries. In theory there could be a problem in that if you were charging a small cell and a large cell in parallel, the total charge current could be too large for the small battery. In practice, the charge rates are normally quite low and limited by the heating of the cell phone or its charger, not the battery. Also the charge current will tend to balance according to the battery capacities, because the voltages have to be the same.

Charging should only be done if the batteries are between 5°C and 40°C. There is only one temperature sensor for several cells in the arrangement that Akikhia suggests, so one cell could be hotter. However, in laptop batteries, there are often 3 cell in parallel, and several inches separates them, so the temperature imbalance is likely to be larger than the temperature imbalance between two flat cells that are one on top of another.

Charging must not be above 4.2 V for Li-Ion. That rule is stuck to perfectly with two cells in parallel.

Charging should be terminated when the current drops to 0.05 C. If you increase the capacity of any Li-Ion cell, then the charger should be adjusted. A charger for a 1 Ah cell will stop at 50 mA. If you charge two 1 Ah cells in parallel, the charging should stop at 100 mA for the pair. However with a 1 Ah charger, the charging will stop at 50 mA for the pair, so it will go on longer. In practice, as long as 4.2 V isn't exceeded, it won't cause a problem. The current will drop quite fast so it won't take many minutes to drop from 100 mA to 50 mA. Also, most charging circuits will restart the charging as soon as the voltage drops to 4.1 V or so. That is not significantly different from holding the cells at 4.2 V, so I can't see there being a problem there.

In all, I can't see a problem with cells in parallel.

 

[audioguru]

I have a Li-Ion cell phone battery that has two cells connected directly in parallel. I also have a Li-Ion laptop battery that has many pairs of cells connected directly in parallel.

 

[Menticol]

I tried this project 3 years ago, but removed the original battery.

After the modification the player turned on and charged normally, but honestly I don't remember how much it increased (or worsened) the player performance. Although the battery indicator remained "full" for longer time, I doubt there was a real improvement. I guess that's because the charging circuit was designed for the older, less capacity battery. Another reason could be that the replacement battery came from an old cellphone and chemicals inside were not at their best days.

I will never know, I killed the player months later connecting it to a faulty home-brew amplifier

I suggest you to take the risk and try! Even if it doesn't work your player will attract a lot of attention

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[Sceadwian]

i am not sure why cell resistance is important here, packs dont need indentical cell resistance to balance out they just need to meet at the same voltage

Which can never occur. Under the same external load there will be a different internal resistance to the cells so there will be a voltage mismatch and power will be lost through this. Also it's not possible to safely charge two Lithium cells of differing capacities in parallel without some balancing circuitry that would waste a lot of energy as heat.

I have a Li-Ion cell phone battery that has two cells connected directly in parallel. I also have a Li-Ion laptop battery that has many pairs of cells connected directly in parallel.

This is irrelevant to the posters goal of using differing capacity cells in parallel. Any commercial device you see that has cells in parallel will always have identical capacities, and quiete frequently (especially for larger packs) have a charge/discharge balance controller even for those closely matched cells.

**broken link removed** to a site that knows it's battery buisness, if you don't trust my word trust their word.

I don't see that there is a problem with different capacity cells in parallel. It is quite common to have a lithium battery in parallel with a tantalum capacitor or two and probably a few dozen ceramic capacitors.

This thought is also faulty. Capacitors are NOT batteries, their charging/discharging principals aren't even based on the same physics and they are in no way shape or from directly comparable to one another, the caps just filter noise nothing more and effect the charging/discharging of a battery in bulk not at all.

If you still doubt me I issue a challenge to find me any commercial product that uses differing capacity cells in parallel properly.

Just because you do not see a reason for this to not be a good idea doesn't mean it is a good idea, and from every bit of research I have ever found on secondary (or even primary cells) I have never once ever come across a site that condones the paralleling of dissimilar capacity batteries.

 

[Diver300]

Any commercial device you see that has cells in parallel will always have identical capacities, and quiete frequently (especially for larger packs) have a charge/discharge balance controller even for those closely matched cells.

I've seen balancing circuits for cells in series. The balancing circuits make sure that the voltages are similar, so that if the capacities of the cells become different over time, no cell will be subjected to a voltage that is too high or too low. The balancing circuits also keep the state of charge or each of the series cells similar, which is especially useful if there is any self-discharge. Without a balancing circuit there is no way that a low state of charge cell can be fully charged without over charging others.

In the largest packs that I have seen, those for laptops, cells are connected directly in parallel. The parallel combinations are connected in series with a balancing circuit for the series combination. So the parallel connected cells have no balancing within the parallel combination.

Can you please tell us what a balancing circuit for cells in parallel does? A series balancer keeps the cell voltages similar by varying the current split between them if necessary. In a parallel circuit, the cell voltages will be kept the same and the current split between them will vary on its own to achieve that. I honestly don't know what else would be done in a circuit balancing cells in parallel.

 

[Sceadwian]

The difference in the capacities of the cells you're talking about are in the single percentage range. This does not apply to packs in parallel within the 10% range of capacity differential. Even with closely matched packs .1% requires balancing for best pack life.

The packs you're talking about are again IRRELEVANT for differentiated capacities (over 1%) due to them all being the same capacity!

There are no balancing circuits required for cells in parallel because you just have to match them carefully. Truly high power discharge packs might have some IC for ballance, but this is far from common.

 

[Diver300]

Can you please tell us what a balancing circuit for cells in parallel does?

Sceadwian, you don't seem to have answered this question.

 

[Sceadwian]

Okay... Let see if I can explain this well enough...

Two packs of differing capacities will under the same current load have different internal resistances, this will de facto cause a cell voltage imbalance under load which will cause the voltage from each cell to be different which de facto means one of the cells will discharge into the other. This will cause premature aging of the pack, and the failure over time will be drastically increased with the increase in difference of capacity. There is no way around these very simple facts.

Generally you do not use a balancing circuit with parallel packs unless they become extremely large though you do have to take slightly greater care with charge and discharge circuits to avoid over stressing the weaker cell (one is ALWAYS weaker than the other even in matched packs)

The primary reason to use parallel packs isn't for increased capacity as the smartest thing to do in that situation is simply use a larger capacity pack, it's to increase discharge current as Lithium is a little more limited than other chemistry.

If you were to use disimilar capacity cells, ballancing would be required to maintain the pack life, problem is the balancing would effectivly waste all the capacity over the lowest one. So if you used a 1000ma pack and a 2000ma pack in order to properly balance them and avoid failure under load you have to waste half the capacity of the larger cell as heat, a balance circuit would just keep the heat from being lost in the battery itself (causing cell degradation) it's massively wasteful.

The only proper way to use two cells of dissimilar capacities together would be to use circuits that effectively isolate them while monitoring both cells charge and discharge seperatly.. Something like a boost converter on each for the output to match the effective load vs capacity of the cells, but even then bam you just lost 10-20% of the pack capacities through converter inefficiency..

This is why differing pack capacities in parallel are never seen in real world devices.