I have 4 cells of 2100 mah and 4 cells of 2650 mah so can I use these 8 cells in my TX

Generally not recommended.  In fact better to avoid it. I would guess that half the cells would get discharged earlier, and the others will start charging them (which is like charging them in reverse polarity) !  This can cause them to pop.

I am not sure even taking precautions like only using 50% of the available discharge would help (thus ensuring that none of them get to full discharge).  Even if this works, it will be short lived.

Simply DON'T DO it.

well it's not a problem as long as all are running in good capacity, you have to keep in mind that if any one of them get discharged that will drop voltage output..

You have to keep them charging before some of them get discharged... also You need to see that they all are giving proper voltage level

I am using mixed cell for quite long and never found a problem...

What type of mixed cells, and for what application ? What are the voltages and mAH ratings of the cells you are mixing ?

I only used AA/AAA battery for different electronics equipments

They all are rechargeable and have MAH rating between 2000 - 2500

I am sorry but if this question was about RC stuff and LIPO's I am not sure whether you should mix them or not.

IMO Mixing AA/AAA cell for radio and other electronics equipment is not harmful if you use them properly

With any type of rechargeable batteries different cells with different capacities should never be mixed together and used as a battery. Even cells of same capacity but different batches should never be mixed. Even same capacity cells but of different brands should never be mixed. Even old and new cells should not be mixed. The reason is if 4 nos of 500 mah and 4 nos of 1000 mah cells are connected to make a battery pack of 8 cells and when they are used the low capacity cells would discharge to 0 charge before the higher capacity cells. Then if we continue to use the pack the higher capacity cells which still contain 50% of charge they start to push reverse current through the discharged smaller capacity cells which results in reversing the polarity of smaller cells. The final result is a completely ruined battery pack.

In any kind of rechargeable battery pack if one or more cells become damaged you can't simply replace the bad cells with new cells and continue to use it. The only option under such circumstances is to remove the bad cells from the battery and use that battery pack for other purposes like a Rx battery pack or for use in rechargeable torch lights, emergency lights or toys like that. Don't abuse these cells. I treat the rechargeable cells with the same respect as I give my shotgun cartridges.

 

Frankly speaking most of people will agree with you, but I don't agree...

As per my knowledge of how current flows and battery works this is not correct...

In fact when you are using such combination not all cell will discharge at same rate.
The lower capacity cells will discharge at higher rates and higher capacity with slow rate as current will flow through entire loop of all cells. thus after some time say you used 2/3 of cell power 500 MAH will have something like 160MAH remaining and 100 MAH will have 320MAH remaining ...

The calculation is ideal condition but it will vary in different situation if you don't believe it you can try by building simple circuit with resistance or Bulb and then test the remaining power of cells...

I have tested it long time back to prove my thinking during college times and results were seen as i mentioned above...  I don't have data sheets with me now otherwise I might have produced them here...


However on the contrary all manufacturers advi9ce that do not use mixed cell and IMO what they mean is don't mix type of cells like NICD, + NIMH + Others as they have different discharge rate + different properties...

Opinion of others is invited..

Also all these are for less precise equipments if you making a battery pack for a heli or for Plane you must make sure that while mixing cells difference should not be very much, small difference like 100MAH will not affect ..

Also finding same capacity 8 cells is very difficult recently i bought 8 eneloops out of which 4 were fake and they all came from same ebay seller, so you can think possibility of getting same batches

The Eneloops I buy have the importer's details on the pack. So far they have proven themselves.

Dear Sir,
I am sorry to say but your knowledge regarding current flow and secondary (rechargeable) batteries is not correct.What you say might be okay if you are discharging only partially and not to fully to 0 charge level. Like you said if you take a 600 mah and a 1000 mah capacity Ni-cd cells and connect them in series to make a 2cell battery pack and discharge it at 100 mA load current level for 5 hrs you will still have 100 mah charge in the 600 mah cell and the 1000 mah cell will have 600 mah of charge left in them. Okay now if you keep on discharging the charge in the smaller capacity would become zero, but the other cell still holds 500 mah of charge and this while pumping current through the load will also charge the smaller capacity cell but it reverses the polarity of that smaller capacity cell and permanently damages that cell. Cell polarity reversal is the largest problem with Ni-cd/Ni-mH batterypacks. Just one bad cell is enough to render a multiple cell battery pack useless.
Also it is incorrect that smaller cells discharge at a fast rate and larger cells discharge at a slower rate. All cells in a battery pack discharge at the same rate.
As you suggested to do a simple test, I suggest you to do the same test once again and see the results yourself.
Take one or two higher capacity cells like 2200 mAh or 2600 mah and take a 900 or 1200 mAh cell and connect them in series. Connect a 3.6 volt, 500 ma torch light bulb to the +ve and -ve terminals of the 3 cell battery pack and run the bulb until it stops glowing and then check the voltage and polarity of each cell. You will find that the small capacity cell's polarity would be reversed. Then try to charge that battery pack and you'll find that the battery pack would not charge.

regards,
Rao.

I agree with Rao. When you have lower mAH cells mixed with higher mAH then you have the problem of reverse polarity on the lower mAH cells. This happens when all the charge in it has depleted.

But then if you think practically, I do not think any sane person would be running their battery down to those dangerouse levels... I would say its better not to mix different mAH cells in a battery pack. If you do better watch your voltage. As jalan said, small differences like 100mA should be fine.

-Ismail

Exactly that's what I am trying to say, you don;t mix cells with too much difference and you don;t allow them to drain to dangerous level