Mixing mAh batteries

Only one problem with all this is that we never really know what the actual or real capacity of these cells are.
Recently I bought 8 x 2300mah energiser Nimh AA cells for my radio Tx.
Now the package said they were "charged for immediate use", wrong they were all over the place. So as I have an Imax B6AC smart charger I had a bit of a fiddle with them charged and discharged a few times individually. NOT ONE of them came up to 2300mah. they were all over the place best one was 2100 down to 980mah according to the Imax. All were brand new to start with. So I put them all together and charge them in series with the Imax and they tend to level out and come up to just over the expected voltage at around 12v.
I've also have similar results with 18650 tabbed cells that are claimed to be 2600mah not being up to spec when tested this way but as they are always balance charged I don't get too excited over that variation.
 
Sarah Winfield said:
The charger and the batteries are all Energiser make. I don't think they have been used previously although the carton they came in is pretty battered.
All 8 batteries look identical. They are marked NiMH.
The other 4 batteries I bought from Lidl are Ni-MH 2400mAh.
SW
Click to expand...
I would expect the energizers to have an accurate mah rating, but I would be skeptical of Lidl since it doesn't seem that well known of a battery brand. You might try testing them to see which ones last longer. I bought some new "XIT" 3100mAh AA NiMH batteries from ebay for a very low cost, and they only ran my train half as long as(and slower than) some old energizer 1500mAh AA NiMH batteries I've had for 4 years. Many makes have inaccurate mah ratings. Here's a web tool which compares the performance of different makes of batteries, but I didn't see Lidl on there: Battery test-review 18650 comparator

I also have some new 1900 mah eneloops, and the old 1500mah energizers run trains almost as long.
What Tony Walsham said about eneloops has me worried about them now.


-bbbb
 
Until they are fairly well worn, batteries will come up to a nominal terminal voltage. But charging is not all about voltage.

The issue is that while you were getting all the cells up to the same voltage, some cells got there sooner, and thus they were overcharged for the rest of the time.

Depending on the chemistry, there is some tolerance for the overcharging, but still not a good idea.

I have a charger that allows testing battery capacity too, and it's not only a good tool to see if you got "taken", but also to sort out weak or defective cells before having them fail to perform completely.

Greg
 
Greg Elmassian said:
If you put 4 discharged batteries in with 2 that are fully charged, AND the charger charges them in series, not individually, you will either undercharge the flat ones or overcharge the full ones.

Does not sound like a good idea.

Greg
Click to expand...
Not sure that you properly read my Post 20, I was asssuming that Sarah's Charger did 4 at a time. Thus only looking at charging 4 of the same mah type at a time the scenario I deacribed is sound.
 
Greg Elmassian said:
i went back and re-read, now I understand what you were saying, and I agree, no issue.

Greg
Click to expand...
Thanks Greg, I probably could have put it better, English was a hated subject at School. The English Teachers in my Senior School were all fascists to a man and woman!
 
PhilP said:
But possibly stop well before the batteries run-down..
Click to expand...
I was under the impression that mis-matched batteries should not be charged together, but it's ok to discharge them together. Am I wrong? Is there a reason to not run mis-matched batteries down together if you intend to charge them seperately?
-bbbb
 
That's only true for batteries in parallel, when one is more discharged than another.

For cells in series, think about it, discharge is current flow in one direction. charging is in the other direction, so series cells cannot charge other cells in the "series string".

only in parallel can positive be connected to positive, how you charge normally.

Greg
 
-bbbb said:
I was under the impression that mis-matched batteries should not be charged together, but it's ok to discharge them together. Am I wrong? Is there a reason to not run mis-matched batteries down together if you intend to charge them seperately?
-bbbb
Click to expand...

Once one cell has hit rock-bottom, the others will follow pretty quickly.. - You instinctively 'push' your system harder to try to keep going, so draining the remaining cells..
It also has to do with internal resistance, but that can get way-complicated very quickly.
 
Having batteries in parallel is not to be recommended. Yes, I know it is done, but the ability to cross charge a duff cell, as noted above, can produce a lot of heat and perhaps a fire.
 
It is against toy safety regulations to have batteries in parallel. So you will be just fine Sarah.
 
stockers said:
Having batteries in parallel is not to be recommended. Yes, I know it is done, but the ability to cross charge a duff cell, as noted above, can produce a lot of heat and perhaps a fire.
Click to expand...

Going to have to take issue with that "blanket" statement.

Before lithiums, this was one of the most common and recognizable battery packs for models:




large__14_09_2010_07_49_NiMH_5000_7.2v_6cell_Shotgun_Pack.jpg

This is a 3s2p pack, meaning there are 3 "cells" in Series, but those "cells" are actually 2 cells in Parallel thus 3S2P

cells are put in parallel all the time at the "Factory" and as long as they are reasonably "matched" this can and has worked well.

And YES, if one cell goes bad, basically if it develops an internal short (if it goes open not much danger)... then things can go wrong.

There is a LOT more to this, but let's watch the overall statement of having batteries in parallel is not recommended.

There is a huge amount of knowledge here, but perhaps it would have been better to say:

don't put batteries in parallel YOURSELF unless you understand them well, and most likely have some overcurrent protection.

There's as much "old wives tales" about batteries as there is fuel for track power vs. battery power.


Greg
 
Agree with Greg's comments above - even with Li-ion packs, you can get plenty of pack types with parallel cells; this one, for example:
Ansmann 2447-3034-01 Battery Pack Li-ion 1S3P 3.7V 7800mAh
...is a 1S3P pack, three single Li-ion cells in parallel delivering only 3.7 volts but a very significant 7.8 Ah capacity. The big difference between these packs and a home-assembled one is, of course, that this is a protected industrial-use pack made up at the factory from high quality matched cells.

Jon.
 
to support that, if cells are permanently wired together, then the possibility to separately charge and discharge them goes away, and of course their capacity is matched at the beginning and through their life.

When I use "loose" cells, I do keep sets together by markings on the cells, and I also periodically use my charger in test/discharge mode to determine mAh capacity... in fact when I used a lot more batteries, I put the date and true mah capacity (as tested) on the battery on a small label. This assured matching and more importantly, if the "new" mah was decreasing as compared to the previous mah, I knew that cell was not long for the world and would discard.

Did this for perhaps 20 years, when I had a lot of things using loose AA and AAA cells. Now virtually all are pre-made liion packs...

I learned a lot about batteries about 30 years ago, thought I was pretty sharp, and then I met a REAL expert, and tried mightily to learn from him.

Still learning.

Greg
 
You must log in or register to reply here.
Back
Top