Mix and match Ni-Mh from different manufacturers but the same 'tested' capacity?

[subwoofer]

I have recently invested in a Technoline BL-700 which has been a revelation with regard to the actual capacity of the batteries I have.

The real shock has been how different the real capacities are within the same batch of batteries. I had always paired up batteries and used them together so they always had the same number of charge/discharge cycles and should age similarly.

Now I have been through a process of running the refresh cycle on all my AAs and noting down the capacity (having numbered the cells). Within one batch of 12 1800mAh AAs the range is from 1185-1783 mAh. I also have some slightly older , but not necessarily more used 1700mAh AAs which range from 1499-1698 mAh.

Because I was trying to 'work' the cells I ran it at the highest discharge rate in the 'refresh' mode, so it may have given a different capacity if run at a lower discharge current.

Having done all this testing, what I would like to know is if it is OK to mix and match Ni-Mh cells with different rated capacities from different manufacturers based on their actual 'tested' capacity?

The cells may then age differently, but if a test cycle is run every now and then they could be re-matched.

Has anyone done this mixing based purely on actual tested performance of each cell?

 

[s0lar]

It's better to use 2 cells with the same capacity then to use 2 with the same label.
In high current applications it's perhaps not always enough that they have the same capacity but by testing them every now and then, you are providing enough care for your cells.

 

[rider]

Now I have been through a process of running the refresh cycle on all my AAs and noting down the capacity (having numbered the cells). Within one batch of 12 1800mAh AAs the range is from 1185-1783 mAh. I also have some slightly older , but not necessarily more used 1700mAh AAs which range from 1499-1698 mAh.

Because I was trying to 'work' the cells I ran it at the highest discharge rate in the 'refresh' mode, so it may have given a different capacity if run at a lower discharge current.

Having done all this testing, what I would like to know is if it is OK to mix and match Ni-Mh cells with different rated capacities from different manufacturers based on their actual 'tested' capacity?

I don't, simply because cells from different manufacturers tend to have different discharge characteristics. You might want to look at Silverfox's AA NiMH shootout from a few years ago as a kind of guide.

Also, I'm wondering if you're using some off-brand cells? All of my quality cells, such as Maha Powerex or Eneloop, etc., track within a few percent even after years of use and hundreds of charge/discharge cycles. In fact, I just ran some refresh cycles on 2 year old Powerex 2700mAh cells that had been getting a little balky, and found that not only did they still exceed their rating, but they were all within 20mAh capacity of each other.

Now with generic Chinese cells that I've gotten "free" with various things, I'll see capacity variations of 10-20% on brand new cells, and after a few dozen charge cycles, the difference is even greater (probably because some of them are breaking down and failing even with little use and proper care).

 

[subwoofer]

Two answers so far with opposite opinions....

The 1800mAh ones are Vanson and the 1700mAh are Uniross, but I do have some others including Fuji and Maplin branded ones.

I would not be using them in high drain applications ranging from torches to DAB radio.

I'll wait for a few more answers and see if opinion goes one way or another.

 

[BatteryCharger]

I have recently invested in a Technoline BL-700 which has been a revelation with regard to the actual capacity of the batteries I have.

Interesting, that same charger is a Lacrosse BC-700 over here.

With the lower capacity batteries you might want to cycle them a few more times, it's not uncommon for a 1400mah battery to turn into a 2400mah battery after a few cycles.

 

[Battery Guy]

I would not "mix and match". Capacity is only one aspect. Cells from different manufacturers are likely to have other differences. You also have to consider the following:

1. internal resistance
2. excess anode discharge capacity (important for voltage reversal situations)
3. self-discharge rate
4. rate of capacity degradation (i.e. cycle life)
5. rate of internal resistance rise

Even if two cells start out nominally the same by every measure you can make, that doesn't mean that in 10, 50, or 100 cycles they will still be the same. Aging characteristics are highly dependent on cell design and manufacturing quality.

If it was me, I would not do it.

Cheers,
Battery Guy

 

[SilverFox]

Hello Subwoofer,

In an "emergency" there are lots of things that you can do, but there are risks that you have to accept when you move outside normal operating procedures.

It is best to use cells from the same brand and batch. When purchasing cells you will generally get cells from the same batch. You can then test them and match them for use.

I think it is OK to use cells that are matched in performance, as long as you understand that matching on capacity and mid point voltage under load are not perfect and you run the risk of ending up with a weak cell after a few charge/discharge cycles.

This is not recommended for demanding use, such as power tools, but less demanding use can often tolerate a slight mismatch. Especially if you don't run the battery completely down and recharge often and follow up with increased testing to check the aging characteristics of the cells.

The rule is to use cells of the same brand and labeled capacity. The reason for the rule is to avoid reverse charge situations in multicell applications. In some cases, and with some chemistries, a reverse charge situation can be explosive.

When you ignore the rule, your chance of having a failure increases. Testing can help reduce those risks, but you have to be prepared for failure.

Tom

 

[Battery Guy]

Hello Subwoofer,

In an "emergency" there are lots of things that you can do, but there are risks that you have to accept when you move outside normal operating procedures.

It is best to use cells from the same brand and batch. When purchasing cells you will generally get cells from the same batch. You can then test them and match them for use.

I think it is OK to use cells that are matched in performance, as long as you understand that matching on capacity and mid point voltage under load are not perfect and you run the risk of ending up with a weak cell after a few charge/discharge cycles.

This is not recommended for demanding use, such as power tools, but less demanding use can often tolerate a slight mismatch. Especially if you don't run the battery completely down and recharge often and follow up with increased testing to check the aging characteristics of the cells.

The rule is to use cells of the same brand and labeled capacity. The reason for the rule is to avoid reverse charge situations in multicell applications. In some cases, and with some chemistries, a reverse charge situation can be explosive.

When you ignore the rule, your chance of having a failure increases. Testing can help reduce those risks, but you have to be prepared for failure.

Tom

Nicely stated! :thumbsup:

 

[HarryN]

Hi, these are NiMH we are talking about here, right? Not Li Ion.

I guess I missed something, but I was not aware that there was all that much risk with reverse charging of NiCd and NiMH based cells. If that were not the case, then there are a whole lot of unprotected Ni based cell packs out there in the hands of unsuspecting consumers.

Think of all of those massive NiMH R/C packs out there in the world running many amps with no balancing between charges, other than the leak-by. I completely agree with the goal of trying to running balanced packs, balanced cells, the risk of Li based cells, etc, but aren't we getting a little too paranoid here to recommend that someone using balanced and tested NiMh cells in a pack mode is still at risk?

 

[SilverFox]

Hello Harry,

The risk during discharge is cell reversal which will destroy the cell. If you are counting on your battery pack to work, you are in trouble.

The risk during charging is venting and sometimes popping which can send cell parts flying across the room. Most of the time you just have a little bit of chemical spill to clean up, but there are some RC people that have lost eye function because they happened to be in the wrong place at the right time.

Of course, we all wear safety glasses while charging...

While this is not nearly as exciting as a Li-Ion vent with flame incident, it is still a risk.

Now tell me, would you feel comfortable using Eneloop cells along with SuperCap 3000 mAh cells that test to 1900 mAh in a multicell application?

I don't think there is anything to be paranoid about, it is just prudent to understand the risks when you break the "rules."

Tom

 

[Battery Guy]

Hi Harry

If a NiCd or NiMH cell goes into full voltage reversal during a high current discharge, both oxygen and hydrogen are generated at rates proportional to the discharge current. As it turns out, you can generate a lot of gas at current levels that would be associated with a power tool under heavy loads. If the vents in these cells do not operate as designed, the pressure inside the cell will rise rapidly, and the cell may "spontaneously disassemble". This is a somewhat legal term for "blow up". It is not technically an explosion because there is no combustion involved, but if you saw it first hand you would probably describe the failure as one.

A typical AA NiCd or NiMH cell has a vent pressure of 500-700 psi. The cell will fail and "spontaneously disassemble" at around 1200-1500 psi. The energy of these events is quite significant. I have seen severe lacerations from NiCd powertool battery packs that experienced similar failures.

So, while most concerns involving using mismatched NiCd and NiMH cells are related to cell leakage, loss of cell performance, etc..., there can also be significant safety concerns.

Cheers,
Battery Guy

 

[45/70]

Think of all of those massive NiMH R/C packs out there in the world running many amps with no balancing between charges, other than the leak-by. I completely agree with the goal of trying to running balanced packs......

Just to add a little bit here. I'm sure most here are already aware of this, but it is not uncommon for R/C'ers to run their NiCd/NiMH packs through a 0.1C 14-16hr forming, or "balancing" charge every once in a while. This is how NiCd/NiMH cells are balanced when in an assembled series pack.

Also, I believe a lot of dedicated tool pack chargers accomplish this by actually applying a small "balance charge" after the charger has indicated that the pack is charged. This way, when the tool's battery pack is inadvertently left on the charger, for whatever reason after it appears charged, the cells in the pack are effectively balanced.

Dave

 

[HarryN]

Hi, thanks for the clarifications. I guess I need to remember that I usually only run setups at 0.5 -3 C discharge rates, which is pretty conservative. I switched to running fairly family safe setups for all of the obvious reasons, and forgot about the 10 - 30C crowd.