Voltage of AA rechargeable batteries suddenly drops

Question

I am having problems with rechargeable AA Ni-MH batteries. We are using 6 AA Ni-MH rechargeable batteries to power our device, but after about an hour of usage suddenly one of the batteries' voltage drops.

We connected all batteries in series as we want 7+ V.

The voltage of all other batteries remains almost the same. I'm not able to find a reason for this.

This voltage drop occurs only in second to last battery in series.

I appreciate any help or suggestions. I have attached the voltage readings of the individual batteries in below image.

Answer 1

"A chain is only as strong as its weakest link"

Due to manufacturing tolerances etc., each cell will have a slightly different capacity. Whichever cell has the lowest capacity will be drained first, and then its voltage will drop rapidly because there are not enough 'charged' ions left in it to maintain the voltage.

Provided the capacity you are achieving is within stated tolerances there is nothing wrong with the cell or the charger. However if a cell is regularly discharged deeply, or even worse drops below zero volts and gets reverse charged, it will degrade and lose even more capacity. Therefore the equipment should be designed to turn off before this can happen.

Your data indicates that by the time the 'weak' cell was fully drained the other cells were also close to it. Here is an example test discharge of some typical '2000mAh' AA NiMH cells. The reason for two discharge curves is that 2 individual cells were tested. Things to note:-

1. at 1.1 V they are almost fully discharged. After this the voltage drops rapidly.

2. One cell managed to exceed its nominal 2000 mAh rating. The other one fell short, but was well within 10% of it (~94% of nominal capacity).

3. When the 'better' cell reached 1.1 V, the other one was below the cutoff voltage (probably would have gone well below 0.7 V if not stopped).

NiMH cells typically require 3-5 full charge/discharge cycles to reach full capacity. If you have some cells which appear to have lower capacity than expected, try cycling them a few times. If that doesn't get them close to the others then they may be faulty.

Answer 2

Is your charger defective? Your starting voltages are far from a full charge voltage of 1.4V to 1.5V per cell. Please replace the weak battery cell.

Answer 3

The voltage of all other batteries remains almost the same. I'm not able to find a reason for this.

This is generally how battery packs with series cells discharge, absent cell balancing. It doesn't even matter which cell type is used: zinc/alkaline/lithium primaries, rechargeables, you name it, they all do it. The only difference is the extent to which it happens. The end-point for comparison would be having extracted 90% of rated capacity from brand-new cells.

You may have a weak cell, and then the question is: where did you get the cells from, and how did you evaluate them?

If you bought the cells from an unauthorized distributor, you'll be potentially wasting a lot of time dealing with consequences of poor quality, etc. Now, in practice a device may need to cope with such cells if the end user can replace them, and that's when you'd really want to provide cell balancing of the type that can bypass fully open cells, so that at least one weak cell won't cause a shutdown.

You definitely need cells that have a known part number, come from a known manufacturer, have a good data sheet, and come from an authorized source. It's safest to stick to large mainstream distributors like DigiKey, Mouser, Newark/Avnet, Allied Electronics, ELFA/Distrelec. Otherwise, first select a cell from a quality manufacturer, then order it from a distributor authorized by the manufacturer.

Answer 4

If you notice such effect the reason is often simple - it has less capacity - or was not charged to full as the others.

If a NiMH cell is dicharged down to 0.24 V it has a lasting damage anyway. You should avoid that somehow. For a good 6S pack you should start with all same cell type, make and age, and ideally with measured same capacity.

Do you charge them externally?

Use a charger with ability to test the capacity. Even new cells may differ by 25%. Also it does not hurt to give them one or two charge-discharge cycles if new, somtimes they level up to each other.

Is there a specific reason to choose NiMH over Li-Ion?

Answer 5

To synthesize bits from two other comments:

Is your charger defective? Your starting voltages are far from a full charge voltage of 1.4V to 1.5V per cell. (Audioguru)

This is generally how battery packs with series cells discharge, absent cell balancing. (Kuba hasn't forgotten Monica)

For NiCd and NiMH cells, the usual charge profile is to overcharge a bit, and often to "trickle charge" (especially with NiCd) at the end. With cells that are built to be forgiving of the practice (and a small-enough current), the already-charged cells just pass the current through while any under-charged cells finish charging slowly.

Over multiple charge and discharge cycles, if that one weakest cell in a pack isn't fully charged, it'll get over-discharged more often until it's damaged -- and cell damage is usually seen as a loss of capacity or overly fast self-discharge. Your voltage vs. time plot indicates that this has happened.

So, start over again with good cells, and make sure that you're properly charging them.

Answer 6

You need to specify load current in mA and cell capacity in mAh.

Cell 5 appears faulty OR is OK but is slightly lower capacity than any other.
One cell HAS to be the lowest capacity one.

If Idischarge in mA is numerically about 2/3 x mAh then what you see is as expected.
eg if cells were genuine 2400 mAh and load was 1600 mA then one cell will "drop out" at about 2400/1600 = 1.5 hours