Why do batteries in series (3 x AA Energizers) not discharge uniformly - Page 1
Forum section: electronics / general

author: stuartlea
author_rank: Newbie
timestamp: August 18, 2022, 02:07:07 pm
content: Hi,We've a ZigBee based product that uses 3 x AA Energizer Ultimates in them and a TPS63031 Buck Boost to give us 3.3v. All fairly straightforward. However when measuring the voltage of the cells when the product has stopped working shows voltages such as 0.2v 0.9v 1.2v for batteries in the same pack. We are now measuring the voltages when they get installed and all around 1.8v.When they are in series I would have expected them to be similar in final voltage.What am I missing???Edit: the batteries are not tapped in any way and are sat in a 3 x AA series battery holder.Stu

author: IanB
author_rank: Super Contributor
timestamp: August 18, 2022, 02:22:47 pm
content: This is completely normal. When there are several batteries in series there is almost no chance that all of them have exactly the same capacity. It follows that one of the three batteries will be the first to run out and so its voltage will be the first to drop towards zero. As soon as one of the batteries drops to zero the product will stop working, and then when you measure voltages the other batteries will still have higher voltages remaining.

author: sokoloff
author_rank: Super Contributor
timestamp: August 18, 2022, 02:53:05 pm
content: Ian has the most likely explanation.Another possibility is that the device is tapping one or two of the batteries to drive some lower-voltage part of the circuit. If that lower voltage circuit is low enough draw, it's not unreasonable to design a device that draws from a subset of the string rather than the whole. (This would manifest itself as the same battery position always dying first.)

author: RoGeorge
author_rank: Super Contributor
timestamp: August 18, 2022, 02:58:36 pm
content: Quote from: stuartlea on August 18, 2022, 02:07:07 pmThey might have the same voltage when new, but they do not have the same capacity. Some cells have more energy accumulated in their chemistry than other cells that otherwise might seem apparently identical. The energy drains almost the same in a series circuit, and the weaker cell will eventually exhaust all its energy first, and start to drop in voltage.Even more, for series batteries with a consumer attached, after a while the weakest cell will exhaust all its stored energy, end the weaker cell will end upreverse-polarizedby the other stronger cells in the series circuit.Use cells from the same lot (which you did), but at some point one will exhaust first. It is uncommon to see 3 series cells exhausting all 3 at the same time.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 18, 2022, 03:34:28 pm
content: Quote from: stuartlea on August 18, 2022, 02:07:07 pmThat spread is too high and not what I've seen from AA alkaline batteries in series, although I've not tested that particular brand. I'd say the batteries are tapped instead of just series connected.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 18, 2022, 03:42:22 pm
content: Quote from: PartialDischarge on August 18, 2022, 03:34:28 pmNote they were discharged quite low - down to 0.76V / cell average.While it's great someone designs a product with a low cut-off voltage (there are crap products that die already at 1.2V/cell, prompting hacky joule thief products to be used), going to 0.76V/cell is maybe a tad too far. The risk is, capacity differences cause some cells to go near zero or negative, increasing risk of physical leak. I would stop at maybe 0.9V average, i.e. 2.7V for 3S.

author: IanB
author_rank: Super Contributor
timestamp: August 18, 2022, 04:10:13 pm
content: If you look at the discharge profile here,https://data.energizer.com/pdfs/l91.pdf, you can observe a very steep "cliff" at the end of discharge.When cells have such a cliff in their characteristic discharge profile, it is almost inevitable that one cell voltage will drop before the others in a series chain.

author: stuartlea
author_rank: Newbie
timestamp: August 18, 2022, 04:15:49 pm
content: Definitley not tapped in any way. In a simple 3 AA series battery holder.

author: james_s
author_rank: Super Contributor
timestamp: August 18, 2022, 04:29:08 pm
content: This is not unusual at all, it is normal and expected. I have unfortunately had a few rechargeable cells damaged when they got reverse charged by the stronger cells in a series application where there was no low voltage cutoff. It is impossible for any group of cells to have *exactly* the same capacity and internal resistance, so it is inevitable that some will be discharged sooner than others.