Strange battery discharge curve - Page 1
Forum section: electronics / projects

author: Kintekobo
author_rank: Regular Contributor
timestamp: August 23, 2015, 06:09:44 pm
content: I have been reconditioning some NiMH batteries and just for shitz and giggles I thought I would do a discharge curve having been following the hoopla around the Batterizer claims. I have four batteries in a holder discharging through a 12V car bulb which pulls around 250mA.When I came to plot the voltage against time I noticed that there were three distinct drop off areas. I am assuming that this was because there were four batteries in series in the discharge and they had different capacities, but I don't remember ever seeing anything like this before. I that a correct assumption or is there some other mechanism in place. It isn't test or equipment error as I have repeated the test twice now and each time was the same.I have attached a picture of the batteries and a plot of the curve.

author: SeanB
author_rank: Super Contributor
timestamp: August 23, 2015, 07:42:13 pm
content: 4 cells with different capacities there, you are seeing them go flat in sequence. If you look at each individual cell you will find each one dropping to 0V then being reverse charged in sequence.Try giving them a C/10 charge for 14 hours and rerun the discharge test. If they are all the same nominal capacity and from the same batch they should match well, if not then the lowest capacity ones are being damaged each cycle.

author: Kintekobo
author_rank: Regular Contributor
timestamp: August 23, 2015, 07:50:02 pm
content: The batteries are all the same and from the same batch in fact, but they most probably have been used in different equipment so have had different charge/discharge profiles I suppose. What I found interesting about the curves was the sudden huge plummet of about a volt over a space of a minute or so, and then continuing the gradual decline until the next major drop. As I said, I hadn't seen that behaviour before. Very curious.

author: SeanB
author_rank: Super Contributor
timestamp: August 23, 2015, 08:04:18 pm
content: Give the equalising charge then, that should get them to all go off the cliff together.

author: Kintekobo
author_rank: Regular Contributor
timestamp: August 23, 2015, 08:18:41 pm
content: I'll let you know if I hear them go wheeeeeeeee!

author: onlooker
author_rank: Frequent Contributor
timestamp: August 23, 2015, 08:28:25 pm
content: QuoteThe discharge curve is basically just an addition (superposition) of several single battery discharge curves with each started at a different discharge state. The somewhat interest part is that you have only 3 steps, instead of, 4. This implies one of the battery was already fully drained before you started to take the curve.

author: Kintekobo
author_rank: Regular Contributor
timestamp: August 23, 2015, 08:33:22 pm
content: Onlooker: That can't be right because the initial voltage was about 5.6 volts which shows that all batteries were at their nominal starting charge, in fact they were at about 1.4 volts almost.

author: onlooker
author_rank: Frequent Contributor
timestamp: August 23, 2015, 09:08:29 pm
content: Ok, then the batteries seem to be at different stages of aging (e.g. damaged by previous reverse charging).I would now guess, the missing step is at outside of your time interval and it will be a much smaller step.The explanation goes like this: The step size for the 3 steps shown are larger than what a single battery's final drop should be (~1V). This is due to the reverse charging that further reduced the voltage about 0.3V. This is why each step had a size about 1.4V.After 3 steps, with only one battery working, the total voltage is about 1.2V - 3*0.3V = 0.3V. If you extend the test time long enough, you may see a ~0.3V drop as the 4th step.

author: Kintekobo
author_rank: Regular Contributor
timestamp: August 25, 2015, 09:38:51 am
content: Further to my last post, and my curiosity being piqued, I decided to do more testing. This time I chose four rechargeable Ni-MH batteries that were from the same batch and had all four been in my label printer since new so they should all have had about the same usage. What I found was that they all discharged at about the same rate to begin with but then one-by-one each battery would suddenly start to drop in terminal voltage until it reached zero whereupon it would then reverse polarity to about -0.125 volts. This can be seen in the attached plot and chart where the marker points indicate a reversal event.This of course begs the question as to whether this is harmful to the battery since this must be happening in any scenario where there are multiple batteries in series. Considering these were four fairly well matched batteries from the same batch which have had fairly light use for about one month in a device where they were used about 10-15 times the spread is quite large. The implication here is that if the reversal of polarity can cause damage to the batteries internal chemistry then there is a need to detect the first voltage drop-off and use that to indicate the "Battery Low" point.

author: mikerj
author_rank: Super Contributor
timestamp: September 02, 2015, 02:26:50 pm
content: Quote from: Kintekobo on August 25, 2015, 09:38:51 amIt certainly is harmful, and is one of the reasons that power NiMH/NiCD tool battery packs tend to have a short life (that plus overcharging at high currents).