0V on 8-Pack NiMH Battery Pack - Page 1
Forum section: electronics / projects

author: joolsh
author_rank: Newbie
timestamp: December 20, 2017, 09:42:42 am
content: Hi forum,Big fan of eevBlog, first time participation on this forumNice to meet you!I am currently investigating a phenomenon occuring on a 8-pack (in series 9.6V) NiMH backup battery.My device (comparable to a Raspberry Pi) uses this battery pack as a backup power source when the main power supply fails.It is recharged during normal operations by verifiying every 72h if the voltage drops below a threshold (1.1 V per cell = 8.8V). If it does, a C/100 charging during 12h is launched. And then this procedures repeats.I did not find the root cause, but it now happens that the batteries fail after a couple of days, and verifying the battery voltage with a multimeter gives a reading of 0 Volts over the battery pack. However, if I try to recharge the battery over a long period of time (24h), the voltage rises above 1V per cell (8V) again.I've read about cell inversion, could it be that once cell is discharged, gets reversed, and kills the battery?Or maybe the battery pack has a protection circuit (trying to find a datasheet, no luck yet, no-name battery pack). That shorts the battery on overload?My question is: what would you suggest to find the root cause of this battery failure, and how is it possible to have 0V on this battery pack?Did any of you have a similar experience with NiMH battery packs?Happy holidays!

author: TMM
author_rank: Frequent Contributor
timestamp: December 20, 2017, 10:13:43 am
content: By '8-pack' do you mean 8 cells in series i.e. a 9.6V battery pack, or 8 cells in parallel i.e. a 1.2V pack with 8 times the mAh rating of one cell?Putting NiMH batteries in parallel is not really recommended. It is certainly a problem for charging unless you have some kind of balance charger which is able to ensure each cell receives the same charging current. Without balancing it will still charge but is very inefficient as the battery with the most charge has the lowest ESR and therefore takes the majority of current, so one cell will reach full charge first and steal away all the charging current from the other cells, causing it to heat from overcharging and the other discharged cells to charge very very slowly. I'm not sure if cell inversion in a parallel pack can cause the whole pack to discharge, and if that does happen if you need to isolate all the cells to recharge them - maybe someone else can chime in.In either configuration (series or parallel), if you charge at 0.01C for 12hours you are only putting 12% of the rated capacity back into the battery pack. Charging is not a 100% efficient process so perhaps you will only bring up the battery pack about 8% instead of 12%. This is bad because your scheme will maintain the battery pack at a low charge (10-20%) and never charge it up to 100% since once you hit 1.1V the pack is at a very low capacity so you should really charge at 0.1C for about 15hours or 0.01C for 150hours to reach 100% charge again.

author: joolsh
author_rank: Newbie
timestamp: December 20, 2017, 10:26:39 am
content: Thanks for the answer TMM, I've edited the question to clarify it is a series configuration.I'll look into you remark about not charging to full capacity!Any ideas about the 0V measurement? Even if the battery pack is fully discharged, I would like to understand how 0V is possible

author: mikerj
author_rank: Super Contributor
timestamp: December 20, 2017, 02:31:32 pm
content: Either the cells are all dead (internal dendrite build up causing very high self discharge) or something in your device is draining the battery. Have you put a meter in series and measured the current into/out of the battery pack?

author: SeanB
author_rank: Super Contributor
timestamp: December 20, 2017, 05:37:32 pm
content: Is your voltage sense divider permanently connected to the pack? This will always draw current, and with the 72 hour sense interval your cell pack will be flat if the current is high enough. try changing charge check to every 24H ,and then do a 6 hour charge instead, which will show if the problem is the cells themselves, or leakage current through either the charge switch, voltage sensing circuit or just a bad batch of cells with low capacity. You might look into having a NMOS switch that only connects the pack when actually sensing the voltage, this was shown by Mike Harrison ( Mikeselectricstuff) in the design he did recently, and is a very nice easy to implement idea to reduce draw when the cell is idle.

author: Audioguru
author_rank: Super Contributor
timestamp: December 21, 2017, 12:42:35 am
content: Your problem is using "no-name" batteries that are cheap junk.Use a name-brand battery pack bought locally so it is not a cheap fake copy made "over there" and it will work well for at least 5 years.

author: joolsh
author_rank: Newbie
timestamp: December 21, 2017, 07:18:30 am
content: Thanks for the answers, I'll investigate furtherThe quality of the battery pack is uncertain, it is "home-made" from a small-town electronics shop, but even then I do not expect a battery to reach 0 Volts. I'll keep you posted

author: TMM
author_rank: Frequent Contributor
timestamp: December 23, 2017, 06:50:03 am
content: I guess the correct way of determining what has happened when the battery pack is at 0V would be to measure each cell individually with a multimeter. If the pack has shrink wrapped plastic you can pierce though the plastic with a sharp multimeter probe to get at the metal tab going from one battery to the next. I would not be surprised if several cells are 'reversed' and if the pack has been permanently damaged if it has been kept in this state for a while.What causes cell voltage reversal of a cell is that the pack is continually discharged after one of the cells has no charge left. At that point, the rest of the cells charge the 'dead' cell via the connected load with negative polarity - consider that you load is just a wire with negligible resistance and one of the cells is dead and no longer functioning as a battery - what voltage is presented across the dead cell? Eventually if half of the cells are say +0.8V and the other half are reversed at -0.8V then you measure the whole pack and see 0V. Low quality packs are more likely to suffer reversal because one cell might have significantly lower capacity than the others.Like others have said you really need to monitor what is happening to the pack voltage between the charging finishing and the voltage being checked 72hours later. I would guess too that there simply is not enough charge in the battery pack, or too much current draw for it to last the 72hours. The pack might be OK if you charge it up to 100% and fix the circuit.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 23, 2017, 07:01:23 pm
content: One other possibility is there is a protection circuit that disconnects the batteries when the voltage gets too low to prevent harmful discharge. At least that's what I'd do.Try measuring the individual cells.

author: ez24
author_rank: Super Contributor
timestamp: December 23, 2017, 09:52:43 pm
content: Watching

author: Andreas
author_rank: Super Contributor
timestamp: December 24, 2017, 04:25:29 am
content: Hello,for me the values choosen do not make any sense:At 8.8V the cells are already almost flat: (perhaps at the 2% charge level) how do you want to make any backup with that charge.At that point charging C/100 * 12 hours will give you maximum about 10% charge level. (and that shall help for the next 72 hours?)And you cannot determine 7*1.3V + 1* -0.xV or 8*1.1V with your strategy.In my battery protection cirquit (10uA current consumption) I measure every second and switch off the pack if the voltage goes down more than 0.5V within 1-2 minutes.(of course this has to be adapted to the load).I am charging CC/CV with C/10 (200mA for AA cells) and voltage limit 1.45V/cell at room temperature (partly with T.C. correction of -3mV/deg C/ per cell).Switchoff is at 1.15V/cell or the voltage drop above.My oldest battery packs with 12*NiMh (ready to use) work since about 5 years.There are 2 scenarios to kill your cells.- reverse polarity of one cell: will kill immediately mostly irreversible. (this cell will be always the weak point in your pack).- cooking a fully charged AA cell with more than about 60 mA: the cell is not dead but the internal resistance increases.with best regardsAndreas