How can I charge some AA NiMh cells with what I have lying around? - Page 1
Forum section: electronics / projects

author: bd139
author_rank: Super Contributor
timestamp: January 24, 2018, 02:04:16 pm
content: My battery charger has just exploded and I have 8xAA Eneloops here I need to charge up. Not going to get a charger until tomorrow which is too late.Can anyone offer any articles or suggestions on how to fast charge these today?I have the usual reasonably well equipped lab available to me, just no bloody charger!I know the basics. The problem here is how do I effectively charge 8 of them quickly without (a) causing charge balance problems and (b) overheating/explosions. They aren't in a pack.

author: tszaboo
author_rank: Super Contributor
timestamp: January 24, 2018, 02:56:53 pm
content: They have unknown charge. So, you need toConnect all of them in parallel with ~ 1 Ohm resistor series with the cellLet it rest for half an hourConnect them directly in parallelPlace it on Lab power supply, and set current limit to maybe 1C-2C voltage limit to charged voltage of the cell (1.3V, I wouldnt push it in this case). Charge it for an hour.

author: bd139
author_rank: Super Contributor
timestamp: January 24, 2018, 03:07:57 pm
content: I see so equalise the charge first then charge. All cell voltages are within about 20mV of each other at the moment and they were from the same pack.Can I charge them all together or two at a time?

author: tszaboo
author_rank: Super Contributor
timestamp: January 24, 2018, 03:22:42 pm
content: If it is 20mV then maybe you can just connect them together directly. They have ESR in the 0.1Ohm range, so current is minimal.Yes you can charge them all together. It will just act as a big battery.

author: bd139
author_rank: Super Contributor
timestamp: January 24, 2018, 03:29:11 pm
content: Excellent thank you. This will only be done once so I shouldn’t ruin them

author: cncjerry
author_rank: Unknown
timestamp: January 24, 2018, 04:41:45 pm
content: any charge rate over about .2C should have some type of temperature sensor and/or a timer cutoff. You would want to limit the total charge to 1.5C with a timer and keep the temperature below around 110F. Otherwise, constant current supply as mentioned.If you have a GPIB controlled power supply and voltmeter, you can make a pretty high-function charger pretty easily with like Labview or Visual basic. You use the voltmeter to measure the voltage during constant current of about .5C. As the voltage increases, at some point it will dip around .010V per cell. At that point the batteries are fully charged assuming you are greater than 1.25C on discharged batteries. You really need a second voltmeter or temperature sensor as batteries can cause a nasty fire. Obviously, we can get carried away here...

author: IanB
author_rank: Super Contributor
timestamp: January 24, 2018, 04:51:09 pm
content: Quote from: NANDBlog on January 24, 2018, 02:56:53 pmNO!!!!!Never charge NiMH cells in parallel. They are like LEDs, the current is unlikely to distribute evenly and some cells are likely to get overcharged while other cells are starved.NiMH cells should always, always be charged in series.If all you have is a bench power supply, the best approach is to charge in series at a 0.1C rate for about 12 hours.Charging faster is possible, but youmustmonitor cell temperature and immediately stop charging any cell that starts to get hot.

author: bd139
author_rank: Super Contributor
timestamp: January 24, 2018, 05:08:29 pm
content: I am charging at 0.5C at the moment, in series.Temperature is being monitored. They're floating around 35oC max, all equal.I'll chuck my GDM-8341 on it now and log into excel and look out for a drop quick. Automation FTW!

author: IanB
author_rank: Super Contributor
timestamp: January 24, 2018, 05:18:04 pm
content: The voltage drop is not reliable with long chains of cells in series unless they are very well balanced to start with (one cell may have a voltage drop while other cells are still increasing--it is quite unlikely that all cells will start to drop in voltage at the same time).However, a sharp temperature rise is a very reliable indicator of full charge. So just watch for any individual cell that starts getting hot and remove it from charge as soon as that happens. Close the gap and let the other cells continue charging until all reach full charge.

author: bd139
author_rank: Super Contributor
timestamp: January 24, 2018, 05:24:02 pm
content: I don't need a 100% peaked charge on these so I'm planning to just charge for 70% of the time required keeping an eye on voltage and temperature.If I'd thought about it more first I would have logged both as I go but I was in a hurry. So far so good.I miss NiCd's though; much easier to charge. I used to charge them off a car battery with a series resistor and a stopwatch

author: tszaboo
author_rank: Super Contributor
timestamp: January 24, 2018, 05:35:48 pm
content: Quote from: IanB on January 24, 2018, 04:51:09 pmYes, they distribute current unevenly. Yes, some are going to be higher capacity/ESR and everything else.But they dont overcharge... Not at 1,3V.How would that even happen? You have the same voltage on their terminals, and you are far far away from the infamous inflection point.

author: IanB
author_rank: Super Contributor
timestamp: January 24, 2018, 05:40:44 pm
content: Quote from: NANDBlog on January 24, 2018, 05:35:48 pmYes, with a low enough voltage limit you might be safe. But we have to be careful about such nuances. Non-experts may not pick up on the details when reading the advice.Also not all NiMH cells are equal. For modern, high quality cells 1.3 V may be far below the peak. But for older cells it may not. You should ideally characterize the charging profile of your cells before deciding on a safe threshold voltage.

author: stj
author_rank: Super Contributor
timestamp: January 24, 2018, 06:20:17 pm
content: mind if i ask what charger blew up, and if you somehow caused it?i'm very safety/reliability conscious of chargers because of what could happen if they are full of lithiums and you arent in the room!!

author: bd139
author_rank: Super Contributor
timestamp: January 24, 2018, 06:25:20 pm
content: Of course. Charger was a Sanyo NC-MQN06U.It blew up because a massive solder blim inside it broke off and shorted something. Basically manufacturing fault. Not impressed really.

author: floobydust
author_rank: Super Contributor
timestamp: January 24, 2018, 06:34:57 pm
content: I manually charge Eneloop pro AA's at about 1/4C and 1.53Vpc. Charge current falls to about 135mA and I stay there for a while to equalize the pack.NiMh seems to really need a high 'top off' end-of-charge voltage, or the cell has low capacity.I see this in all my devices that I changed from NiCd to NiMh. They have low capacity unless I go over 1.5Vpc charging.

author: james_s
author_rank: Super Contributor
timestamp: January 24, 2018, 10:30:15 pm
content: In a pinch they can be charged with nothing more than a resistor and a power supply. It's especially easy if you don't care about speed, charge as many cells in series as your PSU has voltage overhead to support. Set the current to about 0.1C and charge for ~18 hours, at such low current it's not likely to damage them if some reach full charge somewhat before others.Personally I don't like those chargers that come with them, they charge cells in pairs. I use a Lacross BC-700 that charges all cells independently, I've had it for years and been very happy with it.

author: KE5FX
author_rank: Super Contributor
timestamp: January 25, 2018, 03:23:28 am
content: Put them in series on a current-limited power supply set for 0.1C. For AA NiMHs, 1C is usually about 2250 mAh. I set the voltage limit at about 1.5V/cell when charging.Edit: Eveready has a greatapplications manualfor these. I'd heartily encourage about 75% of the people in this thread to check it out (and I'd plead and beg with the remaining 25% to, before they burn their houses down.)Like most components, batteries havedata sheets.

author: IanB
author_rank: Super Contributor
timestamp: January 25, 2018, 05:02:16 am
content: Quote from: KE5FX on January 25, 2018, 03:23:28 amUnfortunately I do not place much faith in this document. For instance Fig 12 shows the voltage decrease (negative delta-V) happening before the temperature starts increasing. This is not reality. The voltage decrease is caused by increasing ion mobility due to rising temperature leading to a drop in internal resistance. The internal temperature of the cell goes up before the negative delta-V is seen. If you actively cool a cell with a cooling jacket or forced cold air flow you will a significantly delayed negative delta-V or no drop in voltage at all.The lack of technical accuracy in this figure leads me to doubt the document as a whole.

author: KE5FX
author_rank: Super Contributor
timestamp: January 25, 2018, 05:32:23 am
content: Quote from: IanB on January 25, 2018, 05:02:16 amArguing with Eveready about batteries?

author: floobydust
author_rank: Super Contributor
timestamp: January 25, 2018, 06:19:49 am
content: The Eveready doc shows 1.60V for end of charge voltage, and 1.50V as pretty much useless for charging beyond ~50% capacity. It seems a bit off

author: bd139
author_rank: Super Contributor
timestamp: January 25, 2018, 07:36:43 am
content: Well they're charged now which is good. Nothing exploded as well which is also good!Thanks for the help everyone

author: tszaboo
author_rank: Super Contributor
timestamp: January 25, 2018, 08:18:55 am
content: Quote from: IanB on January 24, 2018, 05:40:44 pmyeah, at 1.3V you charge it to some 30%, which is perfect for the "I need batteries fast for today". And you are safe if you follow all my instructions. If someone changes the voltage, of course it is not OK. But that's like trying to make pie from a recipe, but instead of 100g sugar, you put a kilo in it, and then complain that the pie is not good.

author: macboy
author_rank: Super Contributor
timestamp: January 26, 2018, 04:02:20 pm
content: Quote from: NANDBlog on January 25, 2018, 08:18:55 amThe problem with your method of parallel charging is that you do not ensure equal current per cell while charging, but do charge all cells for an equal length of time. This means that each will be charged to a different capacity. When subsquently used series-connected, the discharge current will be equal for all cells. The one(s) which received lower charge current will dump first, and if the device is not turned off at this point, the dead cell(s) will begin to reverse-charge due to the discharge current that continues to flow. Cell damage and death is then inevitable.If you plan to partially charge, the I'd always advise series charging. All cells will receive the exact same Ah worth of charging, regardless of when you choose to stop the charge.

author: metrologist
author_rank: Super Contributor
timestamp: January 26, 2018, 05:12:44 pm
content: interesting discussion because I recently disassembled a waterpik to determine its failure. I suspected the battery - 2.4V series cells that are charged via a 3V AC transformer and series diode.What actually happened is one of the motor brushes broke off at the housing.

author: stj
author_rank: Super Contributor
timestamp: January 26, 2018, 05:51:22 pm
content: was that NiCad or NiMh??i'v seen that exact charging method in my old Bosch electric screwdriver.as crappy as it looked from a charging standpoint, it lasted almost 2 decades and then the switch burned out before the NiCad's could fail!!!

author: metrologist
author_rank: Super Contributor
timestamp: January 26, 2018, 06:14:14 pm
content: It says NiMH, but I thought that chemistry would not tolerate this kind of charging.

author: james_s
author_rank: Super Contributor
timestamp: January 26, 2018, 06:35:58 pm
content: It's not ideal but it does work. So long as the current is low it doesn't seem to harm the cells.

author: KE5FX
author_rank: Super Contributor
timestamp: January 26, 2018, 08:45:46 pm
content: Quote from: metrologist on January 26, 2018, 06:14:14 pmThe transformer will have been chosen to saturate magnetically at an appropriate current limit, and I'm not sure why the NiMH chemistry might object to being charged through a half-wave rectifier. This has long been a common approach to charging all (well, most) sorts of batteries.

author: metrologist
author_rank: Super Contributor
timestamp: January 27, 2018, 01:01:08 am
content: I thought an issue was trickle charge, and that pulse type charging, while not shown to be detrimental, wasn't supported as any better (I thought it was bad for nimh).

author: IanB
author_rank: Super Contributor
timestamp: January 27, 2018, 01:45:38 am
content: Constant non-stop trickle charging is an issue, so leaving batteries on constant charge for weeks and months at a time will slowly damage them unless the charge is somewhere around the C/100 rate.However, if you take the batteries off charge once the charge is complete then trickle charging is not a problem.

author: KE5FX
author_rank: Super Contributor
timestamp: January 27, 2018, 01:48:09 am
content: Quote from: metrologist on January 27, 2018, 01:01:08 amTrue, I've never found a way to trickle-charge NiMHs at a steady rate. C/40 doesn't maintain full capacity -- or even prevent slow self-discharge, from what I've seen -- but there are lots of warnings against leaving NiMH cells on full-time charge at higher rates. Of course, I may be mistaking "slow self discharge" for long-term damage as Ian points out.It would be interesting to know the average current from the wall wart and how it behaves at various states of charge, vis-a-vis the rated capacity of the cells. They must be relying on the transformer's B-H characteristic to vary the charge rate with voltage, given how long the batteries in these things tend to last.As far as the diode goes, I imagine they just didn't see the need to spend money on a filter cap. The battery presumably spends most of its time in the fully-charged state where it sees little or no ripple current.

author: james_s
author_rank: Super Contributor
timestamp: January 27, 2018, 07:07:51 am
content: There's a lot of inexpensive gear that has rudimentary less than ideal charging circuitry. The companies making it don't care of the battery life is less than optimal, so long as it's longer than the warranty.

author: Gyro
author_rank: Super Contributor
timestamp: January 27, 2018, 10:28:42 am
content: I remember an article from way back (don't ask for a citation), related to portable domestic appliances. That said that manufacturers had established that C/30 was a safe rate for indefinite trickle charge without degradation. That probably related to Ni-Cd though.