charge NiMh with a NiCd charger? - Page 1
Forum section: electronics / general

author: fatbeard
author_rank: Contributor
timestamp: May 08, 2025, 11:03:37 pm
content: My Festool cordless drill was supplied with 9.6V NiCd battery packs (2.0AH) and a NiCd charger that alledgedly can charge packs up to 14.4V, charge current 1.5A/300mA.After 30 odd years the battery packs are now as dead as the proverbial dodo, but these packs are now unobtanium: all I can find are NiMh replacement packs. Capacity is similar (2.1Ah vs 2.0Ah).New packs should be fully charged before first use, but I need to do a job real soon and have no time to convert the charger ti NiMh (which I intend to do in the near future) before that.How bad is it for the packs and/or charger if I just plug the NiMh pack into the NiCd charger for the initial chargeup and possibly a second charge?

author: tunk
author_rank: Super Contributor
timestamp: May 08, 2025, 11:15:16 pm
content: IIRC it's said that NiCd can take a 0.1C trickle charge, NiMh 0.05C.Is the charger one of the old heavy ones with a transformer?If so, I think it relies on voltage vs. current of the transformerto regulate charging. You could e.g. try to use one or two diodesin series with charger and battery pack.

author: fatbeard
author_rank: Contributor
timestamp: May 09, 2025, 06:20:26 am
content: Old it is for sure: built in 2002: it does have a transformer (and no capacitor on the charging circuit).But quite advanced for its years: it has quite a bit of electronics: temperature sensing, microcontroller and all.Trickle charging is -according to the manual- 30mA pulsed, not 300 so I might be OK if I don't leave the pack on too long?

author: IanB
author_rank: Super Contributor
timestamp: May 09, 2025, 06:46:21 am
content: A NiCd charger will be able to put charge into an NiMH pack just as well as a NiCd pack, so the actual charging should not be a problem. Where the problem may come is the end of charge. The end of charge detection is a bit different between NiCd and NiMH and so the charger has a risk of not stopping when it should and overcharging the pack.You can guard against this by monitoring the charge carefully and stopping it manually if the pack seems to be getting too warm/hot (granted this is difficult if the pack is in a plastic shell).If you have a multimeter you could measure the voltage of the pack before charging it, to estimate its level of charge. If the voltage is around 9.6 V it is discharged and in need of a full charge. If it is around 10.5 V or so, it is already charged and not in need of much more charging.If the charging current is 1.5 A and the pack is 2.1 Ah, then a full charge from empty would take about 1.5 hours. So you could charge for up to 1.5 hours while monitoring, and manually stop if the charger has not stopped automatically by that time or if the battery is getting hot.

author: radiolistener
author_rank: Super Contributor
timestamp: May 09, 2025, 07:29:44 am
content: As I know, both NiCd and NiMh uses -dV as indication of charge complete event.What is the difference?

author: iMo
author_rank: Super Contributor
timestamp: May 09, 2025, 07:36:41 am
content: For initial charge you may wire the battery to a PSU set to 9.60V DC (the voltage of fully charged 8 cells pack I assume) via a, say, 1.5ohm resistor (could be anything between around 1-2ohm 5W resistor).Or set your DC PSU to 9.60V and 1A current limit when your PSU is CC/CV one.It will charge the pack fast and safe.Afterwards tell us what your NiCd charger is like (whether it uses -dV sensing, dT sensing, etc).Btw., your 2.1Ah NiMH pack could be tricklecharged with 100mA DC continuously.PS: below my favorite charger, I've been using it for all chemistries..All you need is to set the PSU to the voltage equal the fully charged battery.It will stop charging "automatically", no need to monitor, the charging time is longer, the batteries will not be charged to full 100% capacity, but the batteries will last longer (more cycles).. Also you may set the "fully charged" voltage as you wish.

author: fatbeard
author_rank: Contributor
timestamp: May 09, 2025, 08:16:07 am
content: Alas, there is no information available on the actual charging metrics, just 1.5A fast chacrge and 30mA pulsed triclecharge.No schematics, but that can be solved by a bit of gninreenigne (reverse engineering) once I bite the bullet for the conversion.But that is then, of course, complete and utter moot...The onboard microcontroller is a PIC12C805 (I think, just screwed the thing back together) which will undoubtedly be locked, so no entrance to the charging metrics there either...

author: iMo
author_rank: Super Contributor
timestamp: May 09, 2025, 08:22:58 am
content: I added above the schematics of the mentioned charger which always works reliably and safely with any chemistry..

author: fatbeard
author_rank: Contributor
timestamp: May 09, 2025, 08:55:53 am
content: It sure does, used it many times to 'revive' a NiCd pack and even LiIon cells. The latter still require monitoring to keep things safe, though.You forgot to mention that it ties up the PSU for the duration of the -incomplete- charge...My NiMh pack has been charging on the NiCd charger for the past hour or so and got warm (around 45C). The charger does not distinguish between full charge and overheat, so I will let it cool down a few hours and try again.

author: iMo
author_rank: Super Contributor
timestamp: May 09, 2025, 09:18:28 am
content: People are possessed with fancy chargers, which mostly kill their accus soon by pushing too much current while at high temperature.For example the famous Hewlett Packard calculators were the luxury items for decades, and the chargers for their NiCd packs consisted from a cheapo wall AC transformer a single diode and a single resistor in series with the accu pack..Today's recommendation is charge to 80% capacity (to save $$ and our environment).

author: Gyro
author_rank: Super Contributor
timestamp: May 09, 2025, 12:16:07 pm
content: Quote from: radiolistener on May 09, 2025, 07:29:44 amThe -dV is much more pronounced / deeper with NiCd than NiMh, which is much flatter. There is a chance that the sensing of an earlier NiCd charger circuit will miss the -dV of a NiMh.

author: WattsThat
author_rank: Frequent Contributor
timestamp: May 09, 2025, 06:43:55 pm
content: Quote from: iMo on May 09, 2025, 09:18:28 amThe actual design flaw with the HP arrangement was that it relied on the cells internal resistance to keep the applied voltage from that transformer below levels that would damage the IC’s in the device. Should you attempt to use the calculator without a battery with the transformer connected to mains, you would fry the IC’s from overvoltage/ripple. The same thing occurred when the cells aged and no longer held a charge or corrosion formed on the battery or contacts creating a poor, high resistance connection. Blown custom HP IC’s. That’s why a working 20 series Woodstock machines are so rare, they all get killed when they get plugged in years after the batteries failed.All because they had to compete in a cost sensitive market and they really had no experience there. Saving a dollar on the BOM was a big deal to them and they chose to cut corners on the worst possible area, the power supply.

author: fatbeard
author_rank: Contributor
timestamp: May 10, 2025, 08:21:27 am
content: An update: after cooldown the pack charged to (presumably) full: the charger switched off and the pack stayed cool.A second new pack is currently charging, will see how it fares.Performance-wise: speed (voltage) OK, torque (amps) lower than I remember, but usable. Enough to get the job at hand done...I will reverse-engineer the charger to get insight on regulating mechanisms and PCB form, in order to change the charging circuit to an intelligent NiMh one. Possibly one that pushes less current and stops at 80% (how does one veryfy that, anyway?).When I decide the performance of the NiMh packs to be to disappointing (they're 'made in China' so the claimed 2.1Ah may be a lot less) I may consider switching to Lithium, but that's a whole other can of worms that I prefer not to open right now...I'll open up one of the NiMh packs to see what's inside, stay tuned.

author: tunk
author_rank: Super Contributor
timestamp: May 10, 2025, 07:04:35 pm
content: What was the battery voltage after charging? It should be 1.3-1.5V per cell.In addition to capacity, you should also consider how much current they can deliver.Cordless drills may draw up to 20-30A. When you say the torque is down, it maybe because the cells cannot deliver enough current.For charging to 80%, you could look at e.g. this (from EEVblog member HKJ):https://lygte-info.dk/review/batteries2012/Ikea%20Ladda%20AA%202450mAh%20%28White%29%20UK.html

author: Gyro
author_rank: Super Contributor
timestamp: May 10, 2025, 09:48:47 pm
content: Quote from: fatbeard on May 10, 2025, 08:21:27 amNiCd cells had lower internal resistance than the equivalent NiMH ones, translating to higher peak torque. This was about their only advantage (other than better continuous trickle charge tolerance).

author: fatbeard
author_rank: Contributor
timestamp: May 11, 2025, 08:03:29 am
content: Quote from: Gyro on May 10, 2025, 09:48:47 pmWell, that explains the lower torque.Sufficient for now, but I have some jobs coming up that may require more.Seems I'll have to get some NiCd's too and refurbish at least one of the dead packs...On the upside: I will not have to change the charger.

author: elecdonia
author_rank: Frequent Contributor
timestamp: June 01, 2025, 04:00:31 am
content: Quote from: Gyro on May 10, 2025, 09:48:47 pm20 years ago this was definitely the case. Battery-powered hand tools may require up to 25A for brief periods of time. But early NiMH cells could supply only 1 to 5A.And yes, older NiMH cells absolutely could not tolerate continuous overcharging from a “dumb” charger. In contrast NiCd cells were much more tolerant of overcharging.Times have changed: Today many current generation NiMH cells support high discharge rates. There is a spec called “discharge rate” which is expressed as 5C (or even 10C) where C is the mAh capacity of the battery. A 2200 mAh NiMH battery pack rated at 10C can supply 22A to a load. The NiMH cells used in the Toyota Prius are rated at 6.5 Ah but support discharge rates up to 100A. This is a discharge rate of 15C.Unfortunately todays NiMH cells can still be damaged by continuous overcharging. Therefore a “smart” charger which terminates charging current upon detection of full charge should always be used with NiMH cells.For most NiMH cells 1.42V represents full charge. During active charging the terminal voltage may briefly reach 1.5V per cell but by this point nearly all charging energy is dissipated as heat. In fact a good way to detect full charge is by observing when NiMH cell temperature begins to increase rapidly. During the first 3/4 of charging the NiMH cell only warms up slightly, perhaps by 5°C. But near the end of charge the cell temperature rises much more rapidly and may increase by 20-30°C within 10-15 minutes. At this point the charging current must be totally shut off to prevent overheating and possible electrolyte leakage due to excessive pressure within the NiMH cell.

author: elecdonia
author_rank: Frequent Contributor
timestamp: June 01, 2025, 02:46:00 pm
content: Over the years I collected a bunch of early 2000’s vintage Skil (Bosch) cordless drills. And also their jigsaw and a small circular saw (6” blade). All have interchangeable 14.4V NiCd battery packs. Not surprisingly their original battery packs were weak or dead. That’s why people were giving them away for free or selling them cheap ($2-$5 each at yard sales). I also obtained a Skil/Bosch “smart” charger which claims to charge these packs in 1 hour. Note: This charger doesnotspecify NiCd or NiMH. However, all of the factory original Skil//Bosch battery packs were NiCd.I rebuilt these packs with 2200 mAh NiMH cells. Form factor of these cells is “4/5 sub C.” I bought cells with the tabs pre-attached and soldered them together. Not pretty but it works. Maybe I will get a spot welder eventually? (a must-have if I move forward to rebuilding lithium cell packs).My first rebuilds eventually weakened because those NiMH cells gradually developed too much internal resistance. Those packs ran hot, both under load and while charging. I suspect this caused their internal resistance to increase with usage. One of my oldest “rebuilt” packs still holds a charge but only supplies enough current to spin the motor in the drill under “no load” conditions. Looking back I discovered that some of my oldest NiMH cells were rated for only a "1C" discharge rate: 2.2 amps for a 2200 mAh cell. This is nowhere near enough for power tools which typically consume 5 amps when moderately loaded and a whole lot more under heavy load.I first obtained NiMH cells rated for “high drain” or "high discharge rate" applications in 2012. Some specified discharge rates of 5C or more. Others indicated their cells could be discharged at 10 to 15 amps. One vendor boasted of"discharge rate up to 35 amps"(I didn't believe this). Cell capacity for these newer 4/5 sub-C size NiMH cells hasn't changed. It's between 2000-2200 mAh, same as the first NiMH cells I bought many years ago. What's changed is that NiMH cells designed for higher discharge rates have become more readily available.Recently I retested the Skil 14.4V battery packs which I’ve been using routinely. Some were having shorter run times. They still had enough power to use, but were weakening. Surprisingly only 1 or 2 cells in each of these packs required replacement. The rest of their NiMH cells (some approaching 8 years of age) still had 85% or more of their original 2200mAh capacity. This is a big contrast to my oldest NiMH “rebuilds” where all of the cells developed high internal resistance, making those packs useless.During the past week I searched for "4/5 sub C” size NiMH cells and discovered a vendor claiming discharge rates up to 22 amps for their 2200 mAh cells. This is a maximum discharge rate is 10C. While searching I discovered the highest claimed discharge rate for a NiMH cell is 15C. I plan to get some NiMH cells rated at 10C (or 22 amps) for testing.

author: elecdonia
author_rank: Frequent Contributor
timestamp: June 02, 2025, 06:46:26 pm
content: OK what I'm trying to do right now is to identify what the open-circuit voltage for a fully charged NiMH cell should be.There's a lot of opinions out there.Some folks state the SOC (state of charge) cannot be determined from a voltage measurement alone.But it's also true that a healthy fully charged NiMH cell will measure between 1.35V and 1.42V, and that "active charging" should stop when the cell voltage reaches 1.5VAnother big question in my mind is whether any sort of "trickle charge" or "periodic maintenance charge" should be applied to NiMH cells, and if so, how much and how often?This issue represents a big difference between NiMH vs. NiCd.NiCd cells accept continuous "trickle charging" without complaining about it.In fact most small and simple NiCd chargers contain nothing but a transformer and a diode (or bridge rectifier). This permits the NiCd to continuously receive 50 to 100mA of trickle charge current for as long as the charger is connected to it.On the other hand, my Skil//Bosch "1 hour" NiCd charger contains an MCU which senses voltage, current, and battery pack temperature. The maximum charge rate is stated at 2.5A and I believe the original NiCd cells in their battery packs were rated somewhere in the range of 1200 to 1500 mAh. For years I've used this charger with my rebuilt NiMH packs without major issues. I have a habit of removing the battery packs shortly after the charger indicates they are fully charged. Now I'm thinking about measuring how this charger actually functions with NiMH and perhaps modifying it a bit.Moving forward to NiMH cells, most folks caution that"NiMH cells should not be trickle charged at all."But then how does one go about keeping their NiMH battery packs fully charged and ready to use?

author: IanB
author_rank: Super Contributor
timestamp: June 02, 2025, 07:50:44 pm
content: Quote from: elecdonia on June 02, 2025, 06:46:26 pmYou can get an estimate from the voltage, but it depends on the ID of the cell (manufacturer and part number). All cells are different.Obtain samples of the cells of interest, charge them, use them, and measure them.Most cells have a high voltage hot off the charger, and when rested the voltage gradually declines over time to a longer term stable voltage.Low self-discharge cells (like Eneloops) have different characteristics from old technology cells.I can tell you that Eneloop cells tend to be about 1.44 V hot off the charger, and over time settle out to about 1.33 V fully charged. You can generally say that 1.28 V is 50% charge, and 1.20 V is 0% charge.QuoteI don't think this is true.QuoteSuch chargers pretty much tend to kill the battery packs over time.QuoteOne answer is to use LSD cells that hold their charge when off the charger. I have had Eneloops sitting on the shelf for years that are still holding charge and able to operate a digital camera.Another answer is to do a periodic maintenance charge, or charge before use.By the way, you may be aware that high power sub-C NiMH cells were very popular in the RC model community. Although they are probably displaced by lithium cells now, they would still be very good for rebuilding power tool battery packs if they can be found.

author: elecdonia
author_rank: Frequent Contributor
timestamp: June 03, 2025, 07:00:46 pm
content: Quote from: IanB on June 02, 2025, 07:50:44 pmI'm doing this now. Although I'm testing cells of different ages and cycle counts, as well as from more than one manufacturer, all the cells I'm currently testing are 2200mAh NiMH and size 4/5 sub-C. None are the "low self-discharge" type.Surprisingly all of my NiMH cells have nearly identical terminal voltages when fully charged.I'm using my HP3456A (6.5 digits) to measure with microvolt resolution.I didnotexpect my cells to have such a narrow range of terminal voltages.Note: I have observed a temperature dependence, so now I'm including "cell temperature" in my data sets.QuoteMy testing confirms this. Immediately after stopping charge I often see1.45V.After 24 hours of resting (no trickle charge, open circuit) this typically drops to1.35-1.39V.I recently started testing fully charged NiMH cells withextremely low(<5mA) trickle charge currents in order to reduce this voltage drop which occursafterremoving a cell from the charger.It appears that <5mA is sufficient to maintain 1.42V.Note that a 5mA trickle charge is C/440 for a 2200mAh cell. This ismuchlower than typical "trickle charge" rates (50mA for example).QuoteMy NiMH cells are also very close to these same terminal voltages.QuoteI agree that long-term overcharging shortens the life of NiCd cells too. But I was just trying to suggest that NiCd cells appear to tolerate overchargingbetterthan NiMH (which don't like overcharging at all).QuoteI agree. Many years ago I learned to remove cells from dumb chargers after they obtained a full charge.QuoteI'll get some LSD "AA" cells and try them out. I have some small devices which I use only occasionally (vintage transistor radios).

author: elecdonia
author_rank: Frequent Contributor
timestamp: June 10, 2025, 10:06:29 pm
content: After several days of testing it appears that a trickle charge current of5mA maintains my 2200 mAh NiMH cells at a steady 1.4V per cell.Dropping charging current to2mA lowers the cell voltage to a steady 1.36V per cell.For a control test I stopped the trickle charge and observed open-circuit cell voltage for 48 hours: It gradually dropped to1.33V per cell.Next I plan to observe open circuit voltage for 2-3 weeks. I want to get an idea of their rate of self-discharge.Note: I'm testing standard NiMH cells,notthe "low self-discharge" variety.

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 01, 2025, 04:41:08 pm
content: Update after many hours charging, discharging, and pondering how NiMH chemistry works.I still don’t understand how or why “cycling” NiMH cells (by repetitive controlled charge/discharge cycles) increases their mAh capacity.I am testing this. However I have no conclusions yet. Several NiMH packs I bought recently had only 50% of rated mAh to start with. Manufacturers often state:“Battery pack won’t have full mAh capacity until it has been charged/discharged several times.”My new battery packsareable to operate my cordless tools satisfactorily. So I can’t complain. Note: My favorite torture test is to use a wire wheel with a cordless drill, where I am running the motor at full speed continuously for 5 minutes or longer.What I learned recently1) Published specs for mAh are almost always based on a C/10 discharge rate down to 1.0V per cell. Unfortunately this doesn’t reflect the way NiMH packs operate in cordless tools. The mAh at high discharge rates is only 25 to 50% of “rated” capacity. However, mAh must be measured under mfg conditions (C/10 with 1.0V cutoff) if the goal is to confirm whether or not the cells conform to the manufacturer’s mAh spec.2) “Fast” charging won’t bring packs up to full mAh. During fast charging the cell temperature must be monitored to avoid overheating. Most “fast chargers” actually do monitor pack temperature. But I also suspect many chargers supply about twice as much charging current as what’s best for the NiMH cells. A popular marketing claim is “one hour charging.” This is a rate of about 1.5C.3) I’m experimenting with slower charging rates, in the range of C/5 up to C/2, in order to determine how fast a pack can be charged while still reaching a full charge without getting excessively hot.4) NiMH charging doesn’t begin to heat the cells until the latter portion of the charging cycle, where cell voltage is >1.4 V/cell. The absolute maximum cell voltage allowed during charging is 1.5V. At this voltage nearly all charging current is dissipated as heat. Cell temperature may rise quite rapidly. Therefore the charger needs to have a hard cutoff at 1.5V per cell.5) When a cell reaches full charge the cell voltage reaches a peak, after which it drops slowly. Most chargers detect this and use it to end the charging cycle. Note: This voltage drop is very small, typically no more than a few mV per cell. NiMH cells have smaller voltage drops than NiCd. Therefore older “NiCd only” chargers may fail to detect the smaller “fully charged” voltage drop of NiMH cells.6) To me it seems that changing the charging algorithm from “constant current” to “constant voltage” near this “end of charge” condition would be desirable. Chargers for Li-ion cells use a similar algorithm. I intend to test this concept.7) Application of “trickle charge” is controversial. However, many chargers continue supplying a small current (typically 50mA) after the charging cycle ends. Perhaps the best method for “trickle charge” is to apply a fixed voltage of 1.4 to 1.45V per cell through a series resistor. I suspect it is important to avoid raising the cell temperature above ambient.

author: tooki
author_rank: Super Contributor
timestamp: July 01, 2025, 05:26:38 pm
content: Quote from: elecdonia on July 01, 2025, 04:41:08 pmThere are two potential mechanisms:1. Recovering from memory effect (when a battery is repeatedly, long term, always discharged to a certain partial SOC and then recharged, the battery will then struggle to be discharged significantly below that partial SOC level). There is disagreement as to whether NiMH cells truly suffer from the memory effect and if so, to what extent.2. Recalibrating a fuel gauge IC, as I described earlier.Quote from: elecdonia on July 01, 2025, 04:41:08 pm4) 1.5V/cell is much too low a limit. It may well be OK forsomecells, but certainly not for all of them, and the peak voltage may change as the cells age. I perfboarded a charger using the MAX713 chip, and IIRC in testing using Eneloops, most cells needed 1.6V or slightly above.The MAX713 datasheet says that some cells may require peak charging voltages of up to 1.9V/cell!!https://www.analog.com/media/en/technical-documentation/data-sheets/max712-max713.pdf(Trying to FULLY understand that datasheet was hugely instructive for me in understanding NiMH charging. I encourage you to do this, too. Read it carefully, do the calculations, etc. Note that the datasheet occasionally makes reference to an otherwise undocumented “linear mode”. This is a remnant of very old datasheet revisions that showed both a linear mode and a switching mode. The switching mode was removed from the datasheet years ago, leaving only the linear mode described. But the editors missed a few spots. Just know that the entire datasheet now describes only the linear mode.)6) Before you test, do some research to learn from the people before you who have already tried it. Google “NiMH cc/cv charging”. One very nice write up is this one:https://www.ti.com/lit/an/slyt468/slyt468.pdf(In a nutshell: it works, and is gentle on the cell, but is slow, and won’t charge the cell to true maximum capacity.)7) Again, that voltage could be VERY wrong for some cells. Most NiMH cells terminate at voltages around or above 1.5V, so constantly applying 1.45V or less means zero current will flow into the cell and you’ll only maintain it at a less than full SOC. That’s why trickle charging (and indeed, NiMH charging in general, usually) is specified using constantcurrent.Basically, using fixed voltages for limits is OK if you’re building a product where you will extensively characterize the cells you use inside it. Otherwise, going by current and -dV/dT termination is better.I would also look at the charger reviews onhttps://lygte-info.dk/info/roundCellChargerIndex%20UK.html— they show how real chargers and real batteries behave at different charging currents. For example, most of them were tested using Eneloop and Eneloop Pro cells, and in most cases, using -dV/dT termination, those have a peak voltage close to 1.6V, sometimes higher.

author: IanB
author_rank: Super Contributor
timestamp: July 01, 2025, 06:22:59 pm
content: Quote from: elecdonia on July 01, 2025, 04:41:08 pmA possible explanation for this is that repeated charging and discharging circulates and redistributes the electrolyte within the cell, reducing starved regions of electrode and allowing more of the available surface area to become active.QuoteOne way to look at this is that the measured terminal voltage during charging can be expressed as V = EMF + IR, where EMF is the electrochemical potential (a function of the state of charge), I is the charging current, and R is the internal resistance of the cell.During the early phase of charging, the EMF increases rapidly and causes V to rise overall. When nearing full charge, the rise in EMF flattens out and the cell gets hotter as more charging current is turned to heat. The increase in temperature causes the internal resistance to fall, and hence the IR term decreases. Combining this with the plateau in EMF, then V overall is observed to decrease.The observed voltage drop is more pronounced at higher charging currents for two reasons:1. The cell gets hotter, so the drop in R is more significant,2. The higher value of I also causes a larger change in IR overall.The reason NiCd cells have a more observable voltage drop is said to be that the charging process is endothermic in NiCd chemistry, but exothermic in NiMH chemistry. As a result, NiCd cells stay cooler in the early stage of charging and the rise in temperature at the end is much sharper.An overall conclusion is that the voltage drop with NiMH chemistry is more pronounced when using higher charging currents. (But very smart NiMH chargers can look at the voltage curve during charging and stop charging when the voltage reaches a plateau.)

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 01, 2025, 08:00:12 pm
content: Many thanks toIanBandtookifor their info and suggestions. Although I've used rechargeable batteries in my cordless tools for many years, I have always wished to learn better ways to maximize their life and performance. I feel like I am on the right track now!

author: elecdonia
author_rank: Frequent Contributor
timestamp: August 30, 2025, 06:08:28 pm
content: I know it's been a while since I provided an update on my experiments with charging small NiMH and NiCd battery packs.I have a new piece of test gear: The Tenergy T180 balance charger/discharger. This unit has a large graphic LCD display (sadly only monochrome). It accommodates several types of Lithium batteries, NiMH, NiCd, and lead-acid. The T180 is a nice upgrade to the SkyRC imax B6AC V2 charger/discharger which I've had for years (The B6AC is still quite useful within its limits). And now that I have two devices which can charge a battery pack I can do more tests faster because I can charge two battery packs at the same time.I have been testing a variety of NiMH cells and multi-cell battery packs. Most are around 2,000 mAh, although I have a couple of new packs which (claim) to be 4,500 mAh. I do not have any "low discharge rate" NiMH cells to test yet. The standard NiMH cells and packs I'm testing range from "brand new" to about 10 years of age.When charging NiMH with an "intelligent" charger (B6AC, T180, or original tool manufacturer's "fast" charger) it appears they all employ -dV/dt sensing to terminate charge. Some also measure pack temperature. Some of my hand tool battery packs contain internal temperature sensors. Also the B6AC and the T180 can be connected to an external temperature sensor which can be taped to the battery under charge. I have their external temperature sensors but as of yet have not tested them extensively.First I'll provide my results from measuring what happens when NiMH cells reach "full charge."After 100's of tests of many different NiMH cells and packs I've observed that "full charge" occurs between 1.40V and 1.42V per cell.If charging is allowed to continue above 1.42V per cell, the battery begins to have a temperature rise which can be quite rapid if the charging current is anything above 1A. I haven't yet observed any batteries which require more than 1.5V per cell to be "fully charged."After removing the battery from the charger the open-circuit voltage gradually decays to between 1.33 and 1.36V per cell.I've experimented with "trickle-charging" and I now agree that trickle charging must be limited in both current and duration.Commercial chargers (such as B6AC and T180) have a "trickle charge" option but the lowest available trickle charge current is 50mA. This is actually too large. 10 hours of 50mA pumps 500 mAh of energy into the battery. Application of 50mA for 24 hours adds 1,200 mAh. Application of so much excess charge has no value and might damage the battery.Therefore I tested considerably lower "trickle charging" currents: 1 to 20 mA. The sweet spot seems to be 10mA.This applies only 100mAh in 10 hours or 240mAh in 24 hours. It seems that 10 hours of 10mA is sufficient to charge multi-cell packs up to voltages which are above 1.42V per cell for every cell in the pack.My conclusion at this time is that application of 10mA for 10 hours after completion of a standard charge terminated by -dV/dt is sufficient to "balance" all of the individual cells inside a multi-cell battery pack.

author: elecdonia
author_rank: Frequent Contributor
timestamp: September 29, 2025, 11:03:17 pm
content: I recently purchased some Tenergy brand 2,000 mAh 4/5 Sub-C NiMH cells. Amazon sells these in lots of 25 cells for $47.99. This works out to $1.92 per cell. Not bad for cells which actually meet their specs (I did test them). Another good thing is these are rated for "high discharge rates" up to 20A. This is 10C.https://a.co/d/8m7zel8In contrast I've purchased generic unbranded NiMH cells (and complete battery packs) which were "rated" at 3,000 mAh or higher, yet they only tested at 1,300 to 1,500 mAh.My worst case is a "generic" 19.2V pack for Craftsman cordless tools, advertised at 3,600mAh, yet my tests revealed only 1,300mAh. They were cheap: $36.99 total for 2 complete Craftsman 19.2V battery packs. Each pack contains 16 cells. 32 cells in total. This works out to $1.15 per cell, not including the cost of the plastic housing and the cost of assembling the cells.https://a.co/d/hK4AwCdBoth of these packs "work." My tools work fine with them except for the low mAh. Amazon customers rated these packs at 4.6, probably because these are as good as original Craftsman NiCd packs ( 1,200 mAh? ). The plastic housings on these generic packs are well made and fit my Craftsman tools perfectly. It's unfortunate the vendor greatly exaggerated their mAh specs while using the very cheapest cells they could find.My generic 19.2V Craftsman packs contain 16 sub-C cells per pack. Tenergy offers several grades of NiMH sub-C cells ranging from 2,200 mAh to 5,000 mAh. Their "top-of-the-line" 5,000 mAh cells are $6.50 each. These have a 50A discharge rate ( Wow! ). But this price makes the cost of upgrading my cheap generic Craftsman packs $104 per pack plus the time and effort to replace their original cells.https://a.co/d/9mcPtHgMy conclusion: You get what you pay for.