My NiMH cell get hot while charging..is this necessary or damaging and wasteful? - Page 1
Forum section: electronics / projects

author: martys
author_rank: Regular Contributor
timestamp: September 18, 2023, 03:46:10 am
content: How best to get a NiMH battery/cell fully charged as quick as possible without shortening the service life?I've designed and built my own intelligent charger, but I think ignorance is a good part of the intelligence of the charger.My battery is being used in an handheld tool, so it is important to get the battery as quickly fully charged as possible to get the most operation use of without wasting energy. It is equally important to do charge the battery without damaging a battery's service life.I've done a lot of research on the web to find out how to determine when a NiMH batter/cell becomes fully charged.Most experts agree that delta temperature rise is the best way to determine/ensure a cell is fully charged and not overcharged, but none of these experts will recommend (that is, they give advice without any supporting data) the exact number of degrees of temperature rise is ok, but most seem to agree on about 10-DegC or else when the temperature rise is beginning to rapidly increase per 10-min interval of monitoring.I've even contacted customer support of a major battery mfg, but only got responses like "it is important to not damage the battery by causing the battery to get too hot while charging."Butthere is something screwy about heating up the battery while charging:Any heating of a charging battery is possibly damaging it.Energy is being wasted heating rather than charging the battery.While fast charging enables a battery's heating to be easily detected by a thermistor sensor I am faced with this enigma:It is impossible to determine the exact mAH of charge the battery gets to determine battery status or in-use operating time because it becomes unknown how much charge had been wasted justheating the large mass of the battery 10 DegC above ambient/start temperature.Here's my questions:In determining when to terminate charging, why heat 10-Deg C..why not just detect a 2-DegC rise from start of charging?Assume the battery-operated tool has been fully charged and then put into use for just a several minutes..still plenty of charge left in the battery. We know the battery has been partially drained, and it is always returned to the charging station. What to do? Should the intelligent charger recharge the battery again until heating determines the battery is fully charged again?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 18, 2023, 04:13:05 am
content: I would guess that there are several issues here. First, battery charging is never 100% efficient. I have heard numbers ranging as low as 60% for some chemistries. I don't know the value for NiCads, and suspect there is some variation with configuration and vendor. So there will be some heating during the charging interval. Second, the designer can't assume that the charging occurs in a precisely temperature controlled room, so there must be allowance for that. Even having two temperature sensors doesn't solve this problem as there will always be thermal lags. Another factor may be that NiCads have been around for a long time, and the standards were developed when temperature sensing was not as accurate and convenient as it is now. Still another factor is the short duration of the time when the temperature is elevated. Damage is related to time at temperature and presumably the cells spend only seconds at ten degrees above ambient, and minutes in the interval between full charge and cutoff. This short time also means the energy loss is small. Even today it may not be enough to warrant salvage, and in the heyday of NiCads we were using 100W of energy to generate 3W of light. The charger efficiency just wasn't an issue.A question for you? If you are going to the trouble of developing a new charger, why are you using NiCads? Newer battery chemistries offer far superior charge efficiency, higher storage density and well defined end of charge.

author: martys
author_rank: Regular Contributor
timestamp: September 18, 2023, 04:17:24 am
content: Catalina: I don't see any reference to Ni-Cads in my posting???Not using Ni-Cads, using NiMH.Measuring temp with thermistors has been easy for over 50-yrs.While the charger may remain at a temp rise of 10-DegC for ten seconds, it takes many minutes for the charging current to raise the battery temperature 10 DegC.

author: Infraviolet
author_rank: Super Contributor
timestamp: September 18, 2023, 04:19:36 am
content: Commercial grade single cell and multiple-cell-in-series NiMH chargers I've used definitely heat up the cells, often by rather more than just 10 degrees above ambient. I measured this once with some level of precision during a charging cycle when I wanted to see how hot cells might get when kept in a confined environment (inside a wheeled robot) while charging. But even with cells exposed to the air on 3 sides for cooling one still gets an appreciable temperature rise, if commercial charger designs can get away with doing this I should think yours can too. Also, plenty of commerical chargers have no means to measure cell temperatures, some are designed as wall warts with a long cable (two conductors only) which you run over to a DC jack connected to your battery pack, the only information the charger can possibly monitor are voltage and current, it has no way of getting a temperature sensor anywhere near the location of the cells. I had thought NiMH charge termination* was usually based on detecting either a small sudden drop in voltage or some small sudden event related to the charging current, terminating by temperature rise, in any chargers which appear to do this, might be more like a matter of pausing the charge if temperature gets too high, then resuming it after the cells cool a bit, not a way to determine when the charging cycle is over but rather a way to ensure that the cells don't get extremely hot during the cycle.*or rather just switching to a slow trickle charge mode which keeps the battery topped up and could go on forever without harming the battery and also could go on forever without increasing the useful energy stored within it

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: September 18, 2023, 04:25:51 am
content: Fast charging and long service life are mutually exclusive, because excessively fast charging shortens the service life. It's up to you to decide how fast you want to charge and how much degradation you can put up with.Likewise, with cells not being 100% efficient, they will undergo some temperature rise just during normal bulk charging phase. 2 degrees since start of charge is too small a threshold.Lots of chargers do dv/dt end of charge, because of a quirk of NiCd/NiMH whereby a fully charged battery actually drops slightly in voltage as you keep feeding current into it. I'm guessing this gives an earlier indication than thermal rise.Either way, do you need to cover the use-case of someone unplugging a fully charged battery and then plugging it back in again? Something to think about..

author: martys
author_rank: Regular Contributor
timestamp: September 18, 2023, 04:27:27 am
content: Catalina and Infraviolet: Precisely because of the known limitations and compromises of "commercial chargers" have I decided to design my own so as to not have these limitations.Geoff-AU: Other charge methods other than Delta T often fail to protect a NiMH battery from overcharging.I didn't consider using DeltaV charge termination when designing my charger because there is much said on the web about this technique, that is, DeltaV fails often enough and does not detect this deltaV condition (by even intelligent chargers) and this results in the damaging overcharge of a battery or premature termination of charging.I have seen many commercial appliances(i.e. handvacs) having destroyed cells by using continuous C/10 "safe, no-damage" charging. Not only must the user wait seemingly forever for a battery to charge, it does eventually damage(kill) most NiMH batteries over time.Fascinating.charging NiMH batteries is exothermic( according to battery mfg's data sheets). However, monitoring the temperature of many NiMH batteries has shown almost no detectable temperature rise during charging at a (C/3) fast rate until the cell is almost completely charged. False info?

author: IanB
author_rank: Super Contributor
timestamp: September 18, 2023, 04:53:25 am
content: I have attached a below a graph I made from fast charging an NiMH cell (at about the 0.8C rate).What you can see is the temperature rapidly starts to increase when nearing full charge. This indication also leads (comes before) the minus delta-V indication.So if you want to charge NiMH batteries fast and safely, you could look for a rapid increase in the dT/dt rate and use that as the end point.If you want to charge the cell to the max, you could put a feedback loop on the temperature and reduce the charging current as the temperature rises, to keep the temperature below some upper limit of your choice. This would allow you to do a reduced rate charge near the end rather than stopping the charge completely. You could then end the charge when the charging current goes below some lower threshold.This would be analogous to the CC/CV charging algorithm for lithium ion cells, except it would be a CC/CT (constant current/constant temperature) algorithm.If you had cells in poor condition, this would also charge them more gently since they would get hot much earlier and the charging current would be correspondingly reduced (early start of the CT phase). An optimum charging temperature would be in the 25-30 C range (80-90 F). The cells need to be somewhat warm to charge efficiently.

author: IanB
author_rank: Super Contributor
timestamp: September 18, 2023, 04:58:30 am
content: Quote from: martys on September 18, 2023, 04:27:27 amThe minus deltaV condition is reliable for good cells and well-designed chargers. However, many chargers are not "well-designed", even when they have a respectable brand name. Panasonic is one brand that seems to do well, in my experience.

author: martys
author_rank: Regular Contributor
timestamp: September 18, 2023, 05:16:47 am
content: IanB:A million thanksfor real data and also thanks for your insightful info instead of the conjecture perpetuated over the web!Even "well-designed" chargers can fail Delta V charging termination unless the user is careful enough to clean the cell terminals before charging.I am now trying a new charging protocol.C/10 charging for cells discharged below 1-V.C/5 charging for same cells until reaching 1.2VC/3 charging (fast charging) for same cells until reaching ~1.45VC/6 charging again until 2-DegC rise above start of charge temperature ends charging(start of charge temperature is updated if cell temp drops during charging).Charge current decreases to keep cell V to < 1.6V at any point in charging.Charge termination if cell/battery V rises to 1.55V at approx C/6 rate after fast charge(if not already terminated by 2-DegC rise).New Protocol: Advise not returning battery to charger station until user noticed degradation of operation.

author: IanB
author_rank: Super Contributor
timestamp: September 18, 2023, 05:52:22 am
content: Quote from: martys on September 18, 2023, 05:16:47 amBear in mind the temperature trace on my chart. The cell temperature rises from its initial cold state to a plateau after about 15 minutes, and remains there for most of the charge. This temperature rise is due to resistive losses inside the cell causing a small amount of heat dissipation. This is different from the end of charge condition where nearly all the supplied power is being rejected by the cell and is instead being converted to heat.As you can see, the temperature rose from the initial state of about 74 F to a more or less steady 80 F, which was a completely normal 6 degF rise. If you look at this rise, you will get a false early termination.So you will want to base your "start-of-charge" temperature on the plateau conditions, and not starting conditions. (Maybe establish the baseline temperature during the C/3 phase?)

author: IanB
author_rank: Super Contributor
timestamp: September 18, 2023, 06:01:02 am
content: Also note the voltage trace. For an Eneloop, a terminal voltage of 1.45 V occurs very early, almost at the beginning of charging.In general, the exact voltages exhibited by different kinds and types of NiMH cell during charging vary widely, such that looking at specific voltages is not helpful for a general purpose charger. Only if you know exactly what cell you are charging, and if you adapt the charger to only that cell, does the absolute value of voltage become useful.

author: martys
author_rank: Regular Contributor
timestamp: September 18, 2023, 06:32:25 am
content: IanB: Thanks again for the most insightful info and advice!I have failed to mention that I am using unusual cells.I am using now a rather unusual 1800mAH NiMH cell type that reaches only ~1.45V when near the end of charging. This is indeed a big difference from the Eneloop charge V characteristic and entirely unreasonable as a standard for fast charge voltage for use in a generalized algorithm.I have visited websites that show charts showing the charging behavior of many consumer chargers and most seem to agree that a rise to 1.55V signals a end of charge condition when the charge current is reduced to C/10 after a fast charge (or even if charged at a steady C/10 charge rate) I don't know why this is so well-agreed upon V in a dozen or more consumer chargers.

author: mikerj
author_rank: Super Contributor
timestamp: September 18, 2023, 07:07:29 am
content: Quote from: IanB on September 18, 2023, 04:58:30 amDelta V is already significantly less pronounced on NiMh than on NiCd, and gets worse as cells age and also with lower charging currents. It can be useful, but needs to operate in tandem with delta T and a suitable timeout for fully reliable charge termination. It's annoying that such a robust and inexpensive cell chemistry is so awkward to charge.

author: IanB
author_rank: Super Contributor
timestamp: September 18, 2023, 07:24:21 am
content: Quote from: mikerj on September 18, 2023, 07:07:29 amI think the awkwardness of charging comes from the need to look at temperature, and not rely solely on voltage. If you have a pre-assembled tool battery, then you can embed the temperature sensors inside the pack and ensure good thermal contact, but if you have a consumer charger for removable cells, it becomes difficult to achieve reliable and consistent temperature measurement.

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2023, 07:47:34 am
content: Why noy use both ΔV and temperature to terminate charge?

author: IanB
author_rank: Super Contributor
timestamp: September 18, 2023, 08:04:00 am
content: Quote from: Zero999 on September 18, 2023, 07:47:34 amMost chargers do. They typically place a temperature sensor like a diode against one of the battery contacts.

author: soldar
author_rank: Super Contributor
timestamp: September 18, 2023, 10:17:21 am
content: I always try to avoid fast charging because it shortens the life of batteries. My philosophy has always been "you do not need to charge fast, you need more battery capacity so you can charge slow". I have had great results with this system.

author: martys
author_rank: Regular Contributor
timestamp: September 18, 2023, 02:08:24 pm
content: The problem is not to charge slow or fast, but to not overcharge to make cells last.

author: martys
author_rank: Regular Contributor
timestamp: September 18, 2023, 02:14:57 pm
content: Karpouzi9: Suppose you have a piece of equipment whose battery space was made to fit the NiMH cells/battery cells. You can't just try to shove into it a Li-IdontCareWhatChemistry cell without instant regret.

author: radiolistener
author_rank: Super Contributor
timestamp: September 18, 2023, 02:48:25 pm
content: Proper NiMH chargers have temperature sensor, they using it to put charge procedure on pause when temperature raise above 45-50 °C, when battery cool down, it continue charge procedure. If you don't control temperature, your battery can be damaged. Also proper NiMH battery charger uses -dV condition to detect full charge state.Recommended temperature to charge NiMH is from +10 to +40 °C

author: macboy
author_rank: Super Contributor
timestamp: September 18, 2023, 02:51:29 pm
content: Quote from: martys on September 18, 2023, 04:27:27 amI concur. NiMH can't tolerate constant trickle anywhere nearly as well as NiCd could do.Quote from: martys on September 18, 2023, 04:27:27 amYes, NiCd charging is endothermic, and NiMH is exothermic. The opposite is true in discharge.Hobby R/C users have a lot of experience with abusive usage of sub-C size NiCd and NiMH cells. We would fast charge NiCd packs at about 4C for around 15 minutes. Yes, 15 minutes for a full charge. The packs could take that due to low internal resistance and endothermic charging. The pack remained cool, unless overcharged. Usually when the delta-V charger stopped the charge, the pack would be only slightly warm. NiMH is completely different. If you tried 4C charging on NiMH, it would heat up very quickly due to the exothermic charging, and generally higher internal resistance than NiCd. So 1C is about the max you can get away with. When I do 1C charging, I stop it at about 80% and switch to a lower rate (C/3) to finish. Otherwise the batteries get too hot for my liking. I've never used nor seen a hobby charger with delta-T end of charge detection, though many have a temperature sensor and a hard upper temperature limit which can stop charge if the usual delta-V failed to trip.Delta-T is a known good method of detecting end of charge, if you can monitor all cells well enough. You don't want to cook the weakest cell at the far end of the pack from the temperature sensor. At least delta-V does reflect the total voltage of all cells, even if you likely won't detect the first cell to finish.

author: Veteran68
author_rank: Frequent Contributor
timestamp: September 18, 2023, 03:22:17 pm
content: Just an anecdote related to NiCd vs NiMH charging. I have an old Norelco rechargeable shaver. I got it as a gift maybe 25 years ago, and I love that shaver. I'm not opposed to buying a new one, I just haven't found one that I liked as much. It may not last me much longer as I can no longer get genuine heads for it, only cheap knockoffs that aren't made correctly and don't work as well. So I have a little sharpening kit that I use on the heads every so often, but you can only take off so much metal by sharpening, so unfortunately its life is limited.Its battery pack consists of 2x NiCd AA cells welded together and soldered from the welded tab to the circuit board. Obviously not designed to be user-serviceable, but not too hard to replace. As you would expect, the NiCd pack develops memory effect and wears out relatively quickly. The original pack lasted maybe 3-4 years. For the first few replacements, I sourced equivalent NiCd packs and soldered them in, only to have to replace them again every couple of years. Eventually I read that some people had successfully replaced their NiCd packs with NiMH packs for longer life and less memory effect. I was skeptical that the charging circuit was designed to also support NiMH, but based on these reports of success I decided to give it a try, figuring worst case I finally destroy my shaver and have to buy another. It had certainly lived a longer and more productive life than the manufacturer intended.To my surprise, the NiMH upgrade worked REALLY well. This NiMH pack has lasted over 5 years (since May 2018, I just checked my Amazon order) and seems to show no signs of breaking down. Battery life is easily twice as long between recharges as the NiCd pack, and when I do recharge it, it doesn't get noticeably warm. I was a bit worried at first and so watched it carefully the first couple of charge cycles, and didn't leave it unsupervised overnight or all day while gone to work. The only downside is the estimated battery life on the LCD is wrong -- obviously calibrated or maybe even hardcoded to the OEM NiCd specs rather than monitoring battery voltage -- but I don't care about that. I now use it WAY past 0% reported life remaining, and know to recharge it when it starts lagging a bit.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 18, 2023, 03:49:08 pm
content: Charging Ni-MH is easy to do "roughly" and difficult to do "correctly".A problem is that the voltage needed to charge them changes with the age of the batteries. When they age the internal resistance increases and the voltage to charge them rises too. On old cells, you may need 2V to get some current into them.Then there is the minus delta V thing. This was one of the differences that was used for a "good" Ni-MH battery charger back in the '90-ies. But the way I see it, it is a secondary effect of the cell getting hot from over charging. And getting hot and overcharging are both bad things for batteries. Ni-MH is quite robust and can take some abuse, but less abuse prolongs lifetime, and thus, quick chargers are not compatible with long life.The "best" compromise is probably to use a moderate charge current, allow for a bit of initial warming, and stop charging as soon as the temperature rise at the end of the charge cycle is detected.