Best way to discharge single NiMH?
xanadu
Posts: 3,347
I've been having some bad luck with high capacity NiMH AA batteries lately. I find a lot of conflicting info on discharging using search engines. My charger recharges them in series, two at a time. I would like to discharge them individually to the same voltage then recharge them to see if it helps with longevity.
All that aside, in general, is there a best voltage to drop a single NiHM AA battery to?
Thanks.
All that aside, in general, is there a best voltage to drop a single NiHM AA battery to?
Thanks.
Comments
I have had good success using Sanyo Eneloops. They are the best rechargeable cells I have ever used. I also use a MAHA charger that charges each cell individually.
Sanyo eneloops are overrated and overtrended. A lot of companies offer the same for much lower price. Sony Cycle Energy series are also great. Recently bought Rayovac platinum, which also does fine so far.
NiMH can be stressed by excessive discharge causing a reverse polarity situation and by excessive temperatures. At some point, it is all about studying the product's quirks and buying a good charger... not the one that is included with the cells in a retail package.
Personally, I think a really good battery charger should monitor the temperature of the cell during charging... not just the electrical characteristics.
http://en.wikipedia.org/wiki/Nickel-metal_hydride_battery
In fact, my $4 chinese charger also does.
People do seem easily convinced to buy a "Fast Charger!" in preference to one that will charge overnight at a more battery-friendly rate
(which is how most batteries get recharged in practice) - if you have the intelligence to have a spare set of batteries there is no need for panic
recharging. Having said that every battery type is different and some NiMH will tolerate a 1 hour recharge a lot better than others, but
few manufacturers print useful information on the battery (and few consumers download the datasheet for a specific battery type).
The foible of battery manufacturers that gets me most is the lack of capacity indications on most non-rechargable batteries - this means
you have no guarantee of battery life at all, other than brand reputation. Still I've never worked out why people buy non-rechargables
in the first place, its like buying only paper plates when decent crockery is available...
But like Lithium, they can be discharged excessively and suffer damage when used in a series configuration. For most of us, that is a big snag as we don't have a way to monitor excessive discharge without adding some circuitry.
I generally buy non-rechargibles because they often provide more AmpHours than the rechargible.. and I may actually need a full 1.5 volts.
BTW If you WANT To do a proper discharge and want to invest in a tool that will be very usefull in the long run.
get a CBA ....... they are under 200 bucks and can do all kinds of usefull tasks .
(one is to determine the Real AH at a known load ! . and also the real AH of a primary cell )
western mountain radio has them .
A good charger (not a 2 or 4 cell cheapy from Walmart) is one of the best investments you can make if you own anything that runs off replaceable batteries. Expect to pay $50 or so (more if you need C and D size), though you can catch them on sale. Individual cell charging by Delta T, Delta V, along with discharge/recharge conditioning, etc. should be considered necessities.
Maybe this is yours CuriousOne?
http://www.amazon.com/La-Crosse-Technology-Battery-Charger/dp/B000RSOV50/
Not a bad deal on sale for $30 with "free shipping"
http://www.dxengineering.com/parts/wmt-58250-1014?seid=dxese1&gclid=CPO-gJTnubkCFWNxQgoddQUAGA
""
West Mountain Radio Universal Computerized Battery Analyzers (CBAs) are much more than simple voltage testers or battery load testers. These computerized battery analyzers test virtually any size or type of battery, of any chemistry or number of cells, up to 55 volts. Test anything from coin-cells to large lead-acid batteries! Now anyone can do scientific professional-grade tests of their batteries for a reasonable price!
By themselves, these CBAs are capable of tests up to a maximum of either 40 amps or 150 watts for short periods of time. ""
Universal and like a good scope and a good meter ...... a Tool for the serious tinkerer and Pro alike .
short of a BK precision 5 Grand programmable load , the CBA's are a good price for the stuff it can do ....
http://farm8.staticflickr.com/7147/6809359817_e5d8f071b9_z.jpg
We use them all the time for all sizes of batts ..
IF some one wants a good challange ! then make a OBJ that does CBA with a Prop1 .
I have a tube of 100 A rated fets that need a home .....:) let it dump to a CSV on a SD card an there will be a good market .
p.s. personally I believe, $200 is overkill for consumables such as rechargeable batteries. Will it last 300 or 1000 cycles, does that matter when 4 pack of 2000mah low self discharge AAs can be bought for $8 ?
I was initially wondering on the per cell fully discharge voltage though. I've heard so far that they can be discharged to 0v, .9v under a load, and .9v resting. That is the part that is conflicting. I'm not sure if the conflict is due to some people talking about batteries in packs vs single, or different manufacturers ratings. If they're all the same chemistry I wonder why some would recommend 0v over 0.9v. Again, I like to work with the individual cells, to balance them for use in series. I also keep the same batteries together, so if two go into a flashlight they don't get mixed around with two that go into an RC transmitter.
The other part is there are a lot of NiMH that claim they don't have a memory effect. If that is true then either my charge or discharge rate is too fast. I don't see how that is possible, usually the only place I use AA batteries is flashlights, ~200ma electronics projects, and my Spektrum transmitter. So maybe somehow I have just had bad luck with some of the cells I've been using. Most of the bad ones a Venom brand.
Anyway I want to try to refresh them. I think I'll drop a couple to 0v under a load and a couple to .9v under a load and cycle that with a trickle charge and see which ones act better. I think I paid around $2-3 each for some of these. The only thing I have to discharge something that low is a small motor which has been dropping one cell about 0.1v an hour so I guess I need to figure out how to drain current a little faster too.
What PN are these FETs? I'm looking for something I can use, and also had the P1 in mind.
I suppose newer NiMH have less of a memory effect, but I still want to discharge them to get an idea of their capacity either way.
that is what I use in the Batt lab at Oregon Tech .
Prior to Oregon tech I Was at UNI in Iowa ....... I made for them a 500 amp programmble load based on a golf car speed con and a huge water cooled resistor .
the speed con took a 0-5V to controll it ....
let me draw up a shematic for yall of the " mega load . " ...
It should be easy to scale..
We used a NI Lab DAQ .... but a prop can be easy for a stand alone model
Attachment not found.
The Sanyo Eneloops are the only cells that I have purchased which have been consistently good - out of many brands - Kodak, Energizer, Panasonic, Everready, Rayovac and other that I cannot recall right now, even after long periods of non use. I rate them better than the rest and do not believe that I am over rating - it gains me nothing to do so. I have used them for years and so far not a single one has failed. All of the others have. I don't know how their cost compares to some others but it is a moot point when the other ones do not function.
The Energizers were not too bad in my experience, but not as good as the Eneloops. I have not tried the Sonys.
-Phil
With a single NiMH, not in series ... it seems that nothing special has to be done. This is discharge.
On the charging side,most charges seem to charge in parallel which effectively distributes the current to something less than 100% between the cells involved. I suppose charging in series is as acceptible if the charging parameters are right.
Unlike Lithium cells that have a minimum discharge voltage even for single cells, the NiMH seem willing to go to 0 vo;ts, it is a reverse polarity condition that causes damage.
NiMH in series can have one cell discharge faster than others, and that is the source of a 'reverse voltage condition'. The 0.9 to 1.0 volt discharge limit is the accepted method to avoid such damage in the context of series discharge. This is similar to what Lithium cells require in terms of monitoring.
%%%%%
Implications.
If you need more than the NiMH 1.2 volts for an application, you might consider using Lithium cells as an alternative.
a. You have at least 50% less cells to monitor for a low voltage shut down.
b. There are situations where one Lithium cell can replace 2 NiMH. It is always easiest to monitor one cell without the series situation.
All newer rechargible batteries in series are more complex than older chemistries. Higher voltages might be better be accomodated via a Boost regulator.
In particular, NiMH and Lithium cells are sold BOTH with low voltage shut down (a good product) and without low voltage shut down (a not so good product). I have been stuck with rechargible batteries that are completely dead after one use due to the lack of low voltage protection. Try to ask for and buy cells with the protection built in.
I am uncertain the so-called memoory effect exists in any battery chemistry at this point. It justifies doing a lot of odd things: a. full discharge before charging, b. zapping dead cells in attempts to revive their utility.
The main thing is that the whole concept of memory effect started with NiCd batteries, not NiMH. But these days, the RC crowd may prefer the NiCd for tolerating deep rapid discharge in series that the NiMH and Li ions require a correct low voltage cutoff for.
Consider the fact that the vast majority of recharible cells are thown on a charger with no idea of that actual state of charge. When we need rechargible battereis we usually start by topping off ones that have been sitting in a drawer for months.
Say you have battery with 10ah capacity. You constantly discharge it with 10A current. After say 5-6 cycles, you'll note that capacity dropped and remains on 40-60% of initial capacity. You have to apply several cycles of low current charge/discharge, to regain capacity.
I think I will switch brands as well. W9GFO and Phil both mentioned batteries that seem to be popular in aviation. For the price, the Venom batteries I have don't leave much choice. They are high capacity so I expect them to be more sensitive and have less cycles, but they are not even close to living up to published spec and I'm only trickle charging. The highest load I've ever put on them is a couple LEDs and BS2 or P1 so I know I'm not abusing them.
I would also like to reiterate that aside from cycling due to memory effect which may or may not exist, my main purpose to discharge is to measure capacity, like an occasional audit.
Peter has some skills in this area and I can definitely foresee a Prop controlled programmable load controller popping up in the project section soon. I would like to be able to load test server power supplies, and recondition SLA UPS batteries as well. This could turn out to be pretty cool and hopefully remain simple enough to replicate easily.
I appreciate the comments and feedback, thanks!
We know that NiMH will not tolerate a reverse polarity, and that onlly happens with discharge in series.
I think there is some lack of clarity in what the documents suggest. You may have been figuring a voltage cut off by assuming that all the cells discharge evenly. They don't.
I think you have to make sure none of the cells except one in series ever go below 1.0 volt, and that maybe going to 0.9 volts is dangerously low in an uneven discharge (where one cell might go to zero and the others stay fully charged).
The solution is to look at an extremely asymentrical discharge of a series of cells, not an even discharge -- an set the limits according to that scenario.
Without active sensing and controls, that would mean a 4 x battery pack that provides a usual 4.8 volts should never go below 3.8 volts. Or that a 6x battery pack that is providing 7.2 volts should never go below 6.2 volts.
Lower voltages than those limits can work in combination to create a reverse polarity condition on one cell. It is difficult to anticipate which cell arrives first in a race to the bottom as internal resistances are not always similar and can change with use.
You may already have one cell that preventing the others from a full charge. The only way to tell is to test and charge individually. I am not sure if NiMH fail to a short condition or an open condition. (Lithium ion can fail to a short).
If one cell is shorted, it will allow others to charge .. but it won't. If you are using a series configured charger, that means the remaining cells are damaged via an over-voltage during charging. So it is best to only charge cells in parallel, never in series.
++++
What does this mean? Well with 4 cells you might have more leaway for discharge, and with 6 cells you may have less if you were averaging 1.0 volts per cell.
NiMH also seem to be a bit more temperature sensitive for overheating than other chemistries. So quick charging and quick discharging might be another source of damage.
+++++
The more I look at them, the newer rechargible battery chemistries just don't behave well when used in series. The more cells in series, the more ways they can wander outside safe limits. One cell is best, two cells is okay, and it gets harder with every one you add.
The only think I'm still not really clear on is what voltage to drop a single NiMH cell to. Under load they drop to 0v, when you remove the load they're at around 0.9v. I have been using that to "re-condition" my "bad" cells over the weekend and it seems to have helped quite a bit.
What I wonder is for how long it take to get from 1.0 volt to zero; and how much useful power is really gained. The issue with NiMH and Lithium Ion cells is that construction has physically gotten into the realm of microscopic, with thin films coating the NiMH center electron and Lithium Ion made of thin sheets of metal wound into a package. These drop-offs can cause damage very quickly, especially under a heavy load.
This means it does not take long for a situation to damage the barriers that make the batteries work well. All manufacturers are aware of the problem, but some are likely more concerned with durable higher quality product. We really don't know who is doing what unless groups of satisfied users share their experiences... advertising means next to nothing.
I guess the real engineering answer with NiMH is to never discharge to 0.0 volts. That may be feasible with using only one cell. But a practical, risk based approach might be to set an arbitrary higher limit such as 0.2 volts or 0.5 volts.
That approach is rather silly as the the NiMH cells have a very flat middle and two dramatic tails in their charge and discharge curves. So little power is harvested by going from 1.0 volt to 0.2 volts that it isn't worth the risk and additional burden to managing a cut off.
In other words, batteries in use seem to do better if they are not fully discharged. And they likely survive longer if always fully charged. (But that is a bit of nonsense as you have to discharge something to use the battery).
About all I can figure is that the lithium cells have less problems with living a yo-yo lifestyle of occasional deep discharges combined with frequent topping off of lighter discharges. Some Lithium cell chemistries, like Li2PO4 claim to improve with age.
But all that is at the 'cell level' and doesn't get away from the fact that BOTH NiMH and Lithium cells suffer greatly when you wander too far from down the tail at the end of the discharge curve. SO limits are usually set by where the flatness clearly ends, and NOT by how close to zero one can go.