Safest charge rate to avoid meltdown?

I've been searching and reading threads for two days here in the battery subforum, and while AA charge rate has been addressed in many threads in regards to battery longevity, I haven't seen a very definitive answer from the point of safety. On one side of the spectrum, people are warning that too low a charge rate could result in too small of a voltage drop, and that termination will be missed, causing overcharging/overheating. One the other side of the spectrum, people are warning that charge rates over 1,000 mA on some of the popular chargers (like the BC-900) can overheat just from the charge rate itself.

So where do you strike the balance?
I see 0.5c-1c being kicked around as a range that is best for batteries, but I'm specifically wondering what you see as the safest charge rate in terms of avoiding meltdown, fire, etc?

Apologies if I missed a thread that already covered this in detail. If so, I'd appreciate any relevant links.

You can charge indefinitely at 0.1-0.2C rate w/o a melt down.

A charger with reliable thermal cut out can stop melt down as well. I've not had single melt down with a Duracell 15 or 30 min charger, but I've had some shrink wrapper distortion from overheating on the BC-900.

Hello TOTC,

The safest charge rate, with respect to cell temperature, is 0.1C. It takes longer, and works best if you do a full discharge first so you have a good starting point to base your charge time on. The charge is terminated with a timer.

Tom

Thanks, guys: I appreciate the quick responses

Handlobraesing said:

You can charge indefinitely at 0.1-0.2C rate w/o a melt down.

A charger with reliable thermal cut out can stop melt down as well. I've not had single melt down with a Duracell 15 or 30 min charger, but I've had some shrink wrapper distortion from overheating on the BC-900.

Click to expand...

So if I charged my 2500 mAH AA batteries at 250-500 mA indefinitely, there'd be no meltdown? I guess that even if they didn't get hot, that this would still not be the best thing to do. I've always wondered how long you could leave NiMH batteries on trickle charge before hurting them...anyone know? For example, my charger trickles at 50 mA....how long could I safely leave them on the charger after the main charge is done?

Thanks for any info

Im confused again.Assume I am charging eneloops exclusively and in no rush.Are you all saying a rate of 2000 amps (1.0c ) 0r as low as 200amps (0.1c) are both ok ? Would the same answer apply using the BC-900 , the maha C9000 or the maha 800S .Please forgive my senility

Hello FlashCrazy,

Correct. There would be no melt down when charging at 0.1C.

As a matter of fact, GP mentions that in mission critical applications, such as emergency lighting, you can leave the batteries on a 0.1C charge for a year.

In general, it is not recommended to leave batteries continuously on a trickle charge. Constant charging warms the batteries slightly and tends to dry out the electrolyte, however this happens at a very slow rate. It also does something with the crystal formation within the cell, and can hamper performance.

A general rule is to only leave cells on the charger for less than 24 hours.

Some chargers shut off after 24 hours, and others have such a small rate of trickle charge that it may be OK to leave them on the charger for extended periods of time, but I still don't recommend it.

The best advise is to pull your cells a few hours after the charge is complete and use them.

Tom

I thought nicads could handle C/10 indefinitely but nimh cells weren't quite so tolerant. Anyone know for sure?

Thanks - Greg

greg_in_canada said:

I thought nicads could handle C/10 indefinitely but nimh cells weren't quite so tolerant. Anyone know for sure?

Thanks - Greg

Click to expand...

http://www.batteryuniversity.com/partone-11.htm about two-thirds down the page.

I've found the Sanyo site to be a good source of information:
http://www.sanyo.com/batteries/lit.cfm

In the Engineering Handbook for NiMH, here:
http://www.sanyo.com/batteries/pdfs/twicellT_E.pdf
they confirm Silverfox's 0.1C charge rate (terminated by time), but they also mention a weird trickle charge pulse method. It's in the last section of Chapter 3, in section 3-2-3.

I always thought a trickle charge was a 1/20 C (0.05 C) charge rate. But that doesn't line up with a traditional (NiCad) trickle charge time being 15 or so hours---at 1/20C, you'd have to charge for at least a day. I'd assume 15 hours would be a charge time for a 0.1C charge.

For NiMH, the Sanyo handbook seems to say terminate at 13 hours of 0.1C charging.

It also says that only specific NiMH cells are designed to take 0.1C charge rate continously, but that you should contact them for information about that. This is in section 3-2-2, and most of that paragraph says you're shortening battery life with continuous 0.1C charging---"continuous" meaning not terminating the charge.

Hello Bp,

The question was "What is the safest charge rate to avoid meltdown?"

The number of meltdowns when charging at 0.1C is nonexistent. The odds of having a meltdown when charging at 1.0C is related to the cells condition and how well the termination algorithm works. It is possible to have a meltdown when charging at 1.0C, so it is not as "safe" as charging at 0.1C.

The NiMh battery manufacturers recommend using a charge rate of 0.5C - 1.0C when using negative delta voltage as a termination method. If you use another method, you can charge outside of that range.

For example, if you have a timer that is very reliable and never fails, you can use time to terminate the charge. However, you will also have to figure out a way to know how much charge is left in the cell to determine the right amount of time to charge. One way around this is to fully discharge the cell before every charge.

Another way is if you can insure that you have good thermal contact and an accurate temperature probe, you can terminate on temperature, but the temperature termination rates are modeled using a 1.0C charge rate. You will have to do some testing to see what values to use when you charge at different rates.

Looking at the chargers you mentioned, the BC-900 and Maha 800S, I believe, both use negative delta voltage as the termination method. With those chargers you should follow the manufacturers recommendations and charge in the 0.5 - 1.0C range. The C9000 started off with negative delta voltage, then improved the termination algorithm. It seems to use peak voltage detection of something like that. The termination signal is stronger following the 0.5 - 1.0C charging rate guidelines, but we have been testing the C9000 at lower rates. So far, we have not had a single termination failure. Does this mean that we should tempt fate and charge everything at the lowest rate we can? If you can live with the consequences of a missed termination, go for it.

I now have cells that will not properly terminate on the BC-900, but do terminate on the C9000 when charging at low charge rates. However, in spite of these results, I still try to stay at a minimum of 0.5C.

Now, to further confuse the issues, and to play havoc on your "senility," :devil:
I have an Eneloop charger, designed for Eneloop cells. It is a 4 slot charger. I have no idea what it uses for termination, but when I put cells in, it terminates. The charge rate is 300 mA for AA cells, and 150 mA for AAA cells.

I just wanted to clear thing up and thought that would help... :devil:

Tom

Thanks. So it seems like C/10 is a bad idea with nimh.

Quote "Nickel-metal-hydride should be rapid charged rather than slow charged. Because of poor overcharge absorption, the trickle charge must be lower than that of nickel-cadmium and is usually around 0.05C. This explains why the original nickel-cadmium charger cannot be used nickel-metal-hydride. "

Second quote "It is difficult, if not impossible, to slow-charge a nickel-metal-hydride. At a C?rate of 0.1-0.3C, the voltage and temperature profiles fail to exhibit defined characteristics to measure the full charge state accurately and the charger must rely on a timer. Harmful overcharge can occur if a partially or fully charged battery is charged with a fixed timer. The same occurs if the battery has aged and can only hold 50 instead of 100% charge. Overcharge could occur even though the battery feels cool to the touch."

Greg

oldvultureface said:

http://www.batteryuniversity.com/partone-11.htm about two-thirds down the page.

Click to expand...

Hello Greg,

If you are worried about the safest rate for charging, 0.1C is the rate to use and is not a bad idea.

If you are using a charger that utilizes negative delta voltage termination, then 0.1C charging is a bad idea.

Tom

greg_in_canada said:

Thanks. So it seems like C/10 is a bad idea with nimh.

Quote "Nickel-metal-hydride should be rapid charged rather than slow charged. Because of poor overcharge absorption, the trickle charge must be lower than that of nickel-cadmium and is usually around 0.05C. This explains why the original nickel-cadmium charger cannot be used nickel-metal-hydride. "

Second quote "It is difficult, if not impossible, to slow-charge a nickel-metal-hydride. At a C?rate of 0.1-0.3C, the voltage and temperature profiles fail to exhibit defined characteristics to measure the full charge state accurately and the charger must rely on a timer. Harmful overcharge can occur if a partially or fully charged battery is charged with a fixed timer. The same occurs if the battery has aged and can only hold 50 instead of 100% charge. Overcharge could occur even though the battery feels cool to the touch."

Greg

Click to expand...

Some of this is due to you cant SMART charge TERMINATE, with the slow rate.
that is why this stuff is so hard to explain sometimes. sometimes they are discussing smart chargers and termination via a overcharge state
and sometimes they are discussing slow dumb chargers that never send the battery into an overcharge it cannot HANDLE.

and i bet that didnt improve the situation.

it also helps to know if your talking single cells individually charged, or a series pack , or even a series parellel pack, each would be slightly different.

we have replaced many Ni-Cd battery packs in dumb slow charging devices, with 0 problems VRSES the Ni-cd they replaced.
these devices sit endlessly on a very lite charge, and lasted LONGER than the original ni-cd batteries they replaced.
after years of use they have depleated in total capacity, but they still work better than the ni-cds they replaced.

I dont understnad how they aquire the data on the present technology cells for this type of information, when after years of doing a slow rate charge, these cells are still in the devices that use them, still working , lasted longer than the ni-cads. still last longer than the ni-cads, and seem like they will go on with this "bad" method for more years.
How do you accelerate the slow charge rate and Years of that?
Well
you actually TEST the cells and see if they still HOLD ANYTHING.

ok so these cells probably suck by now, but that does not explain how the single cells that used the other method, were thrown away because they were not usable.

only one thing to do, test them. and its ALL your fault they are sitting there doing thier job just fine, but now i need acurate data. man i hate that.

this reminds me of "20 year paints" that are tested by acclerated UV exposure and baking in a oven. you dont know if its gonna last on your house till you put it ON your house and need to repaint in 7

0.1C is perfect safety wise, but overcharging is still bad for cell longevity.

Handlobraesing said:

0.1C is perfect safety wise, but overcharging is still bad for cell longevity.

Click to expand...

this is what it gets down to

ok so which is worse? using a fast charger, that must damage the cell to test for termination, BY overcharging the cell till it vdrops.
OR
Continually topping off the cell at a rate that is Sooo low, that it never goes over the specs for overcharge of that cell?

one is purposfully blasted past overcharge levels, and one stays within the rate, and just continually assaults the cell.

and
if your going to be smart charging, do it at a rate that IS fast enough to cause a really visable Vdrop, so the stupid computers can see it. .3C and up.

If something goes wrong with the fast rate smart charger, AT the rate you need to charge it at to achieve "smart" charging, its going to reek havoc on the batteries.

If the power goes down with a smart charger, and it RESETS to some stupid rate for your cells, then it aint smart. and a UPS (uninteruptable power supply) could come in handy.


--------------

if your not going to "smart" charge, do it at a rate that is sooo low, its well within the specs for the batteries "overcharge" rate .1 and down

If something goes wrong with the dumb slow charger, it just sits there and the rarity of the occurance of a meltdown with the slllloooooowww rate makes it rather safe. that is why consumer junk had used that method for eons (many many years)

a slow rate of charge could be concidered more safe by far.
just like a pencil and paper is safer than a computer