Charging NiMH cells with low current?

Hello,

Was anyone successful with charging their NiMH cells
with low current (such as a C/10 rate) and if so, how
did you detect end of charge?

With relatively high charge rates (C/2 to C/1) you
can try to detect changes in the voltage curve when
charged with a constant current (such as minus delta V)
but with low currents this wont show up, so any ideas
out there?

Take care,
Al

MrAl, I have a maha charger (2A4?) that is intended for slow charging cells. IIRC, it does have 4 independent channels & charges at either 50ma (4 cells) or 90ma (2 cells). I primarily use it for topping off or equalizing cells but I have ocassionally fully charged dead cells by leaving them on all week end.
I think this is a good charger for someone just starting out with recharging batteries. It is inexpensive ($7-8) and fairly foolproof. I personally don't like to leave NiMH cells on charge indefinitely, but with this low charge rate a week or even a month shouldn't hurt.
My experience with rechargeable batteries (primarily nicad) over the last 30 years leads me to believe that leaving them on the charger full time does not give the longest life. But with a low charge rate (max C/10 for NiCad or C/20 for NiMH), life isn't hurt much. -RussH

I echo what Russ said. At sufficiently low rates, you're in the trickle against self-discharge regime. I would not feel comfortable leaving cells on 0.1C for a month. A few days, a week or two maybe depending on the size of the cell. As always, check for heating or distortion. In fact, dT/dt is an alternate method of charge termination. For example, while my Triton charger has a TC input to cut off charge at a maximum temperature, a more sophisticated charger may sense dT/dt and terminate at 0.5-0.9deg C/min.

Charge timing is another backup safety as is ultimate temperature.

Note: The Triton has adjustable -dV detection and it has successfully detected peaking at a rate of 0.1C for 3.5Ah nimh 1/2D cells.

Wilkey

Hi MrAl,
How quickly do you want to charge 'em?

I float some NiMH AAs off a small solar charger at very low rates -- about 50mA while the sun shines -- and don't worry about termination. Some NiMH mfrs say not to trickle at all, while others suggest 0.05C is okay. 50mA into a 2AH cell is 0.025C, which hopefully is okay.

I measured the self-discharge of a Panasonic 1,600mAH cell at 1.6mA (=2.4% per day), so the 50mA rate applied for 1/3rd to 1/2 of the day (while the sun shines) *is* enough to slowly, gently, load and top the cells off. Keeps them ready for the digicam.

Jamaica

Hello,

Hey thanks for all the great replies guys!

I do appreciate it greatly, and will pass the info on.

My main concern was if charging for maybe C/10 or C/20
would it be a problem if they OVERCHARGED for say 24 hours?

Sure it would be a slow rate and take some 36 hours to
charge cells (sometimes it doesnt matter) but as long
as they dont overcharge because of some time charging
longer then required. I've read that it's not good
to overcharge NiMH cells (worse than NiCds) but there
was no detail on exactly what is considered dangerous
for the life of the cell.

So i guess C/20 is considered OK if they arent left on
TOO long after they're charged fully?

Take care,
Al

[ QUOTE ]
MrAl said:
My main concern was if charging for maybe C/10 or C/20
would it be a problem if they OVERCHARGED for say 24 hours?

[...] I've read that it's not good
to overcharge NiMH cells (worse than NiCds) but there
was no detail on exactly what is considered dangerous
for the life of the cell.

So i guess C/20 is considered OK if they arent left on
TOO long after they're charged fully?


[/ QUOTE ]

Brushing up on the datasheets, I think a 24hr OVERCHARGE @ C/20 (= >200% of capacity) would compromise the cells' longevity. DuraCell has an excellent treatment of NiMH charging here: http://www.duracell.com/OEM/Pdf/others/TECHBULL.pdf.

For timer-based chargers @ C/10, and -dV=10mV/cell charging at C/4, DuraCell's recommendation is to limit charging to 120% of capacity. The problem is excess evolution of gasses vs: the recombination rate within the cell. If you make gas faster than it's re-absorbed, the cell eventually vents, which is bad. I seem to remember that NiCads can absorb the gas produced by a 0.1C continuous overcharge, hence the common use of 0.1C charging rates. Similarly, I remember (but may be wrong!) said rate for NiMH as being 0.05C.


The Energizer NiMH applications data is here: http://data.energizer.com/batteryinfo/application_manuals/pdfs/nickel_metal_hydride.PDF. There's a great graph of charging characteristics under charge, including internal cell pressure, on pg 17.

DuraCell recommends trickle-charging (for float purposes) at 0.003C (section 6.3.5).

Energizer's info says:

[ QUOTE ]
Finally a maintenance (or trickle) charge rate of 0.025C (C/40) is adequate to counter self-discharge and maintain cell capacity.

[/ QUOTE ]

(Sorry for the long URLs -- I haven't figured out the UBB code to give 'em shorter titles yet.)

Best,
Jamaica

For slow rate charging, -deltaV doesn't apply correctly. There is no easily detectable voltage drop after the end-of-charge ramp-up.
At C/10, you simply interrupt charging at 1.41 Volt at 20 Celsius. You also need a timer to take into account a defective cell or temperature problem.

Anthony

[ QUOTE ]
TheProphet said:
For slow rate charging, -deltaV doesn't apply correctly. There is no easily detectable voltage drop after the end-of-charge ramp-up.
At C/10, you simply interrupt charging at 1.41 Volt at 20 Celsius. You also need a timer to take into account a defective cell or temperature problem.

Anthony

[/ QUOTE ]

Yes, I imagine that's why Energizer's applications data only shows -dV=10mV termination down to C/4 rates, while DuraCell (section 6.2.2) says -dV "...may be absent in charge currents below the C/3 rate, particularly at elevated temperatures."

dT/dt sensing is the termination preferred by both vendors; dV/dt-sensing methods are slower, resulting in greater overcharges.

Fig. 19 of Energizer's data shows the voltage profile under various charge rates. It doesn't look like the voltage ever quite reaches 1.4V at the 0.1C charge rate. For this reason, and because of temperature effects on the cell voltage, I'm not confident 1.41V is a good termination criterion. Might be, I'm just not sure.

Something that does look pretty reliable at low charge rates is that the cell voltage plateaus (stops increasing) once the cell is full. Maybe this would be a good measure?

Best,
Jamaica

Hello again,

Thanks for those references Jamaica.
I've checked them both and found that they each recommend
different trickle charge rates

What i am after is the simplest charge circuit for
NiMH cells even if it takes over 24 hours to charge
them. Of course damage to the cells has to be minimal.
This is why i was looking into alternatives to measuring
various parameters like delta v and delta T.
Both manufacturers seem to imply that any overcharging
damages the cell, but then they both say trickle charging
is ok. They even seem to say C/10 rate is ok for a little
while. Perhaps C/20 or C/30 will be ok then.
The main issue is that when the cell is recharged it's not
possible to know the 'state of charge' beforehand, so you
never know exactly how long to charge for unless maybe you
make sure the cells are completely discharged first, but
it may be even hard to get that if they are in a series
pack.

The way i understood it, using voltage level detection
even with added temperature measurement isnt the right
way to terminate charge because there are too many
variables that influence the voltage level measured
across the terminals...either you end up with too much
charge or not enough. I guess a little experimentation
might reveal some interesting results however.
Is the attainment of 90% of full charge really that bad?
Probably not, if the design of the charger is only a
small bridge rectifier and series resistor
And just how damaging is 20% overcharge?
Might be hard to answer these questions.

Take care,
Al

Greetings.

You can see that they don't make it easy(!), but my best understanding
is still that NiMH can handle a continuous 0.05C overcharge without venting.
To be safe, the solar panel floats mine at .025C for about half of each day.

The self-discharge current I measured (just one sample) was 1.6mA for
a Panasonic 1,600mAH cell, amounting to a self-discharge rate of 0.001C
at room temperature.

Even DuraCell's 0.003C rate handily exceeds this loss, and so would slowly
charge cells. Energizer's recommendations allow "... a maintenance (or trickle)
charge rate of 0.025C (C/40) is adequate to counter
self-discharge and maintain cell capacity."

Interesting tidbit: I just charged a pair of (older) 1,200mAH
NiMH AA cells. The cells were partially discharged, and were charged
with 100mA (=C/12) for six hours before voltage plateau and then -- you
guessed it -- a -dV of about 20mV! My pocket computer did this & I
tried to save the data to post here, but biffed it. Oops. I'll try again
later.

Best,
Jamaica

Jamaica, thanks for that info. I personally haven't had any real problem with trickle charging rechargable batteries, except that I do think even the 10% (NiCad) or 5% (NiMH) capacity limit for full time charging does shorten the life slightly.
Don't we all leave our electric razors on charge all of the time? Those are probably all NiCads, but I've gotten the normal 10 years life out of dozens of batteries so employed. Lots of tools, portable phones (still NiCads) and cell phones (finally mostly NiMH) stay on chargers most of the time.
I've still got my little LM317 current regulator circuit with resistors for 50, 100, and 350ma for this sort of thing, and testing LEDs or luxeons. It costs about 50 cents to set one up for 100ma or less, and you can probably fit it all inside any wall wart you can get into. You can use a 12v wallwart, even if it is actually 21v like some of mine, with no concern about the voltage requirements of your load. It will get 100ma, whether it likes it or not, at whatever voltage it takes to get 100ma, whether it is 2 volts or 24v.
The LM317 won't boost, so max voltage is determined by the power supply. The LM317 can go up to 37v, 1.5 amps with a proper heat sink, but it needs about 2 volts over what is required in order to regulate properly. It's hard to beat for the under $1 total cost, including the standard 3 sq. in. beverage can heat sink, wire, solder, etc. HTH,
-RussH

Howdy Russ,
What's not to love about that versatile LM317? Yes indeed. National
Semiconductor has an old applications note on battery chargers that shows
several clever LM317-based chargers -- I'll see if I can find it. I think
one has a declining current limit as the cell voltage increases, ideal
for getting the cell up then floating it.

Like you, I've gotten years of service from devices that float Nicads gently,
and I've also had a few devices that kept their batteries a little warm. These
latter devices used up their batteries every two years or so.

In short, my understanding and personal experience match yours: continuous
overcharge takes a toll, but if the overcharge rate is modest, so is the toll. ;-)

Best regards,
Jamaica

Ok so I know this thread is old, really old, but I think it still warrants some discussion with the advent of the LSD cells.

Has anyone solar maintenance charged the newer LSD cells?

What have been your experiences?

Or has anyone done a maintenance charge of LSD or "regular" nimh for the past several years?



I want to go the simple solar route with some resistors.
Currently I have 4 AA batteries with a solar cell that puts out about 9.5v and 22ma when it is brightest and then drops down to just above 6v and a few ma later in the day.

I think it will be ok but just don't know about the long term effects?

solarbaby said:

Or has anyone done a maintenance charge of LSD or "regular" nimh for the past several years?

Currently I have 4 AA batteries with a solar cell that puts out about 9.5v and 22ma when it is brightest and then drops down to just above 6v and a few ma later in the day.

Click to expand...

That sounds like it would be safe, certainly for regular NiMH cells. I've heard that LSD cells should not be trickle-charged for long periods of time, so it could cause some damage over the long-term.

Your current looks similar to the 10ma trickle charge that the Maha C9000 charger uses after finishing a normal charge. If there's anyone that leaves their Eneloops in their charger for weeks, perhaps they could answer your question. I'm too paranoid about my cells to trickle-charge for long periods of time, usually it's just a few hours at most.

The rule-of-thumb for trickle charging used to be C/40 or less, or about 50ma for a 2000mAh cell. I think that applied to non-LSD technology, so LSD is probably a lot less than that. An Eneloop is only going to lose an average of 1mAh of charge per day, or less. Maybe more during the first several days after a full charge, but nowhere near the rate of a trickle charge.

Thanks you might be right that those guidelines are for the non lsd batteries.

Has anyone been trickle charging NiMH batteries for a long time?

Is Jamaica still on the forums?