# NiMh charging formula

Started by October 25, 2014
```I have a "dumb" NiMh/NiCd charger.

For 9 volt batteries, it specifies 25 milliamps.

The charger manual gives this charge formula.

Charging time(h) = 1.2 x battery capacity (mAh)
--------------------
charging current (mA)
13.44 hrs.

Is is formula useful or is there something better that factors in the starting Voltage ?

Thanks.

```
```Andy K wrote:

> I have a "dumb" NiMh/NiCd charger.
>
> For 9 volt batteries, it specifies 25 milliamps.
>
> The charger manual gives this charge formula.
>
> Charging time(h) = 1.2 x battery capacity (mAh)
>                    --------------------
>                          charging current (mA)
> 13.44 hrs.
>
>
> Is is formula useful or is there something better that factors in the starting Voltage ?
>

** The "starting voltage" is almost meaningless - with NiMh and NiCd cells it is no indicator of a cells state of charge.

The formula applies to charging a *flat* battery, but long as the computed time is more than 12 hours some overcharging is not harmful.

...   Phil

```
```On Sat, 25 Oct 2014 16:36:50 -0700 (PDT), Andy K
<andrewkennedy775@gmail.com> wrote:

>I have a "dumb" NiMh/NiCd charger.
>
>For 9 volt batteries, it specifies 25 milliamps.
>
>The charger manual gives this charge formula.
>
>Charging time(h) = 1.2 x battery capacity (mAh)
>                   --------------------
>                         charging current (mA)
>13.44 hrs.
>
>
>Is is formula useful or is there something better that factors in the starting Voltage ?
>
>Thanks.
>
None that I know of...
Two popular methods of charging NiMH batteries are Delta V and Delta
T.
Delta V - terminates the charge when the cell reaches full charge and
the terminal voltage rise rate increases. the technique is prone to
noise and may terminate earlier.

Delta T - terminates the charge at a 1degC rise rate. You can also
terminate the charge at 35C. Dont exceed the max cell temp.

Overcharging a NiMH may cause the cells to gas Hydrogen, as the
hydrogen no longer is absorbed into the cell. This happens at charge
rates above 0.1C acording to the wiki.
<https://en.wikipedia.org/wiki/Nickel%E2%80%93metal_hydride_battery>

Adding a thermistor to the charger  and terminating the charge at 35C
would not be a bad idea, since your above the 0.1C rate for the 9v
battery ( assuming its a 200mah )

Cheers

```
```On Sat, 25 Oct 2014 16:36:50 -0700 (PDT), Andy K
<andrewkennedy775@gmail.com> wrote:

>I have a "dumb" NiMh/NiCd charger.

You have my sympathy.

>For 9 volt batteries, it specifies 25 milliamps.
>The charger manual gives this charge formula.
>Charging time(h) = 1.2 x battery capacity (mAh)
>                   --------------------
>                         charging current (mA)
>13.44 hrs.
>Is is formula useful or is there something better that factors in the starting Voltage ?

This should explain how it works:
<http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride>

The problem is determining the EOC (end of charge) point.  It can't be
done using the battery voltage because that varies with the age of the
battery.  As you deduced, the starting condition is also important.
For example a half depleted battery would only require about half the
calculated charging time.  Were you to insert a fully charged battery
into this "dumb" charger, and then charge it for 13 hrs, you would
probably be overcharging it.  It might be possible to safely trickle
charge the battery, but I've had no luck with that method.

NiMH and NiCd batteries also become hot only when overcharged.  Some
chargers determine the EOC point when they detect a rise in battery
temperature.  That doesn't work because by the time the battery gets
warm, it's already overcharged and on its way to the dead battery bin.

So, you're left with two choices.
1.  Be very careful about knowing the state of charge of your battery
and controlling the charge time.  One accidental overcharge and your
2.  Invest in a proper "smart" charger.
<http://www.mahaenergy.com/chargers/>

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
```
```On Sat, 25 Oct 2014 20:45:38 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

>One accidental overcharge and your

Not true.

Early NiMH batteries were quite intolerant of overcharging, but modern
batteries are much more robust. Not quite as robust as NiCd, though.

In fact, some amount of overcharge is necessary in order to keep the
cells in a battery balanced.
--
RoRo
```
```On Sat, 25 Oct 2014 16:36:50 -0700 (PDT), the renowned Andy K
<andrewkennedy775@gmail.com> wrote:

>I have a "dumb" NiMh/NiCd charger.
>
>For 9 volt batteries, it specifies 25 milliamps.
>
>The charger manual gives this charge formula.
>
>Charging time(h) = 1.2 x battery capacity (mAh)
>                   --------------------
>                         charging current (mA)
>13.44 hrs.
>
>
>Is is formula useful or is there something better that factors in the starting Voltage ?
>
>Thanks.
>

Check the manufacturer's data for much more detail.

To determine end of charge there are two methods- one is that the
temperature of the cell is measured and the other is to detect a
slight *decrease* in cell voltage with time (which I think is actually
a side effect of cell heating).

For either of those to work at all you need to have enough current to
get signficant heating in the cell, 25mA won't cut it.

There's usually some other conditions about not starting high current
charge until the per-cell voltage is within range, and shutting down
if the voltage gets too high or too low. Also a top-up charge is
sometimes called for after the high-current charge is done.

The dumb way is to assume that the cells are discharged and use a
timer, but overcharging kills the cells over time, so you should get
it right.

It's also rather easy to make the cells vent hot hydrogen gas and
electrolyte mist, but they don't tend to (themselves) spontaneously
combust like lithium cells.

Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
```
```On a sunny day (Sun, 26 Oct 2014 11:08:32 +0100) it happened Robert Roland
<fake@ddress.no> wrote in <qjhp4ahios600diu52pmbc2338fsnl31ka@4ax.com>:

>On Sat, 25 Oct 2014 20:45:38 -0700, Jeff Liebermann <jeffl@cruzio.com>
>wrote:
>
>>One accidental overcharge and your
>
>Not true.
>
>Early NiMH batteries were quite intolerant of overcharging, but modern
>batteries are much more robust. Not quite as robust as NiCd, though.
>
>In fact, some amount of overcharge is necessary in order to keep the
>cells in a battery balanced.

I have a about 6 or so 'eneloops' that were all damaged by normal NiMh chargers.
And one that was damaged by me trying to charge with a lab supply (at 1/4 C).
All show increased internal resistance and varying internal resistance,
so varying voltage in use, or in other words: noise, reduced power.
Regarding the impossibility of charging eneloops in a safe way i have decided not to buy these
anymore.
The old Panasomic NiMhs I have have been abused and overloaded and what not a million times,
and are ten years old and seem undestructible.
Those do not retain charge as long as eneloops though.
Panasonic bought eneloop recently... so whatinaname.
Will try some cheap NiMh next time I need power,
its not always needed to retain charge a long time.
On the other side of the spectrum the Duracell AA NiMhs I have self discharge in a day or 2.
So it there a general rule? I think not.

Anybody any RTGs for sale?
```
```On Sun, 26 Oct 2014 11:08:32 +0100, Robert Roland <fake@ddress.no>
wrote:

>On Sat, 25 Oct 2014 20:45:38 -0700, Jeff Liebermann <jeffl@cruzio.com>
>wrote:
>
>>One accidental overcharge and your
>
>Not true.

I beg to differ.

>Early NiMH batteries were quite intolerant of overcharging, but modern
>batteries are much more robust. Not quite as robust as NiCd, though.
>
>In fact, some amount of overcharge is necessary in order to keep the
>cells in a battery balanced.

Let's see if I understand what you're saying.  In a series string of
NiMH cells, one must overcharge the good cells in order to bring the
not-so-good cells up to the good cell voltage?  Is that what you're
saying?  If so, this sounds like a variation on "battery conditioning"
which works well at killing batteries.

LiIon battery chargers have the same problem with series connected
cells.  The difference is that while NiMH cells might survive some
overcharge, LiIon cells short and die rather easily.  To solve the
problem of maintaining identical and balanced cell voltages, the RC
(radio control) industry uses a "balance charger":
These have two connectors to the battery.  One pair of terminals for
the high current charge/discharge current, and another connector that
goes directly across each cell to monitor and control the charge
current for each cell individually.  Something like this might also
work with NiMH, but I haven't found any balance chargers that will do
it.  I suspect that's because LiIon and mutations are the future,
while the use of NiMH is decreasing rapidly.

One of my experiments was to see how quickly I could fast charge NiCd
and NiMH cells.  Since these chemistries only get hot when
overcharged, I suspected that I could safely charge them at ridiculous
rates, as long as I didn't overcharge.  Trying to detect the 5mv drop
in terminal voltage at EOC was futile as warm contacts would ruin that
measurement.  Most cells were 2000 ma-hr AA NiMH cells that managed to
deliver about 1200 ma-hr at 1C discharge loads[1].
<http://802.11junk.com/jeffl/NiMH/Duracelll-NiMH-2050.jpg>
I would then quick charge them at up to about 5C (10 Amps) for about
12-15 minutes.  Knowing the initial SoC (state of charge) and cell
capacity was critical.

Of course, I intentionally overcharged some cells, just to see what
happened.  I determined that things change at about 75% of capacity.
Below 75%, the charging efficiency is 100%.  If I charged at any rate
to 75% of full capacity, I could discharge the cell (at C/10) and get
all of it back.  Above 75%, everything would change.  If I counted
coulombs and charged to 100%, the charging efficiency dropped from
100% to about 85% with the loss going up in heat.  The heat was a
waste of charging energy.  It also caused the cell to deteriorate in
both capacity and self discharge rate.

Notice that the cell does not magically stay cold at 99% of charge,
and explode at 101% of charge.  It's an exponential curve, that starts
at about 75% and climbs rapidly.  The 100% point seems to have been
selected to produce a reasonable number of charge cycles, safe
charging temperature, and approximate correlation with the -5mv NDV
(negative delta voltage) EoC point.  If you want your NiMH batteries
to live forever, just set the EoC to about 75% and don't go anywhere
near 100% charge.  I attempted to reproduce the NiMH graph at:
<http://batteryuniversity.com/learn/article/battery_performance_as_a_function_of_cycling>
<http://batteryuniversity.com/_img/content/cycle3.jpg>
which shows loss of cell capacity after 300 charge cycles.  At 100%
charge (1C charge, 1C discharge, 4 hr cycle for about 100 days), using
a commercial smart charger, my results were fairly similar to the
graph.  However, at 75% of charge, there was little decrease in
capacity up to 700 charge cycles, when I gave up.  Unfortunately, I
discovered too late that my battery holder had a problem and that the
contact resistance had ruined my internal resistance measurements.

Anyways, I think you can see what might happen as you approach 100%
charge with an NiMH battery.  I don't think going over 100% is a very
good idea.

"Charging Nickel-metal-hydride"
<http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride>

[1]  NiMH cells are rated for capacity at C/5 discharge rate to 1.0v.
For a 2000 ma-hr cell, it takes about 10 hrs to run the discharge
test.  Typical data sheet:
<http://data.energizer.com/PDFs/nh15-2300.pdf>

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
```
```On 10/26/2014 9:11 AM, Jan Panteltje wrote:
> Regarding the impossibility of charging eneloops in a safe way i have decided not to buy these
> anymore.

Lol, have you considered using the charger built to charge them?

> The old Panasomic NiMhs I have have been abused and overloaded and what not a million times,
> and are ten years old and seem undestructible.
> Those do not retain charge as long as eneloops though.

Duh, that is the point of the Enloops.  I believe Enloops are some 50 to
100 times slower to self discharge.

> Panasonic bought eneloop recently... so whatinaname.

Uh, are they dropping the Enloop name, I doubt it.  Does Panasonic have
a line of slow self discharge NiMH?  I expect not.

--

Rick
```
```In article <af4q4ahnkem0ojcv7ot8o3k5pm0ecstg1f@4ax.com>,
Jeff Liebermann <jeffl@cruzio.com> wrote:

> On Sun, 26 Oct 2014 11:08:32 +0100, Robert Roland <fake@ddress.no>
> wrote:
>
> >On Sat, 25 Oct 2014 20:45:38 -0700, Jeff Liebermann <jeffl@cruzio.com>
> >wrote:
> >
> >>One accidental overcharge and your
> >>battery is dead.
> >
> >Not true.
>
> I beg to differ.
>
> >Early NiMH batteries were quite intolerant of overcharging, but modern
> >batteries are much more robust. Not quite as robust as NiCd, though.
> >
> >In fact, some amount of overcharge is necessary in order to keep the
> >cells in a battery balanced.
>
> Let's see if I understand what you're saying.  In a series string of
> NiMH cells, one must overcharge the good cells in order to bring the
> not-so-good cells up to the good cell voltage?  Is that what you're
> saying?  If so, this sounds like a variation on "battery conditioning"
> which works well at killing batteries.

Series cells (and capacitors) naturally unbalance due to slight
variations in self-discharge.  There's eventually a risk of some cells
being reverse-charged while others still have enough voltage to make the
pack seem usable.  That's immediately the end of the battery pack's
life, maybe in a violent way.

Balancing overcharge is very mild; just enough to compensate for varying
self-discharge.  NiMh, NiCd, and lead-acid have no problem at all with
this.  It ages lithium-ion batteries more rapidly so some packs have a
shunt voltage regulator on each cell.

--
I will not see posts from astraweb, theremailer, dizum, or google
because they host Usenet flooders.
```