# 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 battery is dead. 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:

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 >>battery is dead. > >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 >>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.