And Some General Comments on Cheap NiMH Batteries
In this opportunity, I want to present my recent discoveries on the measurement results of some old NiMH batteries. These batteries are old. I think they are in the order of 3 – 4 years old and some have undergone medium duty usage, while some has gone under heavy duty usage. A set of new batteries is also to be measured. The objective being to get some kind of estimate on how long I need to put them in my 100 mA trickle charger. It is critical to get the right mAh number as excessive charging will ruin these NiMH batteries.
The measurement is made possible by the recent invention of a relatively sophisticated contraption, as presented in my article “Using 68HC908QB8 As a Stable Measurement Platform”.
A Brief General View on the Effect of Usage on NiMH Batteries
A quick search on the internet will provide a general view that is widely held, that the impact of long term usage on NiMH batteries will be reduced capacity and increased internal resistance. My question actually revolves around the details of this statement; exactly how much reduction and how big of internal resistance is created? Documents that I found very useful, from leading brands like Eveready and Energizer, have this blanket statement, but both omit the specific details.
Measurement Method
There are some clever measurement methods that people have suggested on the internet. Some things that come my mind is to boost the voltage using the available +5 Volt supply, and feeding it into an LM317 constant current regulator. This will create a constant current discharge of the battery.
In the circuit idea above, LM317 will create a constant output voltage of about 1.25V. Going through R1, we will get a constant 500 mA current discharge of the battery throughout the measurement period. Care must be taken that when the battery is depleted, we do not drive it into negative charging region which will ruin the battery. Boosting the battery voltage with the 5V supply is needed because the minimum input voltage to the LM317 will be in the area of 3.5V (not 1.2V — the battery voltage). Also, tracking the battery voltage is tricky because it is above the +5 Volt rail. Maybe a voltage divider is needed and it will introduce the need for some math in reading the result.
All these ideas are nice, but for the sake of keeping it simple and stupid (the KISS principle), we will do it brute force method.
We will directly load a 2.5 Ohm resistor into a battery, and keep track of the voltage using the ADC reader. We will analyze the resulting voltage and do all the calculations from there.
Measurement Results and What We Learn From It
The first candidate for this measurement is an Eveready NiMH battery. It has been in light to medium use for approximately 4 years. The resulting graph is as follows:
As you can see, my notes indicate that the old battery starts with Rin (internal resistance) of 1.13 Ohms and at the end of measurement, the calculated capacity is about 1400 mAh. This is a battery that is advertised as 2000 mAh battery. So it is shown that the capacity is reduced and internal resistance is increased (from some zero point something Ohms to 1.13 Ohms) as the battery ages. My guestimate of the Rin at the end of measurement is Rin of approx. 2.5 Ohms (my experience is that most batteries will go back to 1.2 V when load is disconnected).
Another candidate is a Gold Peak GP1300 battery. It is a cheap battery that is advertised to have 1300 mAh capacity. This battery has been through abuse. I believe that it has been overcharged many times in my VHF radio. Age is probably close to 10 years old now. The resulting measurement is as follows:
We can see that the starting internal resistance is 1.75 Ohms, and at the end of measurement cycle, Rin is 6.10 Ohms (!) Also battery capacity is severely degraded from what’s advertised as 1300 mAh battery into just a mere 31 mAh. So this is a severe case.
Observation
There we have it. So, indeed, it is true that the capacity is reduced and internal resistance is increased in old NiMH batteries. We get to see the real values of these degradations.
It is worth noting that the internal resistance is a function of battery discharging process. I have seen that the internal resistance goes down as the battery heats up, and towards the end, the internal resistance increases to a final value. In that severe case above, this final value is a killer in the battery application. With these kinds of internal resistance, we really can’t get much of an application of the battery. I mean what do you want to do with a 0.6 Volts voltage source?
However, in the Eveready EV-2000 example above, we can see that the hardy battery lives up to its reputation. Its capacity is changed only from 2000 mAh to 1400 mAh over time. Also, internal resistance is about 1.1 Ohms to 2.5 Ohms. It is still usable as a power source in some applications.
My Comments on Cheap NiMH Batteries
In closing, I will present a new battery. It is branded as WPeak 2800 mAh battery. Really, if it is a cheap Chinese brand (about 50 US cents each), what kind of battery do we get?
This new battery boasts a decent starting internal resistance of 0.45 Ohms. The ending internal resistance is also good at 0.15 Ohms. However, the capacity, from measurement, is a “little” off from the advertised 2800 mAh. The actual measurement is about 700 mAh. So, oh well, the marketing people must have a very aggressive boss, I am thinking.
So what kind of speculative general observation do we make on cheap NiMH batteries? My opinion will be these batteries will have worse internal resistance value (the above of 0.45 Ohms should actually be better with good batteries), and as we can see, the whopping 2800 mAh is actually a puny 700 mAh. A lot more fluff for a lot less stuff. This 700 mAh is a real face value of the battery capacity. If I had followed the advertised capacity to estimate charging with a trickle charger, I would ruin the battery by overcharging it.
There you have it. I hope you have found this writing very educational. Thanks for reading.
Ketut's Electronics Technology Blog 
Interesting conclusion about the start and end internal resistance of the batteries. But if you measure the voltage of the battery at the end (open circuit), is the actual battery voltage less? Thanks for your research. We need more people to validate whether things are actually true, like that 2800 mAh rating.
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Hi Rod, at the end of my measurement, the open circuit voltage of the battery is slightly less than a freshly charged battery voltage. If I remember correctly, the freshly charged open-circuit voltage is 1.2V, while discharged battery open-circuit voltage will show something like 1.1V, but it will drop quickly to much below 1V if you put a load again. Interesting, huh?
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