Batteries And Capacities
Truth or Lies
NiMH (Nickel Metal Hydride) Batteries
11th February 2021
Hello fellow RC enthusiasts from around the world.
In this post I will be talking about batteries and their advertised/promoted capacities.
We all know that in this hobby, batteries are the power plant of our beloved RC vehicles. We also know that they are NOT cheap, especially Lithium - Polymer or otherwise known as Li-Po batteries. They also need special balance chargers and fire retardant charge bags. You are also not advised to store a charged Li-Po battery as they are considered too volatile and could/will cause a fire hazard if stored fully charged. Therefore you are advised to use the correct balanced charger and bag to charge all Li-Po batteries.
I will mostly talk about Nickel Metal Hydride batteries in this post.
These batteries have been used and proved by pro racers and street racers alike for many, many years. Before the NiMH batteries we had Ni-Cad (Nickel Cadmium). The NiMH batteries simply destroyed the Ni-Cad battery due to increased capacity, recharge times and recycle uses. It was also cheaper to produce from a manufacturing point of view, or so I am led to believe. The Ni-Cad battery also suffered with a charge memory problem. If the battery is left discharged for any length of time, the battery had to be charged and discharged many times before it would hold a decent charge.
At this point I decided to build my own discharger. I already have a discharger built into my iMAX B6AC but the discharge times are way too long. This is due to the fact the the discharger cannot handle the current flow due to its limited power rating of around 2 watts maximum. Therefore the current will be highly limited to reduce the power requirements and safely discharge. I quickly set my power limit of 100 watt and current limit of 10 amp. I then worked out a resister value to suit my limits taking into account the highest voltage battery I could safely discharge. I designed the discharger to discharge up to and including a 4S LiPo which has a nominal voltage of 14.8 Volts. I also designed the power rating to increase at lower resistances (higher current). Therefore at 2 Ohms the maximum power handling is 300 Watt. This discharger took me a long time (spare time) to complete. It was also quite expensive but it has many more safety features and has a shorter discharge times than an off the shelf unit. I also built in a variable power supply to test motor, charge batteries and many other electrical tests etc.
I started cycling my NiMH 7.2 Volt 3000mAh battery with my iMax and then discharged it on my discharge rig.
Above. The custom discharger I designed, during it's development stages. It is a very simple, yet effective rig, bearing 300W and being fully fan assisted is perfect up to a 3S pack. The problem being is using over 3S would be acceptable for the electronics. However, at this point the heat build up is a problem that I cannot eliminate. Anyway, 3S is fast enough for my old eyes!
After discharge, I was absolutely shocked to find it had only 680mAh charge stored after discharge. The scanned results lists shows how weak this battery was...
And after 10 recharge-discharge cycles later, here are the final set of hand written results below....
As you can see, I have added another 600mAh to the battery, giving a total of 1460mAh. This is still a long way from the 3000mAh stated by more than 50%!!!!!!!!!! WOW!! not wow??????
At this point, I grabbed a quick picture before switching off R3. Capacity at this point is 1110mAh discharged. If you look at the data in the hand written picture above the last picture, at 25 minutes and 58 seconds, you can see that when I disconnected R3, the voltage went from 7.19 V back up to 7.29 Volts.
You can also see the current flow has dropped to 1.69 Amps as I increase the resistance. You can also see the load resistance (bottom right) reading 4.31 Ohm's.
The picture above shows the total capacity the battery held which was 1.47Ah or 1470mAh. It took 44mins and 52secs to discharge. It also gave 10.96Wh of power. As you can see, this is supposed to be a 3000 mAh battery. As you can see, it is not. It is not even half of what is advised to be.
Can Of Worms
At this point I doubted the accuracy of the readings, but after plugging in various other test instruments it became clear, they were quite accurate and within +/- 5%, which is good enough for this application. Taken to the extreme it would be 300mAh maximum and not the 1540 mAh which is missing. I also did some tests on a 2S LiPo and it was also short of capacity. However it is not as bad as the NiMH pack. I am doing further tests with the 2S pack and I will give you my findings in my next post.
It seems I may have opened a can of worms for battery manufactures here, however I need to test many more types and voltages of batteries to build a clearer picture. Obviously this will take some time to compile. I have started taking results from a 2S pack, which will conclude soon. I will also be testing a 3S, some 18650's and maybe a cordless tool battery in the future too, temperatures permitting!
I hope this post has been of some use and I hope you return for further updates.
Thank you for reading my humble words.
<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-6000357390305251"