About Nimh Batteries

[hobbyteam 0]

Hello everyone, as a newbie i have a question....

Is there a way to measure the Voltage on an empty 7.2V NIMH 4500mAh battery ?By measuring the Voltage can you tell if the battery is full,half empty or empty?I am using a multimeter and although the battery is empty it still showing me 7.4Volts but it's empty and won't move the car.

Thank you.

[Mighty flex 0]

it shows 7.4 because there is no load - the multimeter maybe pulls 0.0001A which cause no drop in the voltage. If you look a full bat will show around 8 or more. Testing the voltage under load- maybe when connected to a motor, or something, is more accurate. You will then notice that a flat battery will show a very low voltage.

The relationship between charge and voltage is not linear though - http://shdesigns.org/batts/battcyc.html - this page may give you an idea of how the voltage varies as the battery discharges.

[94eg! 834]

The pack is empty when resting voltage is at 7.4v. It can go as high as 9v when full depending on your charging current. The higher the charge amp rate, the higher the full voltage will be. The higher charge current gives the battery way more punch, but makes it hotter and reduces the lifespan. This is why racers go through nimh batteries so quickly. Since the voltage is totally depending on how you charge & treat the pack, there is no good way to tell how full it is via voltage.

BTW: If you let the pack sit too long after charging, the resting voltage will drop considerably. This is why you want to run them straight off the charger. Nimh self-discharges at very high rate. You know the pack is damaged when the total resting voltage is under 6v. That means all or some of the cells have gone below 0.9v (from self discharge). Once this happens, the internal resistance of that cell will rise and force the entire pack to flow less current. That means less punch and less run-time (cause it takes less of a charge) and they will generate more heat when discharging. If you can measure individual cells, you can pinpoint problems and replace cells. You need to be really good at soldering, and have a high watt iron.

[hobbyteam 0]

Thank you for the help.Since i am using my Ni-MH battery pack very often iam not discharging them.Do you think is that good?

[94eg! 834]

Thank you for the help.Since i am using my Ni-MH battery pack very often iam not discharging them.Do you think is that good?

Usually no need to unless you store it for a long time. When you take them out of storage, the false peak like crazy so you can't get many amps back into them. Cycling it a few times usually brings them back to life. If you don't have a discharger, you can just cycle it by running it in your car a few times. This is helpful because you can see how the power and runtime come back.

Example: We took an old 3000mah Nimh out of storage yesterday and tried to run it in a 4 minute race in a Grasshopper. I had to restart the charger 5 times because it kept false peaking and the pack only ran about 1.5-2 minutes of race before dumping. Second charge, the pack kept false peaking, but managed to take a lot more milliamps in the process (finished the race, but ran slowly). Third time The pack false peaked twice, but managed to run stronger. Fourth time, it false peaked once, and stayed strong the whole race.