NiMH chargers - Math formula - Battery capacity and Required Charging times

[Kiriakos GR]

There is a simple way to account for the charging times.

First, check the charger label to get the charging (output) current (120mA for AA cells).
Then Charging Time (hours) = Cell Capacity X 1.2 / Output Current

For example, for AA 2000mAh rechargeable battery
Charging Time (hours) = 2000mAh X 1.2 / 120mA = 20 (hours)
 

Efficiency Factor: NiMH batteries are not 100% efficient at converting electrical energy to chemical energy.
You must put in more energy than you get out.
A factor of 1.2 to 1.4 (representing 120–140% input) is typically used to account for this inefficiency

[Andrew-88]

What it does missing from the market, this is smart chargers them able to calculate End of Charge time in minutes.

Thanks mate !!

[Kiriakos GR]

What it does missing from the market, this is smart chargers them able to calculate End of Charge time in minutes.

Thanks mate !!

If this charger it is able to add and calculate it own DC current output error, then this it can happen.

At most battery datasheet, at the standard charge, they calculate hours by using the worst Efficiency Factor 1.4
On top of that they add 15% error,  this is charger output error (fluctuation from a standard DC current value).
The outcome becomes for example: 30mA  for 14 Hours up to 16 Hours.

Only if Japan this engage at design and manufacturing of NiMH chargers, only then we might see more accurate electrical measurements at NiMH chargers.
 
 

[Kiriakos GR]

This period of time I have start gathering my own experiences, in the regard of variables them able to influence charging factor (Accounts for charging inefficiencies).

For example:
a) Begging of winter time, with indoor temperature at 19.5C (excellent temperature for NiMH testing)
b) DUT this is bright new 9V NiMH PP3 battery (lowest internal resistance), 280mAh (tested). 
c) Bright new battery charger, with accurate delivery of charging current at 35mA, actual delivery 28mAh per hour.

At such all three, ideal conditions, Lower charging factor of 1.2 this seems as applicable.
Mathematical estimate 9H 36M.
Actual charge process this required 9H 52M 42S 

Due the fact that its impossible for us to control all three variables ( these known at causing charging inefficiencies), the industry selected charging factor 1.4 as an more fault tolerant choice.

From the other hand, I do believe that the designers of chargers and NiMH battery makers, they do use at their own testing lab, the Lower charging factor of 1.2 as a benchmark.