Battery charger thinks batteries are bad...?

[lee321987]

I just bought a new NIMH battery charger from Radio Shack:
RS part # 23-787
Enercell Battery Charger with Rechargeable Ni-MH Batteries : Rechargeable Batteries | RadioShack.com

It came with 4 AA 2500mAh batteries, and 4 AAA 850mAh batteries.
It charges AA's at 1A.
My old charger is 900mA.


It charges the batteries that came with it fine, but when I insert my older (~1.5 years) AA 2000mAh, I get a flashing LED, which according to the instructions, means "the batteries may be bad". I get full voltage and plenty of capacity out of these old batteries when I charge them with my older charger.

What would cause NIMH batteries to be detected as bad?

 

[andre_luis]

Maybe required sink current exceeded predicted limit value at enviroment temperature.
Check if charger support amperage above.

+++

 

[KerimF]

Some chargers check (measure) the internal resistance (impedance) of the battery. If it is relatively high (according to the battery type), the charger stops charging it.

 

[lee321987]

Okay, so what's the worst that could happen if I used a resistor to trick the charger into thinking the battery has a lower impedance? (assuming I can remove the resistor after the check is done, and it will continue to charge the battery).

 

[KerimF]

Okay, so what's the worst that could happen if I used a resistor to trick the charger into thinking the battery has a lower impedance? (assuming I can remove the resistor after the check is done, and it will continue to charge the battery).

Sorry, I couldn't get your point on how to let the charger see the battery as having internal low impedance.

By the way, I supposed that measuring the voltage of the old battery gives a nominal value (for example like 1.2V or a bit less). If so, please skip the next paragraph.

If its voltage is too low then we should try charging it with a simple method till it restores its nominal voltage. This can be done via a resistor and a DC supply. The charging current (I_ch) doesn't have to be high since we are not in a hurry :wink:
R_ch = (Vcc - V_bat) / I_ch (worst case is when V_bat=0)
The power dissipation by R_sh:
P_dis = (Vcc - V_bat)*(Vcc - V_bat)/R_ch
From the formulas above it is better to choose a low Vcc to decrease dissipation (not higher than 5V). But if dissipation is not a problem (by using a power resistor or many resistors in parallel, series or both, to get R_ch), using 12V for example can let the charging current be rather constant while V_bat is increasing with time.

If the charger refuses to continue charging the battery though its voltage is nominal then we can conclude that it detects its internal resistance (R_bat) first which usually increases with time. Of course we can measure it too. One easy method is to wire the battery as if we like to charge it via a resistor. We measure the two voltages; V_res on the resistor (R_ch) and V_bat1 on the battery. Then we remove the resistor and notice how much the battery voltage drops (Vbat1-Vbat2):
R_bat = R_ch * (Vbat1-Vbat2) / V_res


On the other hand, perhaps we can check the validity of this 'internal resistance' idea for a charger by doing just the opposite as it follows:

First, we find a good battery that the charger can charge it normally.
Then we insert externally a resistor R_ext (as if it were internal) in series between one terminal of the charger and the corresponding pole of the battery.
The value of the resistor is likely small (we can start with 1Ω for example) and find out from which value the charger starts to complain and signals it is a bad battery. Please note that the charger will see now the sum R_bat + R_ext.

Edited:
If you will find out that the charger keeps charging for any value of R_ext and the charger signals an error even if the battery under charge has a nominal voltage... then...I will be confused too. :wink:

 

[JohnRocks]

Choosing the right charger makes good sense since we are also ensuring that our battery and boat will perform excellently for long. As such try not to make impulsive decisions when buying. It might help to shop online for batteries so that we can be more relaxed and able to decide rightly.