Author Topic: AA Ni-MH battery charger output current mA  (Read 1463 times)

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Offline jayboydogTopic starter

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AA Ni-MH battery charger output current mA
« on: April 23, 2018, 01:37:07 am »
Thinking about different cheap AA Ni-MH 1.2v battery chargers.    two look identical. both have 120v AC input.   different brands yet both have label reads "200mA x 4".  I measure current mA by opening the circuit with a piece of paper between charger and battery then close the circuit with meter probes adjacent to each side of the paper,  but each outputs different mA to each battery depending on 1,2,3,4 batteries charging at same time.  #A  120mA    90mA    72mA    60mA,   #B  60mA   60mA    60mA   60mA.

what is "200mA x 4" supposed to mean?


A third charger is called "smarter".   will charge 1,2,3 or 4 batteries at same time.  I do not get a mA output as whichever individual channel I am in series to measure mA causes "error" on the charger display for that channel.  the other batteries continue charging.  why is the meter measuring mA in series with a battery causing "error"?
 

Offline dacman

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Re: AA Ni-MH battery charger output current mA
« Reply #1 on: April 23, 2018, 02:08:37 am »
"Smart" chargers can usually charge both NiCad and NiMH.  They take measurements on the batteries, they do not just source current.  Placing the leads in series would not be something the charger would understand.
 

Offline Audioguru

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Re: AA Ni-MH battery charger output current mA
« Reply #2 on: April 23, 2018, 03:26:33 am »
Your meter measures current by connecting a resistor in series and measuring the voltage across the resistor. A smart charger sees the battery with too much resistance in it and shows an error.
 

Offline IanB

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Re: AA Ni-MH battery charger output current mA
« Reply #3 on: April 23, 2018, 03:46:26 am »
Thinking about different cheap AA Ni-MH 1.2v battery chargers.    two look identical. both have 120v AC input.   different brands yet both have label reads "200mA x 4".  I measure current mA by opening the circuit with a piece of paper between charger and battery then close the circuit with meter probes adjacent to each side of the paper,  but each outputs different mA to each battery depending on 1,2,3,4 batteries charging at same time.  #A  120mA    90mA    72mA    60mA,   #B  60mA   60mA    60mA   60mA.

what is "200mA x 4" supposed to mean?

Whatever the marketing people want it to mean  :)

A more useful number to look at is how many hours it should take to charge a typical battery. If the charger is really charging at 60 mA then a typical 2000 mAh battery would take about 36 hours to charge, which is silly. Nobody would wait that long.

Make and model number of the chargers would be helpful.
 

Offline Circlotron

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Re: AA Ni-MH battery charger output current mA
« Reply #4 on: April 23, 2018, 04:14:55 am »
would take about 36 hours to charge, which is silly. Nobody would wait that long.
 

Online paulca

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Re: AA Ni-MH battery charger output current mA
« Reply #5 on: April 23, 2018, 06:45:48 am »
The 200mA will be peak current.  It is only likely to deliver anything like this when the batteries are virtually flat.  As they charge their resistance changes and the current falls away.

NiMhs are notoriously hard to charge correctly.  So most cheap chargers just back off the current quickly and effectively trickle charge them from 75% or so, meaning they can be just left on overnight or for a whole day without overcharging them.  By the time the batteries are full the charging is only putting in the power which is being dissipated as heat.

The smart charger will push current into the battery for a period of time, then it will pause and watch the voltage which will relax back a bit.  They keep repeating this looking for a specific drop in voltage.  I can't remember if the delta-v happens when current is flowing or when it's removed.  However the delta-V is very small and hard to spot, I've never had a NiMh charger which gets it right, so I typically rapid charge them on a timer then switch to trickle charge.
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Offline jayboydogTopic starter

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Re: AA Ni-MH battery charger output current mA
« Reply #6 on: April 23, 2018, 08:07:26 am »
thank you for the responses.    My AA Ni-MH  2500mAh  1.2v batteries could not power my radio scanner using 4 x AA  when each battery's no load voltage dropped to 1.24 - 1.3v.
I inserted two batteries into a charger which outputs  90mA to each and two batteries into a charger which outputs 60mA to each.  at 4 hours of charging, the batteries no load voltage measured  1.35 - 1.37v.  at 8 hours of charging, no load voltage measured 1.38 - 1.39mA.  at 10 hours of charging the no load voltages were 1.4  1.4   1.37  1.37v.

what charging is needed to increase the no load voltage measure from 1.24 to 1.4v ?

I read statements such as  "a 2400mAh battery being charged with 100mA will require  28.8 hours to recharge."  (1.2 x mAh/mA).  but how does such a calculation take into account the battery's voltage when starting the recharge?  is a different mA x time charging required to raise the battery's voltage from 1.25 to 1.3v  from raising the voltage from 1.3 to 1.35v?
 

Online paulca

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Re: AA Ni-MH battery charger output current mA
« Reply #7 on: April 23, 2018, 08:55:45 am »
If you want peace of mind, just plug them into the charger and leave them for a full day to charge.

If that is too long then you'll need to invest in a much more expense (or cheap and dangerous) fast charger.
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Current Open Projects:  STM32F411RE+ESP32+TFT for home IoT (NoT) projects.  Child's advent xmas countdown toy.  Digital audio routing board.
 


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