Author Topic: Battery Charging  (Read 1593 times)

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Offline PolybusTopic starter

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Battery Charging
« on: September 08, 2016, 08:07:40 am »
Howdy

I've just bought myself a SkyRC Model RS16 Battery Charger and Power Supply - I am trying to charge NiMH cells/batteries with it.

Just to test it out, I attached a single AA NiMH cell rated at 3000mAh (or so it claims) and set it to the NiMH program and set it charging.

I set the charge current to maximum of 3.0A, but in the default NiMH programs there is nowhere to set the voltage.

When the cell starts charging - I can see the display showing a charge current heading up to about 2.4A, but the Voltage goes was up to over 7 volts - that can't be right can it?? Obviously the cell nominal voltage is 1.2V - so why would it be charging it at over 7 volts - maybe I have forgotten to set something???

There are user programmable modes - and in their I can set the NiMH to 1 cell, which shows as 1.2V, but again, as soon as I begin charging - the voltage goes up to 7volts or more.

Am I worried over nothing (I'm sure the battery charger knows more than me about this) or am I just not setting this thing up correctly?

Cheers.

  :)
 

Offline jeroen79

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Re: Battery Charging
« Reply #1 on: September 08, 2016, 08:54:06 am »
AFAIK NiMH is always charged in constant current mode with no constant voltage phase following.

Charge termination is then determined by time or by detecting that the battery is full by sensing a drop in voltage or sudden rise in temperature.
What does your charger's manual say about that?

7V does seem high though.
Does the cell get really hot?
 

Offline PolybusTopic starter

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Re: Battery Charging
« Reply #2 on: September 08, 2016, 09:09:37 am »
Its gets hot alright!!!   :-\

When I start the charge program I can watch the Charge Current ease up to the Max Charge Current (I did have it set at 3A, but now reduced it to 1.2A) and over that same period the Voltage edges up to over 7 Volts.

I think I had best leave this thing alone until I better understand what's going on :)
 

Offline amyk

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Re: Battery Charging
« Reply #3 on: September 08, 2016, 10:44:15 am »
NiMH is charged with constant current so your charger is trying to force the current to 3A by raising the voltage as high as it can. A 1C charge is extremely fast ,and especially if the cell isn't actually 3Ah but less, explains why the charger is having trouble with getting that much current through it; the chemical reactions that take place when charging can only go so fast. Also, numerous other references I've found on the Internet suggest that the absolute maximum voltage should be 1.8V/cell...

Try charging with 0.5C (1.5A), I think the voltage will be a lot lower. I can't easily find any graph of charging voltage vs. current for NiMH but I bet it looks like a gradual slope upwards until the C rating of the cell at which point it shoots up extremely steeply.
 


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