# Nimh battery related thread ?

Discussion in 'General Electronics Chat' started by curry87, Apr 7, 2011.

1. ### curry87 Thread Starter Member

May 30, 2010
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0
How many amps can i draw from a fully charged 2100mah 1.4 aa (nimh)cell at a given time say a few minutes without it being an equivalent of basically shorting the cell with a thick piece of copper wire ?

2. ### Audioguru Expert

Dec 20, 2007
10,606
1,182
Battery manufacturers have datasheets for all their batteries.
An older Energizer AA Ni-MH datasheet shows that at 2.5A the voltage drops quickly and at 5A the voltage drops like a rock.

• ###### AA Ni-MH cell high currents.PNG
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3. ### curry87 Thread Starter Member

May 30, 2010
101
0
How do i find information like the lowest safe load resistance you can place across a aa,aaa,1.4nimh cell i looked on google without success?

4. ### Audioguru Expert

Dec 20, 2007
10,606
1,182
Didn't you see my post 12 hours ago?
Battery manufacturers have websites with plenty of technical information about everything they make.
I showed Energizer's AA Ni-MH cell datasheet's graph that shows its voltage
quickly dropping with a 2.5A load and its voltage is very poor with a 5A load.
Duracell's datasheets will also look similar.
They have graphs for AAA cells too.

Can't you use Ohm's Law to calculate a resistance?
On the graph I posted the voltage when the cell is freshly charged and with a load of 2.5A is about 1.35V. Ohm's law calculates a resistance of 1.35/2.5= 0.54 ohms.
After 1 hour the voltage is 0.9V so Ohm's Law calculates a load resistance to make 2.5A to be 0.9/2.5= 0.36 ohms.

You can do the calculations for any other load current.

5. ### curry87 Thread Starter Member

May 30, 2010
101
0
Im sorry im not being clear to start with, basically how low does the load resistance,how high does the current have to go for a nimh cell (aa) to be considered to be short circuited ?

6. ### Audioguru Expert

Dec 20, 2007
10,606
1,182
I showed that the cell is nearly useless with a 5A load and used Ohm's law to calculate the resistance.
If the current is too high because the resistance that shorts it is too low then the battery cell will get hot, build pressure inside then vent the pressure (and some chemicals) to avoid an explosion.