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Battery internal resistance (BIR) truthful knowledge

Nigel Goodwin

Super Moderator
Most Helpful Member
Don't know where you get 7.7 ohms from?, internal resistance of workable Nicad batteries is EXTREMELY low, so low it makes them dangerous.

If the internal resistance is 7.7 ohms, then it's easily solved - send it to recycling and buy a working one.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The only way a NiCd (or NiMH) cell can have high internal resistance, and not just be wrecked, is if it has not been discharged fully for a very long time.

The metal in the cell plates can gradually crystallise, which makes it much less reactive and the cell current drops off, making them appear flat when used in high current devices. When in regular cyclic use, the metal stays in an amorphous state, which is much more reactive.

If it is that effect, the cure is very simple - connect a load, eg. a low value resistor with adequate power rating, eg. 2.2 Ohms 1W, directly across each cell, and leave them for a week or so to totally discharge.

That MUST be done on a cell-by-cell basis, if you try and "deep discharge" a battery as a whole rather than the individual cells within it, some cells will be wrecked.
(As long as you connect a resistor directly across each cell, they can still be assembled in a pack)

Once the cells have been fully discharged by that method, just charge them and use them!
 

alec_t

Well-Known Member
Most Helpful Member
MM tested it 1.53 V 0.17 A
There's a problem with the cell or your MM if it is reading 1.5V. The nominal voltage of a NiCd cell is 1.2V when discharging through a load (except perhaps immediately after a full charge), but the charging voltage needed is ~1.4V.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The test method in the Quora link is also completely wrong and dangerous.
In essence it says "Measure the cell voltage then short the cell via a high current ammeter". Then divide measured voltage by the (separately) measured current to get the cell resistance.

The cell voltage will drop significantly while it is loaded. The voltage that matters is the voltage while it is under load.

Off-load voltage is irrelevant; a discharged cell will recover to show around 1.2V but drop to near zero once under load.
Plus deliberately connecting what is effectively a dead-short load to a cell or battery like the article suggests is likely to damage things - the cells, meter or both!
 

crutschow

Well-Known Member
Most Helpful Member
To measure the cell resistance, you connect a resistive load (say about 500mA).
You measure the open-circuit battery voltage and its voltage under load.
The internal resistance is the difference between those two voltages divided by the load current or (Vo-Vl) / (Vl/Rl) where Vo is the open-circuit voltage, Vl is the loaded voltage, and Rl is the load resistance.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
To measure the cell resistance, you connect a resistive load (say about 500mA).
You measure the open-circuit battery voltage and its voltage under load.
The internal resistance is the difference between those two voltages divided by the load current or (Vo-Vl) / (Vl/Rl) where Vo is the open-circuit voltage, Vl is the loaded voltage, and Rl is the load resistance.

Yes, it's very simple to measure, just requires a little common sense and simple maths. What you don't want to do is short a NiCad out :D

You also need to consider that it's not a fixed value, and will vary with the state of charge and the actual load - however, it's not really much of a concern, as it's stupidly low and quite capable of destroying itself or a load that's too high.
 

audioguru

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Most Helpful Member
The last time I used a Ni-Cad battery cell was about 20 years ago. They came in cheap solar garden lights. The batteries lasted only for a couple of months and ended shorted so I replaced them with Ni-MH cells ever since. For at least 10 years, cheap solar garden lights come with a Ni-MH battery cell.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The last time I used a Ni-Cad battery cell was about 20 years ago. They came in cheap solar garden lights. The batteries lasted only for a couple of months and ended shorted so I replaced them with Ni-MH cells ever since. For at least 10 years, cheap solar garden lights come with a Ni-MH battery cell.

OK for solar lights, but NiMh have far less current capability than NiCd, hence they weren't used in drills etc.
 

audioguru

Well-Known Member
Most Helpful Member
An obsolete little AAA consumer Ni-Cad cell was asked about.
Ni-Cad batteries for consumers are banned in the EU and not sold anymore in North America because they are severely poisonous and have a low capacity. A tool battery is much larger and more powerful than a little AAA cell.
Most of my AAA Ni-Cads had a dismal capacity of only 200mAh, the Ni-MH ones are 800mAh. The Ni-Cad charge quickly faded away, it had a "memory" and it failed soon.
Here are specs for an Energizer AAA Ni-MH that has modern Eneloop technology so that it is sold pre-charged and holds a charge for one year:
 

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Nigel Goodwin

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Most Helpful Member
You're rather missing the point - NiCd provides high current, NiMh doesn't.

Obviously NiCd batteries in power tools have been replaced by Li-Ion, which also provide high current capacity, where NiMh were never used due to their relatively low current capability.

The capacity of NiCD and NiMh are fairly similar, your experiences were obviously crap quality ones poorly used.
 

audioguru

Well-Known Member
Most Helpful Member
The thread starter is probably measuring the resistance of his meter leads in series with the contact resistance of his breadboard.
Either his AAA Ni-Cad battery cell is 20 years old or is is a poor quality Chinese one from a cheap solar garden light.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The thread starter is probably measuring the resistance of his meter leads in series with the contact resistance of his breadboard.

If you had bothered to follow the original link - the post, in essence, advises measuring the cell voltage [open circuit] then short the cell via a high current range on a multimeter, then divide the open circuit voltage by the short circuit current & call the the cell internal resistance!

It takes no account of the cell voltage during the short circuit part, so will give wildly inaccurate readings, as well as being plain dangerous.
 

audioguru

Well-Known Member
Most Helpful Member
Cheap little old Chinese AAA Ni-Cad battery cells could be shorted with no problems happening because they were very weak.

Just now I shorted an Energizer AAA Ni-MH cell and got a reading of 5.1A when the meter leads measure 0.24 ohms in series with its burden resistance of 0.02 ohms.
5.1A x (0.24 ohms plus 0.02 ohms)= 1.33V so the current without the leads would be much higher than 5.1A.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
We used to test old dry cells (before alkaline batteries existed) by first measuring the voltage across them, then 'blipping' an Avometer set to the 10A range across it, looking for a decent deflection. However, we wouldn't test NiCd like that, as AA's (or larger) were likely to considerably exceed 10A.
 

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