# Resistance???

Discussion in 'General Electronics Chat' started by New123, Jan 4, 2013.

1. ### New123 Thread Starter New Member

Jan 3, 2013
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How do I calculate the resistance of a lithium ion battery

2. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Just assume it is zero, which is close to the real value for any charged battery.

What are you intending to do with this information?

3. ### GopherT AAC Fanatic!

Nov 23, 2012
6,071
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Measure voltage across the terminals.

Then connect a 100ohm resistor across the terminals and record the voltage again. If the voltage is different, then assume the drop is because of internal resistance of the battery and calaculate the voltage divider (i.e. two resistors in series).

If you get the same voltage reading, move to a 10 ohm resistor (or get a volt meter with more significant figures). Repeat with smaller resistors until you get a reading but be careful, your resistor will get hot, your battery may over heat and vent. By "Fast", I mean 2 or 3 seconds at most.

Do these measurements quickly, behind a sheild and while wearing gloves and eye protection with no flammable/combustible materials near the battery.

4. ### JMac3108 Active Member

Aug 16, 2010
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You can meaure the internal resistance of any battery as follows.

(1) Measure the open circuit voltage of the battery

(2) Add a resistive load and measure the battery voltage. The battery voltage will be lower because the internal resistance of the battery forms a voltage divider with the exteral resistor.

(3) Calculate the internal resistance ...

But what are you going to do with this information?

5. ### JMac3108 Active Member

Aug 16, 2010
349
66
Hahahah .... Gopher beat me to it while I was writing my post

I assume you mean a LiIon BATTERY, not a LiIon CELL. If its a LiIon battery then no need for any of the protections mentined by Gopher because the battery will be protected with an internal safety circuit. If its a LiIon cell, then don't mess with it.

6. ### Audioguru New Member

Dec 20, 2007
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No.
Electric model airplanes, helicopters, boats and cars use Li-Po multi-cell BATTERIES that have NO protection built-in. The protection circuit is inside the product and charger.

7. ### New123 Thread Starter New Member

Jan 3, 2013
13
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So if I were to disconnect the battery from the board ( my laptop battery) there would be zero resistance?

8. ### Audioguru New Member

Dec 20, 2007
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The lithium-Ion battery for your laptop has a protection circuit inside it that prevents over-charging and over-discharging.
The internal resistance of the charged battery is extremely low so a very high current will flow and start a fire if it is shorted without having the protection circuit.
The internal resistance of a battery is low so that its voltage does not drop much when it has a normal load.

9. ### New123 Thread Starter New Member

Jan 3, 2013
13
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Right so if I remove the protection circuit?

10. ### WBahn Moderator

Mar 31, 2012
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Would you PLEASE describe what it is you are trying to accomplish?

Trying to find out how to remove the protection circuits from Li-ion battery packs falls into the realm of the type of unsafe practices that are not discussed here on AAC.

11. ### Audioguru New Member

Dec 20, 2007
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I used the Li-ion cells from a laptop battery in my RC electric model airplane for one year.
It was easy to remove the protection circuit.

In the airplane the cells got pretty hot then the power faded until I cooled them.

Of course I made my own 8.4V current-limited charged and I was lucky that the cells were well-balanced.

12. ### JMac3108 Active Member

Aug 16, 2010
349
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Yes, because the these devices have batteries with relatively small capacities. You certainly wouldn't want to do this with a laptop battery. I know lots of people do this and have no problems. But the cells really are dangerous. There are numerous cases of explosions and fires due to 18650 LiIon cells (laptop cells) with faulty (or no) proptection circuits. In general, I don't recommend it at all.

I designed products with LiIon batteries for more than 10 years and we always included a complete safety circuit (had to or you can't ship the product due to FAA and DOT regulations). I remember once we got a call from a customer in France that one of our products exploded and burned in their warehouse. It was right around the time of several high profile laptop fires and subsequent battery recalls. We were VERY concerned, but it turned out that they had accidentally crushed the product with a forklift. Not even the best protection circuit in world can protect against breaking open the cell and exposing it to air!

Last edited: Jan 7, 2013
13. ### New123 Thread Starter New Member

Jan 3, 2013
13
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I wanna use the Li-ion laptop battery for something else..but with the protection circuit it doesn't produce enough amps..if I remove it wouldn't it only be unsafe if I were to use it on a device that couldn't handle the output of the battery without the protection circuit?

14. ### New123 Thread Starter New Member

Jan 3, 2013
13
0
I wanna use the Li-battery for something else..but it doesn't produce enough amps with the protection circuit..and wouldn't it only be unsafe if I were to use the battery on a device that couldn't handle the output of the battery without the protection circuit?

15. ### WBahn Moderator

Mar 31, 2012
17,777
4,805
No, it would be unsafe if it were used on a device that drew more current than the battery can safely deliver, regardless of whether the device could handle it or not.

16. ### Audioguru New Member

Dec 20, 2007
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But you do not know the maximum allowed output current of the lithium cells in a laptop. It does not need to be high in a laptop so maybe they will catch on fire if you overload them in something else.

They flew my RC model airplane until they got hot then lost their power until they cooled. If the airplane motor stalled then the current is much higher and they probably would have started a fire.

17. ### New123 Thread Starter New Member

Jan 3, 2013
13
0
I wanna use the Li-battery for something else..but it doesn't produce enough amps with the protection circuit.wouldnt it only be unsafe if I were to use it on a device that couldn't handle the output of the battery without the protection circuit?

18. ### kubeek AAC Fanatic!

Sep 20, 2005
4,670
804
Yes it will be dangerous. The protection is there for the exact purpose of limitng the maximum current. If you disable it and exceed that current, the battery will burst on fire.
If you need more current, get a ni-cd or lead-acid battery.

19. ### New123 Thread Starter New Member

Jan 3, 2013
13
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So there's no way to have a Li-ion battery produce around 200amps safely. Only for a very short period of time

20. ### Potato Pudding Well-Known Member

Jun 11, 2010
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There is a very simple way to get the 200 Amps. It is called a filter capacitor. You don't want to exceed the current capacity of the batteries but you can allow them to charge a suitable electrolytic capacitor without exceeding their rated current capacity and then drain the capacitors for your short short pulse.

Maybe you work in the pits for a stock car racing team wanted to make a portable spot welder for emergency repairs, Capacitor discharge is how you would need to do it.

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