# How to limit current without parallel leakage

Discussion in 'General Electronics Chat' started by naveed, Jul 24, 2010.

1. ### naveed Thread Starter Active Member

Mar 12, 2008
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To limit current from a battery, a resistance in parallel is used. But this resistance causes leakage and soon the battery will be discharged even if no load is applied to the circuit.
Is there any way to prevent this leakage?

I have a 6V 4.5Ah battery. And I applied it directly to charge my mobile but the charging ic of my mobile blown off. Then someone gave me a circuit to limit the current of the battery. But that circuit has a parallel resistance to limit current and this way the battery discharges very soon even if I don't charge my mobile.
Is there any circuit to charge my mobile with the battery for long time??

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
This is not correct. The resistances in parallel will make an equivalent resistance that is less than the smaller of the two discrete resistances.

For charging the battery, the correct means depends on the battery type. Merely applying a current from a higher voltage source is not a particularly good way to do it. There are IC's that can correctly control a battery charger.

3. ### tom66 Senior Member

May 9, 2009
2,613
214
Are you talking about voltage regulation? You need to drop the voltage of the battery to the 3-5 volts the phone requires. You can readily buy a voltage regulator IC for this purpose, for less than \$1.

4. ### Markd77 Senior Member

Sep 7, 2009
2,803
594
Look for a low dropout regulator if you are trying to get 5V from a 6V source. The phone itself contains all the charging circuitry and all you need to do is provide it with the correct voltage. A good regulator will use less than 5mA if the phone isn't connected.

5. ### naveed Thread Starter Active Member

Mar 12, 2008
42
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But I think it is not the voltage but the current which matters in charging. For example, a travel charger which is applied in a car battery gives 12V as an output. And car battery is itself 12V. It means the charger is not limiting the voltage but the current.

Also the issue is to prevent the leakage of current(i.e., no current flow if no load is applied)

6. ### tom66 Senior Member

May 9, 2009
2,613
214
All the charging circuitry is in the phone. The travel charger will supply 3-5 volts from a 12 volt input.

Mar 12, 2008
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8. ### tom66 Senior Member

May 9, 2009
2,613
214
I've never seen a phone which takes a 7.2 volt input, though I suppose they could exist. What does the mains adapter say?

9. ### naveed Thread Starter Active Member

Mar 12, 2008
42
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I tested these cells on nokia n80 and nokia 1200. The charger is 5V 800mA but these cells(7.2V) charge them.

10. ### naveed Thread Starter Active Member

Mar 12, 2008
42
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I want to make a rechargeable mobile charger which could charge my mobile more than 20 times. Is it possible???

11. ### tom66 Senior Member

May 9, 2009
2,613
214
First, the phone boosts the 5V to 7.2V (or whatever voltage the cells are charged at.) If you put in 7.2V then you will destroy your phone, or at least damage the charging circuit.

You must supply the same voltage that the adapter provides.

Since your supply is 6V and you require 5V you will need what is known as a low dropout regulator.

You probably won't be able to recharge it 20 times. If I assume your phone charges in 1 hour, that's 500mAh per charge. You only have 4500mAh, so you could go a maximum of 9 times.