# Forum: Analog Circuits Battery protection unit

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Hi,

I want to protect my battery when I am charging it.
I realised that most protection Ic need two external transistors, which
limit the charging and discharging current.
My maximal chargin current it 1C which i 60 mA.
Most IC have overcurrent detection voltage of aroun 100mV.

To find the right transistor I am calculation Ron.
R= U/I  = 100mV/60mA = 1.6 Ohm.

Here the Ron seams to me quiet big. Traistors which I found have an
resistance in the mOhm range.

What can be my error here?
Am I calucating correctly?
Are there other ICs which meet the requirements, or other transistors?

I hope to get some help.

Thanks a lot.

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> I realised that most protection Ic need two external transistors, which
> limit the charging and discharging current.

They don't limit it, they completely turn off on overcurrent (usually
discharge; don't know about charge, don't think so).

You seem to have a misunderstanding about these ICs: they are not there
to handle normal operation conditions - they are protection devices to
avoid catastrophic failures (fire, explosions etc).  When they activate,
the battery may already be damaged.

That may explain the rather high trip voltages - they are meant for some
Amps, not Milliamps.  Btw, there are two transistors in series, so each
one takes half the resistance.  And nobody stops you from adding an
additional resistor.  But low power MOSFETs are easily in the hundreds
of Milliohm, if not Ohms range.

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foobar wrote:
> They don't limit it, they completely turn off on overcurrent (usually
> discharge; don't know about charge, don't think so).

That's true, thank you for reminding me.

foobar wrote:
> When they activate,
> the battery may already be damaged.

Does this not depend on how I choose the transistor?
As I want to protect the battery, I need to choose the transistors so
that they switch off before too much current can damage the battery.
Is this not the purpose of a battery protection circuit?

foobar wrote:
> there are two transistors in series, so each
> one takes half the resistance.

But usually there is one transistor for charging and one transistor for
discharging the battery.

foobar wrote:
> low power MOSFETs are easily in the hundreds
> of Milliohm, if not Ohms range

I will check this.

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>> When they activate, the battery may already be damaged.
>
> Does this not depend on how I choose the transistor?
> As I want to protect the battery, I need to choose the transistors so
> that they switch off before too much current can damage the battery.

You can try, but it's not what they are made for.  I.e., the tolerances
are rather high (DW01: slow trigger: 120-180mV, quick trigger: 1.0-1.7V;
overcharge limit: 4.3V; etc).  For charging you need an additional
current limiter anyway and only use the (too high for normal use)
overcharge protection.

> Is this not the purpose of a battery protection circuit?

No, think of them like fuses or RCDs.  They protect you from an
exploding battery ;-)

If you want a charger, why don't you take a charger IC?  They know how
to charge batteries (CC/CV mode), they have a current regulator, they
have precise references and they will stop at the right time.

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