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'[EE]: battery charge ?'
2011\11\21@113802 by

Hi all,

what can happen if 2.5v connected to 3v  2xAA none rechargeable battery.
I did some mod on the board but there is a condition where the user can make a mistake and 2.5v will connect
to battery line.

any idea?

thanks

AA
On Mon, Nov 21, 2011 at 11:06 AM, Andre Abelian <abelian.andreyahoo.com> wrote:
> Hi all,
>
> what can happen if 2.5v connected to 3v  2xAA none rechargeable battery..
> I did some mod on the board but there is a condition where the user can make a mistake and 2.5v will connect
> to battery line.

My take on it:

If battery voltage is > 2.5V and no diode is present on the 2.5V,
current goes into the 2.5V line.

If battery voltage is < 2.5V and the 2.5V line isn't current limited
and connected abruptly to the battery, depending on exact battery
chemistry, it could leak or burst. BUT alkalines CAN charge a limited
amount with low currents (less than 10mA I think).

If the 2.5V line is always connected to the battery line, I think
battery voltage won't dip below 2.5V as both sources provide the
power.
> what can happen if 2.5v connected to 3v  2xAA none rechargeable battery..> I did some mod on the board but there is a condition where the user can make a mistake and 2.5v will connect> to battery line.

If this is a transient or short term condition it is unlikely to hurt
the batteries (for most values of unlikely :-) ).
I'd expect you to "get away with it" for tens of seconds
"occasionally". ee below re charging and possible high reverse
current.

If batteries are relatively new the terminal voltage can be as much as
2 x 1.6V = 3.2V. Say 3V. You then have 3V Alkalines connecting to a
2.5V line. Internal resistance of a new AA Alkaline is well under 1
ohm (perhaps as low as 0.2 ohm best/worst case?) so IF the 2.5V line
will accept current you may get up to hundreds of mA from the battery
into the 2.5V line. This would only be if it was "stiff" and low
impedance. In many cases a circuit's voltage will rise to near the
applied voltage and current will be lower or not flow.

What does the 2.5V circuit consist of?

Alkaline batteries in many cases will accept substantial charge rates
and may charge with variable degrees of efficiency depending on
circumstance. Some Alkaline batteries in poor condition may be driven
into catastrophic failure Venting may occur but is far more likely to
be a whimper than a bang.

All the usual disclaimers: YMMV / Caveat Emptor / Don't try this at home /  etc.

Russell
Thanks you all

From: RussellMc <apptechnzgmail.com>
To: Microcontroller discussion list - Public. <piclistMIT.EDU> Sent: Monday, November 21, 2011 9:13 AM
Subject: Re: [EE]: battery charge ?

> what can happen if 2.5v connected to 3v  2xAA none rechargeable battery..> I did some mod on the board but there is a condition where the user can make a mistake and 2.5v will connect> to battery line.

If this is a transient or short term condition it is unlikely to hurt
the batteries (for most values of unlikely :-) ).
I'd expect you to "get away with it" for tens of seconds
"occasionally". ee below re charging and possible high reverse
current.

If batteries are relatively new the terminal voltage can be as much as
2 x 1.6V = 3.2V. Say 3V. You then have 3V Alkalines connecting to a
2.5V line. Internal resistance of a new AA Alkaline is well under 1
ohm (perhaps as low as 0.2 ohm best/worst case?) so IF the 2.5V line
will accept current you may get up to hundreds of mA from the battery
into the 2.5V line. This would only be if it was "stiff" and low
impedance. In many cases a circuit's voltage will rise to near the
applied voltage and current will be lower or not flow.

What does the 2.5V circuit consist of?

Alkaline batteries in many cases will accept substantial charge rates
and may charge with variable degrees of efficiency depending on
circumstance. Some Alkaline batteries in poor condition may be driven
into catastrophic failure Venting may occur but is far more likely to
be a whimper than a bang.

All the usual disclaimers: YMMV / Caveat Emptor / Don't try this at home /  etc.

Russell
On Mon, Nov 21, 2011 at 12:13 PM, RussellMc <apptechnzgmail.com> wrote:
> Alkaline batteries in many cases will accept substantial charge rates
> and may charge with variable degrees of efficiency depending on
> circumstance. Some Alkaline batteries in poor condition may be driven
> into catastrophic failure Venting may occur but is far more likely to
> be a whimper than a bang.
>

I have seen accidentally-trickle-charged Alkalines spew electrolyte
all of the surrounding components so I would think that one should be
very wary of charging Alkaline batteries even at very low currents.

Sea
On Wed, Nov 23, 2011 at 10:43 AM, Sean Breheny <shb7cornell.edu> wrote:
> I have seen accidentally-trickle-charged Alkalines spew electrolyte
> all of the surrounding components so I would think that one should be
> very wary of charging Alkaline batteries even at very low currents.

I found that it depended on the company/specific chemistry on how well
they tolerated charge

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