Strange battery

[waz]

Good day.

Last week I came across one of those emergency light power supplies, the
type with a 6 volt lead battery inside that are constantly hooked to a
charger.

I took it home and plugged it in after attaching the battery (6v 7Ah).
The yellow charge light came on. I left it for several days.
Today I took the battery out.
I measured the voltage.
It read -3.x volts (lets say -4 volts).
I had the common lead on the meter connected to the black post (clearly
marked negative) and I had the other lead connected to the red post on the
battery (also clearly labeled positive).
I tried again.
-3.x volts. As if I had the meter connected backwards.
"This isn't right" I thought. Impossible. Something is backwards here"

I took my meter and tested the voltage across a battery I knew was good, a
12v 7Ah I had been using for years.
Red to red, black to black I read +12 volts or so. No problem with polarity.

BAck to the 6v battery and I see -3.x volts.
I place a 6v lightbulb across the battery leads.
It lights up.
Not bright, but there is current flowing.

I check the charger. The dark wire (blue) connects to the circuit board at a
spot clearly marked - bat.
The red wire connects to the board at a spot marked +bat. I connect the red
wire to the red post of the battery. The blue wire to the black post. Just
like I did when I got the thing home last week.

Turned on the charger.
I read -1.85 volts across the battery terminals.
When the battery charger is removed the voltage reads -3.3 volts but slowly
climbs to -3.7 volts (within 2 minutes).

What the hey is going on???
Is this battery shot?
If so, can someone explain the chemistry going on? It is like I have not
only drained the battery, but actually drained it past the 0.0 volt level
and taken it into a state where a reverse reaction is going on.

Anyone?

 

[Don Bruder]

What the hey is going on???

At least one cell of the battery has been discharged too far and
polarity-flipped.

Is this battery shot?
Yes.

If so, can someone explain the chemistry going on? It is like I have not
only drained the battery, but actually drained it past the 0.0 volt level
and taken it into a state where a reverse reaction is going on.

That's a reasonable (though highly simplified) explanation of what has
happened - The battery has flipped polarity due to having been drawn
down too far before being recharged. Not *REALLY* common, but a known
issue, particularly with NiCd batteries.

As to describing the chemistry, it's beyond my own
understanding/education, and therefore outside of my ability to explain.

Suffice it to say that you're not seeing anything new - It's not common,
but it is a well-known failure mode of most rechargable battery types
when subjected to heavy/prolonged discharge, with NiCds being the most
commonly seen "victims".

 

[waz]

Thanks for the reply.
I'll take your word that the battery is dead.
I still wonder about the chemistry though.
One day I'll figure it out.

Thanks again.

Waz