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Strange battery

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?

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

Yes.

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".

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Reply to
Don Bruder

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

Reply to
waz

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