Alcaline batteries subjected to higher voltages

[Roger44]

Hello

I have just repaired a childs toy that runs on rwo 1.5V alcaline batteries or an external 4V DC pack. Some thing was wrong with the its external voltage regulation/protection circuit. As it has surface mounted components, I dropped the idea of examing all that, and linked the external source to the battery connections via a resistance and then a 3.05 zener diode to bring the voltage down. I think you know what I mean. It works fine.

I've just got one question. Sopposing the alcaline batteries are down on voltage, and I then subject them to 3.05V, how are they going to react?

 

[Sceadwian]

They're going to heat, vent gas, and possibly leak caustic compounds into the battery compartment with a slight possibility of fire.... 1.5V is nominal full charge for an alkaline... Please be aware of the use of the word nominal!



It works fine for now. It's rigged not sane engineering.

 

[ccurtis]

They will not react well. In lieu of repairing the regulation circuit, I would buy rechargeable Nicad AA's (or whatever the size is) and a separate charger for them from Radio Shack, or wherever, if the idea behind your contraption is to avoid having to constantly replace non-rechargeable alkaline batteries. The other option is to remove the batteries from the compartment and get a clean 3V DC power source to hook-in their place.

 

[Roger44]

Spot on, Sceadwin.
While awaiting your answers, I connected it up to the mains last night and 4 hours later both batteries were hot, and one was also leakiing. Fortunately the leakage was confined to the battery compartment, springs etc, easily cleanable. There may be some vapour spread from the hot liquid leaking out, but I can do nothing about that.

Not good engineering you're right, I've just added a Schottky 0.24V to stop the external source current flowing into the batteries, elementary, I should have thought of that.

Two further questions arise :
A. Are non-rechargeable batteries manufactured with precautionay venting or would that just be mechanical failure under internal pressure?

B. The unusual 3.05V zener voltage. I'll develop this point in detail as it may be of interest concerning aging of electronic equipment. This componant came from a box containing zeners recuperated in the 70's or 80's from old boards (TV,radio, cassette tape recorders etc). Selection by measuring with a 4.1V batt supply supply + 1k resistance, so we can say 3.05V at 1ma. There was a second zener, pratically identical in appearance, which came out at 3.4V, so less suitable. Here are its marking :

http://hpics.li/0ba2b7b

TFK BZY 6v2. TFK telefunken means it's old. It should be 6.2V. Something tells me that it must have been around 6.2V in the past or the circuit would have been rejected, not like a capacitor at half value which may go undetected. I googled on zener aging but got pratically nothing other than that their voltage does go down with the years, but to half value? Do you have any further knowlege on the subject?

 

[Sceadwian]

All batteries are generally designed to vent in a specific spot in case of failure. This is as simple as an intentional weakenening in the casing so pressure doesn't build up to the point of explosion. You see it on capacitors all the time in the form of a X shaped dent on the end, used to blow them up when I was a kid with a massive reverse voltage, they go off like a cross between a shotgun shell and a party popper ;>

The zener could have been damaged by a current spike but still functional? That's definitly too much of a drop from nominal to be aging, outside of power electronics anything not abused and kept cool will age very little.