WARNING on backwards battery

[Juggernaut]

I don't know about you guys but the last time I checked you could put one battery in you light backwards so it will not allow the rest to discharge / drain them selves, allow the light to accidentally turn on. Well that's what I have heard anyways. Well it's not TRUE! As I sadly found out yesterday. Just before I was about to install a new 30 watt sealed beam bulb on my 10 D rayovac I decided it was best to switch the battery in the back around so that when I was installing the bulb it would not accidentally turn on in my face "the switch can be finicky" . So just as I screwed on the tail cap the contacts meet and to my horror the 4.75 volt 4546 bulb originally in the light "earlier I was using it with 6 volts" turned on and instaflashed! :mecry:Noooo I yelled as I realized that my vintage bulb from the early 70's with a slight dimpled light defused glass face was gone forever. I will never again find such a bulb as I am quite sure they stopped making them and a normal clear lens replacement cost $10 with out shipping. So I just wanted to worn people that it is possible for your light to still function with a revered battery. Maybe I'm the only one here that thought that worked though.

 

[Probedude]

So I just wanted to worn people that it is possible for your light to still function with a revered battery. Maybe I'm the only one here that thought that worked though.

Reversing one battery will only drop the output voltage by 1 cell. It works for a 2 cell light.

In your case it was

10D but you reversed 1 so it was now
9D - 1D = 8D * 1.5V = 12V

You needed to reverse 5 of the cells for zero output voltage.

 

[Mr Happy]

Sorry about your bulb, but you have been misinformed. The way to prevent a light accidentally turning on is to remove one or all of the batteries from it.

The voltage of batteries adds up in series. So if you put 4 x 1.5 V in series you get 6 V. If you turn one of the batteries around you get 1.5 + 1.5 + 1.5 - 1.5 = 3 V. (You subtract rather than add the reversed battery because its voltage is now negative.) Furthermore there is nothing about a battery that will stop current going through it the wrong way, so a 3 V bulb would light up for this arrangement.

 

[Juggernaut]

Thanks for the clarification, I wish I had knew this sooner.

 

[TorchBoy]

Reversing one battery will only drop the output voltage by 1 cell. It works for a 2 cell light.
...
You needed to reverse 5 of the cells for zero output voltage.

+1. We live and learn - sometimes the hard way.

Of course, turning half the cells around doesn't work with torches with an odd number of cells.

 

[Probedude]

+1. We live and learn - sometimes the hard way.

Of course, turning half the cells around doesn't work with torches with an odd number of cells.

True but at least he'd only be at 1.5V worst case.

Today it dawned on me when I was in the attic (and Mr. Happy mentioned it), instead of turning half the batteries around, just take out one! No way it'll turn on then.

 

[2xTrinity]

In general reversing the polarity on a cell is asking for trouble anyway -- as the voltage of the other batteries can actually "reverse charge" it if it gets switched on and essentially ruin it. In the case of a NiMH or alkaline, this could lead to nasty leakage inside your light. In the case of a LiIon without a protection circuit, cell reversal could lead to "venting with flame". Improper balancing circuits in computer battery packs led to cell reversals, which is what led to the the infamous battery venting cases.

 

[Mr Happy]

Reversing one battery will only drop the output voltage by 1 cell.

Just thought I'd mention since nobody else remarked on it, but reversing one cell will drop the voltage by 2 cells.

In general reversing the polarity on a cell is asking for trouble anyway -- as the voltage of the other batteries can actually "reverse charge" it if it gets switched on and essentially ruin it.

In fact, not reverse charge it, but forward charge it just like a battery charger. NiMH cells will be cool with this as long as they don't get overcharged, but alkalines won't like it and might be caused to leak.

 

[2xTrinity]

Just thought I'd mention since nobody else remarked on it, but reversing one cell will drop the voltage by 2 cells.


In fact, not reverse charge it, but forward charge it just like a battery charger. NiMH cells will be cool with this as long as they don't get overcharged, but alkalines won't like it and might be caused to leak.

<pictures positive and negative signs in head>



You're right, I should think more before I post... At least I don't make those kinds of dumb mistakes when working with extremely expensive equipment... <hooks up $10,000 amplifier with incorrect polarity>... err, nothing to see here... I was just, smoking in the lab... yeah, that's it

Of course, if you have an odd number of cells, it's very likely that one will be overcharged, as there will be a higher number of cells summing their voltages and "charging" it.

Cell reversals take place when the cell is in a series stack in the correct orientation, but more drained than the others. Its voltage drops, and gets charged in reverse by the others.

in the RIGHT order, but gets overdrained, yet deu to the fact that there are other cells in series that aren't drained, gets charged by the others.

 

[electromage]

Consider yourself very lucky that you didn't make that same mistake with Lithium batteries! You could have much bigger problems than a blown bulb!

 

[Probedude]

Just thought I'd mention since nobody else remarked on it, but reversing one cell will drop the voltage by 2 cells.
[\quote]

Depends on if you're counting the reversed cell, I wasn't as you can see from the calc. Confusing - it's like when I'm programming - am I counting from zero or from 1? My brain knows what it meant, but the words and typing translate it all wrong!

 

[uk_caver]

Assuming cells are all in series, to prevent a light turning on without removing any cells, it's possible to insert a piece of paper or plastic to prevent a cell from making contact.

If cells are in holders, it's easy to place a small tab of paper or plastic between a cell pip and its contact in the holder. If cells are loose in a tube, a disc of material could be placed between two cells, (probably easier to do with thick cells like D cells than with thinner cells),.
Alternatively, with AAs, a piece of thin sticky tape could be run up the cell side, over the top and down the other side, maybe leaving a doubled-up tab at the tape end for ease of removal.
It's probably best not to have anything between the last cell and a tailspring since something there may be more likely to be damaged or displaced when the tailcap is screwed on.