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Normal Aa Battery Vs Led Light Question

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Ok here is one for you electrical technical experts. Nothing to do with boats, but I have asked other knowledgable people I know and googled this and not come up with an answer. 

We have 2 sets of led lights powered by 3 aa batteries each. One has worked faultlessly for a month, before I had to change the batteries, the other had 3 sets in it. Each set has 3 positions, off, on, timer (on for 6hrs, off for 18).

So, batteries (of a well known copper colour) were installed, tested and worked and both units left off. After a week or so I wanted to put the timer on. One set didn't work. Checked batteries and 2 were very flat, one was actually reverse polarity!! Assumed faulty batteries

new set of batteries and timers set. 4 days later same set of lights dead. Batteries checked and same problem. 

New batch of batteries purchased. Again after a few days same set of lights dead and batteries same issue. 

Put batteries from good set of lights into dodgy lights and new bats into other lights. Next day same set of lights has killed the batteries. 

So, we get a new set of lights as they have to be faulty. although how is it only one battery ever goes reverse polarity?

anyway, the set of lights that had played up I can now dissect. See below. 

First my meter is not quite zerod, but it was good enough. I'm on the 10v scale as well.

Next is the battery voltage off load.

see next post for what happens when on load.


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Part two. On load batteries go reverse polarity. This is the same whether in the off, timer or on mode.


This picture I have reversed the probes to show the reverse polarity voltage is about 1.6v


So I have now also checked the older working set and new set and as soon as you put batteries into them they both go reverse polarity too!!! Although the other original set seem to last a fair while.

i have never seen this before and was expecting the voltage to drop slightly when they were turned on, but not this. 

So any ideas you bright sparks? 

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In post 1 you are measuring from the positive of the top battery, through the light circuitry to the negative pole of the lower battery. Giving you 3volts.

In the second post you are effectively measuring the middle battery, with your meter connected firstly in reverse, then correctly.

Assuming I'm seeing the battery pack connections correctly. Blue are links, White is Neg, Red is Pos?


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it has been known for the wiring inside components to be faulty, I once bought something that did not work until I put the batteries in backwards, ie with the positive towards the spring end, generally a lack in the quality control department.

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looking at your meter connections, the bottom picture, you are correctly testing the middle battery, in the top picture you are once again testing the middle battery but with the probes reverse polarity, you have your positive probe connected to the positive end of the top battery (which is the negative end of the middle battery, and your negative probe connected to the negative end of the bottom battery (which is the positive end of the middle battery).

Clear as mud - probably

are the lights both the same colour? some colour LED's use more current than others, so this may be a contributing factor.

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I agree with grendel and regulo, although what neither have said (but both implied) is that the battery box is not wired as you assumed!  The top-right corner is negative and the bottom left positive.  So:  With the top cell displaced you are measuring across that cell, through the circuit and then through the bottom cell hence the 2.5v (there's probably a diode in the circuit hence 0.6v dropped).

The "negative" reading is because you are measuring across (just) the middle battery backwards.  With the leads reversed it's just over 1.5v as expected.

My guess is there's a "short" somewhere allowing a continuous discharge even when you think it's "off".

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1 minute ago, Jonathan said:

My guess is there's a "short" somewhere allowing a continuous discharge even when you think it's "off".

I think that's almost certainly the case, perhaps an errant piece of solder shorting two contacts


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Cheers guys.

just to clarify, I am measuring across the 3 batteries in series. It might not look it, but I am. All 3 are in series.

Stumpy, my initial thoughts as the batteries came from eBay!!!!! But using a known good set, didn't last 24 hrs. 

As I said, taking the same measurements on a known good set of lights shows the same voltages which totally bemuses me. 

Regulo, correct. Measuring across all 3 batteries, but should be 3.6v not 3 so looks like the batteries are rubbish. But doesn't explain the negative polarity when connected up. 

Going to measure each battery. 

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Ok school boy error. I'm a plonker!!! 

Thanks to you guys for making me double check!!!

the colts are all good and 4.5 volts when I measure across all 3!

volts stay positive when connected up. 

But......the dodgy lights turn one of the batteries reverse polarity after a while. So I reckon dodgy imitation batteries or dodgy lights. Maybe all from china!!!

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