Naked Science Forum
Non Life Sciences => Technology => Topic started by: graham.d on 12/04/2008 15:15:43
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I bought one of these high power torches (more of a searchlight really) but got a cheap one on offer at B and Q. It was made by Draper; well it had their name on it but was very cheaply made in China actually. It was fine for a few weeks but then stopped working. I took it apart and found that the VRLA (Valve Regulated Lead Acid) battery was high impedance. The crude charge circuit developed 18V across this nominal 6V battery. Out of circuit the battery produced all of a few 100mV only on a meter. I took the battery out and found that it was easy to remove the rubber caps on the three cells. There appeared to be no electrolyte, which would certainly explain the lack of functionality.
Does anyone know why the battery should have suffered sudden death or if it can be rejuvenated by adding deionised water? I confess I have added water to see what will happen but I am not expecting a good result. It was only £12 and the construction, mechanical and electronic, is commensurate with the low price. A false economy I expect. I think the battery is of the fibre-packed sort rather than gel based, but I am no expert on battery construction.
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Your big mistake was not to take it back as soon as it failed. You did what I tend to do: you poked about inside and invalidated the guarrantee!!
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Yes. You are absolutely right. But for £12 against driving back to B and Q and arguing the case... Well, I thought there was a chance I could fix it. :-)
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I saw them in B&Q too. Glad I resisted the temptation. TNSTAAFL, remember.
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Yes, no free lunch but look at the enjoyment I got from dismantling it.
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I can't imagine a better 12 quid's worth of fun.
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Just to confirm: adding DI water did not fix the battery. Apparently, if the battery voltage falls that low, it is considered not recoverable. I am not sure of the chemistry, but I tried charging it for about 30 hours with no effect.
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Two simple plates of lead should be chargeable, to some extent, but you need a big surface area for a lot of charge. The surfaces in accumulators are usually made 'spongy' to provide this surface. Once they have been wetted and then dried, this surface is lost. This seems to be what has happened to your battery. I would suspect that you would detect a small charge, initially?
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Yes, it charged to all of about 200mV. Not very good for a 6V accumulator. I did read that it may be possible to recover the battery with a whole set of chemical treatments and renewal of the electrlyte, but I gave up at this point as it wasn't worth the effort. Even then it was suggested that discharging to zero volts was bad news, so your analysis may be right. Surprisingly I have got a few litres of Conc Sulphuric in the garage, acquired for other purposes, and that could have been suitably diluted. A new torch is much easier though and with much less chance of dissolving my fingers.
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I believe it is possible to get some of the cells reverse biased or possibly short circuited?
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I can't tell completely because the three cells are series connected internally. I can see that the output impedance is rather high though (charging voltage of about 18V). The cells had all dried out judging by the amout of water I could add to them. There was no sign of this being through leakage so I can only assume it was due to outgassing through the valves. I did notice that one of the wires that connected to the battery terminals was nearly cut through by being pressed in contact with something. I can't be sure, but if this had been trapped against the other terminal, it could have resulted in a short circuit. This could result in dry-out, or worse, could have resulted in a fire from the likely heating.
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I have tried rejuvnating several of those batteries from my UPS and emergency light system - nothing worked including adding Bismuth Sulphate etc. So, best solution is to buy a new one. Most of my betteries last about 4 years and then just die.
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Thanks, kmguru. I have binned it now. It was only that it was new and, as Sophie suggested, I couldn't resist taking it apart rather than, more sensibly, putting it back in its box and taking it back to B and Q.
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I find it hard to understand why anybody would make a torch with a lead acid battery. It would make it heavy and lead acid batteries don't cope well with being completly discharged.