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Posted by Piccolo Pete on July 5, 2008, 4:18 pm
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>
> ====snip====
>
>> So I'm not sure what you're saying here. Sounds like the really old
>> batteries could deal with their polarities being reversed but the new
>> ones
>> can't. By the way, I already blew up one charger by accidently reversing
>> polarity. I don't want to blow another one.
>
> The preformed plates type of battery can be reverse charged but you
> need to totally flatten them first and use some sort of current limiting
> at the start of the reverse charge cycle since you can't totally flatten
> a battery within the 24 hour grace time allowed before the lead sulphate
> starts to crystalise into a form that no amount of charging will be able
> to completely reverse.
>
> This sort of treatment should only be resorted to when all else fails.
>
> In practce, a lead acid battery can tolerate having one or more of its
> cells reverse charged provided the whole battery isn't left in the
> resultant 'flattened' state for very long (usual advice is to put it on
> charge within 24 hours of being fully discharged to avoid significant
> permanent damage).
>
> Batteries assembled with other forms of secondary cell such as NiCads
> and NiMH types cannot tolerate such abuse where any such reverse
> charging would permanently reduce their AH capacity.
>
> Equipment designed to be powered by such batteries have battery voltage
> monitoring circuitry to disconnect/switch off once the per cell voltage
> gets down to around 1 volt in order to avoid this harmful condition
> (lowest capacity cell being reverse charged by the discharge current
> from the rest of the battery pack).
>
> The problem of reverse charge protection increases as more and more
> cells are used to create higher battery voltages. Equipment powered from
> a 2 cell battery pack (eg. a digital camera) often use the battery
> voltage monitoring primarily to trigger a controlled shutdown to protect
> the equipment rather than for the sake of the battery pack's health. It
> just so happens (rather neatly) that such equipment protection happens
> to save the battery pack as well.
>
> The ideal "battery" would consist of just one cell since this avoids
> the problem altogether. The problem with this ideal is that very little
> equipment can be directly powered from 1.2v and the losses in even the
> most optimised of switching converters required to generate the higher
> voltages required become unacceptable when more than a watt or two is
> required.
>
> The problem is eased somewhat when a 2 cell pack can be used. At a 2
> volt end point, it's virtually impossible to reverse charge a cell and
> the two cell battery pack represents an optimum battery cell count size
> for a lot of portable equipment which can be designed to work off a 2.4
> volt supply.
>
> Considering the reverse charge issue with NiCad and NiMH batteries,
> there's a lot to be said for the good old fashioned lead acid battery,
> especially when 12 or more volts at high power levels are required. You
> just have to make sure you're using the right type of lead acid battery
> for the type of service they need to provide (and the appropriate care
> and attention).
Tilt... God, I've been doing electronics for over thirty years and you guys
make me feel like a total idiot when it comes to batteries. I'll try
rereading your post a few more times to see if I can get a grip on what you
are saying.
Thanks for the info though.
Bart
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