MickCheese":3414lmrz said:
You've made me think.
I have a 12v intelligent trickle charger that I use for my motorbikes.
I wonder if that would do the same?
Sadly, if it works at all, it will most probably cause a loud bang and a mess.
The different chemistries need different charging techniques:
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Lead-acid cells are 'constant voltage'. They are charged at about 15V for a nominally 12V battery (a battery is several cells wired together, in the case of a car battery 6 x 2V gives the nominal 12V of the battery).
Trickle charging involves maintaining the voltage (at, say, 14.7V) and limiting the current so that only a trickle flows through the battery. "Intelligent" chargers detect both voltage and current flow during the charge, and switch to trickle mode at the end. It's also the method of charging standby lead-acid systems for emergency lights, etc.
Lead-acid cells like to be kept fully charged, and are damaged if deeply discharged (even the ones specifically designed for that - it shortens their life!).
You can get LA batteries packaged as 'torch' batteries (in 'C' and 'D' sizes: they used to be branded 'Cyclon' and have slightly different characteristics to the normal 'wet' cells, and are found in specialist industrial applications.
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Nickel Cadmium (NiCd) cells need constant current charging. The design of the charger system is (or should be) quite different to lead-acid. Weight-for-weight, they usually have smaller capacity than LA but can deliver higher currents at a given size) - this is useful for motor applications, which is why they're used in power tools (where current delivery is more important than voltage).
NiCd cells dislike being left charged. Persistent topping-up is the most common cause of cell failure. Tool batteries should be run till
virtually flat (the point when the drill struggles to turn a No. 10 screw into softwood is a good indicator), and only recharged at that point. Don't let them go totally dead (I won't bore you with why).
Resuscitation techniques:
Lead-acid: those tablets from Halfords - they suffer from sulphonation ('sulfation' across the pond), after being deeply discharged, and if the sulphates are chemically removed the battery may function again. If it's a liquid battery, you can detect the failing cell (one always goes first) with a hydrometer. Just do the bad one first and see if it improves. Sometimes draining that cell and refilling with sulphuric acid works, but it's not for the faint hearted (and you need acid!).
NiCd: They are often over-charged by cheap consumer chargers. This will cause one cell to fail first, rendering the battery useless (flattening completely also causes this). The endpoint (fully charged) is marked by a detectable heating-up of the cells, and a duff one will heat earlier than the others. So, if you can get into the battery without destroying the connections, or (better) you can work out which cell is which from the outside, charge it and feel for a warm spot. Replace that cell.
You can sometimes revive cells by putting them in the freezer for a short while, letting them warm back to room temp, and recharging them. But sometimes they freeze and split!
A different aspect to cell failure is dendrite formation. Dendrites are pointy conductive crystals that grow out from the battery plates. They grow in the electrolyte, and will punch through the blotting-paper spacers used in the 'dry' cells of power tools (think Superman movies on a tiny scale). Once dendrites from opposite plates touch, that cell is short-circuited and useless. They can sometimes be literally be blown apart by a sudden short-circuit: charge the battery and carefully connect two fairly chunky wires with bared ends to the battery contacts (car wire works well), and just brush them across each other quickly a couple of times - there should be sparks! You're not aiming to flatten it, just surprise it! Charge the battery again and see if it has recovered.
Cells that have grown dendrites have a propensity to do it again, so the above is at best a temporary fix. It also will not work if the battery has an internal fuse (common in 'phone batteries and other similar appliances) - you'll just kill the battery completely and the fuse isn't easily replaceable.
Note that NiCd are nominally 1.2V per cell (not even 1.5V!) and completely incompatible with lead-acid chargers.
Don't get me started on
Lithium hydride batteries. They're used in laptops, phones, etc. and they hate frequent charging. They do, however have a high power density (which is why they're used in that app.) - they work well for the first month or so then tail off markedly. In a nutshell, I don't think there is much you can do to revive them.
Sorry - the above is a bit scrappy. I'm sure Wikipedia has more info and probably more tips, but the above all come from personal experience. I used to rely on rechargeable kit, long before the laptop was popular and needed to know how to keep the batteries fettled. My dad also designed and successfully sold a battery reviver for the radio controlled model market, back in the 1970s - it worked by deep-cycling NiCds in a controlled way. Anyway, I think I've avoided most of the old wives' tales in circulation.
HTH.
PS:
CAUTIONS: High-capacity NiCds in good condition can discharge almost explosively (you can weld with them, although I don't recommend it!). If you dismantle any battery you have to be very careful not to short out individual cells as they're often connected by thin metal strips that can touch each other.
The liquid contents of either type of battery is nasty too, in different ways. If you have one or more leaking cells, put the battery into a plastic bag and recycle it carefully. If you think you're opening a battery with burst cells, wear surgical gloves or similar.
PPS: A few commercial battery suppliers will re-cell the more expensive industrial NiCd batteries. They usually send them away, but it can be done for less than the cost of a new one. It's worth enquiring.
. The power supply was a car battery, with a handle, connected via a short lead to the drill, 12V I assume. Shortly after Ni-Cad cordless drills turned up.
