(1) Does it really harm batteries to over-charge them?
(2) If so, where can one get a charger that switches off when the battery is charged?
(3) How can one tell with a multimeter if a battery is charged? Can one tell from the voltage? If so, at what voltage would one say the battery needed charging?
(4) Similarly, for non-rechargeable batteries, can one tell from the voltage on a multimeter if the battery should be thrown out, and if so, what is the critical voltage for 1.5v batteries?
What type of batteries? NiMH, NiCd, Sealed Lead Acid, Vented Lead Acid?
The answer is generaly yes, though.
You need the right type of chrger for the battery type you are charging. Usually you get them from the same place you getthe batteries, or try Maplin, Rs, of TLC...
You can't really tell with just a multimeter. A battery right off the charger might well read above full voltage, but drain a little bit of current from it and it might appear flat again. (Although lead acid types are more tolerant of this test)
The critical voltage is the voltage at which your device stops operating.
Different types of battery will behave in different ways - Old Zinc cells and Lead Acid types will degrade gently through their life, alkaline have a flatter discharge curve and Lithium even flatter, going from almost full voltage to next to nothing in a matter of minutes (or less!) depending on the application.
Sorry if this answer doesn't help much.
Battery technology is forever changing though, so who knows what will come next year...
Gordon
Here's one for you - why do batteries on quality tools (nicad and nimh) seem to go from almost full power to nothing very quickly, whereas on cheaper power tools they gradually get slower and slower as their charge runs out?
Alex
I'd guess that the cheaper ones might use NiCd and the expensive ones use NiMH these days. NiCd have more of a discharge curge than NiMH. And I'm sure that within the battery types there are expansive types and cheaper ones which don't perform so well.
They might also have better internal circuitry - eg. a 12V battery, but only run the motor at 9V with a power regulator inbetween that will cope with a volt or 2 drop then shut off before draining the battery completely. Hard to tell though without knowing exactly whats inside.
In the past few years I've seen NiMH AA capacity go from barely 1AH capacity to over 2, so as I said, who knows! It wouldn't surprise me if NiCD development had all but stopped though.
Gordon
It depends on the battery type and the *rate* at which you overcharge them but all rechargeables will suffer eventually. NiCd will tolerate a low rate of overcharge without too much damage (most will anyway, it does depend on the design). A 'low rate' of overcharge means something less than the 10 hour rate, preferably something like the 20 hour rate. Many 'overnight' NiCd chargers run at this sort of rate and won't do
*much* damage but it's still better not to overcharge.
Lots of places, but they need to be designed specifically for the battery type. It's quite difficult to tell whether the 'automatic' chargers you see advertised are really intelligent ones or whether they are just timed, it's a bit of a minefield.
Not easy from the voltage.
Yes, probably, one of my multimeters even has markings for 'good', 'marginal' and 'bad' 1.5 volt and 9 volt batteries. However it's not perfect and, as others have said, it does depend very much on the application. You need to apply a load to a bettery when measuring the voltage to see if it's any good too.
Probably somewhere in the 1.0 to 1.2 volts area, but sobject to so many caveats that doesn't really help you very much.
The cheaper tool might well have one or more faulty cells.
Sorry to be persistent, but if you can't tell with a multimeter how do the intelligent chargers know when the battery is charged?
varies with battery type but the main technique used is measuring the current flowing through the battery with temperature sensing often used as a backup.
NiCd and NiMH chargers are constant current, indeed the only battery technology that I can think of that uses constant voltage charging is Lead Acid...
Detection of the rate of change of the terminal voltage is normally used. Once the rate starts to fall the battery/cell is reaching fully charged, the temperature will also start to rise more rapidly as the supplied energy is no longer stored but dumped (for want of a better expression).
Defnitely it does some.
Lithoum polymer and lead acid can both explode...
What battery?
Yes fopr lead acid and lithium polymer, yes, but its not that simple with NiMh and NiCd.
That depebnds entirely on teh chmistry
About 0.1v. After that they won't even power the meter needle.
NiMh. It runs out like a dog. Also cheaper batteries maintain voltage less well under discharge.
Umm. Lithium uses conatnt curent/ voltage limited, the same as lead acid...
Nickel chemistry uses peak detection. At some point in th1e charge the voltage starts to FALL - that's time to stop charging or at least switch to trickle charge.
>
When a NiCd or NiMh battery reaches full charge there's a little blip in the voltage, it drops slightly by a few tens of millivolts. The intelligent chargers detect this and turn off. (Old fast chargers used to have temperature sensors which stopped charging when the battery got hot, really good chargers use this method as a sort of backstop)
Lithium I don't know. Lead acid is constant voltage, with the voltage effectively setting the "fast", "standy" etc, any current limit would be just a limit from the chargers power supply. The current will fall as cell voltage increases but the applied voltage is constant. "Smart" lead acid chargers will automaticlly switch the voltage output in response to the detected charge state.
It is down to the quality of the cells, and how well matched they are.
In addition to the better cells having lower internal resistance (hence able to deliver more current) and more capacity, their discharge characteristics will be far more consistent between different examples of the same type of cell.
This is quite significant in a battery pack since ideally you want all the cells that make up the pack to run out at the same time. If they don't you get into the situation where the performance drops off in steps with each cell that goes flat. (there is also the possibility that you will start to damage the flat cell if you keep drawing current from the battery in this state. Hence you should try not to wring the last few drops of power out of it as it goes flat).
The "fast electric" radio control enthusiasts will often go as far as computer matching cells, so as to make up packs from cells with as near identical drain characteristics as each other.
It makes a significant difference to the overall performance of a cheap cordless drill too. I re-celled a pack on my PP 18 volt, and it made a huge difference to it even compared to a new standard pack. Trouble is it cost nearly the price of a new drill complete...
Well if you really want to fall over, look at the price of a Lithium pack that will do a kilowatt for 15 minutes.
At about $2.80 for a watt hour, thats arond $1000.
Mind you, you have the equivalant of a moped engine and totally verical performance on a fair sized plane :D
I priced up a 'town car' on rechargeable lithium. Target was about 250 bhp peak, 300 mile range and ability to recarge fully in 10 hours on 13A supply (domestic) on night rates.
Last year it was $150,000 for the batteries. This year its more like $50,000.
Trouble is the safety circuits and crash resistance are horrendous, and the likely lifetime of the battery is about 3 years.
Mind you against £1.20 per 300 mile tank of electricity, it starts to look better..
A 300 mile tank of petrol costs about £30 these days, so at 10,000 miles per year, thats £3000, and perhaps another £500 on maintenance that the electric shouldn't need. So...ways to go yet, but its not so far off.
places that sell battery chargers? Look for a multistage charging one.
yes, though it does depend. For small household cells and carbon zincs (which can indeed be recharged), an analogue meter on 10A range, touch the tips on the battery /very/ momentarily, and speed of needle movement indicates charge. Fast = full, sluggish = empty.
Also a 1.5v cell delivering 1.2v off load is a bit of a giveaway.
Forlead acids you need more than a mulitmeter.
whatever voltage the appliance drops out of function at. If a carbon cell is down to 1.4v off load, its not got much life left.
NT
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