Thread: Discharging cordless drills before recharging.

sydneychicken read this
from battery university.....http://www.batteryuniversity.com/parttwo-33.htm

How to restore and prolong nickel-based batteries
Crystalline formation is most pronounced if a nickel-based battery is left in the charger for days, or if repeatedly recharged without a periodic full discharge. Since most applications do not use all energy before recharge, a periodic discharge to 1 volt per cell (known as exercise) is essential to prevent memory.

Nickel-cadmium in regular use and on standby mode (sitting in a charger for operational readiness) should be exercised once per month. Between these monthly exercise cycles, no further service is needed. No scientific research is available on the optimal exercise requirements of nickel-metal-hydride. Based on the reduced crystalline buildup, applying a full discharge once every three months appears right. Because of the shorter cycle life compared to nickel-cadmium, over-exercising is not recommended. Exercise and Recondition - Research has shown that the crystals ingrain themselves if no exercise is applied to nickel-cadmium for three months or more. A full restoration with exercise becomes more difficult the longer service is withheld. In advanced cases 'recondition' is required.

Recondition is a slow, secondary discharge applied below the 1 volt/cell threshold. During this process, the current must be kept low to minimize cell reversal. Nickel-cadmium can tolerate a small amount of cell reversal but caution must be applied to stay within the allowable current limit.
Tests performed by the US Army have shown that a nickel-cadmium cell needs to be discharged to at least 0.6V to effectively break up the more resistant crystalline formation. Figure 2 illustrates the battery voltage during a discharge to 1V/cell, followed by the secondary discharge to 0.4V/cell.

cheers rileyp

I want to complain, I had my thoughts all worked out then Crisp got in before me. Yes the idea used to be to discharge but as Crisp states its likely that one will go reverse polarity and that kills them. So charge when low not flat.

Ahh the dead keep rising

chicken chicken chicken............

NiCads will recover from discharge down to 1 volt quite happily and repaetedly........not a recommended every day occurance but quite permissable..........a blinker lamp is 15 watts not 5.....and charging thru light bulbs while cheap does not work anywhere near as well as a simple fixed resistor..... because the resistance of the lamp varies greatly with the ammount of current flowing thru it.......the result is inconsistent and you have very little choice of component value..........done quite a few experiments and had some discussions on this one......

consider also that most cordless power tool batteries are fast charge items and will stand much higher charging currents often in the vacinity of C=1 charge rate to achieve a 1 hour charge.... but you have to terminate charge before the battery gets too hot, or it will fail.......some times in a quite spactacular way.

for the most part NiMh batteries can be charged on exactly the same equipment as NiCad.
In fact there are many direct NiMh replacement batteries that work very well in equipment with very crude charging circuits.

Um..... how many people have a 24 volt "car" battery charger hanging arround.

as for polarity reversal......seriously you have to very deeply discharge a pack and that pack has to be made of poorly matched cells to achieve a genuine polarity reversal.

back when NiCads were seriously expensive I did quite a bit of battery pack recusitation, right down to pulling a pile of packs apart and rebuilding them with the surviving cells.

I have seen plenty of Open circuit cells and quite a few high resistance cells, but I cant say I have ever seen a reverse polarity cell.....I know it is suposed to be theoreticaly possible....but it is much less common than some would try to make you believe.

as for all the battery revival methods.....I have tried them all on nicads......there are only two that I have found to work at all........deep cycling...... and freezing.
And then only if the battery is only mildly sad...........if a battery is Bu####d.....its.... Bu####d.


cheers

If anyone has worked in the TV trade there you would find half the processors we replaced in the good old days was caused by a nicad going reverse polarity. At a peak I would be replacing 2 or 3 nicads a day, split them open and half would have a reverse cell If that went above 0.4V the micro was cooked. We could revive a cell most times by doubling the voltage for 10mins or so, some revived some did not but that was only an academic exercise. The only thing I never understood was a voltage rise, A 2.4V could read 2.7V and it was stuffed and didnt hold memory. Seems to defy Ohms law.

I gather we are talking about memory back up batteries, which are often not replaced even when the battery will not hold up the memory....a because the machine will still work with some fiddle or the machine is never powered down so the issue does not arrise.......I guarantee you this sort of incident would be far more common after a series of blackouts.
So I put it to you any appliance with a memory battery caused problem is very likley to have a battery that has been dead walking for some time.

Ahh but was the polarity reversal caused by the normal charge discharge cycle or was it because the battery was well stuffed and had started to decompose.

Most dry batteries will at some point in their decomposition reverse their polarity...... but this is not the same as a cell reversing during satisfactory operation.

cheers

After watching a thing on batteries a few months ago the answer is dont discharge and dont run till flat. The reason for this is the stack of batteries doesnt discharge evenly so you can get flat batteries fed by half charged batteries and creating reverse current flow. Same with TV , leave a dead cell there and up goes your micro at $80 plus fitting or your motherboard.

All very interesting
I have an 18v makita nicad battery drill I use it for driving screws, dilling small holes and other general use it is my first one and is a good gadget
I use mine until that first sign of power loss then recharge straight away, didnt really consider that the battery might be too hot to recharge has never felt hot stays in charger until at least recharged, might stay there for longer but only for the remainder of the day.
Charger has about 5 lights indicating things like broken battery,too hot, fully charged charge going in, ready to accept battery
Itis only reasonably new and doesnt get worked every day but at this point going strong!
Peter.

Thanks for the link .I worked in the tv trade for some time.I made a zapper for nicads.I would charge an electrolytic condensor with a resistor to about 30 volts or thereabouts.I would then connect the reversed battery with the capacitor for a second to zapp the little sucker.And 90 % of the time it worked and the battery could be recharged and put back into service.I know I lived dangerously but the cost a few years ago made the risk seem worthwhile.lol.I only ever zapped a single cell not a bank of cells.And the blinker globe filament is selected to supply the correct milliamp for the battery under charge so the voltage ,so long as it was 20 % greater than the pack ,could be utilised as a charger unit.As the voltage of the battery increases ,the current will reduce so that no danger of excess current is possible.Only the time has to be watched .The bulb has a certain amount of thermal regulation in its very mode of operation.

My experience with nickel cadmium tools has been that, even when you go exactly by the book, not recharging until they're dead flat, the batteries still die, and I've thrown away three of them. And these were Bosch drills, not el cheapo stuff. As a result, I'm a convert to lithium ion, which I hope will last for many years.
Keith.

I have 4 of the old 7.2 volt makita battery drills, the oldest one is well over 15 years old.
At one time ( before I discovererd compressed air) I used to give them a daily hammering.

Now note that the chargers that came with these drills are not all that clever.... constant current, 1 hour charge and terminate charge when the pack is hot enough to trip the little "Pepi" switch in the pack.

Over the years I have gone thru a good handfull of batteries ,and always got pretty good service out of them. ( I'm a battery dater, so i know)........I still have working batteries dated 2000, 2003, 2006......admittedly the 2000 battery wont hold its charge for long, but the others aren't too bad... but some new ones are due.....Even in heavy use I was getting 3 or 4 years out of a good pack and 2 years topps out of a cheap one.

Often I have been asked why I get such good performance out of my packs.....now we are talking like for like.....there was a time when this was the most common battery drill you saw.......in one company I worked for there would have been 3 or 4 other bolkes with exactly the same machine.

After schooling many of these blokes up, they reported definite improvements in the performance of their machines.

things that definitely reduce battery life and performance are.

short cycling...... charging before the battery is anything like flat, just incase.......Repeated top up charging.............recharging at the first sign of a slow down... and the like.

charging when hot, letting the battery get hot, NiCad's do not like heat....it degrades their performance in every way.

leaving the battery flat for extended periods

discharging the battery and leaving it under load when flat.



things that did bring back performance time after time.

Ocasional very deep cycling......the standard rig involved a drill and some electrical tape.....the battery was allowed to run the drill UNLOADED till it stopped or nearly , where upon it was immediately removed from the drill put aside to rest for a time.....then charged.

Now this seems more drastic than it is.....the current drawn by an unloaded drill is quite light and the load reduces with voltage.......

I have seen reasonably young batteries come back from not lasting a single service call to being nearly as good as a new one with this method..... it might take a couple of cycles.

Allowing the battery to cycle to a reasonable depth during normal use.......running the machine till it is no longer practrical before replacing / charging the battery.

The overnight deep freeze.......I didn't believe this one when it first came to light..... but it works, particulay in combination with other methods



Things that I have found to improve single time performance.

Letting the battery cool.......the limiting factor for fast charging NiCad batteries is heat.......if the battery is warm you have lost charging time and stored charge before you start.....the colder the battery before you start the better....... below zero is fine........A nicad will cope with any low temperature you can generate with domestic refrigeration.
NiCad performs at its best around zero C.

Charging in a cool place.......the cooler the better.........I conducted an experiment charging in the workshop fridge once.........( charger and all)........considerable increases in stored charge.............everybody laughed...........till I said prove me wrong, give me your battery...........That one put a big smile on my face.


Now I have probably said it before but....... batteries have improved considerably in the last few years, memory is much less of a problem than it was in the eighties. Capacities possible in the battery sizes has gone up 20-30% and the price has come down.

I can remember paying well over $100 for a non genuine 7.2 makita pack.

But the fact remains... batteries ain't batteries.....if you want fast charge NiCad batteries that work and last they have to be Sanyo Cadnica, the others simply do not stack up. The better of the non genuine batteries are are built on Sanyo cells, some say so.
Remember also increased capacity packs are available that take advantage of the new technology......the original "sub C" fast charge cell was 1.2Ah, I am currently seeing 1.5Ah and 1.8Ah cell being offered and ocasionaly NimH.

Now the trap of the new tool
New tools especilay cheap ones often come with cheap, low capacity batteries, even the 4 genuine Makita batteries that came with the last pair of machines I baught were only 1.2Ah.... when I was being offered 1.5Ah after market batteries at the time.....and I have to say they did not last as long as some of the after market packs I have had.

Now onto the lesser brands......If we are talking Bosch.....if the case is green....it is a domestic mass market tool and it is trying to compete with the real cheapies.....so don't expect real flash batteries.......we have not seen the BLUE Bosch ( industrial range) in this country in volume for a few years now.

So don't blame the technology.

cheers