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An Easter Tale of Cordless Batteries.

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Mike London12/04/2020 11:53:28
24 forum posts
1 photos

A good few years ago Screwfix had a special offer of Titan 18volt cordless drill with a charger and TWO batteries in a case at a very competitive price. The drills were probably old stock, end of line clearance but the price was so good and as I had a discount voucher as well, I bought two sets. So I had two drills with two chargers and four batteries.
The sets have been a very good buy. I have worked them hard and they have lasted well and I have been well pleased with the purchase.
The drills haven’t been used for awhile and a couple of days ago I had a job on, got the drills out. The batteries were dead. No problem, put them on the chargers and start work. Except none of the batteries will charge!

Four batteries dying at the same time? Bit unusual!
Two chargers dying at the same time? Bit unusual!

So with Easter lockdown I've got time to investigate.

Measured battery voltages, They were flat as anticipated. Each showing between 1.5v and 3v.
One battery hadn’t been able to charge previously so I had already marked it up as dead. But at least all four showed no direct short or open circuit.
Now as mentioned, this is old kit. The chargers are heavy, and probably have transformers. There are two LED’s, a green indicating the charger is powered and a red which lights when you plug the battery in for charging. When the battery is charged the red LED goes out and the green lights.

Plugging the batteries in for charging, nothing happens. The green LED just stays on, so the battery isn’t charging.
Measured charger output voltage. There are plus and minus 18volts terminals and a third connection marked “T”. Plus and minus show no output but are probably switched on by battery condition. “T” terminal shows 5 volts. Can’t imagine both chargers dying simultaneously, especially as there is output on the “T” terminal.

Measured resistance on the battery between minus and the “T” (Thermistor) terminal and all four batteries show 11.5Kohm. Again, can’t believe all four batteries dying with exactly the same fault at the same time
So we are in stalemate.
I have an old 12 volt car battery charger purchased at a boot fair, many moons ago for £2. It is so old that it only pushes out 13 volts open circuit voltage but it has charged many a car battery and most importantly it has an ammeter.
I have nothing to lose, a dead cordless battery is a dead cordless battery, I can’t kill it any further. What happens if I put the car battery charger on it? The ammeter will show me if things are really dangerous. With trepidation I do so. A quick flick of the ammeter and it settles down to 1 amp. I monitor the battery, no signs of distress. I leave it for a few minutes then put the voltmeter across it. It shows 19.5 volts, not bad for a 13 volt charger output!

Maybe I have invented perpetual motion?
Putting the cordless battery back onto its proper charger everything then works perfectly. I repeat the process with the other batteries. All behave the same way and all are able to be fully charged.

Question: Can anybody tell me why this should happen? What is going on?

Suggestion: If you have a dead cordless battery (minimum 12 volt) try this and see if you get the same results!
(A word of warning if you do this. Rechargeable, heavy duty batteries if mistreated can catch fire and/or explode. Do be careful and take precautions.)

Steviegtr12/04/2020 12:07:51
1969 forum posts
268 photos

It's possible that the T trerminal you mention , has to see a residual voltage to start charging. If it does not see a min voltage of whatever volts then it thinks the batteries are dead & will not charge. Most smart chargers are like that now. Dead flat battery & it will do nothing.


Clive Foster12/04/2020 12:47:26
2625 forum posts
91 photos

Judging by the Makita NiCad batteries I have re-celled in the past the T connection is for a temperature monitoring thermistor.

Batteries get hot as they charge and will be at best damaged and at worst almost explosive if allowed to get too hot. The thermistor tells the charger circuit to reduce the current if things are getting too warm. On simple systems end of charge, which depending on the battery age and condition may or may not be full charge, is detected by a combination of voltage and temperature. Effectively the simple systems detect the voltage and switch to a low current, sufficient to keep the battery warm enough for the thermistor to hold the current down low. Although it helps to keep things safe if the battery is left on the charger its not really very good for the battery and will reduce both life and capacity if indulged in on a regular basis. Even expensive, Makita, DeWalt et al systems don't seem to switch NiCad and NimH chargers off once full charge has been detected. Which seems a poor decision. Li-on batteries have to have properly sophisticated charg control so its not so much an issue.

To ensure things stay safe the T terminal needs, as Steve says, to see a bit of residual voltage from the battery before it will allow charging to start. Power tool battery chargers deliver high currents for fast charging. Enough for there to be a very real explosive risk if maximum current is applied to a damaged and therefore inherently dead, as opposed to over-discharged, battery. If the battery is just over-discharged but inherently sound then the tickle up a bit with a basic charger works fine.

Best to limit the current and not do it for vey long to be safe. I amp is higher than I'd ideally like but almost certainly fine for 5 minutes or so. I think the one time I did such I used something like 0.1 amps for about 5 minutes. But I have proper lab bench power supplies to hand so setting a desirable current was easy. Also the knowledge to sort out what the likely specifications of the batteries involved were.


SillyOldDuffer12/04/2020 12:56:45
7019 forum posts
1549 photos
Posted by Mike London on 12/04/2020 11:53:28:

The drills haven’t been used for awhile and a couple of days ago I had a job on, got the drills out. The batteries were dead. No problem, put them on the chargers and start work. Except none of the batteries will charge!

Four batteries dying at the same time? Bit unusual!
Two chargers dying at the same time? Bit unusual!


What happens if I put the car battery charger on it?... It shows 19.5 volts, not bad for a 13 volt charger output!

Maybe I have invented perpetual motion?...

Without knowing exactly what the batteries are it's difficult to comment beyond noting that rechargeable batteries tend to go flat if left alone and self-discharge is thoroughly bad for them. They perform best and last longer when they are used and fully recharged regularly and often.

In the good old days secondary batteries and rechargers were both basic; a pair of right thickos, fairly easy to understand.

Unfortunately a simple battery/recharger combination doesn't get the best life or performance. Big Lead Acid batteries were originally managed by a man who carefully adjusted the charger and acid density manually. Later chargers cut the man out by making the charger sensitive to the state of the battery (an imperfect arrangement, but cheaper than paying men.) Improvements in Lead Acid technology optimised motor car batteries for simple recharging and low maintenance - we don't even have to top them up any more.

Over the years chargers got ever more complicated. Quite a few recent battery technologies are intelligent, including built-in electronics monitoring the state of the battery to improve discharge, and cooperating with the recharger to extend battery life. Some contain buck converters that alter and regulate the output voltage and current.

Hard to know if the batteries in Mike's drill are smart or stupid, and if smart, what on earth they're thinking.

I suggest the batteries flattened to the point were the internal intelligence and/or the charger lost the plot, failed to recognise they were batteries, or simply decided they were dead and gone. Putting the batteries on a deeply stupid car charger may have put enough energy in to wake them up again. But I doubt they are in 'as new' condition.

The 19V / 13V thing is easily explained. A car battery charges at about 13.8V, so the charger's mains transformer is wound to output 13.8V on average, which is 19.5V peak on an AC or half wave rectified DC. When the charger is connected to a capacitor or battery, the DC voltage floats up to peak, which is read by the meter. It's not perpetual motion.

Lots of battery 'fixes' out there. Fair enough when used as a way of squeezing more life out of a deceased battery, but the battery isn't restored to full capacity and won't be up to specification reliable. I've rejuvenated car batteries for personal use with great success - getting another 6 to 9 months out of them. Passing them off as part of a second-hand car deal is dishonest.


Clive Steer12/04/2020 13:41:21
41 forum posts

If the cells in the batteries are Lithium and have discharged below about 2V as far as the charger is concerned their viability is questionable. So the charger checks each cell and if any are below the 2V min it will not continue with charging. Unlike NiCad/NiHi batteries Lithium batteries need complicated charging profiles including cell balancing and the chargers are not simple devices especially if they are designed for rapid re-charging where charging currents can be high. If discharged below the lower viability threshold voltage the cell can develop a number of issues that subsequent charging at high currents can cause fire and/or explosions which is a bad thing.

It may be possible to revive a cell by carefully charging it with a current limited( say 100mA) bench power supply to bring it above say 3V so the normal charger will see the cell as viable but care is needed.

If the cell shows any signs of swelling then the cell is definitely beyond recovery. I've noticed this condition in several phone and camera batteries where the device hasn't been used for some months. Although Lithium cells have very low self discharge rates they have protection circuitry that can worsen this and the devices they are fitted too may add a background discharge rate which could be just a couple of months.

The other Clive

Pete Rimmer12/04/2020 13:54:05
929 forum posts
57 photos

I had this problem with a dead Dewalt 14.4v screw gun. The battery was so flat that the charger would not detect that there was a battery connected. My solution was to very briefly connect a 12v motorbike battery across the terminals. As soon as I did that the battery charged up just fine and has been working this past 18 months.

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