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Old 23rd Oct 2020, 3:20 pm   #1
avocollector
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Default 18v Li Ion chargers and going quietly bonkers

My wife has elected to do some trimming of bushes and I have a nice Tektool brand cordless hedge trimmer. So I went and excavated it from the shed, then checked the battery which has those little green lights to show how much charge. Only 2 of the 3 light up so I think righto give it a charge. So far I've spent about 6 hours looking for the charger with nothing in sight - the trimmer has been in the shed unused for the past months of winter.

Now I do recall the charger was a bracket to slide the battery into and a wallwart transformer connected to it with a lead. I doubt if there were any sophisticated electronics as in say a DeWalt charger. The battery has 3 connections marked + - and C.

Dare I use a simple 18v dc power supply to recharge it?? can anyone tell me what the C connection is for? In the DeWalt cells they seem to have a temperature sensing element but this is so simple I doubt it has anything like that. Any advice gratefully received.
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Old 23rd Oct 2020, 4:04 pm   #2
Bazz4CQJ
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Default Re: 18v Li Ion chargers and going quietly bonkers

Simplifying the question a little, if you just have a battery of lithium cells, you could certainly charge that from a bench supply. You'd just to set a maximum rate applicable to the cells and maximum voltage. You'd need to verify the exact voltage on your 18V supply when charging.

I built a 12V battery consisting of 3 off 18650 cells and the current limit for those is 1.5A and for the battery, 12.4V. For the voltage limit, there is a strong trade-off on Li cells between charging to the absolute limit and decreasing the battery life; staying a little low on V is good.

In the tool battery pack, I imagine you are right that the third terminal could be a temperature sensor, maybe less likely, part of battery management system, but there's a small element of uncertainty there. With the 18V (nominal) batteries, you need to find out the actual max voltage. So, I'd guess you can charge from a 18V supply, but monitor I and V and include a fuse in there; if ever they get shorted, the sparks really do fly.

Of course, in the UK, we are now putting the garden tools away

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Old 23rd Oct 2020, 4:51 pm   #3
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Default Re: 18v Li Ion chargers and going quietly bonkers

It did probably have a charge controller chip, but as Bazz says you can charge it with a simple fixed voltage. You'll need to work out what that is - it will be some multiple of 4.2V, probably 21V in this case. Include a current limiting resistor, and charge it somewhere where it won't matter if it bursts into flames - very unlikely, but possible.
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Old 23rd Oct 2020, 5:08 pm   #4
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Default Re: 18v Li Ion chargers and going quietly bonkers

LiIon cells are remarkably easy to charge properly and also easy to make a dog's breakfast of it. You apply a current limited voltage of 4.1 to 4.2V* per cell, the current can be almost anything as lithiums are nearly 100% efficient but I would not go much further than 1C**. then wait until the current goes down to about 5 to 10% of C** and stop charging.

Simple but needs good control to stop (full stop) charging at the end. The temperature sensor is there if it all goes tits up, such as one cell going bad and the other(s) getting hot, it is not an indication of being charged but a way of preventing a bang!

*4,1V gives many more charge cycles at the loss of a bit (10 to 20%) of capacity, the military do this a lot.
**C is the ampere hours ignoring the hours bit.
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Old 23rd Oct 2020, 7:30 pm   #5
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Default Re: 18v Li Ion chargers and going quietly bonkers

Just out of curiosity, I just search google for "how many cells in 18v li ion battery", and up come quite a few hits showing people opening up such batteries. Many had either 5 or 10 18650 cells. Every one I looked at did have a battery management system (BMS) PCB inside it.

I guess that doesn't change the view that you can use a general purpose PSU for charging, as I think the BMS is mainly there to equalise cell charging?

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Old 24th Oct 2020, 9:23 am   #6
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Default Re: 18v Li Ion chargers and going quietly bonkers

Thanks to all for your contributions/help. Having looked over You tube videos and recalling the charger I'm fairly certain the battery C terminal is in fact one of the charging terminals and probably simply puts in a diode as the wall wart appears to be a small transformer and the socket for the battery had no space for electronics. So will give it a go at the nominal voltage of 18v limiting the current to 300-500ma to start with. Just wish the charger would appear though!
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Old 24th Oct 2020, 6:54 pm   #7
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Default Re: 18v Li Ion chargers and going quietly bonkers

Just a random thought!

Is it possible that you could currently be using the wall-wart charger plug as a power supply for something else?

Might have been a 'quick temporary fix' for something, that's subsequently been forgotten about.

It's happened to me - more than once!
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Old 24th Oct 2020, 7:27 pm   #8
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Default Re: 18v Li Ion chargers and going quietly bonkers

Quote:
Originally Posted by Bazz4CQJ View Post
I guess that doesn't change the view that you can use a general purpose PSU for charging, as I think the BMS is mainly there to equalise cell charging?

B
It serves more than one function, it controls the charging, discharge shut-off, cell balance charging and even lets the power supply know to stop outputting, if it's electronic.

In most cases, there will only be 2 pins/wires used for charging from the charger itself.

Any power supply will work as long as the voltage is a little higher than the pack outputs, as has already been mentioned by someone else and is regulated.
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Old 24th Oct 2020, 8:41 pm   #9
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Default Re: 18v Li Ion chargers and going quietly bonkers

Most, if not all, BMS chips only check for over and under voltage on each cell, and merrily turn off the output if either are wrong. None I have seen control charging or do cell balance. Cheap chargers that consist of a USB plug and series resistor rely on a cells BMS chip to terminate the charge at about 4.35V (high voltage trip), way too high. Not the best way of doing it. They also trip at a low voltage about 2.8V per cell but allow a low current in to revive the cell(s), sometimes this works.
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Old 26th Oct 2020, 4:44 pm   #10
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Default Re: 18v Li Ion chargers and going quietly bonkers

Quote:
Originally Posted by Lancs Lad View Post
Just a random thought!

Is it possible that you could currently be using the wall-wart charger plug as a power supply for something else?

Might have been a 'quick temporary fix' for something, that's subsequently been forgotten about.

It's happened to me - more than once!
Thanks for the suggestion but no - it's all hard wired together with the socket for the battery so you cannot use the wall wart for another purpose.
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