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Cordless tool electronics

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Cornish Jack27/10/2020 10:13:59
1184 forum posts
163 photos

It seems that most (all?) li-on battery powered tools incorporate some sort of electronic interface between the battery and the motor. They seem to be a potential (and actual) source of trouble. The only information I have found demonstrates by-passing this interface and just making a 'normal' battery-switch-motor circuit - which worked. This being so, what is the purpose, and importance, of this extra bit of circuitry?

I am aware that in the aviation world these batteries are viewed with suspicion and caution. Should we be similarly concerned?

rgds

Bill

PS I am considering 'repairing' a cordless angle-grinder using this method.

Andrew Tinsley27/10/2020 10:22:13
1208 forum posts

I may well be missing the point here, as far as I am aware the electronics between battery and motor is usually a speed controller for brushed motors. For non brushed motors then it will be the DC "chopping" circuit that presents pseudo 3 phase to the motor and gives speed control too.

For a brushed motor you can bypass the electronics, but the motor will then run flat out. For a brushless motor bypassing the electronics will not work.

Andrew.

JasonB27/10/2020 10:22:17
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19087 forum posts
2100 photos
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I think it is to stop you completely draining the battery as my tools cut out suddenly as the battery starts to get low where the older Nicad etc ones just slowed down until they came to a halt. Not had any problems with mine used for work and given the cost of batteries would rather not risk damaging one by full discharge.

fizzy27/10/2020 10:35:30
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1737 forum posts
116 photos

These batteries are Very sensative to over disscharge - then they are permanently damaged (I use lots with my rc stuff). A typical so called 20V call is 5S. Full charge is 21V, Critical disscharge is 18.5V which is just 10% full capacity, the electronics are there to ensure you dont go below 18.5V

Dave Halford27/10/2020 10:57:07
1011 forum posts
9 photos

Yep, as Fizzy says total discharge of a lithium battery = death. It's the reason for all the fire hazard warnings, they have to be shipped charged.

Andrew Johnston27/10/2020 11:42:23
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5716 forum posts
658 photos
Posted by Dave Halford on 27/10/2020 10:57:07:

It's the reason for all the fire hazard warnings, they have to be shipped charged.

Never heard that one? In my experience most rechargable lithium cells come discharged. For shipping lithium batteries are divided into two basic categories. Those containing lithlum metal (non-rechargeable) and lithium ion (rechargeable). Both are capable of causing a fire if faulty or abused, but the regulations are much stricter for batteries containing lithium metal. Combustible electroyte is one thing, an alkali metal fire something else.

I'd concur that the electronics between battery and motor is for controlling the motor. The battery protection functions are normally built into the battery iteself and are not accessible. Otherwise some people would defeat them. The protection circuits normally protect against short-circuits and undervoltage, but not over-voltage.

Charging lithium ion batteries is somewhat involved. If the battery voltage is below a threshold (deep discharged) the charging cycle starts with a constant current charge at a reduced rate, often C/10. Once the battery voltage exceeds the threshold then charging switches to constant current at a higher rate, 1C or more. As the battery voltage nears full charge the charging switches to constant voltage so the charge current falls off as the battery voltage rises. At a set voltage charging is turned off. Lithium batteries don't like trickle charging as often used with lead acid batteries. The voltage thresholds are critical, they need to be better than 1%. There are some really nice linear and switcher based charging ICs available. The ones I've used seem to perform well over long periods without fudging the batteries.

There have been some reports in the press recently about fires caused by replacement batteries bought on the cheap. In general you get what you pay for but with batteries you may well get an additonal "bonus" surprise.

Andrew

Cornish Jack27/10/2020 12:07:42
1184 forum posts
163 photos

Thank you to all responders - I understand the potential problems.

The angle grinder has been used on two occasions only and it just stopped with no warning and no smoke. Opening it up shows no signs of internal component distress, hence the query. I suspect that the electronic, as distinct from the electric, bits are the problem, but I wouldn't know where to start in repairing such. Are these obtainable as 'off-the-shelf' items or are they 'home brew'? All the mechanicals, plus charger and battery are fine, so, any suggestions other than the bin?

rgds

Bill

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