Li ion battery charger

[avz]

Hi, I would like your help with an issue I have with my new cordless impact drill's battery. I bought it from Aliexpress about four months ago. the battery (picture attached) was loaded, so I used it 3 or 4 times and left it without use for about two months. 3 day ago, I wanted to use it but found that the battery is dead. I've tried to charge it with the charger supplied with it (picture attached). it was connected for 24 hrs, but the battery is still dead. I suspect the the charger is faulty because the green led is on as soon as it's plugged into the mains, even without the battery connected to it, as it's seen in the picture, the plug is disconnected. I thought that the led should light in red until the battery is charged, but that's not the case. the green is constantly on. one more point that brings me to believe that the problem is with the charger and not the battery, is the fact that it worked. now, after all this long story, I would like to ask for your help to find a simple as possible charger circuit - the battery is rated 20V and 3Amps. as far as I know, the charging current should be 10%, so 300mA. I'm not considering to address the seller about the issue because it will be an endless saga.

 

[rjenkinsgb]

Green constantly on with chargers such as that can often mean no current is being drawn by the battery - so either no battery in it. or a fully charged battery, when everything is working OK.


The problem you describe can happen if a battery is left flat too long; the cells discharge to the point a charger does not recognise it as a correct battery, so makes no attempt to charge it.

Some type of charger will apply a very low trickle charge; if the voltage comes back within limits, it will then start to charge normally.

Either, your charger does not attempt to recover the low battery, or it has discharged to the point where one or more cells fail - lithium cells develop internal leakage or shorts if discharged to too low a voltage.


An 18V or 20V battery (same thing, different tool makers) uses five cells in series. The battery voltage should never get as low as 15V. If it's near or below that, that would explain the charger locking out. If it's at 12.5V or lower, some cells in the battery will almost certainly be wrecked.

Do you have a multimeter you can measure the battery voltage with?

 

[avz]

I wasn't aware of that. I've measured the battery voltage - 0.59V. is there a way to revive it?

 

[rjenkinsgb]

I've measured the battery voltage - 0.59V. is there a way to revive it?

Unfortunately, no.

Once the cells have discharged too much for more than a very short time, they start to short themselves out.
Trying to charge them once that happens can cause them to leak, burst or catch fire.


It's not obvious from the photo if the battery is a normal power tool style removable one, or built in?

If it is removable, can you post some good photos of it from different angles - the sides, end and top - to see if it is a recognisable type used by other tools?

 

[Nigel Goodwin]

I wasn't aware of that. I've measured the battery voltage - 0.59V. is there a way to revive it?

There's probably a BMS in the battery pack, which has detected the battery is too low, and disconnected it to prevent damage (hopefully). So the 0.95V reading probably isn't the battery voltage, but the (switched-off output from the BMS).

Does the battery pack come to pieces easily?, if so try measuring the voltage actually on the batteries.

You might need to reset the BMS in some way?.

 

[avz]

Thanx for your replies. I've attached 3 pics of the battery. as you can see, it's removeable, the drill itself seems to be a clone of Makita but the battery is charged via a connector that it's socket can be hardly seen in the center of "bat1".
As per Nigel Goodwin's question - as far as I can tell, the battery can be taken apart. there are 4 screws at the bottom. is there a way to reset the BMS in order to enable a direct charging to the cells, at least to a point where the battery can be charged normally?

 

[rjenkinsgb]

If it is the BMS that's cut out, temporarily connecting power via current limiting resistor will allow you to see the battery voltage, and that will reset the BMS if / when the battery voltage is adequate.

You could connect the charger to the battery terminals with eg. a 1K resistor in series with the positive, then measure the battery terminal voltage while power is being supplied.

The BMS "switch" should always be on while charge power is present & the voltage is below the over-voltage protection limit, so you should see the actual battery voltage under those conditions.

If the voltage gradually creeps up while charging via the resistor, see how high it gets to??

 

[Nigel Goodwin]

We replace a LOT of batteries in industrial equipment, and often after replacing Li-Ion ones the BMS prevents it working - a quick 'blip' with a 9V PP3 battery usually resets the BMS. But of course that's for lower voltage batteries.

 

[avz]

The BMS "switch" should always be on while charge power is present & the voltage is below the over-voltage protection limit, so you should see the actual battery voltage under those conditions.

If the voltage gradually creeps up while charging via the resistor, see how high it gets to??

Could you please elaborate how do I cause the BMS switch to be on? only by applying the charging voltage via the 1K resistor?

 

[rjenkinsgb]

only by applying the charging voltage via the 1K resistor?

Yes, if they shut down from undervoltage, charging back to above the minimum re-enables it.
It can charge regardless of the output circuit being enabled, so the resistor provides a safe way to apply some voltage.

That will be limited at the battery, to whatever the internal voltage is, with it just taking a few milliamps.
It allows you to measure the battery voltage even if the BMS has the output part switched off.

 

[avz]

I've connected the charger to the battery via 1.5K resistor (I haven't found a 1K) about 3 hours ago. I've checked the voltage on the battery terminal and it seems that it dropped even lower than yesterday. the reading is 0.405V. I've expected that after 3 hrs it will increase a bit, not decrease. is it the reqviam?

 

[Nigel Goodwin]

I would suggest you need to connect it directly to the battery, and not through the BMS.

 

[rjenkinsgb]

Are you measuring while the power is connected (via the resistor) ??

That's the only time you could get a sensible reading through the BMS while the cells are undervoltage.

Otherwise, take it apart and check the voltage across each cell directly, as Nigel suggested.

 

[avz]

Are you measuring while the power is connected (via the resistor) ?? yes.

I would suggest you need to connect it directly to the battery, and not through the BMS.

I'll try that.

That's the only time you could get a sensible reading through the BMS while the cells are undervoltage.

Otherwise, take it apart and check the voltage across each cell directly, as Nigel suggested.