Help with a impact drill Li Ion battery.

Hi, I would like to ask for your advice and opinion regarding a 18V Li Ion battery that I have. it was purchased from Aliexpress along with a clone of Makita impact drill. it came as a set of three - the drill, the battery and it's charger. I was only able to charge the battery twice and then it went dead. when I connect the charger's plug to the mains, it's led is immediate lights green (as can be seen in the attached pic), even without being connected to the battery's charging socket. as I know, it should be red until the battery is fully charged and then turn to green. could it be that the problem lies with the charger that doesn't provides current to charge the battery? I've measured the voltage on the socket - it's 21.58V and the voltage on the +B and -B battery terminals is 1.71V. on the battery's top there is a JY-MT-5S-V1.0 pc board, which I guess controls the charging. if my assumption is right, and the charger doesn't provide the current to charge, is it possible to connect an "external" power supply (with the correct values, of course), even if only to assure the assumption that the problem is with the charger, and see if it charges the battery? I'll appreciate any idea.

avz said:

purchased from Aliexpress

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This is a big part of the story.

Is this the same impact drill as you had trouble with last summer? Or a different one?

The same one. I haven't used it since and I would like to know if there is a way to fix it or at least to find out which part of it is the faulty one.

Typically, the charger indication on simple chargers is based on current sensing; if the battery is not drawing current - so either no connection or fully charged - it may be green.
Red means charging current is being drawn.

The most likely problem is the battery is over-discharged.

If any cell is down to near 3V, the charge current should be limited to a few milliamps until all cells are within working voltage range - then normal charging can start.

Some simple charge controllers don't do the initial test charge current, so a low voltage cell just stops everything working.

With meter negative on the tag that has the black wire, measure the voltage on every other terminal tag on the battery casing & give the readings??

rjenkinsgb said:

Typically, the charger indication on simple chargers is based on current sensing; if the battery is not drawing current - so either no connection or fully charged - it may be green.
Red means charging current is being drawn.

The most likely problem is the battery is over-discharged.

If any cell is down to near 3V, the charge current should be limited to a few milliamps until all cells are within working voltage range - then normal charging can start.

Some simple charge controllers don't do the initial test charge current, so a low voltage cell just stops everything working.

With meter negative on the tag that has the black wire, measure the voltage on every other terminal tag on the battery casing & give the readings??

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Deja vu!

rjenkinsgb said:

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?

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As mentioned, the green light is just the charger "thinks" the battery is charged. (no current or volt above set value)
It is likely the BMS board (seen in bat2.jpg) is not allowing the battery to be charged.
The photo is not very clear so cant see for sure, it is fairly common the BMS board in cheap battery packs does not balance the cells or if it does the balance current is too low relative to the charge current so it will terminate charge when any 1 cell reaches Vmax. & some other cells are not near full.

A problem I get with my 22V battery fans is they don't get used when summer is over & the cells discharge at different rates.
If I charge the fan with its charger(1.5A) the top charged cell trips the end of charge but there are still very low charged cells.
1 fan would not charge at all (some cells were too low & no trickle charge in BMS.)
So I had to remove the battery, charge each cell with a CC/CV power supply to the highest cell volt (was about 3.4V) & current drops to some value(100mA). Then charge with the usual charger or my bench supply & checking each cell Volt.
With "Good" battery packs I prefer to not full charge for storage but don't let them get flat.

Having said all that, it might be easier to get a balance charger, you still have to open the battery case to attach cell sense clips.
Even better don't let the pack discharge too much in storage, this should reduce the time to when manual balance is needed.

So in summary: - to balance the battery pack
1. Measure & record Volt for all cells.
2. Start with the highest volt cell, & a CV bit above its measured Volt... (don't full charge it)
2.5. CC/CV charge to a set drop current value.
3. Repeat 2.5 above for all other cells.
4. Charge the battery as usual & verify cell balance.
___ I use 1A for 2500mAh battery & less current for very low cells.
!! Always monitor cell temperature

Handy hint:
If you have a lithium battery that is not holding charge like it used to, try charging with a "same cell count"eg. 5S,6S charger but much lower current, if it restores pack capacity then Yay, if not open the pack & check cell Volt balance, it maybe the BMS does not do balance.

Warning!
Lithium battery packs can be hazardous so research the risks & adopt safe working procedures.
Always charge & work in a fire safe area.

CC = Constant Current
CV = Constant Voltage

Thank you all for your replies. could you please direct my to a cc/cv power supply circuit diagram that I will be able to use to charge each cell separately - as suggested.

Your charger is too high for longevity of the cells but increases capacity temporarily.
4.316 is the average cell charge voltage if balanced should be 4 to 4.2V max. I suggest 4.1 max.

Most likely 1 cell is dead and thus B+ to B- is reading < 2V. It should turn Red and flash if the design is decent when the battery is plugged in after testing it with a load test.

Tony Stewart said:

Your charger is too high for longevity of the cells...

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21.58V would be the open circuit volt of the charger as the BMS is not allowing charge, no volt reading on the battery would be discharge is disabled as well due to some very low cells.

avz said:

Thank you all for your replies. could you please direct my to a cc/cv power supply circuit diagram that I will be able to use to charge each cell separately - as suggested.

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Better off getting a small bench supply, plenty of these on aliex too.

Any very low charge cells may end up not recovering useful capacity if the battery has been "flat" for too long, but its always worth a go.

As requested, please reply all the cell volt measurements when you get a chance.

tonigau said:

Any very low charge cells may end up not recovering useful capacity if the battery has been "flat" for too long, but its always worth a go.

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Lithium cells can develop internal shorts it left at below around 3V

Charging them (at anything above a few milliamps) can end up with them bursting or catching fire.

Any at very low voltages should always be discarded!

tonigau said:

Any very low charge cells may end up not recovering useful capacity if the battery has been "flat" for too long, but its always worth a go.

As requested, please reply all the cell volt measurements when you get a chance.

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Attached a drawing (please forgive my poor graphic abilities) with the measurements of the battery voltages. all measurements were taken from the battery pcb. please note that 3 readings are of resistance (the values are measured when connecting the probes, the increased during the reading). please keep in mind that I've neglected the battery issue for quite a long time. I guess that it might effect the readings since it was untouched about 9 months.

avz said:

Attached a drawing (please forgive my poor graphic abilities) with the measurements of the battery voltages. all measurements were taken from the battery pcb. please note that 3 readings are of resistance (the values are measured when connecting the probes, the increased during the reading). please keep in mind that I've neglected the battery issue for quite a long time. I guess that it might effect the readings since it was untouched about 9 months.

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Are the Ohms symbols a mistake? Did you mean Volts for all of them?

Diver300 said:

Are the Ohms symbols a mistake? Did you mean Volts for all of them?

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No, they are ohm symbols. I've also mentioned the the resistance reading increased during the measurement. again, maybe it's because it was left for 9 months.

Is there no voltage on the cells that you measured the resistance of?

Diver300 said:

Is there no voltage on the cells that you measured the resistance of?

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I guess not. the multimeter is an auto function one. I guess that since it wasn't able to measure voltage, it jumped to measure resistance.

avz said:

I guess not. the multimeter is an auto function one. I guess that since it wasn't able to measure voltage, it jumped to measure resistance.

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As others have said, the cells are so low that they are probably useless. Even the highest voltage ones are so low that there is a big risk of damage.

The "resistance" of the cells increasing when measured is probably the multimeter starting to charge them very slightly.

You could try charging each cell separately and very slowly to around 3.2 V, and seeing if you can then get the pack to charge.

Do that where nothing else is damaged if the pack catches fire. Also there could be a fault after a cell has been charged from nothing, so don't trust it not to go wrong until it's been working at least a month.

Diver300 said:

As others have said, the cells are so low that they are probably useless. Even the highest voltage ones are so low that there is a big risk of damage.

The "resistance" of the cells increasing when measured is probably the multimeter starting to charge them very slightly.

You could try charging each cell separately and very slowly to around 3.2 V, and seeing if you can then get the pack to charge.

Do that where nothing else is damaged if the pack catches fire. Also there could be a fault after a cell has been charged from nothing, so don't trust it not to go wrong until it's been working at least a month.

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Thanx. does it makes a difference that the stack of the battery constructed so that every individual cell has another cell parallel connected to it? practically, when I'll charge one cell, it's actually two cells, it's done so to increase the A/h capacity of the drill battery.

It doesn't really make any difference that there are two (or more) cells in parallel. The parallel pair (or set) will be at the same voltage.

Letting Li-Ion run flat completely ruins them - I would VERY strongly suggest not attempting to charge such cells, just replace the cells with new ones.