Cordless Screwdriver Battery Meter Modification

Thread Starter

JoeCK

Joined Mar 20, 2020
33
Hello Guys,
Hope you all are going well:)

Here I have a question about;
How to modify the "battery meter LEDs" of a NiCd battery-powered screwdriver after Li-ion 18650 battery conversion pls?
The screwdriver was working on 4x 1.2v = 4.8v NiCd battery but recently I have replaced all the 4 NiCd batteries with 1 Li-ion 18650 battery and it's working fine.

WhatsApp Image 2021-05-17 at 10.29.48.jpeg

However, the only problem is the battery meter LEDs. It only shows the last bit of red light always, even if the battery is fully charged.

I assume it's something to do with the resistors next to the LEDs, which control the flow of voltage according to the battery capacity...
Earlier the NiCd volt was around 5.2v and now the Li-ion is 4.2 when fully charged. So will it work if I reduce the resistor values for each LED accordingly? If so which values should be changed with pls?

WhatsApp Image 2021-05-17 at 10.29.49.jpeg

Pls find the attached images for clarity :)

Any help would be appreciated.
 

Audioguru again

Joined Oct 21, 2019
3,173
Did you know that a Li-Ion cell is ruined if you allow its voltage to go less than about 3V?
Some cells or products have a "protection circuit" that disconnects the battery before its voltage gets too low.
The battery meter LEDs you are fiddling with surely will not show a warning that the voltage is nearing 3V.
 

Thread Starter

JoeCK

Joined Mar 20, 2020
33
Can you show a picture of the other side of the circuit board or describe what is on the other side, connected to the B+.
Hello Keith,
Thanks for the reply.

I have attached the picture of the front side here.
B+ is connected to one side of the push switch in the picture.

WhatsApp Image 2021-05-18 at 09.27.08.jpeg
 

Thread Starter

JoeCK

Joined Mar 20, 2020
33
Did you know that a Li-Ion cell is ruined if you allow its voltage to go less than about 3V?
Some cells or products have a "protection circuit" that disconnects the battery before its voltage gets too low.
The battery meter LEDs you are fiddling with surely will not show a warning that the voltage is nearing 3V.
Thank you for the input.

However, I'm using these indicator LEDs to understand how much juice is left on the battery.

So maybe is it possible to set the value of the resistors in such a way, the last red led (at about 3v) is lite when pressing the button?
Like;
Green = 4.1v (or Green + Yellow + Red)
Yellow = 3.6v (or Yellow + Red)
Red = 3.1v (or only Red)

What do you think?
 

Audioguru again

Joined Oct 21, 2019
3,173
Anything using a Lithium rechargeable battery needs an automatic low voltage sensing and disconnect circuit.
If you do not use a Lithium rechargeable battery properly then it might explode and/or burn down your home.
 

KeithWalker

Joined Jul 10, 2017
1,779
The low automatic voltage cut-off is used because if a Li-Ion 18650 cell is discharged to below 2.7V, it will not recover and will no longer take a charge.
 

Audioguru again

Joined Oct 21, 2019
3,173
One Lithium battery charger IC datasheet says when it senses a battery voltage that is too low then it attempts charging with a very low current. If the voltage does not rise then it assumes the battery is shorted with a fragment of Lithium plating and stops the charging and issues a warning. If a normal charging occurred then the shorted battery will explode and/or catch on fire.
 

Thread Starter

JoeCK

Joined Mar 20, 2020
33
Anything using a Lithium rechargeable battery needs an automatic low voltage sensing and disconnect circuit.
If you do not use a Lithium rechargeable battery properly then it might explode and/or burn down your home.
Thank you for your advice.

However, here I'm looking to sort out just the "battery level indicator" section.

And taken care of the charging and discharging sections and using it without any issues for some time now.
 

Thread Starter

JoeCK

Joined Mar 20, 2020
33
The low automatic voltage cut-off is used because if a Li-Ion 18650 cell is discharged to below 2.7V, it will not recover and will no longer take a charge.
Thanks, Keith.

I have used TP4056 for the battery charging/discharging cut-off functions and working quite well.

I'm looking to sort the battery level indicator bulbs.
Hope you could suggest the value of resistors that are in series with the LED bulbs.
 

Audioguru again

Joined Oct 21, 2019
3,173
A resistor in series with a LED determines its brightness, not the voltage where it turns on.
There appears to be a zener diode that produces a threshold voltage for one LED and a voltage divider for the remaining two LEDs.
 

Thread Starter

JoeCK

Joined Mar 20, 2020
33
A resistor in series with a LED determines its brightness, not the voltage where it turns on.
There appears to be a zener diode that produces a threshold voltage for one LED and a voltage divider for the remaining two LEDs.
Wow, that's a step towards the result Thank you.

The above circuit having only a few components!!!

The fully charged voltage of the NiCd battery is 4.8v.
Whereas the LiIon is 4.2v

In this case, can we make it work for 4.2v?
 
Ni-cd to Li-ion conversion will take a lot more work. Li-ion dies when you sneeze on it wrong.
I've dropped Ni-mH batteries in for Ni-Cads for cordless phones and i have not seen any issues in 20 years.
TI has an app-note that this even isn't a good idea.

You need to read the stuff at www.batteryuniversity.com seriously.
 

KeithWalker

Joined Jul 10, 2017
1,779
I tested the circuit and made the modifications. The resistor values may need trimming a little bit because the forward voltages of your LEDs will not be exactly the same as the ones I used, but they should be close. The forward voltage of the yellow LED is 1.7V and the green is 2V. With the new values, the green LED extinguishes at 3.7V and the yellow at 3V. I could not figure out the circuit for the red one. I think D2 may be a zener diode but I don't know where the motor wires connect to the circuit.
Modifications:
R1=510 Ohms
R3=510 Oms
R4==510 Oms
 
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Thread Starter

JoeCK

Joined Mar 20, 2020
33
I tested the circuit and made the modifications. The resistor values may need trimming a little bit because the forward voltages of your LEDs will not be exactly the same as the ones I used, but they should be close. The forward voltage of the yellow LED is 1.7V and the green is 2V. With the new values, the green LED extinguishes at 3.7V and the yellow at 3V. I could not figure out the circuit for the red one. I think D2 may be a zener diode but I don't know where the motor wires connect to the circuit.
Modifications:
R1=510 Ohms
R3=510 Oms
R4==510 Oms
Thanks, Keith for your help. Much appreciated

I could not figure out the circuit for the red one.
As of now the red one only working when pressing the switch on this board. Maybe connected straight?

I think D2 may be a Zener diode but I don't know where the motor wires connect to the circuit.
"AudioGuru- previously mentioned:- There appears to be a Zener diode that produces a threshold voltage for one LED and a voltage divider for the remaining two LEDs."

Also, there are no motor wires on this board. Only 2 wires straight from the battery.

When I checked the resistors, the values as of now are;

R1 = 361 - 360 ohms
R2 & R4 = 50 - 50 ohms
R3 = 621 - 620 ohms
R5 = 202 - 2 kilo ohms
 

Thread Starter

JoeCK

Joined Mar 20, 2020
33
Ni-cd to Li-ion conversion will take a lot more work. Li-ion dies when you sneeze on it wrong.
I've dropped Ni-mH batteries in for Ni-Cads for cordless phones and i have not seen any issues in 20 years.
TI has an app-note that this even isn't a good idea.

You need to read the stuff at www.batteryuniversity.com seriously.
Thank you for your input.

However, from my experience;
Li-Ion (not Li-Po) is far more tolerable and dependable than NiCd or Ni-Mh. I have salvaged lots of them from various sources and using them successfully even on my electric bike

The problem with NiMh is;
1. It's good for low current drain applications like clocks, phones, etc. But not sure about high torque applications like screwdrivers.
2. Also, NiMh will self-discharge to a certain level after some time on the shelf and will end up charging it before we use it. On the other hand, Li-Ion is always ready to rock.
3. Last is the number of cells we need to use to achieve the motors working voltage. We need to use 4 NiMh cells whereas only need 1 Li-Ion battery and have enough extra space inside these small gadgets.
 

Thread Starter

JoeCK

Joined Mar 20, 2020
33
Aren't all Western Ni-MH battery cells sold pre-charged today using Eneloop chemistry so that they hold a charge for 1 year?
In relation to my specific requirement;

I have to use 4 NiMh cells to reach the target volt,.. Whereas 1 Li-ion would do the job with more energy density.

As well the cost of the number of the batteries used and their maintenance.

Pls correct me if wrong.
Thank you
 
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