On most lithium ion batteries they have a protection board that shuts off discharge at around 2.5 volt per cell.

2.5 volt per cell is too low for me, but the problem is you can't buy a PCB with anything higher. ALL protection boards basically stop at 2.5 volts.

I would like it to cut off at 3.2 volts so I get more cycle time out of my batteries.

I have an idea about how to modify a PCB to cut off sooner and wondering if this will work.

The microchip (8254aa) on the protection board I bought has voltage sense lines (VC1, VC2, VC3) as shown in the pictures below. I will cut into one of the sense lines and put a resistor or pot between the connection to lower the voltage reading. This way when the cell is at 3.2 volts the sense connection will read 2.5 volts and cutoff?

I still have the other sense lines connected as normal for overcharging, but I will balance charge the batteries with my hobby charger anyway.

Will this work? Thanks

Datasheet for battery protection S-8254A series
http://datasheet.sii-ic.com/en/battery_protection/S8254A_E.pdf

 

I will cut into one of the sense lines and put a resistor or pot between the connection to lower the voltage reading. This way when the cell is at 3.2 volts the sense connection will read 2.5 volts and cutoff?

That may well work for the discharge protection, but raising the discharge voltage threshold like that will also raise the overcharge threshold by the same factor, if I'm reading the datasheet correctly. Methinks the battery cells would suffer from overcharging .

 

Because of the way the circuit works, with Vc2 sharing the connection between two cells, I would use the VC1 input which connects to only one cell terminal.
You could then put, say a 100kΩ pot, between the positive and negative of the top cell and connect the pot wiper to the VC1 input.
The 3μA VC1 maximum pin current (below) through the pot wiper will have, at most, a 75mV effect on the pot setting voltage (the maximum equivalent wiper resistance is 25kΩ for a 100kΩ pot).
You then adjust the pot to get the cutoff you want.



To prevent overcharging of that cell you could add a switch to connect the top cell directly to VC1 when charging, bypassing the pot.

 

Thanks for the responses, I will give this a go and let you know how it works. For the overcharging, I am not exactly worried about this because I will use a hobby charger and that taps directly into each cell with its JST/XH connector and bypasses the protection board, so the hobby charger will deal with the balancing and overcharge.

The other safety will be that 2 of the cells will still be active with proper cut/off, and since all the cells generally stay within 1% of each other, that will act as a secondary safety. I also never charge to full voltage of 4.2 volts, but 4.1 volts, this will give a huge buffer.

crutshow: Will a 100K pot be enough to lower the voltage by .8 volts (from 3.3 to 2.5 volts) at 3μA? I couldn't find a decent calculator but it seems that 100K pot may be too small? Would it be possible to check this for me or do the calculation. Thanks.

EDIT: One other question, I just tested the resistance from my VC1 connection pin on the microchip to the battery solder pad, There is a 1.009 K resistor between the connection or between the battery and microchip. With this resistor in place, should the pot still be 100K pot? Do I even need this 1.009K resistor or can I just remove it to make it easy to solder in my pot?

Edit 2: One other question I was thinking about, if I need to connect the pot wiper to VC1 and the other ends to the battery cell positive and negative, won't there be a parasitic load going through the 100K POT all the time? Is this a problem?

 

Just to give you an update on this. I actually just built the battery and it works exactly how intended.Thank you Crutschow for the help and idea about using the voltage divider.

The only small issue is that how the protection boards work is that once the board hits the low cutoff, which is 2.7 volts, they need to bounce back to over 3.00 volts to work again. The problem is that normally a lithium battery will jump from 2.7 to 3.0 volts if it is empty after you remove the load. But if you are faking the voltage as in my case, it won't bounce back from 2.7 to 3.00 volts, because my low real voltage is 3.3 volts and it isn't going to jump from 3.3 volts to 3.7 volts.

But this is easily worked around by adding a special charging port on the battery that bypasses the board and charges the pack to over 3.00 volts faked voltage. I use a hobby charger with balance leads so I don't need the protection board for charging. The battery and protection board will also work as normal in my voltage range of 3.3 volts to 4.2 volts, but after it hits the cutoff, it will need to be charged for the protection board to work again and allow the 12 volts battery output.

Here is a picture of the completed pack. You can see the 100K pot, the on / off switch. The on / off switch will shut the PCB off and will shut the load off also. If I put the switch back on and the faked voltage is above 3.00 volts the PCB will turn back on.

The first 2 pictures were before I wrapped the pack in tape, I have the battery charging in the picture. The other connector is for the connection to my heated clothing. The last picture is the pack wrapped in clear packing tape, this is how I will use the battery. The clear packing tape works really well and is easily removed, I've used batteries like this for over a year without any issue and they saw lots of use.

This may be the first hacked Protection board on a lithium ion battery known. I've searched extensively and people have asked about how to do this but never have actually done it.

Batteries are 20700 4200 MAH Sanyo cells.

 

eddie500 can you please provide a scheme for this?