I got sick of picking out battery protection IC (I’ve ordered the wrong version 2x) so I just worked out a Schmitt trigger with VLTP of 2.7v and VUTP of 3.2v. Currently I have the sensing on the bottom battery. The Circuit draws about 200uA is it better to put a divider on the full battery and sense it? I’m concerned that drawing power out of just the bottom might unbalance them. I don’t want two circuits.

 

I got sick of picking out battery protection IC (I’ve ordered the wrong version 2x) so I just worked out a Schmitt trigger with VLTP of 2.7v and VUTP of 3.2v. Currently I have the sensing on the bottom battery. The Circuit draws about 200uA is it better to put a divider on the full battery and sense it? I’m concerned that drawing power out of just the bottom might unbalance them. I don’t want two circuits.

We are only given a very small part of the story, so how can anybody provide a useful response when what is needed is rather unclear.

 

Was it unclear?

2 S lipo is 2 lithium ion battery cell. each battery is 3.6v nominal actually anywhere from 2.5-4.2v or total of up to 8.4v for the 2 cells. The batteries get damaged when they get below 2v and there’s really no useful current below 2.9 or so.

ive Placed the voltage sensor between the bottom battery and ground sensing for voltage

Im wondering if the sensing circuitry which draws approx 200 uA will throw the batteries out of balance and is it better to put a voltage divider on the full cell voltage rather than just the bottom battery.

actually the sensing current is about 30uA, in any case I wonder If it would throw the batteries out of balance eventually.

There is no charging system as the battery is charged out of circuit. Even after cutoff there is a small current draw from the comparator.

does it need a picture?

 

If the sensing circuit is always drawing current from only one cell, it would eventually discharge the cell. How soon would depend on the capacity of the cell.
If the battery is used at regular intervals and is charged using a good balanced charger then you should not have a problem.
If the battery small and is idle for long periods of time, you may run into problems.
Regards,
Keith

 

I’ll try to get the sensing circuit down to a few uA. It should be recharged regularly but designing for worst case scenario. It may need a power switch. I’m tempted to go back to the protection IC it draws 4 uA and 2 uA at standby. Lower current means slower switching but it shouldn’t matter that much.

 

How about using JUST a series resistor from the top of the stack? Since you know the current that the circuit draws you should be able to calculate the resistor value. You will need a very high impedance meter to verify that it is working correctly, though.

 

It should work well enough as a battery cutout. The divider is simple divide by 2. I will start with couple megs and for testing it would be best to connect a variable power supply to verify cutout at correct voltages.

Some more information about the protection ICs, the variations come with different cutoff and release voltages, over charge, over discharge and over current conditions. Most will not release from over discharge without connection to charger which is the issue Im having. It seems the auto release ones are more rare.

this is what it looks like and each chip has their own peculiarities

just tired of waiting and picking wrong ones until I finally read this... comparator will work and I don’t need the charge protection.

 

I got it down to just under 4 uA at cutoff. This is good enough, thanks for helping me think through this.

 

With multiple lipo cells, you need to balance charge them anyway. One will be stronger and have more internal leakage than the other naturally so they will get unbalanced over time if not balance charged.

 

I got it down to just under 4 uA at cutoff. This is good enough, thanks for helping me think through this.

You are welcome. I usually include a power switch to disconnect the battery from all circuitry for when my projects are on the shelf.
Regards,
Keith

 

I've ordered the correct chip.. there are too many issues with the comparator... I keep chasing issues... the system uses 5V source but it's not dependable - can be anywhere from 4.5-5.5V so a comparator won't work reliably without additional work. I believe the IC uses an internal bandgap and has 6 comparators built in.