Balance accuracy problem on a Thunder Power TP820CD LiPo charger

[ilium007]

Hi guys - I have a decent (supposedly) quality dual port LiPo battery charger. I mainly use it to charge 3 cell LiPo's but have found cell#1 on port 2 is always balanced lower. I have calibrated both ports with a decent meter (to the mV).

The charger is a Thunder Power TP820CD. I have had a look inside and I know that this charger is essentially two discrete chargers. I can see the two banks of through hole 10 ohm 1% load resistors (for discharge as well as balancing ?? I am not sure) and a bunch of surface mount resistors used in the voltage divider parts of the circuit.

I am thinking that the low resultant cell voltage on cell#1 is because (maybe) the resistor tolerances are not so great. I was thinking that if I could narrow down the locations of the resistors used in measuring the individual cell voltages I could replace them with lower tolerance items.

I have attached a pic of the board. Port two is on the right hand side and the balance lead plug is the white external connector on the right side. The uppermost pin (pin 1 in picture) is ground and the next pin down (pin 2 in picture) is cell#1. I haven't taken the board out of the case and given it will be a multilayer board I assume that because I can't see the trace for cell 1 it may be underneath.

The battery packs are in series so cell 1 voltage is measured between pin 1 and pin 2, cell 2 voltage measured between pin 2 and pin 3 etc.

Not sure if it's worth investigating but thought it would be fun to give it a go. Happy to pull the board all the way and get pics of the PCD traces.

I don't know 100% how the circuit works but the LiPo charger has a built in balancer the compares each cell voltage and attempts to keep them balanced. This charger can handle an 8 cell battery, looking at the board there are opamps there that I assume are being used as comparators ? Each side (left / right) has its own discrete set of balancer components.

Can anyone help !! I would like to keep this charger and given cell balancing on port #1 (left side) is spot on I know it has the potential (no pun intended  ) to be a good unit.

[Prime]

Hi,

I can't comment on the individual charger designs - I haven't pulled enough of them apart but yes, there should be a compare method. They could either be check the cell voltages against a comparator or reading the cell voltages with the ADC. My guess is that the comparators are for safety and the analog input of the micro is doing the level checking. My reason for saying this is that the LCD displays cell voltage.

I'd trace the measurement circuits back until you find some form of voltage divider or current limiting resistor on the measurement circuit. You'll probably find an out of spec resistor or bad soldering.

[ilium007]

Thanks. I'll open it back up tomorrow and post some more pics.