Cut-off Voltage when charging

[changfrancis]

I am using a power meter to monitor my charging of Airwheel X3 battery, I noticed the cut-off will only happen after 68.1V, which is about 4.256V per cell.

I assume my power meter may have small measurement error, but idea charging cut-off should be 67.2v or 4.2V per cell.

This is happening to anyone charger ? or my battery/charger having issue

[Chriull]

Chargers have tolerances, the charger could have been trimmed to a bit too high voltage and there could be a voltage inversion protection diode in your bms, which will drop something between 0.4-0.8v, so you could be at almost 67.2v for the batteries,,,

[changfrancis]

After charging complete I immediately unplug and measure the terminal using a digital multimeter. It shows 68.1v too.

If i leave the wheel untouch for 1 day, I notice it will slowly discharge to 67.2V which is the BMS doing its job within the batteries.

Sad to say if I have one cell dead, the charger design will likely cause fire or over charge to remaining cells

[Chriull]

47 minutes ago, changfrancis said:

After charging complete I immediately unplug and measure the terminal using a digital multimeter. It shows 68.1v too.

With terminal you mean the charging port or the charger plug? If you can measure a voltage at the wheels charging port than you have no invers polarity protection diode and indeed 68.1V directly at the batteries. (Be carefull with measuring at the wheels charging port, especially if the BMS has no protection diode - short circuits are easily possible!)  Which is a bit high - max 67,2V would be the recommended charging voltage. If you dare and know what you do, ?some/most? chargers have 3 different trimm potentiometers inside - one to set the max output voltage, one for the max charging current and one for the cutoff current. Here you have a description of one specific charger and the trimming posibillities: http://forum.electricunicycle.org/topic/2247-decrease-charging-time-5a-high-current-charger-mod/?do=findComment&comment=25662

Normally you should not unplug directly after charging, but let the charger at the wheel with the green led on (charging complete). At this point there is still some smaller current flowing from the charger and enables the BMS to balance the cells.

/Start Edit:

could be, that this balancing with 68,1V is not really recommandable for the battery pack health - you should get a replacement charger or calibrate yours (if possible).

Striked out the previous sentence, because it's bullshit  Once the green light goes on at the charger it should switch to constant current mode again (~100 mA) and regulate the voltage so that exactly this current is flowing. However the charger should be replaced or calibrated. The surplus of 0.9V should not be really dangerous but will reduce the lifetime of your pack.

/End Edit

The balancing capabilities of a BMS board can be very different - one hint is if the components for the external balancing circuitry are populated like here: 

One example of this components not populated is shown here: 

With the external components populated a BMS should to a fine job balancing the cells - but only if the charger is left plugged in with the green led on (after charging completed!)

Without the external components polpulated most BMS will still balance, but some/many of them with way too low currents and so it could be that they cannot really balance the cells - for these boards one should be carefull with the pack, bad/dead cells are more likely with these BMSs than with BMSs with the external balancing circuitry!

Quote

If i leave the wheel untouch for 1 day, I notice it will slowly discharge to 67.2V which is the BMS doing its job within the batteries.

Thats normal, that cells set their voltage after charging. That has nothing to do with the BMS - if the charger is not plugged to the wheel, the BMS cannot do anything with the cells (balancing)!

Some more information about li ion chargin and li ion lifetime:

http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

 

Quote

Sad to say if I have one cell dead, the charger design will likely cause fire or over charge to remaining cells

With 68,1V over all the 16 cells or after some while once the cells settled "their voltage" to 67,2V it is highly unlikely (?impossible?) that you have a dead cell.

Your cells settle to 4.2V per cell which is the normal behaviour. If one cell would be bad you would have a lower voltage than the 67,2V after fully charging and letting the wheel rest for a day! (Could be a good test for checking the pack condition!)

[changfrancis]

Hi,

Thank for the help.

after charging I measured the voltage of battery terminal at the wheel 68.1V.

as for the charger it is less then 60V, the led is green hence no correct voltage can be measured.

I will disassemble my charger later, but i dont recall my chinese charger have the variable resistor trimming port.

 

I will do some experiment and update the result here.

[Jason McNeil]

68.1v is too high. Batteries are supersensitive to overvoltage. I would consider getting another charger (as Chruill suggested) or buy an inline ChargeDoctor.

[changfrancis]

Hi,

This is the charger and setup i have in my hostel.

I have a power meter connected in series with the charger and wheel (Charger -> Power meter -> wheel battery)

when i turn ON the charger it will charge at 1.93A which is normal then, I tried tuning the trim port highlighted in red arrow but I see no changes in the charging current reading(I guess it is for something else). I did not attempt the experiment with this setup when voltage is near the 67.2V, as I am afraid of over voltage the batteries.

Currently, i don't have access to a load tester. Do you have any idea for using simple resistor or capacitor setup to trick the charger, so I can test what is this trim port for ?

 

 

[changfrancis]

I continued with the test.

This is a Airwheel X3 charger made in 2014.

After turning the trim pot for 11x counter clockwise(from factory setting), I manage to bring down the cut off voltage to 67.2-67.3V where the ending charge current is about 0.23A.

I guess the trim pot is for output voltage control.

 

Thanks all for the help.

[Chris Westland]

I added a charge monitor that I purchased for $14 from Banggood to my charger to keep track of such things dynamically.  I screwed it to the plastic case, and cut the output cable to attach to the monitor.  This provides me continual useful feedback on the battery condition without any additional equipment.  I think I am using the same AiLi charger as @changfrancis, and it may be worthwhile to drill holes to make the set screws accessible.  But currently my charger cuts out at 67.1v so there is no pressing need to fiddle with it.