I need some help with this project

[ZEUS-FL]

I have built a 3S7P 18650 battery pack using Sony VTC6 cells to be used in a 12V 7AH battery case kit from AliExpress for a UPS. I have two UPS models, one with a 350VA capacity and another with 750VA. Both UPS units have a charging voltage of 13.5V at either 10W or 20W.

The issue I am facing is that with that UPS charging voltage my battery will exceed the voltage of 12.6V, which is the maximum I want for each cell (4.2V per cell, with 4.15V being my preference). This high voltage could potentially damage the batteries since the BMS cutoff is at 4.3V or 12.9V.
Is there any way to regulate the UPS charging voltage or modify the BMS to cut the voltage at 4.1V, for example?

 

[DIYrich]

Is there an app to control the BMS? Can you re-wire for 4s5p? 4s is more standard.

 

[Vigo]

You need one of these:

$15 Adjustable step down converter

I have a 5amp model, they also sell a 3 amp model for $13. It has very fine resolution on the voltage so you should be able to get exactly what you want out of it.

 

[sunshine_eggo]

I have built a 3S7P 18650 battery pack using Sony VTC6 cells to be used in a 12V 7AH battery case kit from AliExpress for a UPS. I have two UPS models, one with a 350VA capacity and another with 750VA. Both UPS units have a charging voltage of 13.5V at either 10W or 20W.

So less than 2A of charge current.

The issue I am facing is that with that UPS charging voltage my battery will exceed the voltage of 12.6V, which is the maximum I want for each cell (4.2V per cell, with 4.15V being my preference). This high voltage could potentially damage the batteries since the BMS cutoff is at 4.3V or 12.9V.
Is there any way to regulate the UPS charging voltage or modify the BMS to cut the voltage at 4.1V, for example?

You NEVER want to rely on a BMS cut off for ROUTINE disconnects. It will wear the cheap ones out. Best to set the charge/discharge limits inside of the BMS limits.

any concern with cell C rate?

Is there an app to control the BMS? Can you re-wire for 4s5p? 4s is more standard.

True for LFP. Not for 3.6-3.7V Li-ion chemistry. That has an operating range of 12-16.8V

You need one of these:

$15 Adjustable step down converter

I have a 5amp model, they also sell a 3 amp model for $13. It has very fine resolution on the voltage so you should be able to get exactly what you want out of it.

+1... assuming that little brass-looking flat blade screw is the output voltage adjustment for 12.45V output.

 

[Vigo]

It is. It takes MANY MANY turns of the pot screw to change even 1 volt so i would hope that even if its not 100% repeatable it shouldnt wander far? I admit i havent tested the one i picked up to see if it output the exact same after power cycling.

 

[ZEUS-FL]

I am still thinking on solutions. Will this work?

 

[sunshine_eggo]

I don't think so. You'll get voltage drop across the diodes, but the resistor only drops voltage based on a current. Once the target voltage is achieved, the current will taper, and the drop provided by the resistor will decrease thus increasing circuit voltage.

On second thought, the linked product won't work because it's output power is greater than the input power. The battery will draw as much as the converter can deliver. Since the converter is fixed output at 3A, it will attempt to deliver 3A overwhelming the input and likely shutting down the UPS charger. You need a DC-DC converter that can also lower the output current.

This appears to be both voltage and current adjustable output:

https://www.amazon.com/DROK-Adjustable-Converter-Transformer-Protective/dp/B07JZ2GQJF

A wee bit cheaper and looks cool.

 

[JBertok]

Those APC UPS are as you know set up to use VRLA batteries. They are expecting a voltage/current relationship and health consistent with lead acid chemistry, and they periodically test that the battery is within serviceable range. (Some models have an LCD that shows state of charge expressed as percentage, but this is a translation from voltage not actual state of charge.) The small desktop models have no battery temperature sensors for fault detection. I don't think you're going to be able to adapt the charging stage strategy of one of these to Lithium Ion, at least not safely and reliably. Maybe it would be possible to use an external Lithium-Ion specific charger and disable charging from the UPS. Another option could be to use a Lithium-Ion UPS to begin with; APC does make several models, include true-sinewave at the upper end.

 

[ZEUS-FL]

Thanks for these info. I burned one of my converters trying this and I desist.