12V lithium pack + charging

[wavey]

Hi everyone,

I'm building a lithium battery powered device driving a bunch of 12V motors at around max 3A, but now wondering if I made the right choices. In my view there are two options:

1. Parallel cells:

• Each cell gets a separate battery charger ICa boost converter. Correct voltages are easily handled by standard lithium charger ICs
• Boost converter to go to 12V for motors
• 5V power supply

2. Serial cells

• 3 cells in serial. 12.6V for charging and 11.1V when discharging
• Buck converter from power supply to 12.6V with current limit to protect charging.
• >13V power supply

So far I went with option 2, but I noticed that I'm not that good at making a reliable buck converter, some of them fail to supply enough current. Also the feedback voltage divider needs 0.1% resistors to have enough accuracy and those can be expensive/harder to source. Should I have gone with 1 instead?

For reference this is what my current buck converter schematic looks like

[wavey]

Thanks, I guess you're right. I was a bit too optimistic with rolling my own. I haven't found any 3S IC's but there are plenty of 12V lithium packs on taobao that have charger ICs on them. And I'm never going to make anything cheaper than that..

[KL27x]

I noticed that I'm not that good at making a reliable buck converter, some of them fail to supply enough current.

What's enough current? This is your maximum charging rate, yeah?

It shouldn't be too difficult to get an amp or so out of a buck converter. If you're not good at making your own, maybe just buy an IC and follow the datasheet? Even a linear regulator could work fine, if your Vin isn't too high.

[KL27x]

2. Serial cells
•3 cells in serial. 12.6V for charging and 11.1V when discharging
•Buck converter from power supply to 12.6V with current limit to protect charging.
•>13V power supply

So far I went with option 2, but I noticed that I'm not that good at making a reliable buck converter, some of them fail to supply enough current.

Maybe?

I like the mental image of 3 cells in cereal.

[wavey]

Yeah KL27x got it right. Indeed my buck regulator needs to supply at max charging rate. Some work fine, some only work after I touch them. I guess that suggest some dodgy oscillator that does not start properly. But no clue yet how to fix it.

I just have to accept that all the time I spent so far is wasted and should just buy complete 3 cell batteries.