Managing 2 x 3.7v with 2 battery babysitters (series)

Components & Accessories Power
1

Dear community,

I need for my project a 7.4v supply (in order to successfully power a thermal printer and other devices). My device has to work both on battery, and on USB +5V wall power while it is being charged - it should not power off. In my personal stock I have 2x3.7v (10.000mAh) batteries, 2 Sparkfun battery babysitters and also a 2S balanced charger module.

What I tried so far:

  • Output batteries in parallel and use a step-up module (https://www.pololu.com/product/2570) to get the required voltage: stepper ups don’t seem to source enough power (?) so the thermal printer will not run.

  • Output batteries in series, using balanced charging modules, but it seems like only one battery is discharging and not charging up completely. The balanced modules are this:

balanced.png

Now, my final try was using the Sparkfun battery babysitter (love those modules). So I build this setup:

logbook power1-01.jpg
logbook power1-01.jpg1617×846 185 KB

Now, sorry for my limited electronics knowledge, but this is what happened:

  • The setup provides 7.4v, but only if i do not connect the supply (dotted) lines. Once I do this, all grounds are the same, and one module will be shorted (I think) and leds turn off. So I said, ok i will add some diodes on the negative terminals on the supply lines and this sort of worked, but only as long as I do not plug in a 5v supply. Once I plug in a 5v supply, only one babysitter is firing the charging led (and charging its battery).

Any advice would be of great help. Thank you very much!

George

2

I would try using a different battery and/or re-test the Pololu step-up…it says it can run @ up to 5A; how many A does the thermal printer require? Those Lipos are generally < 0.5c discharge rate (so, ~750mA each), so I’m imagining they won’t be able to provide enough amps for a thermal printer…try using a known-good power source and going from there after it’s working

3

Thank you. When the batteries are in series (7.4v) the printer works perfect with both 7.4V and after using Pololu step down to 5V. But when I use them in parallel and step up to 5V the printer does not work any more even though the stepper up is rated at 5A. So I guess I have to design this series system with a power path. I am currently trying something with relays and check if it works.