Charging up series of three 3.7V 110mAh batteries by using battery babysitter(from sparkfunk)

Thread Starter


Joined Aug 29, 2016
Hi all,

I have a question about charging up series of three 3.7V 110mAh batteries by using battery babysitter(or charger) from sparkfunk.

Here is my goal of this project:
There is a device that I want to use as cordless, like a cellphone battery system. When I'm not charging, the device rely on the battery power, and when I connect the device to the charger then, battery will be charged and the device will rely on wall outlet power.

About the device:
The device originally get 12V 110mAh from an adopter. The input will be split in two ways by LDOs: 5V low drop output (LDO) and 9LDO in the circuit.

What I have:
I purchased three 3.7V 110mAh lithium polymer batteries and a battery babysitter takes input up to 28V, and output range is between 3V and 5.5V.

When I purchased the babysitter, I didn't pay attention to the output voltage range. Thus obviously, I cannot charge 11V battery with it.
I read somewhere that If I have three individual chargers and connect to each battery in parallel then I should be able to charge them up.
But I wasn't quite sure about this.. Because I'm going to use single power source from the wall outlet, and split the input in three ways to feed individual chargers. Then would they have same ground though?

Here is a link of battery baby sitter in case..

Please advice me.. If you have any suggestion please do.

thanks in advance.



Joined Jul 11, 2016
in order to implement your circuit you must gonnect the GND-s to SYSTEM-s -- such may become a problem source

if your [+ charger -] has isolated output then you just maybe can implement your original layout in #1 - but this must be studied
... for combinations [supply][supply][battery] etc. - what are the odds is that the switched mode supplies develop some synchronous oscillation pattern that actually may lead to serious instability or other unpredictable malfunction . . . if one of the batteries becoming to end of it's life suddenly then such also may rise issues that the charger probably but - not for sure - can handle
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Joined Jul 11, 2016
i just checked the random battery similar to yours'
what the following means ...

(now the javascript malfunctioned with win8.1 - the site technical is indeed an idiot (read a beginner - good if there even is one . . . no comments) )

Standard DischargeUsing 0.2C(assuming! 750/5=150mA!!! - that verifies - if it does - your batteries being valid for intended application)constant current discharge to the Discharge Cut-off Voltage. Quickly discharge current: 1C(750mA) )) (◄that thing)
Description This is a very small, extremely lightweight battery based on Polymer Lithium Ion chemistry. As with usual LiPo cells, it generates 3.7V with 110mAh capacity. This cell comes pre-wired with 2mm JST-PH connector. Make sure the polarity markings match your particular application. This cell also features built-in over-voltage, over-current, and minimum voltage protection circuitry. These are small cells, and so don't expect them to offer the huge 2C discharge rates larger cells offer. (( different product (◄ should be in here - the script somehow voluntarily navigates to a mouse cursor not the text cursor - if i reached to notice it correct . . . the computers have been around for decades - and they still can't program them)

Specifications ● 110mAh capacity (if not paying attention to terminal voltage the battery can be drained in one hour outputting 110mA - if it's capable to that e.g. it's a theoretical/statistical figure) ● 50mAh standard charging rate (usually the battery is cycled in between 50%-80% of it's full capacity = 30%*110mAh/50mAh = 39.6 minutes charging time approximately to recover the 30% charge loss); 1C (110mAh) maximum ((don't reach) don't exceed!) ● < 8% discharge per month (recharge at least 1-ce per 6 months or critical per 9 month) ● -25 to 60 degree C (-13 to 140F) (i assume it's operating parameters don't much change in that range - but it also may be not damaging range !!! - and the parameters do change widely)
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Joined Jul 11, 2016
tested "Sony Ericsson CST-60" + "NoKiA AC-3E" in series (output isolation is small transformer - the I/O coils of which likely have some inductive coupling with their 240V~AC inputs !!!) ::
(the jumpy cursor ghosted around again - not as awful as in WinXP where it takes longer time to re-find it - but it still feels like going on seas with a holey boat . . . :rolleyes:)
  1. From SE(-) to (-)Nk terminal was connected ~AC voltmeter that didn't exceed 2.7V ac
  2. From SE(-) to (-)Nk terminal was connected full-wave rectifier bridge that had 47uF(400V) on it's output in parallel with in series uA meter and DC Voltmeter
    • the voltage climbed up to 6V with the (uA) and (V) meter disconnected
    • the voltage stabilized at 3.95V , 300nA with the (uA) and (V) meter connected
  3. From SE(-) or (+) was connected Nk(-) or (+) the (L) (N) was swapped
    • during 8 combinations the AC voltage varied from ~2.3V to ~2.7V
  4. Connected the SE and Nk in series - output loading was the DC voltmeter only
    • resulted in 13.5V and 13.49V with swapped (L) (N)
    • the meter was connected in series with 10nF 600V capacitor in between in series SE and Nk say to SE(-) and Nk(+)
      the meter showed ~430mV and ~240mV AC with (L) (N) swapped for Nk (applies for above (L)(N) swappings as well)
so - so far we can say such is safe for short period - supervisor monitored - operation with less than (13.5V)² / (1MΩ) load in the output
we can't say anything else here → what is the trouble here is unlike multiple I/O coil transformer the primaries may develop over 800V difference at input side that is passed through the inter coil capacitances to isolated side ← this rises questions . . .

if your wall adapters manufacturer provides you information how to connect the adapters' outputs in series - and verifies such as safe - you may follow their instructions

otherwise you have to reconsider your setup - and or sell the equippement you - purchased - or swap it in a local dealer (usually with commission fee loss)
. . . most likely you want to connect all your 3 babysitters to the one and the same wall adapter and use a load balancing "controller" before 3.6 to 4.2V ... to ... 12V output converter . . . or consult and rethink the whole thing (as using single 12 battery babysitter e.c.)
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