Parallel 18650 supply question

Thread Starter

Bobsone

Joined May 20, 2022
2
Hi
I want to power a 5V project from rechargeable batteries.
I am not sure what the day to day draw will be but my guesstimation is 80mA (in standby) to 200-250mA when under full load. I can fit up to 8x18650 cells with some room for charge/boost pcb(s) like the Adafruit PowerBoost 1000 Charger (https://www.adafruit.com/product/2465).

My plan was to start with a moderate mA supply and add to it if-when it turns out the recharge interval is a bit short. I thought this could be achieved by adding a second charger-boost board with its own batteries (2-4 per board) then connecting the boards in parallel with a schottky diode on each supply output, I think this is called Oring. Unfortunately there is the drop across the diodes and heat to contend with.

I have found an active load balance chip, the LTC4370 (https://www.analog.com/media/en/technical-documentation/data-sheets/4370f.pdf) but although this looks like the perfect solution it is hard to find and is crazy expensive.

So I am wondering, because the batteries would be behind the Adafruit board(s) and putting aside the diode voltage drop-heat, am I over complicating things, do I really need an active load balancing system?
Thanks.
 

MrSalts

Joined Apr 2, 2020
1,762
Be careful with those batteries. Answer the following question please...

One 18650 cell, when fully charged, has the energy to lift...
A) a 20-Liter (5-gallon) bucket of water 1-meter off the ground​
B) a 90kg (200 pound) man 1-meter off the ground​
C) a 4-ton elephant 1-meter off the ground​
 

Thread Starter

Bobsone

Joined May 20, 2022
2
r.e. "Be careful with those batteries" from what I have found out that is sound advice, which is why I am still exploring the possibility.

perhaps b...

I went with 18650s because they are available in the Adafruit store as prepackaged matched combos (e.g. https://www.adafruit.com/product/354) and connect directly with their charger-boost boards.
 

MrSalts

Joined Apr 2, 2020
1,762
r.e. "Be careful with those batteries" from what I have found out that is sound advice, which is why I am still exploring the possibility.

perhaps b...

I went with 18650s because they are available in the Adafruit store as prepackaged matched combos (e.g. https://www.adafruit.com/product/354) and connect directly with their charger-boost boards.
Answer: C

3500maH 3.7v lithium battery energy:
3.5 ampHours * 3.7V * 3600sec/Hour = 42kJoules

4-ton elephant Potential energy at 1 meter:
4000kg * 9.8m/sec2 * 1 meter = 39.2kJoules
 

KeithWalker

Joined Jul 10, 2017
2,351
If you are connecting the cells in parallel, you don't need a load balance chip. The batteries will automatically share the load. and will all charge to the same voltage together.
When you add more cells, you can charge at a higher current or use the same current which will then take longer to charge.
When you connect cells in parallel, make sure first that they are at the same voltage. They have very low internal resistance so connecting cells with different terminal voltages can result in enough instant current to cause an explosion or fire.
Keep it simple!
 
Last edited:

BobTPH

Joined Jun 5, 2013
4,897
Answer: C

3500maH 3.7v lithium battery energy:
3.5 ampHours * 3.7V * 3600sec/Hour = 42kJoules

4-ton elephant Potential energy at 1 meter:
4000kg * 9.8m/sec2 * 1 meter = 39.2kJoules
So the correct answer is A+B+C. I.e. a man carrying a 5 gallon bottle of water riding an elephant.

Bob
 

Audioguru again

Joined Oct 21, 2019
4,675
I did not look-up how many Joules is in a stick of dynamite which would happen if the fully charged battery is over-charged or shorted. Maybe instead of exploding, the abused battery would simply catch on fire?
 

KeithWalker

Joined Jul 10, 2017
2,351
In my experience, an extremely excessive current overload simply causeg the PTC to limit the current, and in my case it turned the battery into a doorstop. I don't recommend that you verify my findings though.
 
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