18650 li ion series/parallel balancing

Batteries and Chargers Rechargeable Batteries
1

so i have a 4 cell 18650 battery holder thing

it puts 2 cells in parallel, then connects the 2 banks in series (output is 8.x volts, nominally)

if i have cells that are not new and probably not matched, what is the best way to do it?

i can check capacity with my charger

should a parallel bank be close to each other in capacity?

or should the voltages of each pair be balanced?

what happens if they are slightly mismatched?

the holder also has a charger, which requires you put 8.4V into it

so i want charging to work also (as opposed to having to take all 4 cells out and charge separately)

thx
wle

2

Each series bank must have the closest capacity possible and resistance.

3

And different internal resistance or discharge curves for parallel pairs will result in uneven load. Just use identical cells for your pack.

For charging make sure to use balancing. Otherwise your series pairs will over/undercharge, especially with different cells.

4

but then you are putting an unbalanced pair in the parallel side

is that ok?

are you saying this so the voltage stays balanced on each half of the series pair?

wle

5

That’s fine, as long as they are the same voltage when the parallel pack is made.

A parallel bank will just act as a large battery with its own internal resistance and capacity.

For a 13S pack for example, just make sure every series bank is 3Ah and X mOhms. Nothing else matters really.

TLDR: Only make sure series capacity and resistance is the same. Everything else does not matter.

6 
  +-- A --+  +-- C --+
--+       +--+       +--
  +-- B --+  +-- D --+

The series pairs A+B and C+D must have the same total capacity and similar discharge curves. Otherwise one will be discharged too much.

Parallel cells (like A and B) should have very similar characteristics, otherwise the load won’t be distributed evenly.

7

so to start with, if i have 2 high capacity and 2 lower, you would say ‘parallel a low and a high, then the other pair, then connect those in series’.

right?

8

I’d say: get four identical cells. But yes, given these cells, that’s how it should be done.

9

ok thanks

too cheap to do it right

if that kills them, then i will have to :slight_smile:

wle

10

@wle

Let’s just say you have:

2x 2.5Ah cells and 2x 3.0Ah cells.

You ideally want to do this:

2.5Ah + 3.0Ah 1S
2.5Ah + 3.0Ah 2S

What you don’t want to do is this:
3.0Ah + 3.0Ah 1S
2.5Ah + 2.5Ah 1S

11

That’s the moral of this story.

If the zombies are scaling the wall, then have at it with what you have.

One shouldn’t skimp with anything lithium-ion, IMO.

Chris

12

Don’t do this inside your house or garage, take it outside away from anything that could burn your house down. Don’t do this while sleeping.

Don’t try to fully charge (8,4V) any configuration of these bastard cells.

When you connect 2 cells in parallel, then they need to be equal in voltage to +/-10 mV. If not then you can have a big spark and huge current flowing between them. Cells in parallel always try to keep themselves at the same voltage. So if you keep charging to 8.4, then the smaller capacity cell of a parallel group will get overcharged.

When you connect cells or groups of cells in series, then they should have the same capacity and be balanced to the same voltage also. If not then one group (with lower capacity) will fill quicker and get overcharged while trying to bring the other group up to the final voltage. Cells in series always have the same current flowing thru them.

If in the example, you put a 2Ah and 3Ah in parallel, then it doesn’t make a 5 Ah—it makes a 4Ah capacity group. When you charge this you must use coulomb counting (current x time) to determine and control when it it “full” with 4Ah.

But a 3Ah cell that only has 2Ah worth of energy will not have the same voltage as a full 2Ah cell. So i don’t think mixing different size (capacity) cells in parallel is a good idea.

The best way to do this is to drain all the cells to the same voltage, e.g. 2.8V. Then connect all of them in parallel and let them sit on the bench to equalize and balance the voltage. Then connect in any series/parallel configuration you want, but you can only charge to add 4Ah worth of energy to the pack. Charging must regulate by current and capacity, not by voltage. Measure the voltage after completing the 4Ah charge, then this will be your reference Upper Voltage to never exceed when charging.

Without a BMS then YOU are the battery management system.

13

@kennybobby, everything you said is correct except these bits about different capacities in parallel:

“But a 3Ah cell that only has 2Ah worth of energy will not have the same voltage as a full 2Ah cell. So i don’t think mixing different size (capacity) cells in parallel is a good idea.”

That’s not true. The 3Ah cell will charge at the same relative speed as the 2Ah one, just at a higher current.
Putting cells in parallel just makes for a larger capacity cell.

“If in the example, you put a 2Ah and 3Ah in parallel, then it doesn’t make a 5 Ah—it makes a 4Ah capacity group. When you charge this you must use coulomb counting (current x time) to determine and control when it it “full” with 4Ah.”

No no no. 2Ah + 3Ah = 5Ah.

14

Consider this Experiment:

Take your 5Ah paralleled cell and connect a load to draw 4 Amps.

After 1 hour the electrolyte of the 2Ah cell is depleted of ions, but the 3Ah cell is still 1/3 full.

As you continue with the load the 2Ah cell will continue to supply current, but it will not be from Li+ ion transfer but rather from elemental copper being pulled out of the anode current collector. This is operating in the dendrite growth region and the cell will be permanently damaged.

This is the reason for bottom balancing this oddball configuration before connection and charging based upon the smaller cell, such that both cells will be depleted at the same time. The chain is only as strong as the weakest link.