Making Own Li-ion Pack and Charging from Different Sources

I'd like to build my own 3-cell lithium-ion (11.1V) battery pack because it needs to fit into an oddly-shaped enclosure. I understand that I need to use a protection circuit module board (like this one) to prevent over-discharge. I believe the maximum charge for 3 cells is 12.6V, which the protection circuit will also protect against.

I'd like to be able to recharge the pack without opening the enclosure and I'd rather not run a whole bunch of wires out, so balance chargers are not an option. It would be nice if I could charge the pack from a variety of sources: 110V AC, 12V sealed lead-acid batteries and 12V solar panels. My understanding is that 12V SLA batteries and solar panels will actually deliver more than 13V.

My question is: if I have a protection circuit, can I charge the pack directly off a "12V" power supply without any additional charging circuitry?

No, you need a LiIon charger and with that circuit you also need balancing leads to the charger.

HKJ said:

No, you need a LiIon charger and with that circuit you also need balancing leads to the charger.

Click to expand...

Can you explain what the charger will be doing that the dumb power supply won't and why balancing is required? What about using a protection circuit with equilibrium functionality? (eg, although that one's too big for my enclosure)

Okay, I looked into this a bit more. My understanding is that if I use a dumb power supply I'll only get stage 1 of this diagram, while a smart charger will include stage 2 yielding a more complete charge and longer lifespan. But from what I gather, it's not considered necessary to do balance charging on 3-cell packs (balance chargers mostly seem to be used for larger packs).

So I believe all of these will work together:

• 3 x 14500 lithium ion cells
• 11.1V protection circuit module
• smart DC -> 11.1V charger
• AC -> DC converter

I'll be using it to power an LED strip.

HotterRod said:

Can you explain what the charger will be doing that the dumb power supply won't and why balancing is required? What about using a protection circuit with equilibrium functionality? (eg, although that one's too big for my enclosure)

Click to expand...

A dump power supply will not necessary limit the current at the correct level and will never turn the charge current off.

HotterRod said:

But from what I gather, it's not considered necessary to do balance charging on 3-cell packs (balance chargers mostly seem to be used for larger packs).

Click to expand...

Without balancing the pack will get out of balance after some time, this will reduce the capacity of the pack.
When the batteries get out of balance depends on the quality of the batteries and how you thread them.

When they get out of balance, it is possible to use a balancing charger and restore them to full capacity again.

HotterRod said:

Okay, I looked into this a bit more. My understanding is that if I use a dumb power supply I'll only get stage 1 of this diagram, while a smart charger will include stage 2 yielding a more complete charge and longer lifespan. But from what I gather, it's not considered necessary to do balance charging on 3-cell packs (balance chargers mostly seem to be used for larger packs).

So I believe all of these will work together:•3 x 14500 lithium ion cells
•11.1V protection circuit module
•smart DC -> 11.1V charger
•AC -> DC converter

I'll be using it to power an LED strip.

Click to expand...

Here are a few comments without getting into great detail.

• The batteries you selected already have a protection circuit built into them. So get unprotected cells to utilize with your protection circuit module that you have selected.
• The charge current on the charger that you selected chargers at a higher level (1A) then what is the maximum charge current recommended by the battery manufacturer for that particular battery (.75 A).
• The charger you selected is a smart charger and in theory should stop charging at the proper level.
• Balancing is definitely better and safer, but in my opinion it is not a necessity for this type of set up.

Thank you both for the feedback! One question:

Knight_Light said:

The charge current on the charger that you selected chargers at a higher level (1A) then what is the maximum charge current recommended by the battery manufacturer for that particular battery (.75 A).

Click to expand...

Doesn't the charging current get distributed between all the batteries in the pack so each one is receiving 333 mA? For example, this pack made with 3 x 14500 cells has a max charging current of 1.4 A.

HotterRod said:

Thank you both for the feedback! One question:



Doesn't the charging current get distributed between all the batteries in the pack so each one is receiving 333 mA? For example, this pack made with 3 x 14500 cells has a max charging current of 1.4 A.

Click to expand...

That is a parallel circuit you are thinking of. In a series circuit if you put one amp in all of the batteries will get one amp.

Knight_Light said:

That is a parallel circuit you are thinking of. In a series circuit if you put one amp in all of the batteries will get one amp.

Click to expand...

That is a series circuit "Li-Ion 14500 Battery: 11.1 V 750mah" not parallel.

It's not recommended to charge Li-ion cells in series.

Capacity: 750 mAh
Voltage: 10.8V (Peak at 12.6V, avg. at 11.1V )
Max. charge current: 1C (1.4A)
Max. discharge current 2.0 C (2.8A )

Click to expand...

It looks as if part of there description is wrong. charging at 1.4A would be 2C. Max. Discharge at 2.8A is 3.73C

Norm

Norm said:

That is a series circuit "Li-Ion 14500 Battery: 11.1 V 750mah (8.32Wh" not parallel.

It's not recommended to charge Li-ion cells in series.

Norm

Click to expand...

Norm what do you mean it's not recommended to charge lithium cells in series? People do it all of the time. I also don't understand your comment about it not being parallel. I never said it was parallel.

Norm said:

It looks as is part of there description is wrong. charging at 1.4A would be 2C.

Norm

Click to expand...

There definitely is a mistake somewhere. But if you look at the datasheet on the cells that are utilized in that pack they claim they could be charged up to 2C, but you are correct 1.4A would be 2C.

Knight_Light said:

That is a parallel circuit you are thinking of. In a series circuit if you put one amp in all of the batteries will get one amp.

Click to expand...

Knight_Light said:

I never said it was parallel.

Click to expand...

greatest danger using li-ion cells occurs during re-charging

it's not possible to charge li-ion in series and stay balanced without individual leads going to each cell. This requires charger with multiple channels or balancer module. safest way to charge li-ion cells is in singles! multiple li-ion cells can be charged in parallel. make sure cells are within .5v of each other to prevent excessive currents as cells start to balance each other. charge rate should be set to 1c of lowest rated cell in stack. http://candlepowerforums.com/vb/showthread.php?t=161299

Click to expand...

if you charge those same series of unbalanced cells. charger will charge up to correct voltage/termination point for number of cells. but internally some cells will be undercharged, some will be overcharged. if unbalance condition becomes bad enough you will reach thermal runaway (explosion).

Click to expand...

4. don't charge in series without balancer leads going to each cell. w/independent channel for each cell.

Click to expand...

Norm said:

greatest danger using li-ion cells occurs during re-charging

Click to expand...

The parallel comment was in regards to his post about splitting current not about the battery pack.

And although it is better to balance charge 3S packs it is not always done nor is it always necessary. If it was they wouldn't sell that battery pack that we are discussing.

Knight_Light said:

The parallel comment was in regards to his post about splitting current not about the battery pack.

Click to expand...

Sorry, I misunderstood.

I stand by my charging in series comment.

Norm

Norm said:

Sorry, I misunderstood.

I stand by my charging in series comment.

Norm

Click to expand...

Norm no problem I've made similar mistakes especially after a long day. I respect your position on the series charging so let's just agree to disagree.