18650 Questions.

[marcopolo]

I'm trying to understand how the protection systems work in an 18650 both in discharge and charge. I can't seem to find the answer here.

1. I presume unprotected cells are just 'raw' 3.7V li-ion batteries with no other circuitry?

2. Does a protected cell go open circuit if the voltage drops below a specific level and if so I take it this means that if you have a few in series it will cease to operate - but surely the voltage pops back up above the voltage cut off level once the load drops away? Do you get some sort of rapid toggling effect?:thinking:

3 Does a protected cell go open circuit when it is charged above a certain level?

4. What happend when the protection circuitry blows up - does it normally go O-C or short?

5 If you use a flex driver from task LED is it better to use the built in Vcutoff programming with unprotected cells or just use both protected cells and the flex?

6. Are cells from specific manufacturers made in both protected and unprotected flavours or is it just 'pot luck' whether the cells you buy are or are not? Are the tagged cells usually unprotected while the 'nippled' ones protected?

7. One more question - how do series paralell strings handle being charged eg 14.8v series with 2 srtings in paralell (for around 4Ah)?

Any pointers appreciated,

Marco.

 

[marcopolo]

After much digging around I think I can answer most of my question but a few are still outstanding.

1. Yes.
2. Yes, butcutoffs differ between manufactures - some not even working.
3. Yes
4. Usuallly O-C
5. Use both as Vcutoff on flex is only for whole string.

6. Still not sure about this
7. Get conflicting stories.

 

[Fallingwater]

>1. I presume unprotected cells are just 'raw' 3.7V li-ion batteries with no other circuitry?
Yes.

>2. Does a protected cell go open circuit if the voltage drops below a specific level and if so I take it this means that if you have a few in series it will cease to operate - but surely the voltage pops back up above the voltage cut off level once the load drops away? Do you get some sort of rapid toggling effect?
No. If the protection is tripped in any way, you need to put the cell in the charger in order to reset it. It stays open-circuit until you do that.

>3 Does a protected cell go open circuit when it is charged above a certain level?
Depends on the protection, but usually yes.

>4. What happend when the protection circuitry blows up - does it normally go O-C or short?
Depends on the damage to the circuit, but I haven't heard of excessive voltage or such causing protection to short.
What can cause shorts is the metallic strip that goes from the positive terminal to the protection board. It's normally separated from the negative body of the cell by a thin strip of paper; this doesn't usually cause problems, since everything is so tightly pressed together, but sufficient abuse (or user modification) could conceivably break the paper strip and cause a short.

>5 If you use a flex driver from task LED is it better to use the built in Vcutoff programming with unprotected cells or just use both protected cells and the flex?
If the driver has protection, it's always better to use that. Some testing I conducted months ago revealed that cell protection circuitry doesn't always work right, and in some cases it's practically inert.
Also, driver protection allows you to use cheaper unprotected cells.

>6. Are cells from specific manufacturers made in both protected and unprotected flavours or is it just 'pot luck' whether the cells you buy are or are not? Are the tagged cells usually unprotected while the 'nippled' ones protected?
You specifically buy protected or unprotected. I haven't seen tagged protected cells, and I don't think anyone makes them - if they're tagged they're meant to be used in a pack, and in that case protection is needed for the whole pack, not for each single cell.

>7. One more question - how do series paralell strings handle being charged eg 14.8v series with 2 srtings in paralell (for around 4Ah)?
Cells in parallel behave as if they were a single cell with the aggregate capacity of all of them. 40 2Ah 18650 = 1 huge-*** 80Ah LiIon cell, as far as the devices attached to them are concerned.
The parallel configuration autobalances, but you need to make sure you're only paralleling cells at the same charge level, otherwise one will try to charge the other with a whole lot of current, potentially doing damage to both.

Serially connected cells need to be of the same capacity (or reverse charging can occur), and configurations with more than two cells (or parallel connections) need to be balanced. Ideally you'd want to balance even two-cell configurations, but it's not as badly needed as in 3+ ones.

 

[45/70]

6. Still not sure about this
7. Get conflicting stories.

To my knowledge, there are no individual "loose" cylindrical cells made by any manufacturer worldwide, that are protected. The protected cells offered by xxxxFire etc. are done in house. These companies are not manufacturers, merely distributors. Yes, I believe all tabbed cells are unprotected, as these cells are intended for assembling packs. Packs made from these cells have an external (from the individual cells) protection PCB.

Li-Ion cells can be charged in parallel (unlike nickel based cells). Charging them in series requires charging with balancing leads, as with a hobby charger. You can not (it would be very dangerous) charge Li-Ion cells in a simple series configuration.

Dave

 

[marcopolo]

You can not (it would be very dangerous) charge Li-Ion cells in a simple series configuration.

Nearly all power hungry commercial electronic devices are either 2,3,4 or 5 cell li-ion and they only have simple 2 way DC jacks to charge them so how could this be the case? Are you talking about 18650 cells only?

 

[ifor powell]

The balancing circute can be enbeded with the battery. It costs more that way if you have multiple batteries but for the sort of electronic devices you are talking about that is not the case. It's us light makers and the RC croud how have multiple batteries so don't want to pay for multiple balancers.

 

[buickid]

AW stated that his IMR cells are safe to charge in series without balancing circuitry, just an idea. They're also inherently safer.

 

[45/70]

Nearly all power hungry commercial electronic devices are either 2,3,4 or 5 cell li-ion and they only have simple 2 way DC jacks to charge them so how could this be the case? Are you talking about 18650 cells only?

As ifor powell said, the charging circuitry (with a means to balance the cells) for these devices is usually incorporated into the device itself or, into the battery pack, not in the power supply.

Dave

 

[Fallingwater]

Li-Ion cells can be charged in parallel (unlike nickel based cells)

How is NiCD/NiMH parallel charging any different? There are many cheap chargers (admittedly usually the dumb kinds) that charge Nichel-based cells in parallel.

 

[45/70]

How is NiCD/NiMH parallel charging any different? There are many cheap chargers (admittedly usually the dumb kinds) that charge Nichel-based cells in parallel.

I think you are confusing chargers that charge NiMH/NiCd cells only in pairs. These chargers are charging the two cells in series, not parallel.

Nickel based chemistry cells are not conducive to parallel charging. The only parallel charger that I know of, for NiMH/NiCd cells, is the C.Crane/Saitek. It has a sophisticated algorithm for charging Nickel based cells, more or less, in parallel.

Dave