Pros and cons of parallel cells setup?

I've read often in the old threads that parallel setup of cells is considered safer than series.

Well, Series can have its drawbacks as if the cells are unbalanced the one that goes discharged first is in risk of being damaged from the current the other cells still can provide through the circuit, up to the point of reverse charging, which is damaging the cells and could be dangerous too, if unprotected.

But parallel is not really good for the cells too:

Any imbalance between the cells voltage, even minimal difference, will result in higher voltage cells to flow current to the lower voltage ones, trying to equalize the cells voltage. But this is not free, the internal resistance of cells always takes its toll on this current flow. Also the cells cannot really selfbalance to perfection, there will always be minimal difference, thus there will always be a low current flow between them still trying to equalize. In the long run, cells will all be depleted.

I cannot say in how much time, but know this is a sensible problem in other fields: in automotive or nautical, the multi battery setup in parallel always have electronic or mechanical (manual) separators to isolate each battery and avoid this self-equalizing risk.

What about our cells?

Is Lithium ion (any of the available variations) maybe less sensible to this than the lead accumulators used in the above mentioned fields? Don't think so.

I would like some experts thoughts on the subject.

In the meantime, I would consider a serial setup safer over a parallel, because PCB can trip and avoid problems in a serial setup, but it's totally ineffective in a parallel setup. It will not trip untill all cells are depleted. Will not really damage the cells (maybe, unsure) but one will end finding the torch totally depleted instead of fully charged. Nice when you need it, eh?

Laptops have been using Li-Ion 18650’s in parallel (usually 2P, sometimes 3P) for over a decade, and it has never been causing any issues. So I guess you’re worrying too much. :stuck_out_tongue:

Difference between laptops and torches is that the cells in the laptop are permanently connected, so voltage imbalances are effectively irrelevant, if a cell dies, they both go down. Fundamentally, if a cell is the same manufacturing type/date/run as another, a parallel setup is realistically not an issue, as its effectively the same cell (just imagine if the cell was bigger, same thing anyway, rate of failure is the same)

In torches, cells are fundamentally individual, so potential issues may include - Voltage matching, internal resistance matching, capacity matching.

Cell initial voltage balancing when connecting/replacing batteries is the sole responsibility of the user, and therefore potentially an issue in the short term until voltages balance themselves, hopefully uneventfully. The potential for issue here is that a large voltage differential, in combination with low resistances (cables, as well as cell internal resistance), can create large cross charging currents. This is of potential concern to the cell being discharged, but is most concern to the cell being charged for exceeding max charge current.

The voltage differential is an initial issue, but if uneventful, will tend toward balance, and as you say, its not perfect and some energy will be lost, but voltages will tend toward a common value, because as the voltage differential drops, the current flow drops, and a dropping current flow for a given internal resistance will reduce the effect of the internal resistance, allowing the cell to tend toward 0 voltage differential. This can happen in a relatively short time (seconds, maybe minutes with our small torch cells) So ultimately you will Not end up with cells discharging each other faster than the self discharge rate of the cell with the highest self discharge rate.

Capacity matching is not an issue in parallel setups, because the parallel connection means power will be drawn from the cell with the higher voltage. Therefore capacity is not the issue. Its like having 2 tanks of water, one is a big wide tank, while the other is a narrow tall shaped tank. Water level represents voltage, so if both tanks have the same voltage/water level, then if you were to drain from the tanks, the voltage will be paired and drop together, water level remains the same in both tanks.

Parallel setups have their merits, and are common practice with lithium cells. My RC car lipo pack is labelled as a 2S pack, however opening it up reveals that there are 4 cells in the pack, wired as a 2S2P setup. All you need to do in the context of torches is ensure cell matching interms of voltage and health.

Personally, I would prefer serial setups as they give you a higher voltage (ideally above Vf of LED (s)) which is easier to work with electrically (buck drivers, rather than boost/buck/unregulated) and means less resistive losses through switches/connections. This is in the context of torches we play with of course.

I read and understood your points.

Agree on most of them, but still unsure about the part:

"So ultimately you will Not end up with cells discharging each other faster than the self discharge rate of the cell with the highest self discharge rate."

Rechargeable cells are rechargeable cells, there are many discussions about the differences between inner chemicals and how they affect charging/discharging characteristics, but voltage is voltage, on any cell or electric circuit. This to say that if you are right, then what is the difference in the parallel setup of other application fields, which require separators instead?

Going more specific with real world examples, why do I require a separator for my Nissan GQ double battery setup for the winch, and at the same time 4 parallel cells in a SkyRay King are ok?

There is something that I cannot catch right...

The difference with your winch setup and any other dual battery setup for eg tailgate, have what you have called a seperator is to keep the two circuits separate on the one charging circuit so that you dont flatten the battery operating your car. If you were winching without the separator and flattened both batteries you have no current left to start or operate your car.

Makes sense.

It will separate the batteries to prevent the charged one to recharge the other.

I re-read some articles about parallel connection and all them point that if one of the cells is less charged than the other, given the low inner resistance of the cells, the current that the higher voltage one can supply to the lower voltage one can easily reach critical numbers, but then they state that hence the voltages approach, the current becomes negligible.

I see... They start with an unbalanced setup, where one of the batteries supply current for auxiliary items, and then they need to separate them to guarantee cranking power to the main one. That's why I got it wrong. I was thinking they started with a parallel fully charged setup, in which every load is splitted into the two cells, so I was mistakenly thinking that the difference in voltage was due to the parallel setup, meanwhile their setup was parallel for the recharging circuit, but kept separate when discharging! This is completely different then!

Come to think of it, then also the guy that suggested me to use a separator on my winch setup is wrong, because I keep the batteries parallel when winching in order to have more Amp for the winch, and then the engine will recharge them both at a time, like they were a single bigger capacity one. The more I think about, I DO NOT need a separator then!

I know I have to stop winching before depleting completely the batteries. But that is normal, I always did it when on a single battery too. I just want more juice.

Thanks for pointing me the light

Looks like I was wrong and I can leave parallel cells unattended into a torch without them flatting out all of a sudden, LOL!

Well, learnt one more thing today!

I read about tests (with LiIon cells) where they connected a full one and an empty one. Initially, the current reached about 8 amps but dropped steady and fast to uncritical values. I charge some of my cells in parallel..

So there certain risk involved when using unbalanced cells in Lights like Skyray King?
I read that we should use same cells from the same batch…

...and preferably new ones, so there's no different number of cycles or age.

And they MUST be all charged to the same level.

Better yet if the charger can equalize them (RC dedicated chargers do it)

Buy a volt meter, check you cells after charging and before using.

Never buy the bare minimum of a battery, check the voltage before and after each charge and number your cells. If ones not keeping up with the others, that goes in a single cell option.

I prefer single cell lights, and bringing extra batteries because I’m lazy :wink:

Being cautious with multi cell lights is worth it for the extra brightness, run time and wow factor but if your not using them every day, you won’t need to dig out the dmm all the time :stuck_out_tongue: