protected 18650 in series safe?

[eyeeatingfish]

What are you trying to say? That the PCBs will ensure they are evenly charged? No.

no, i mean why would you need to charge them individually if the PCBs would prevent any one of the batteries from falling too low.
Ive never seen a warning on any 2x18650 light about making sure both batteres are charged evenly. Like the MVP page on batteryjunction.com

 

[shadowjk]

The PCB is the last-resort protection. You don't want to actually rely on it or use it more than necessary.

When you charge the pack, the battery that first gets overcharged will trip its protection circuit. That interrupts the charging of the entire pack, and some cells will not be fully charged.

When you then put the pack back into use, the cell that got the smallest charge will trigger its protection circuit when it gets overdischarged, interrupting the entire pack.

When you do this again and again, the most fully charged and the least charged cells will be repeatedly hammering into overcharge and overdischarge, adding wear and tear making them perform even worse, as well as limiting the total performance of the pack.

 

[eyeeatingfish]

So if one cell has the PCB interrupt it then whole light will stop working?

How would the batter get overcharged with the PCB? Or are you saying if the PCB fails?

And if this is the case then are the side by side chargers like the ultrafire one bad then?

 

[eyeeatingfish]

Whats the difference between current draw and voltage?

And why would the batteries need to be charged evenly? I dont have to worry about that for NIMH batteries, there are no warnings on 2x18650 lights.

 

[mikeh]

Whats the difference between current draw and voltage?

Oh dear. What are schools doing these days?
Perhaps you could get a thing called a "book" and learn the basics first.

 

[VegasF6]

The wattage, amperage, or current is what kills you

Think of it this way, you can have a 120 volt light bulb in a 60 watt or a 100 watt. Which one pulls more current? Think of a hair dryer or a microwave at well over 1000 watts, but still the same voltage. That's current.

So, if you have a 6 volt bulb at 35 watts, or a 18 volt bulb at only 5 watts, which would be brighter, but pull more current?

2x18650, or any 2x li-on lights often don't have warnings, but they should. Some more responsible manufacturers do mention things like at the first sign of dimming stop using. (This applies more to unprotected cells though)

All battery packs should be balanced, be they nimh, nicd, li-on whatever, but the difference is nimh doesn't explode, you just risk damaging one cell. If you have ever had a pack go bad in a tool battery, cordless phone, whatever, odds are that one cell was more damaged than the others and that is what drags everything else down.

Since we aren't using pre built packs with balancing circuits we are responsible for that balancing ourselves. If you buy 2 new cells and always dedicate them to the same light and always charge them together, you will be fine.

 

[eyeeatingfish]

Are li ion batteries limited at what amount of watts they can give? The bulb doesnt dictate that does it?
I understand that a higher volt bulb without the wattage. But why would a mag mod running 27-35 volt range not be possible, surely there must be some bulb that would work with that voltage and wattage, or is it a limitation of the battery?

I was trying to find a good book about electronics but havent found one yet. I read reviews about electricity for dummies and stupid etc. but they didnt get good reviews.

 

[Fourinchdragslicks]

There is a fundamental misunderstanding of electricity here.

voltage adds
-This means that as you place cells in series, you add the voltages. think of it as stacking blocks on top of each other. The more you use, the taller it gets (more voltage) However as far as each block (cell) is concerned, it still maintains the same height (voltage) as it originally had. It does not "see" the rest of the blocks (cells).

currents passes through
-This is like joining sections of hose together and running water through it.

wattage is a combination of the two
Watts=volts*current (amps)

wattage is dependent upon resistance (incan) once the resistance has been determined, the current passing through the bulb is determined buy the voltage applied to it.



The primary benefit of increasing voltage, is that it reduces current requirements from each battery, thereby reducing the "strain" on each individual cell, So long as bulb is being regulated. Every cell has its own characteristics, so the more cells you use, the more "unique" characteristics you throw into your pack. if one cell fails and for whatever reason its voltage drops, it increases the current demanded of each cell. In the case of protected cells, this could cause the pack to "stop working" for the reasons mentioned above.

 

[mikeh]

There is a fundamental misunderstanding of electricity here.
...(incan) A 20watt bulb will still consume 20 watts of power independent of the voltage applied to it

So if I plug in an American 20w light bulb to my 240V socket, it will still work fine?

Q. Whats black and hangs from the ceiling?
A. An electrician who learned his job from reading internet forums.

 

[Fourinchdragslicks]

So if I plug in an American 20w light bulb to my 240V socket, it will still work fine?

Q. Whats black and hangs from the ceiling?
A. An electrician who learned his job from reading internet forums.

yeah, it will work very briefly. here on cpf in the incan forums, users regularly do what you are talking about... overdrive their bulbs. go look at the wiki for your bulb life calculations. for the duration of time that you would have that 120v 20W american bulb hooked up to 240, it will draw nearly 80W.

 

[mikeh]

120v 20W american bulb hooked up to 240, it will draw roughly 20W (as it heats up, it will draw more) A light bulb is merely a resistor what happens to emit light.

Are you taking the ****? Stay in school, kid.

 

[qwertyydude]

If you take a 20 watt US bulb, and run it on 240v you almost quadruple the power output. Power is I^2*R. Not exactly quadruple since the bulb has a positive temperature coefficient of resistance but given a fixed resistance, if you double the applied voltage you double the current. So it will burn out because you'll be running nearly 80 watts!

But protected li-ions is necessary in series. And in series they do not have a common ground. They way these pack work out is that the strong cell ends up trying to reverse charge the weak cell as the battery dies. So the weakest cell whether nimh or li-ion will die an early death if you keep taking the pack below it's rated minumum voltage. Oh yeah and you can "balance" nimh batteries, it called trickle charging charge at less than c/10 overnight and you bring all cells to max charge.

In li-ion's case you do not want battery voltage dipping below 3 volts under load. I don't know why the protection kicks in at 2.75, that's conducive to early cell failure. I try to never take my cells to that level and I have some lithium batteries already 5 years old still running strong. They rarely get below 3.3 volts/cell and are balanced every time they're charged.

 

[Fourinchdragslicks]

Begging pardon, correcting posts as appropriate. The last involved electronics project I had was a regulated design where my constraint was the wattage. It was (relatively) constant even as the input voltage was changed.

 

[EngrPaul]

When I buy a light that uses 2 rechargeable lithiums, I look for one that has the appropriate shut-off at a safe voltage. That way, I can use unprotected batteries in series, which I feel is optimum.

This was a good thread until it went off on a tangent.

 

[Tythagoras]

eyeeatingfish, (or anyone else) I have found that "Teach Yourself Electricity and Electronics" by Stan Gibilisco is an exellent book to read. It goes into some mathematics, particularly in AC circuits, but it keeps it minimally simple and explains all of the math before you get into it. It's also not very expensive, compared to other books of its quality.

http://www.amazon.com/dp/0071459332/?tag=cpf0b6-20

Forrest Mims is another good author whose books I have experience with. He explains things very simply, and illustrates clearly. I don't have any of his books handy, unfortunately.

 

[lyyyghtmaster]

The ability to connect more and more protected cells in series is not unlimited. At some high enough pack voltage, a tripped protection circuit has the potential of being burned out due to excessive reverse voltage being applied across it by the other untripped cells. I don't know what this voltage is, or how (or if) the circuits are designed to fail in this case, but the results could conceivably be very bad indeed.

Does anyone have more specific information regarding this failure mode?

Thanks!

 

[friendly1_uk]

The ability to connect more and more protected cells in series is not unlimited. At some high enough pack voltage, a tripped protection circuit has the potential of being burned out due to excessive reverse voltage being applied across it by the other untripped cells. I don't know what this voltage is, or how (or if) the circuits are designed to fail in this case, but the results could conceivably be very bad indeed.

Does anyone have more specific information regarding this failure mode?

Thanks!

I'm here looking for this very information. I have aired the fact on forums over the years, to an equally show stopping effect.

As soon as the first protection device opens, it see's full pack voltage across it.

Example, at 10S the disconnection device would see around 30 volts across it. Presuming the cells were around 3v each when disconnection occurred.

I have seen people getting away with 8S packs. Having looked at cell spec lists, I have to wonder if this idea has escaped the manufacturers.



My interest is EV's so a high cell count is normal.


I have noticed, that generally speaking, power electronics falls within certain thresholds. Around 35v being a common maximum value for many devices. 50v less common. 100v and your list of devices is getting short. All cells are going to carry different devices, but there should be some sort of industry wide expectation, and higher voltage versions available I would of hoped.

I'm not seeing anything still, and this is not a new thread.