LiFePo4 18650

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Barry Lyndon
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LiFePo4 18650

Hi. New here.

Interested in these BLF-designed Sofirn lights, but I want to use LiFePo4 cells. From a cursory search here, I understand that the low cutoff voltage for some of these are around 2.8-3V, which wouldn’t allow much depth of discharge with my chosen cells.

My questions are:
Can the software that comes with these lights be edited to lower the cutoff?
Or, which light do you recommend that could use these cells — either 18650 or 26650? [ in the middle ($30) to high power ($60) range]

I have lots of cheap LED lights from Amazon, some use 18650 Li-ion, some use AA… but none are quite powerful enough for certain tasks; and I’d kinda like to move away from exploding battery technology.

Any ideas?

Virisenox_
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You may be able to edit the software, but the hardware is the real problem. 2.8V isn’t enough voltage to power an LED at a high brightness.

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Barry Lyndon
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Good point; and I expect some degree of lower brightness — unless some LEDs are out there which are designed to use lower voltages (without using a boost converter)?

Also, what if I took a longer light designed to use 2 × 18650s in parallel but flipped one around in series instead to output ~5V+? Would that be a problem?

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Virisenox_ wrote:
You may be able to edit the software, but the hardware is the real problem. 2.8V isn’t enough voltage to power an LED at a high brightness.

Yeah, even changing the firmware, you will find that the hardware will be your next limitation. There are LEDs available now that can give max rated output at 2.8V or even lower, but then the MCU on the driver will not function with too low voltage. If you can mod, a good workaround might be to put a 2AA or CR123A capable 3V boost driver into your light of choice. Or, put 2 or more of the LiFePO4 cells in series and use a buck driver to step it down.

EDIT: cross posted Silly

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wstrachan
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why not just use 18650 Li-Ion cells? I’m not trying to be a jerk or contradict what you’re asking for but was just wondering?

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Barry Lyndon wrote:
Also, what if I took a longer light designed to use 2 × 18650s in parallel but flipped one around in series instead to output ~5V+? Would that be a problem?

Shocked .. YES… it would.

Why not just use 2 in series or parallel, whatever the light is designed for ??

….
Edit: Oh yeah, the “longer light” you are describing is most likely set up for 2 batteries in series… not parallel.

Why not just stick to 18650’s??
…….
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Barry Lyndon
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DavidEF wrote:

Yeah, even changing the firmware, you will find that the hardware will be your next limitation. There are LEDs available now that can give max rated output at 2.8V or even lower, but then the MCU on the driver will not function with too low voltage.

I suspect, if they’re moving in the direction of lower-voltage LEDs, we’ll start to see lights designed for LiFePo4s coming out then.
Quote:
If you can mod, a good workaround might be to put a 2AA or CR123A capable 3V boost driver into your light of choice.

Last time I tried to solder a Line In onto an old car stereo PCB, I completely mangled the joint and nearby traces. Not sure if I just suck or my 30 year old cheapo soldering iron is worthless.

Quote:
Or, put 2 or more of the LiFePO4 cells in series and use a buck driver to step it down.

So it would have to be stepped down from 5-6V to 4V?

Barry Lyndon
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teacher wrote:

Shocked .. YES… it would.

Why not just use 2 in series or parallel, whatever the light is designed for ??

Well the idea is to bring the_ too low_ 3.2V of the LiFePo4 cells up to the typical 3.7V+ of the Li-ion. I’m just not sure if running overvoltage is a big problem.
I mean, in my mind (not being an electrical engineer), it could either fry something or it could not be a big problem. I just don’t know.

Barry Lyndon
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Well… I just don’t like the idea of thermal runaway, fires, explosions, etc. It’s a hassle to worry about. I want the worst that can possibly happen to be “oops I killed the battery”.
There are better cells out there and I think they will be the future. I don’t want to invest too heavily in what I feel is planned obsolescence.

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Overvoltage is a huge problem with any rechargeable battery.

Do not forget that the max charge voltage of most lithium-ion chemistries is 4,2V/cell, while LiFePO4 is 3,65V/cell.

And besides, lithium-ion 18650s are actually very safe. If you get legit cells, you will actually never ever had a problem with them unless you short them out for a long period of time.

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wstrachan wrote:
why not just use 18650 Li-Ion cells? I’m not trying to be a jerk or contradict what you’re asking for but was just wondering?

Great point man. The EXACT reason why I DID NOT buy Mactac Rechargeable. I ready like the light but use of LiFePo4 18650 made PASS!!!

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I wouldn’t be overvoltaging the battery. I mean the flashlight — by running two 18650s in series (3.2V nominal x 2 = 6.4V)… in a light designed for 3.7V Li-ion.
The idea is just to be able to run the light on lower voltage cells without it cutting out due to low voltage.
So the question is just: would 5 or 6V fry it or not?

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mmalive: So the idea of double the discharge cycles, deeper depths of discharge, and no chance of fire doesn’t appeal to you? I don’t get it.

I just looked at the Mag-Tac (wasn’t aware of it). My impression is — way too expensive, and don’t like the proprietary 18650. Otherwise, looks pretty cool.

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Barry Lyndon wrote:
mmalive: So the idea of double the discharge cycles, deeper depths of discharge, and no chance of fire doesn’t appeal to you? I don’t get it.

I just looked at the Mag-Tac (wasn’t aware of it). My impression is — way too expensive, and don’t like the proprietary 18650. Otherwise, looks pretty cool.

The answer is “ yes and no “. Yes to points mentioned. BUT no to lowered power capacity with limited run times as opposed to Li ion cells.

Very expensive and proprietary cell = NO BUY. No matter how cool it looks.

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mmalive wrote:

The answer is “ yes and no “. Yes to points mentioned. BUT no to lowered power capacity with limited run times as opposed to Li ion cells.

Very expensive and proprietary cell = NO BUY. No matter how cool it looks.

Yeah, I wouldn’t buy anything that doesn’t accept standard cells.
But as to capacity and price of LiFePo4 18650s… I came across some 1400mAh last night for $2.50 apiece — which seems worth it to me.

Brand: K2
Model: LFP18650E
Capacity: 1400mah
Watt Hours: 4.48
Nominal Voltage: 3.2
Fully Charged Voltage: 3.65 (recommended), 4.1 (maximum)
Fully Discharged Voltage: 2.5 (recommended), 2.0 (maximum)
Continuous Discharge (A): 5.6
Pulse 30 Seconds Discharge (A): 9.8
Charge Current (A): 1.5

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Barry Lyndon wrote:
mmalive wrote:

The answer is “ yes and no “. Yes to points mentioned. BUT no to lowered power capacity with limited run times as opposed to Li ion cells.

Very expensive and proprietary cell = NO BUY. No matter how cool it looks.

Yeah, I wouldn’t buy anything that doesn’t accept standard cells.
But as to capacity and price of LiFePo4 18650s… I came across some 1400mAh last night for $2.50 apiece — which seems worth it to me.

Brand: K2
Model: LFP18650E
Capacity: 1400mah
Watt Hours: 4.48
Nominal Voltage: 3.2
Fully Charged Voltage: 3.65 (recommended), 4.1 (maximum)
Fully Discharged Voltage: 2.5 (recommended), 2.0 (maximum)
Continuous Discharge (A): 5.6
Pulse 30 Seconds Discharge (A): 9.8
Charge Current (A): 1.5

Price is OK. BUT look a mAh. Most guys running 18650 lights these days have 3400-3500 mAh. These only running 1400 mAh. Lower than my Duracell AA 2500 mAh.

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mmalive wrote:
Price is OK. BUT look a mAh. Most guys running 18650 lights these days have 3400-3500 mAh. These only running 1400 mAh. Lower than my Duracell AA 2500 mAh.

It’s a trade-off. I view them as objectively better cells (due to safety) with twice as many lifetime cycles — even though cycles last half as long. The lower capacity is also somewhat mitigated by the fact that you can take them to a lower relative depth of discharge.

And for $2.50 each… heck, just throw a couple in a pocket if you really anticipate needing more Ah during a particular outing (with no worries of a pocket bomb).

PLUS… 4 in series makes an excellent 12V battery for car or solar applications… Can’t do that with Li-ion.
Yep, I’m sold.

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LFP cells are pretty constant at 3.2V until the bitter end, then drop like a rock. So at 2.8V or whatever’s the cutoff for regular Li cells, LFP cells will also get rather abruptly cut off.

You’ll just never see the maximum brightness at the LED in that range from 4.2V down to 3.2V.

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Lightbringer Yeah I’ve looked at the discharge curve. I just wasn’t sure if, under load, it would dip below the low voltage cutoff prematurely — like at only 10% depth of discharge or something.

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Whether a driver circuit that powers the LED in a torch can handle more than 4.2v (the fully charged voltage of a LiPo) is dependent on how it drives the LED, and the voltage tolerance of the components in the driver.

We understand that LEDs have a very narrow voltage band within which they will draw a reasonable amount of current. Increase that voltage slightly, say 10% above what it likes, and suddenly you go from reasonable operation, to drawing Crazy amounts of current and the LED burning up almost instantly. The ideal voltage for an LED is usually around the 3.2-3.5v range (not sure on exact figures, but this is where it generally is) and is somewhat determined by the energy required to move an excite an electron sufficiently to release a photon of light of the blue wavelength which we then convert to white light through phosphors.

Connecting a LED directly to a battery is only done in carefully managed cases because the LED generally draws almost too much power to survive normally. We use drivers to keep the voltage a little lower to keep things safe. A driver takes your input voltage, and presents the ‘ideal’ voltage to the LED. How it does it is the answer to your question of whether your 5-6V source will work.

Fancy drivers that have buck (AND) boost circuits are able to handle any input voltage, and change it into a lower (or higher) voltage that the LED wants, and the LED will only ever be exposed to a safe voltage. These drivers can be designed to handle one, two or more cells, and are generally stated as having compatibility to 8.4v, or 12.6V (or similar). These drivers you would be able to use, as simply supply it a voltage within what it can handle and your fine. They will only give the LED a voltage that is generally safe for most LEDs.

Many cheaper LED torches use Linear drivers and are fundamentally connecting the LED directly to the battery. For brightness control, its simply connecting power to the LED on and off really quickly to create a lower “averaged” voltage. The LED and driver need to be able to bear the effects of the full supply voltage. In a single cell light, this is at most going to be 4.0V or so even with a really top notch cell, and your going to be drawing a high amount of current for a short period. Given that its maybe only 0.5-1.0V above what the LED wants, its generally doable. But if you increase your voltage to 5-6 volts, thats HUGELY outside of what the LED itself can handle and you get Instant POOF!

This is assuming the components in the circuit are able to handle a higher voltage too (which is likely, but not guaranteed)

In short, cheaper linear drivers – No Go!
Switching drivers that have a wider input range, – Most likely Yes!

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okwchin Thank you for that quick lesson.

I guess the way I’ll proceed is to buy the bulk (48 cells) LiFePo4 18650 cells I’m eyeballing and stick one in my [1x]18650 flashlights designed for Li-ion and see what happens. I’ll see how long it lasts before low-voltage protection kicks in, and I may be able to live with whatever the results are, that is — if it runs at all.

See, I just really want to make an auxiliary car trunk battery out of these things, and also a handy “pocket” solar 12V pack… but I’m trying to justify them for other uses too — like my flashlights.

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If its a well regulated torch that has current controlled outputs to the LED, then brightness will be the same, but these are generally found in higher price point type torches.

If its a direct drive, you can expect lower light outputs, but it should be able to maintain that until the cell is near cutoff. Deep discharging is never a good thing for the cells anyway. I personally prefer higher cutoffs to extend cell life.

If your buying these cells for other uses, then it makes a lot more sense Smile

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Got it.
So which of the Sofirn (or similar) lights in the $20-$30 range would you buy if you planned to use a LiFePo4 cell (at least some of the time, but not always), with an eye for high lumens and few modes.

I’m looking at this Sofirn SP32A V2.0 at the moment. I need one “high power” flashlight which I don’t have. I’d like to get a Q8 or SP36… but can’t really can’t justify the expense right now.

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mmalive wrote:

Price is OK. BUT look a mAh. Most guys running 18650 lights these days have 3400-3500 mAh. These only running 1400 mAh. Lower than my Duracell AA 2500 mAh.

Ummm… Unless I missed something…..

AA nimh are running between 1.3 to 1.42 volts.

Li-ion are 3 to 4.2 volts.

LiPo 2.8 to 3.65 volt.

Volts x amps.

Even at 1400, the li-ion have more power then the aa. Not much more, but still.

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Barry Lyndon wrote:
Got it.
So which of the Sofirn (or similar) lights in the $20-$30 range would you buy if you planned to use a LiFePo4 cell (at least some of the time, but not always), with an eye for high lumens and few modes.

I’m looking at this Sofirn SP32A V2.0 at the moment. I need one “high power” flashlight which I don’t have. I’d like to get a Q8 or SP36… but can’t really can’t justify the expense right now.

The Q8/SP36 and SP32A can do the same job, but different usability. Small, compact, light versus powerful, heavy, huge lumens.

Comment based on that I’m a recent buyer of a Q8 and probable buyer of the SP32a type. Different uses.

Price aside, it’s what you want/need/desire out of a flashlight.

Moses came from the mountaintop carrying a tablet. The Words were....WITH GREAT LUMENS COMES GREAT REPONSIBILITY.

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Just some ideas, not solutions:

Would lights with a 6v LED be better suited to 2xLiFePos?
The Convoy L2 can run 1 or 2×18650s so should be able to handle 2xLiFePos (most likely at the expense of LVP), are there other lights with this driver in them?
Lights that can run a CR123 that have a readily available extension tube might work, such as the current Armytek lights which are designed for interchangeable battery tubes so their 18650 lights will run from a single CR123 and also from 2×18650s with the right battery tube. (There’s also a promo on until Feb 10th as advertised in this thread: http://budgetlightforum.com/node/21925)

And you could always look at 2, 3 or 4xAA lights if you’re going up in size anyway Smile

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The perfect candidate for 2xLiFePO4 cells would be a light using a Buck driver and a 3V LED.

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Very much like everything else, “safety” is in the eye of the beholder. I see no issues with using li-ion cells in flashlights or even my hair clipper without BMS (I am the battery management system). But this is me, The Battery Whisperer LoL.

Using LiFePO4 in flashlights designed with regular li-ion cells in mind won't happen without a whole new or at least reprogrammed driver setup with buck, boost or buck/boost topologies. Good luck with this.

The LD-29 buck driver is designed to drive 1S emitters and could be reprogrammed for 4V cut-off (stock features 3V and 5,8V cut-offs for 1S and 2S li-ion cells operation). Available at FastTech (take a look at the currently available coupons in the website's front page; when nothing else, “BLF” Wink is your friend).

 

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@Barry Lyndon what did you end up with? Which lights did you choose to be used with lifepo4?