LiFePO4 18650 Safer?

Scotcha · Apr 12, 2014

Hi,

This is my first thread here and apologies if this has been answered previously. I have tried to read through several threads but getting more confused.

My question is are LiFePO4 18650's safer?

I intend to use them with the following lights :

Prometheus Alpha Ready made (Single 18650)

Steve Ku 18650 Gavina (Single 18650)

I have already purchased a voltmeter and Xstar VP1 charger with Eagletac protected Li-Ion's.

Looking at switching to LiFePO4's if they are a safer alternative.

Thanks in advance

[h=3][/h]

ChrisGarrett · Apr 12, 2014

Higher thermal runaway temperature as I understand it, than the others--Li-Co and Li-Mn, but you can still get them to pop.

Protected li-cobalt cells and naked li-manganese cells are safe if you undertake a common sense approach to using them and not let them run down to zero and sit for weeks, before you charge them up.

I don't bother with LiFePO4 cells, personally.

Chris

BVH · Apr 12, 2014

If I can deal with the lower nominal Voltage in a given use, I will always choose LiFeP04 over Lithium Ion/LiPoly/LiMn. While LiFeP04 will vent, they won't vent-with-flame. The others will and it's not just the fire hazard that is a potential danger, the fumes given off during burning are extremely dangerous and do lung damage fairly rapidly. I still have a few LiPolys but as they wear out, I do not replace them.

Scotcha · Apr 12, 2014

Thank you. Any particular cells that you would recommend?

mcnair55 · Apr 12, 2014

BVH said:
If I can deal with the lower nominal Voltage in a given use, I will always choose LiFeP04 over Lithium Ion/LiPoly/LiMn. While LiFeP04 will vent, they won't vent-with-flame. The others will and it's not just the fire hazard that is a potential danger, the fumes given off during burning are extremely dangerous and do lung damage fairly rapidly. I still have a few LiPolys but as they wear out, I do not replace them.

A right happy chap you are,reading that makes operating a torch a total nightmare scenario.

StorminMatt · Apr 12, 2014

Speaking of the lower voltage, it might not be a bad idea to look for a two cell light. One LiFePO4 battery probably won't be able to drive an LED that hard unless it is directly wired to the battery. Also, given the generally lower capacity of LiFePO4 compared to other lithium chemistries, two cells will greatly help with runtime.

gravelmonkey · Apr 13, 2014

Nice pair of lights you have! They're both single cell, you've got a volt-meter, a decent charger and protected cells, you'll be okay with what you've got.

slntdth93 · Apr 13, 2014

StorminMatt said:
Speaking of the lower voltage, it might not be a bad idea to look for a two cell light. One LiFePO4 battery probably won't be able to drive an LED that hard unless it is directly wired to the battery. Also, given the generally lower capacity of LiFePO4 compared to other lithium chemistries, two cells will greatly help with runtime.

Well 2 cell in the sense they are wired in parallel, but not sure many lights do 2xCR123 in parallel (usually they are in series)

StorminMatt · Apr 13, 2014

slntdth93 said:
Well 2 cell in the sense they are wired in parallel, but not sure many lights do 2xCR123 in parallel (usually they are in series)

Two LiFePO4 in parallel won't be any better than one since the voltage is no higher. Basically, the problem with LiFePO4 for flashlight use is that the voltage is not much higher than the Vf of the LED. Running them in series and using a buck driver takes care of this issue.

slntdth93 · Apr 13, 2014

StorminMatt said:
Two LiFePO4 in parallel won't be any better than one since the voltage is no higher. Basically, the problem with LiFePO4 for flashlight use is that the voltage is not much higher than the Vf of the LED. Running them in series and using a buck driver takes care of this issue.

Running 2 in series would provide the same runtime as running 1 in terms of capacity. The voltage would be doubled, but the runtime would remain the same (relatively).

ie. given two 750mAh Lifepo4 16340's, and a light that draws 750 mA:
in series: 6 volts, for 750mAh/750mA = 1 hour
in parallel: 3 volts, for (2*750mAh)/750mA = 2 hours.

You would be correct in the voltage for parallel is no higher, but given the same current draw scenarios, a series connection does not provide more run time. Although, we could also be comparing the efficiency of circuits, but I have no idea how buck and boost circuits work.

HKJ · Apr 14, 2014

slntdth93 said:
Running 2 in series would provide the same runtime as running 1 in terms of capacity.

That is only true for linear drivers, a buck driver will give longer runtime.

Scotcha · Apr 14, 2014

Thanks for all the suggestions and answers, I will toughen up and use the 18650's I have and make sure that I test the cells regularly.

snakyjake · Apr 14, 2014

ChrisGarrett said:
Higher thermal runaway temperature as I understand it, than the others--Li-Co and Li-Mn, but you can still get them to pop.

The way I understand it from the information I received from K2's US DOT tests: There's no thermal runaway. There's no "pop". There's no fire. There's no explosion. LiFePO4 is safe.

Another advantage to LiFePO4 is not having the circuit cutoff your light.

BVH · Apr 14, 2014

And you can leave them fully charged and unused for long periods of time without damaging them unlike Lipo's

Scotcha · Apr 15, 2014

snakyjake said:
The way I understand it from the information I received from K2's US DOT tests: There's no thermal runaway. There's no "pop". There's no fire. There's no explosion. LiFePO4 is safe.

Another advantage to LiFePO4 is not having the circuit cutoff your light.

BVH said:
And you can leave them fully charged and unused for long periods of time without damaging them unlike Lipo's

Thank you. A123 cells and Xstar VP 2 ordered

jtr1962 · Apr 15, 2014

StorminMatt said:
Basically, the problem with LiFePO4 for flashlight use is that the voltage is not much higher than the Vf of the LED. Running them in series and using a buck driver takes care of this issue.

That's actually an advantage in that you can use a super simple linear current regulator while still getting decent efficiency. For example, let's assume the LED Vf is 3.0 volts. Hot off the charger LiFePO4 are 3.65 volts but this almost immediately drops to ~3.3 volts once you start discharging. That means a linear pass regulator is ~90% efficient with freshly charged batteries, and it only gets more efficient as the cell(s) run down. Once the cell drops to a bit over 3 volts the current will start dropping but by then over 90% of the energy in the cell is used. Of course, if you have LEDs with Vf significantly above 3.0 volts then you probably need to run a pair of LiFePO4 in series with a buck regulator, but the trend with LEDs nowadays seems to be towards lower Vf in the 2.8 to 3.0 volt range.

LiFePO4 is also quite amenable to direct drive provided the current from freshly-charged cells isn't too high for the LED.

StorminMatt · Apr 16, 2014

jtr1962 said:
That's actually an advantage in that you can use a super simple linear current regulator while still getting decent efficiency. For example, let's assume the LED Vf is 3.0 volts. Hot off the charger LiFePO4 are 3.65 volts but this almost immediately drops to ~3.3 volts once you start discharging. That means a linear pass regulator is ~90% efficient with freshly charged batteries, and it only gets more efficient as the cell(s) run down. Once the cell drops to a bit over 3 volts the current will start dropping but by then over 90% of the energy in the cell is used. Of course, if you have LEDs with Vf significantly above 3.0 volts then you probably need to run a pair of LiFePO4 in series with a buck regulator, but the trend with LEDs nowadays seems to be towards lower Vf in the 2.8 to 3.0 volt range.

LiFePO4 is also quite amenable to direct drive provided the current from freshly-charged cells isn't too high for the LED.

Speaking of direct drive, it might also be possible to have a driver that goes to direct drive (in the highest mode) once the battery reaches plateau voltage. Unlike other Li-Ion chemistries, in which the voltage has a VERY strong dependence on state of charge, the voltage of LiFePO4 is constant within a tenth or two of a volt until completely discharged. With this kind of voltage stability, regulation is more of a power waster than a necessity. Doing away with regulation entirely when the light is on its brightest mode can do quite a bit to make up for the lower capacity of LiFePO4.

Canuke · Apr 19, 2014

jtr1962 said:
LiFePO4 is also quite amenable to direct drive provided the current from freshly-charged cells isn't too high for the LED.

LiFePo seems almost designed for direct drive. One cell can run an XM-L close to its limits (very low Vf examples in lights with minimal contact resistance might go :poof:

) while two cells in series can run a 6V MT-G at around 3A with lots of headroom.

Dr. Mario · Apr 28, 2014

There is some risks you should consider also: some LiFePO4 batteries are very powerful in term of short-circuit current consumption. A123 18650 LiFePO4 can easily dump 70 Amps in a dead short (and blowing the short up in the process). Just mind your battery connections, and you will be fine. Of course, the safety aspect of LiFePO4 batteries have me sold on A123 18650 LiFePO4 batteries (currently using one in a handheld 520nm Gallium-Nitride green laser).

Scotcha · Apr 28, 2014

Dr. Mario said:
There is some risks you should consider also: some LiFePO4 batteries are very powerful in term of short-circuit current consumption. A123 18650 LiFePO4 can easily dump 70 Amps in a dead short (and blowing the short up in the process). Just mind your battery connections, and you will be fine. Of course, the safety aspect of LiFePO4 batteries have me sold on A123 18650 LiFePO4 batteries (currently using one in a handheld 520nm Gallium-Nitride green laser).

Thanks. No suppliers in Australia have A123 in stock but managed to get some K2 batteries. Just waiting for the charger now.

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