Shouldn't flashlights use Li-polymer cells?

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cclight
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Shouldn't flashlights use Li-polymer cells?

Modern high-end flashlights drain a high current; in turbo mode it can reach 10A or even more. This forces these lights to use top-notch high-drain LiIon cells, which are difficult to find of good quality. Maybe this is one of the reasons that are driving makers to use proprietary batteries, as we can see in several recent models.

Another issue that bothers me is the contact resistance between the battery and the light, usually at a spring. I’ve seen posts saying that for maximum performance sometimes a spring bypass is necessary (with the BLF X5). This avoids the spring resistance but not the contact one because the contact area remains quite small. I’ve just made a measurement with 2 wires joined by 2 alligator grips, that have a small contact surface, to have something that resembles a contact between a spring and a flat surface. I found a resistance of 0.03 Ohms, which produces a drop of 0.3V at 10A. Such a drop is important using LiIon cells.

So, shouldn’t lights use a better contact, such as a soldered connection on the cell that joins with the light via pins, like a wall outlet?

I’ve seen that practitioners of helicopter/drones, who have higher current requirements, use LiPo batteries. These cells have a better performance at high drain than LiIon cells, and they have pin connectors. Why not use these cells in flashlights?

It seems cylindrical LiPo cells are difficult to find but they’re flexible, so could be put in rigid cylindrical cases to fit in flashlights, at least in modified ones.

The format may be a significant obstacle for single cell lights but less for 4-cell, big models. These could do well with a battery pack to improve contacts. The recent Acebeam X65 already uses a pack. Maybe this trend has already started?

I’m trying to buy 14500 cells for an Astrolux SC and searched for LiPo cells but it’s too much hacking for me and I gave up… An additional advantage of LiPo is that they’re less dangerous to ship, so it’s better than the mess we’re experiencing with LiIon.

Jerommel
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Contact surface is just not that important as you would think. Smile
The wires between your alligator clips have the real resistance.

I think an 18650 has a cilindrical LiPo cell inside.
Rolled up and put in a metal tube.
This adds safety and protection.

Get the 840mAh Keeppower protected 14500.
They have the Sanyo 14500 under the wrapper, which is the best 14500 on the market.

Maybe i’m not aware of the difference between LiPo and tubular cells, but both are Li-ion batteries.
Both can be dangerous when you handle them wrong.

leaftye
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The problem is that there aren’t strict size classes. Shipping lithium polymer cells is more dangerous since there’s only a soft pouch instead of a built in steel case to protect its contents.

Another big potential problem is lack of quality. With name brand Sony, Samsung, LG, Panasonic batteries, it’s expected that identical models will have very nearly identical performance. How consistent will identically labeled LiPos perform from the same batch, different vendors, wildly different purchase dates?

I do like the idea of using pouched batteries instead of cylindrical cells for large flashlights, both because it makes better use of space, and because contact losses can be lower.

The low mode should be lower.

Lexel
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A spring bypass is easy to make
Higher quality lights use special material for springs and are gold plated for low contact resistance

I have found that steel springs are really bad

Good 3000mAh IMR cells can easily handle 15-20A on a continous discharge, the main problem is too much heat in a small light body
If the light can cool 30W heat its so big they often use multiple cells anyway

wle
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I think it’s more important to have choice of what cells to use, and ability to change the cells, than a few milliohms that may or may not be there.

Plus polymer is less safe, prone to puncture, and more expensive, you usually only use it when weight is important {wireless headphones, flying things}, or you only have an oddly shaped space to put the battery {phones, headphones}.

wle

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cclight
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Jerommel: “The wires between your alligator clips have the real resistance.”

No, not with these wires. Their diameter is at least 1mm, so the resistance is about 20-30mOhm/m according to the table in en.wikipedia.org/wiki/American_wire_gauge. The length is nearly 1m, so the wire resistance is about 30mOhm, one order of magnitude smaller than what I found. The cable is high quality, it’s the one that comes with the Skyrc Imax B6acv2. You can see a picture in the manual: www.skyrc.com/index.php?route=product/product/download&download_id=146, page 23. I joined cables 3 and 5.

Lexel: “Higher quality lights use special material for springs and are gold plated for low contact resistance”

I’m asking specifically about contact between spring and cell. Even with gold on the spring the cell doesn’t have it and the area of contact is really small, which is the main factor.

I agree that there are LiIon cells can give enough current but only a few ones. Also I’m worried with turbo mode; for short periods heating is not a limitation but a voltage drop of 0.3V is quite significant compared to the 4.1—3V voltage range of the cell…

leaftye

It seems that the LiPo chemistry is much less susceptible to “ignition” and is not dangerous even if beaten or perforated. About the quality assurance, don’t your arguments apply also to current LiIon cells?

Enderman
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What you see labelled as “LiPO” is actually just a lithium ion battery.
The original LiPO battery type using a polymer electrolyte was never commercialized, and what you buy is a typical lithium ion battery.
https://en.wikipedia.org/wiki/Lithium_polymer_battery

An 18650 battery is just a lithium ion battery, just like a regular RC “LiPO” battery that can do several hundred amps discharge rate.
The reason it cannot do such high discharge rates is simply the size and the way it is built. It is not some completely different chemistry like nimh vs lipo.

If it says 3.7v then it is lithium ion, if it says 3.2v then it is lithium ferrite polymer (lifepo4).
https://en.wikipedia.org/wiki/List_of_battery_sizes
You can call an 18650 cell a lipo and it is just as correct as calling a lipo cell a lithium ion battery.

cclight
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Jerommel: “Get the 840mAh Keeppower protected 14500.
They have the Sanyo 14500 under the wrapper, which is the best 14500 on the market.”

Thanks for the tip, noted. However at the moment I’m looking for high-drain. I think the windyfire 6500mAh (red) is one of the best, as reported by HKJ at http://budgetlightforum.com/node/33434.

Jerommel
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cclight wrote:
Jerommel: “Get the 840mAh Keeppower protected 14500.
They have the Sanyo 14500 under the wrapper, which is the best 14500 on the market.”

Thanks for the tip, noted. However at the moment I’m looking for high-drain. I think the windyfire 6500mAh (red) is one of the best, as reported by HKJ at http://budgetlightforum.com/node/33434.


Well, thank you for the tip too. Smile
Lexel
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It is not that hard to measure the contact voltage drop
Usually 0.3V comes from a steel spring resistance not contact resistance

In a tight light there is quite some force pushing the battery to the spring

The batteries are also plated so with a good spring there is very small voltage drop

I measured bypassed springs at 5A with 17.5mV using my banana plugs also measuring the contact and plug resistance, not pushing hard on the spring I got about 20mOhms

P.S.
I measured today the spring again, but using 4 seperate wires to eliminate the voltage drop within the 5cm plug an contact drop

I measured the spring with 5.5mV drop, so for 5A I get 12mV drop for losses in the plugs + contact drop

Just pushing the 2 plugs as hard together as I could on a big surface I measured 10.5mV drop within the plug

That leaves 1.5mV @5A drop for two contact surfaces as you have within a flashlight

cclight
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Enderman: thanks for the very informative links! This removes the doubts about performance and regulations.

Lexel: “I measured bypassed springs at 5A with 17.5mV using my banana plugs also measuring the contact and plug resistance”

Thanks, you did measure resistance of a spring, just what I wanted. I also measured voltage drop at 5A (coincidence), including a T connector, and found 175mV at the banana plugs. So my value excludes the drop at the banana plug-power supply contact, and includes only the plug-wire resistance. Still it is 10x your value, showing that a spring contact is 10x better than the alligator grip one. Just this first result already shows that spring contact resistance is negligible.

“I measured today the spring again, but using 4 seperate wires to eliminate the voltage drop within the 5cm plug an contact drop

I measured the spring with 5.5mV drop, so for 5A I get 12mV drop for losses in the plugs + contact drop”

I didn’t understand this, I’d just have directly joined the wires connected to the plates that the spring connects to. Anyway this is another order of magnitude improvement.

Thanks guys for enlightening a newbie!

Lazy-R-us
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cclight wrote:
Jerommel: “Get the 840mAh Keeppower protected 14500.
They have the Sanyo 14500 under the wrapper, which is the best 14500 on the market.”

Thanks for the tip, noted. However at the moment I’m looking for high-drain. I think the windyfire 6500 mAh (red) is one of the best, as reported by HKJ at http://budgetlightforum.com/node/33434.


Before this gets repeated too many times, or anyone gets too excited, you have an extra digit in there. That cell is a 600 mAH cell.

Lazy-R-us

Lexel
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cclight

The 5.5mV is measured with the voltmeter directly holding on the tip of my babana plug and on the base of the spring
To separate the measurement from the banana plugs that are delivering 5A

If I stack the banana plugs so I handle only 2 probes I get additional 12mV drop within the plugs measured as well, leading me to 17.5V with the spring
If I shortcut that setup I measure 10.5mV

So I get 3 voltage measurements
17.5V with plug, spring and contact drop
10.5mV shortcut probes
5.5mV 2 probes on spring with seperate current tips

So to get the real contact voltage drop I need to calculate
full drop -spring -plug
17.5-5.5-10.5mV=1.5mV

I know there is a possible error so that is maybe 2 or 2.5mV at 5A loss
That leads to 1mOhm resistance for contacts inside the light

While I measured on my bronce alloy 6 and 12mm double springs in my Klarus XT11GT 15 and 36mOhm

Conclusion contact drop on gold plated springs can be ignored as it is less than the AWG20 bypassed spring
The voltage drop on switch with PCB, FET and driver lanes is a lot more

dw911
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Lazy-R-us wrote:
cclight wrote:
Jerommel: “Get the 840mAh Keeppower protected 14500.
They have the Sanyo 14500 under the wrapper, which is the best 14500 on the market.”

Thanks for the tip, noted. However at the moment I’m looking for high-drain. I think the windyfire 6500 mAh (red) is one of the best, as reported by HKJ at http://budgetlightforum.com/node/33434.


Before this gets repeated too many times, or anyone gets too excited, you have an extra digit in there. That cell is a 600 mAH cell.

Lol I thought rubbish that’s impossible and would normally of laughed it of, then I saw the “ one of the best as reported by ‘HKJ’”
So thought hmmm maybe I should check out this amazing 6500mah when I got home.
Thanks you saved me looking at the review Smile

cclight
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Thanks for the explanation Lexel.

Sorry for the confusion with the capacity… I intended to write 650. Still not right though, because I mixed up the Windyfire with another battery I was looking at, a Nitecore (http://www.nitecorestore.com/Nitecore-14500-650mAh-Battery-p/bat-imr1450...).

Jerommel
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Yeah i knew it was a typo.
Still, they’re the best high drain 14500s.