Is there any reason not to use them, do they have similar power density, how is the voltage sag under load?
bump
Yes, I think for the short run times and high power that some members seem interested in they should be better. I can’t remember what Wikipedia said, but I have fooled a little with them for model airplanes. Last I noticed, they were the main power source for model airplanes, including very large ones, which implies that they have more power per weight than any other small non-fuel burning source. The little ones I have are stacks of flat cells. They charge in about 15 minutes with a voltage peak detecting charger and discharge in less time than that. Each cell is charged separately, and they discharge in series.
I think some folks use them for their bike lights.
I think the main reasons they are not used is shape, most flashlights are cylindrical and most lipos are rectangular. I think that the lipos may be more sensitive to physical damage but as they are used in absolutely daft rc cars and helis I can’t imagine them being that sensitive. They do have some impressive discharge rates.
If i pull 3A from a lipo battery, how much will the voltage sag?
mostly depends on the size and quality of the cell
I can pull 60A+ from my R/C car batteries, the bigger ones can draw way more.
fair enough, but at 3A how much will the voltage drop from 4.2V?
I’m not sure, I don’t have anything to measure it and I only have S2 and S3 packs (I assume it would be close to nothing though).
The HobbyKing forums would have the information you are looking for, I did a lot of research there years ago but forget most of it not apart from what I need to know specifically for my car.
You are thinkig of LiFePO4 cells, those are at 3.2V. LiPos are just LiIon cells built a little different.
LiPos have a lower energy density, their power comes from the size. You would need a lot of wiring and a lot of 18650s to get the same performance.
so why can they do so many C then?
I’m guessing that if i took an 18650 and tried to get 50C from it it would not do so well
Yes. shape is the main reason… High end LiPO cells are in the * 60C * range while good LiPOs are in the 35-40C range. For a 5000mA pack, that’s about a max current ouput of 200A for a 40C rated pack! Yup, 200A! I bought a gen ace, 5500mA, 2S pack about three years ago. (Good budget pack) Internal resistance per cell was something like 2-3 mili ohms (estimated using an iCharger 106b+). Now, after practically no use and being stored at close to full quite often, internal resistance is something like 7 and 8mili ohms and capacity about 4800mAh… Haven’t tested it in a year or so though.
Because of the low internal resisistance, when something goes wrong, it goes wrong FAST so LiPOs are dangerous (LiMn are safer) and can catch fire if the cell is damaged in a crash… I once had an old, 3300mAh, 2S, LiMn pack that was drained too low in a car that heat up like crazy! It didn’t catch fire though…
One reason is: because they are flat. They dont heat up that much. You can draw more C from a 14500 than from an 18650 just because its smaller. But thats just a little easy explanation. Its also a matter of the ingredients. If you are interested in technical stuff, read on. If not, dont. ;)
LiIon is just the "umbrella term". LiPo is just a type of LiIon where the electrolyte (the stuff where the Ions are moving) is not liquid but in a "solid" stuff, the polymer. They are used in a lot of technical gadgets because you pretty much have no limits in how you build them.
So much for the form factor/the way the cells are built. Next is the real stuff: chemics.
LiIons we usually see (Sanyo UR18650FM for example) are LiCoO2, with a lot of Cobalt. Those are the "dangerous" ones: the thermal runaway occurs at about 150°C.
Then there are the safer LiMn2O4 ones. We usually call them IMR cells (Sony US18650VTC2 for example). Thermal runaway kicks in at about 250°C, they deliver a lot more energy and can heat up much more before it gets dangerous. They are less prone to abuse than LiCoO2 ones.
So, what else is there? LiFePO4 of course, they will go into thermal runaway at about 270°C, hold much less energy than LiMn2O4 ones but there is pretty much no chance to destroy them by abusing them. And they can put out much more amps.
Then we have the various cells containing Nickel. I dont know the exact formulas, but its usually LiNiO2 and some extra stuff. (Panasonic NCR line, Samsung INR18650-20R for example). Those are somewhere between LiCoO2 and LiMn2O4, depending on some other factors. Very versatile: a lot of capacity and low discharge rates or still a nice capacity and high discharge rates, but still limited compared to true LiMn2O4 cells.
And you can vary the voltage by changing the chemistry. The first LiIon cells had a charging voltage of 4.1V and could be discharged to 2V. Nowadays we have cells anywhere in the range from 2V to 4.35V.
So whenever you compare "LiIon cells" to "IMR cells", you are "literally" killing proper speech. ;)
Even "LiMn is safer than LiPo" is technically wrong. Its colloquial and most people accept it. I dont really mind either, but in this long post, the difference is important.
You can build LiPo cells containing any of the above. My best guess is that there is usually Nickel involved.
Are the flat packs of LiPo cells flexible? Can they be curved to fit the larger canister style lights? I have a LiPo pack I adapted for use to power an older Pro level camera that takes NiMh cells. The pack is switchable from 11.1 to 13.1V and the camera seems to like the higher setting. Where a OEM NiMh cell in new trim might take 1000 pictures, this pack will take (estimated) well over 3000. I’ve run it a 12 hour day taking 600+ pictures and it was barely discharged. It is in a cordura belt pack with a cord running to the camera, acting as an external pack.
I’ve wondered if an external power supply like this might work well for a light. Power pack on belt, small very well heat-sinked light module in hand. If building a custom light, this would allow virtually any design to work, and easily allow multiple high voltage cells like the MT-G2 to put out a truly awesome amount of lumens!
How about a six pack of MT-G2’s in a comparatively small “brick” of aluminum with copper cores under the emitters and a cooling fan pushing air through the finned backside of the “brick”. Top mounted handle with switch for easy carry and aim allowing stand use or placement almost anywhere, velcro strap (included on the pack) the battery pack to the stand or back of “brick”. 18,000 lumens anybody?
Just a thought…
i made a cobblejob light using a 3000mah 4s turnigy pack, one of those flex driver things (luxdrive maybe), a pentium 4 heatsink and 4 luxeon rebel neutral white led’s driven at 700mah.
the light pulls around an amp depending on the input voltage.
ill test it next time i have it out to see what the voltage drops to when i plug it in. i have one of those balance plug voltage alarms too so it will be easy to measure the current and voltage drop.
Brian
I notice my LiPos have air gaps between the flat cells. Very effective if they are out in the wind or if some air goes through the fuselage to cool them, but for a flashlight that further reduces the run time for a given volume.
Why is it the fire-resistant charging bags are called “LiPo guard” and not some other chemistry? It seems to suggest they might be more susceptible to fire. Or maybe it’s targeting RC hobbyists who use LiPo?
LiPo’s are charged very fast, the bag is probably just a good idea when dumping a charge into them at such a high charge rate.