21700 battery advice needed - BudgetLightForum.com

Someone correct me if I'm wrong but I don't believe there is a protection circuit in existence that will allow that kind of current flow through it. Seems like the max is about 10A, maybe a little more for some on larger cells like this? Come to think of it I don't think I've even seen a with protection yet although there are with it. Samsung 40T will get you there and I see a Sony VT6A I didn't even know about...based on the performance and longevity I'd go for the Sony (in fact I think I may order a pair right now), but it looks like HKJ is showing the Samsung to perform a little better overall in his tests. Last year I got the Samsung 40 and 50 for I think $7 but right now they're about $10, as is the Sony. They are flat tops and a true flat top which is actually very slightly recessed. I just received one of the 50E-2 from Liion which was a rewrapped version with their house wrapper and a "semi button top" they added (only ordered it because they were out of the standard factory wrapped cell). That semi button top is flat and it also isn't really raised like you might expect - just enough to permit series contact.

Standard Samsungs I have measure 70.8mm, the semi-button rewrap measures 71.5mm. I have a few Sofirn cells that say mAh and 40A on the wrapper (capacity is actually more than stated and they are good cells...not sure how they've tested for current but I kinda doubt they can do 40)...those measure 71.2mm.

If somebody has a protected cell then that should surpass your length requirements by a few mm, but you'd have to be sure it would fit, and also fit in your charger. Probably easiest to alter your light by soldering a brass button to replace the driver spring, and/or swap for longer springs. I would not try to use magnets to increase the length for this kind of higher current draw. Maybe a copper slug on the bottom end if you don't mind fiddling with that every time you open the light or replace cells but that would need to fit snug and stay snug. This is also a good case for the high current beryllium springs and a bypass wire mod if the light doesn't have either of those now.

Here's the IMR link for the Sony...they make you buy two, no onesies: https://www.imrbatteries.com/sony-murata-vtc6a--mah-40a-battery/

HKJ's handy comparator (select your cells from the drop down, pick your current(s) in the checkboxes)...he has other lists for small and large cells, NiMH, etc...and you can click on each cell you choose to be taken to the individual review and data: https://lygte-info.dk/review/batteries/Commoncomparator.php

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Been pondering this and I don't think I have great answers, but I do think this is worthy of doing it the right way and thinking it through carefully because we're not talking about a trivial amount of current. Are you modding/creating in a host, or altering an existing light, or just trying to save pennies on the batteries it was designed for?

Ultimately, it makes so much more sense to change the host parts in a way that they will take ordinary-length cells, rather than having to change each cell each time and attempt to get their lengths consistent.

If the tail spring board is easy to remove, totally try a longer spring there that can handle the current and retain all or most of its height while doing so over time. If that can/should be done on both ends, assuming the driver board is compatible and accessible, then do that...or perhaps a button/slug can replace a spring.

I really really do not recommend soldering cells. Spot welding is the way to go, when done correctly, but soldering adds a lot of risk especially on the negative end, and you need to add a considerable amount here, not just a little dab. If soldering is the way you want, then do it on the positive end/cap and be careful about it so you don't short, and quick about it so you don't damage seals or cause it to vent. Overheating the negative end can spell bad news for the cell either in performance or safety and that's true of these high drain cells that can take a lickin' as well as it is for ordinary lower drain cells that are somewhat more sensitive. And of course if you were to use protected cells then you wouldn't be able to solder on the negative end anyway. I've soldered buttons and blobs and tabs on dozens of NiMH and NiCd batteries over the years, but I won't solder lithium...did once, but all things considered it's not worth it to me unless it's just a blob on the top to make a small button but I haven't needed that in any of my lights in a very long time. I just wonder about all of the heat and current in this application. Plus soldering to nickel can be unreliable even with scuffing. We've seen springs literally melt from high current that was done wrong, and cells that overheated to a dangerous level through current or accidental shorts, so I wouldn't put a lot of faith in solder here even though their melting points are above what we'd expect to see.

I searched the forum for a bit but couldn't find posts that I think I'm remembering from the past about magnets and resistance/current limit. It may have been from Dale (DB Customs) in some thread, really can't remember. But I was thinking that there was a medium-level of current that was a practical limit for magnets but of course most who use them are using tiny ones as buttons/spacers, not thicker ones to fill up space and cover a larger cell. I would try some kind of slug to use as a spacer on the tail end if you don't want to modify the light itself. If nothing else, a copper pipe cap could be trimmed and filled with solder - a 1/2" is around 5/8" o.d. and I think a 3/4" will be too wide without a fair amount of sanding. I looked on Amazon to see if there were any convenient discs/slugs and there are lots of jewelry stamping blanks but most are way too thin - thickest I saw was 1/8" and they were pretty expensive for a smaller quantity of 10 or so. Washers and bushings present the problem of the spring going in the hole, so you don't get the effect you're after. If you have a very well stocked hardware store there are copper bolts that exist and you could cut the head off of one and grind it to suit....a little pricey but you could do the same with an electrical split grounding nut thingy. Heck, really for this purpose I think aluminum would be fine and if you can score a short length of 3/4" bar you could just trim pucks from it.

So the cost of parts and consumables, repeatedly for a trio of cells and more cells in the future, plus risks that are not zero...sounds cheaper and better to just spend a couple extra bucks on the ridiculously priced cells that fit the design of the light...and don't buy lights like that again. :)

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I´m planning to build a 72v 20ah (only around 20ah due to limited space). I have never build a battery pack before so I have been doing quite a lot of research but I can´t seem to figure out the best cells to use.

I´ve been looking at Samsung 40t cells because a lot of people recommend them and because I will be able to draw a lot of amps from a batterypack made by these cells using the copper nickel sandwich method. However if I use these cells (or any other / cells) I will have to invenst in a spotwelder. I have been looking at a kWeld with the Turnigy lipo battery recommended on the kWeld website and the total cost for welder and battery including freight cost will be around 400 euros. Since I don´t know how many battery packs I will be building this seems a little bit expensive and it got me thinking that there might be other ways of building good battery packs without spot weldning? I´ve seen people using eig c020b cells for example with bus bars that screws on to the cells. Eig c020b seems impossible to find and order but maybe there are other similar cells? I have been looking at some Lifepo cells that also allows bus bars to be screwed in place but it seems hard to find Lifepo cells that are as space efficient as the Samsung 40t? Another thing is that I live in Europe so I would prefere a EU (or possibly a Chinese) supplier.

Any help is appreciated! To help select cells, we need to know what the battery needs to do for you.

How much capacity is required? (not the 20Ah you note that you're limited to, because with different cells that might be a different number)

How many amps peak will your system draw from the battery?

How many amps continous will your system draw from the battery?

What are the measurements of the space you have to put it into?

Are you willing to use alternate locations / mounting methods than whatever you've presently settled on?



Some other notes:

--Most pouch (flat) style cells (not the plastic box style like CALB / etc, but the ones like the EIG, Leaf, etc) will be less volume for the same amount of cell, because all of the cylindrical cells will leave an unavoidable gap between each other that is just air. So generally flat cells will fit more battery in the same space than cylindrical cells, as long as your space is a box-shaped space that accommodates that type of pack (where cylindricals can be rearranged in a number of shapes that larger flat cells can't do efficiently).

All the EIG C020 cells will be a little on the older side (AFAICR they haven't been made in a while; mine are over a decade old, I think), and though they do last a long time, all cells do degrade with age. If you do want some NOS unused cells, Jimbob01 has a thread in the items-for-sale-new section; he's in the UK so if you're not it may cost quite a lot to get them shipped to you (probably more than the cells cost). They *are* nice cells, easy to DIY with.

There are plenty of similar cells out there, though not many have premade cell holders and interconnects from the manufacturer like EIGs that make them very easy to DIY with. But as long as they have long enough tabs, there are quite a few ways to clamp them together without soldering or welding, many of which are in various threads around this forum. Some are titled with A123, JonesCG has at least one thread with smaller cells in very large packs for motorcycles/racing/etc, etc.

There's plenty of Chinese Li-Ion / LiPo flat pouch cells out there from Chinese vendors; the catch is finding a cell that will do what you want *and* that you can actually be sure to get from whcihever vendor you choose as actual new top-grade cells. There are numerous places selling non-top-grade cells as if they were, some of them literally recycling garbage cells and even remarking them as different brands / models than they really are (a big problem in stuff), so you have to vet whatever vendor you use to be sure they really will sell you the real thing you're after (or be sure you don't care and will just use whatever you get).


Note that flat pouch cells will generally need some form of physical compression across the flat faces of the end cells of a stack, yo'ull need the specs for how many lbs/sq in from the cell manufacturer to do this "right", to ensure that they don't puff up while in use (charging or discharging). If they do puff up, they're generally damaged and don't perform as they should (and potentially are a fire risk, but mostly just performance issues), and you'll end up replacing them to fix that. There's info on this in the various pouch-cell build threads, among others.

Cylindrical cells don't need that because their casings do it for them.