Spinning Batteries.... really fast... - Page 1
Forum section: electronics / general

author: mrcrud5
author_rank: Contributor
timestamp: October 08, 2019, 06:25:11 pm
content: Hey guys,I'm looking to power something thats spinning at around 8000 rpm. Power consumption is about 300mA at 9 volts. I know there are solid core batteries which should generally be used for this, as the electrolytes inside dont separate under high speeds. Does anyone know of a cheap source for these solid core batteries? Alternatively, does anyone know where I can find any data on spinning various kinds of "off the shelf" batteries like Nicad or NiMh? All batteries are spun on center.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 08, 2019, 06:52:37 pm
content: Can't really help, but the question is interesting. Centrifuging electrolytes at high rotational speeds...https://news.utexas.edu/2017/02/28/goodenough-introduces-new-battery-technology/

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 08, 2019, 07:13:39 pm
content: Have you verified that normal paste electrolyte batteries have a problem with your environment? The capillary forces holding any liquid in place in these are quite strong, and by my rough calculations they will see "only" about 700 g's at the outer periphery of the battery. There is a time component to this also. Does your battery have to operate in this environment for seconds, minutes or hours at a time? Is there time for fluid to redistribute between operating cycles?While not an exact match for your situation I have seen fairly conventional batteries operated in high g environments successfully.You might also look into slip rings, a rotary transformer, or even photocells as a method for powering your device.

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: October 08, 2019, 07:15:17 pm
content: How far is the battery from the axis of rotation?

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 09, 2019, 11:32:13 pm
content: What about use a BLDC motor as a generator?

author: Red Squirrel
author_rank: Super Contributor
timestamp: October 10, 2019, 02:50:21 am
content: I'm actually kinda curious myself about this... never really something I gave any thought.I would try to avoid that situation and find a different way to transmit power, but I would still like to see what happens if you put a battery in a centrifuge powering an active circuit.Could maybe look into how some brushless alternators transfer power to the rotating coils.

author: Rick Law
author_rank: Super Contributor
timestamp: October 10, 2019, 05:49:54 am
content: Quote from: pardo-bsso on October 08, 2019, 07:15:17 pmI was waiting for a reply to Pardo-Bsso's reply asdistance from axisis an important factor.Frankly, I was hoping to see your battery's mass (m) in milli-gram, and distance to axis (r) in milli-meter. Otherwise,you don't have a prayer.It is late at night, so I may have math error... Since we also don't know how much your battery weights, lets work this thru and see how many g's you may be pulling. This will tell you if it is possible. Your 1 gram battery will weight 2 grams if you are pulling 2 g's.F is forcem is mass (it cancels out in how many g's you are pulling)a is accelerationR is Round perSECONDwhich would be 8000/60r is the radius, your distance from axis of rotationn is the number of g's you are pullingg is the gravitational acceleration (9.8m/s2)v is your linear velocityFor each rotation, it moves the circumference of the circle = 2*pi*radiusI am going to take pi out, so 2*3.14*r = 6.28rYou are doing 8000RPM, so RoundPerSecond R = 8000/60So, your linear velocity v = R*6.28rYour Centripetal Force is f=mv2/ rYour Centripetal Acceleration is a=v2/ rExpress Acceleration a in number of g's would be n = a/gWe know a from above, so now n = a/g becomes n = v2/ rgWe know v from above, v = R*6.28rSo,n = v2/ rg becomes:= (R*6.28r)2/ rg= R*R*6.28*6.28*r*r/rg= 39.44*R*R*r/g (Note one of the r cancel out)= 39.44*R*R*r/9.8= 4.02R2rSince R=8000/60= 4.02*(8000/60)2r= 4.02*(133.3)2r= 71467rSo, if you are rotating at1 meter from axis, you are pulling 71467g's.If you are rotatingat 1mm, you are pulling 71g's;at 1cm, you are pulling 714.67g's.If you have a mere 10 gram battery, rotating at say 2 cm, at 2*714=1428g's,so at 2cm from axis, your 10 gram battery "weights" 14280gram or 14.28kg(weights as in outward centrifugal force). Unless you use very thick steel plates to rotate and to bolt the battery down, the whole setup will tear itself to pieces before your electrolytes stops working.Check my math, it is late, and contrary to my own believe, I do make a mistake every now and then.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 10, 2019, 06:06:42 am
content: Quote from: Rick Law on October 10, 2019, 05:49:54 amYour math is right. Got the same numbers I did. But he did say (maybe) that the batteries are on the axis of rotation. AAA battery would see 350 g's at the skin (0.5 cm) if long battery axis was along the spin axis. AA gets close to the 700 g's. Doesn't seem out of line to me. Does need to have a very strong battery holder, and pay attention to centering. Small geometry errors lead to big forces.He will also have the same kind of worries on PWBs, harnessing and anything else that is involved in his project. But since this sounds much like a projectile, and since people have put all sorts of things from radars, cameras and gps in projectiles it can certainly be done. For some open source info just Google the VT fuze. WWII project.

author: Psi
author_rank: Super Contributor
timestamp: October 10, 2019, 06:14:20 am
content: Have you considered if an ultra capacitor might work?

author: Rick Law
author_rank: Super Contributor
timestamp: October 10, 2019, 06:57:53 am
content: Quote from: CatalinaWOW on October 10, 2019, 06:06:42 amYeah, I am constantly amazed by how those electronics in projectiles handle the huge g force. The Anti AirCraft Artillery shells is probably the hardest. It would have been under a huge g force to be able to reach 10,000ft and more. The proximity based shells would require a bit of electronics beyond the altitude based fuses.For the OP, rotating at 8000RPM, everything has to be precision balanced dynamically with center of mass right at rotational axis. If even a small 10gm PCB is installed on one side at 2cm from axis, it would "weight" 14kg pulling against the shaft. Dead-bug soldering is definitely out... Everything is 700+ times it weight at just 2cm. It is quite a challenge to get that setup to work and lasts. To OP, I wish you the best....

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 10, 2019, 07:01:22 am
content: Quote from: Psi on October 10, 2019, 06:14:20 amIt's still a mass full of electrolyte, the same problems would apply with the only benefit being a rapid charge.If the device does have a mount and is not some sort of mortar shell or something then slip rings or a rotary transformer are the best options. All of the previous calculations are for a single AA battery mounted dead center, when the power requirements in the OP are for 9V meaning either a PP-3 cell (which won't last too long at 300mA) or worse a multicell holder (which would not survive the forces very long). You could use a single cell and a boost converter but due to the high voltage gain and relatively high current again it won't last long with a standard small battery. Of course, this wouldn't matter if it's only in brief use. If a battery is required (no external power available) than a small tubular lithium-ion cell stood on end in the center axis with a boost converter mounted directly on top might work (though keep the cell well within the safe voltage range to prevent excessive ion migration or membrane damage).

author: BrianHG
author_rank: Super Contributor
timestamp: October 10, 2019, 07:16:27 am
content: (A wild guess, make your own judgements or research) You can try 3x 500Mha lithium coin cells mounted upside-down facing outwards. The coin cell's + side facing out makes it a simple circular tin can retaining the electrolyte so long as the internals aren't flat interleaved fins, it may be small, flat and light enough to survive. But for how long, I don't know.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2019, 08:36:20 am
content: Didn't mention if there's a shaft or not. So...Even if there isn't, if there's a range of position for the thing, inductive coupling may be practical. Uh, heck, these days with the availability of wireless chargers, just use a set -- reshape the coils as needed (make sure they're about the same inductance as original, though).If there is a shaft and everything is consistent, slip rings are obvious, but there are other options for induction too. I've seen a design where there's a series of concentric disks, giving capacitive coupling (electric induction). (I don't know that this is actually any better than inductive, or that it can be used for anything else, e.g. as a Tesla turbine to move cooling air. Maybe it can save copper cost, since a winding isn't needed?)Anyway, again yeah, distance from axis is important. Don't know of any battery rated for such acceleration, you'll have to ask a bunch of manufacturers. You may not get a response until you try some really specialty ($$$) brands. No idea...Regarding supercaps -- they at least are filled with the same electrolyte through and through, no air space, no  separator (there is still a porous insulating barrier between electrodes, of course). I would be worried about damage: these things are made of different density materials, you can't rely on buoyancy to keep things in place. Eventually, enough force or acceleration will push the electrodes together or tear the separator or something, and the part will fail.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: October 10, 2019, 11:03:49 am
content: Quote from: BrianHG on October 10, 2019, 07:16:27 amCoin cells are not a "solid piece of metal" they could very well fall apart in large accelerations.OP: I assume you want rechargeable battery, and not primary?

author: ChristopherN
author_rank: Unknown
timestamp: October 10, 2019, 02:10:58 pm
content: We build a sensor for the end of a shaft once, that one used a single CR2032 sized cell which worked well. We did tests up to 10k rpm, but as others said, you have to be really, really careful.We used one half of a standard motor coupling and embedded the sensor in there. It was all potted in flexible compound and balanced afterwards. Get someone with a newer CNC mill center for testing, the housings are "projectile proof" in most cases and newer machines are able to measure the xyz force on the spindle. This lets you test in a controlled environment and really helps balancing.

author: Red Squirrel
author_rank: Super Contributor
timestamp: October 10, 2019, 08:36:35 pm
content: Depending how complex this can be one option might be to mechanically rotate the battery 180 degrees back and forth at intervals throughout operation. If there is any separation of materials inside it should maybe help cancel it out? Not really sure if it would though.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 11, 2019, 02:57:13 pm
content: "But it's got electrolytes!"

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 11, 2019, 07:53:47 pm
content: Quote from: Red Squirrel on October 10, 2019, 08:36:35 pmAt these forces the big concern is the batteries physically flying apart not so much electrolyte seperation.

author: Rick Law
author_rank: Super Contributor
timestamp: October 11, 2019, 09:03:25 pm
content: Quote from: Rick Law on October 10, 2019, 06:57:53 amFirst, CatalinaWOW, thanks for confirming my math. It was late that night and I was nearer to brain-dead than I thought. My immediate reply right after yours that night was to say thanks but I totally forgot about it in the reply.The other reason for the reply which I also forgot: The one advantage a projectile (as in bullet) has is that one can likely place the electronics parallel to the base of the projectile - therefore, the entire set up is subjected to exactly the same g force. In a spinning setup, you can't do that. The force is proportional to the distance to the axis. For every 1mm off-axis, the g force is 71.467g. However one arranges the components on the PCB, and however one arranges the PCB itself, one part of the PCB will be closer to axis than other parts of the PCB. The base of the PCB is a line but the rotational is an arc; so, different part of the PCB will be subjected to different magnitude of force thus bending or pulling the PCB.My saying "haven't a prayer" is an exaggeration. But this is going to be hard for sure. The arrangement of components will be both an electronics problem and a mechanical problem.Quote from: ChristopherN on October 10, 2019, 02:10:58 pmGreat to hear from someone who has done it before and at 10k RPM no less.At 9V 300mA requirement that the OP needs, the required battery is probably going to be larger than CR2032.It is unlikely that the battery will be mechanically perfect, so, a geometrically perfectly aligned battery may still be unbalanced statically or dynamically. For every battery change (or recharge), the darn thing would likely need to be retested and perhaps re-balanced.I think the setup could use a design-in means of mechanical-balance adjustment. Rechargeable has an advantage here - assuming a charging would not change the battery mechanically (mass distribution)....The induction method is looking better all the time. Or, if stealing a little from the rotation ok... It is worth looking at making your own dynamo - your setup is rotating anyway. Just A few loops of wire on the PCB plus static magnets (arrange as a ring) that your PCB can rotate over - loops of wire is pure solid. Now one has to worry about the electro-magnetic force slowing the rotation down. That justmay beless than the problem of securing a properly-sized battery.