Potting a battery? - Page 1
Forum section: electronics / beginners

author: Lee697
author_rank: Regular Contributor
timestamp: September 10, 2019, 01:44:39 am
content: I'm planning a small project - an ATtiny85, LED, mercury switch and a battery. The battery I'm planning to use is a primary cell, 3.6V 1200mAh, although am open to similar suggestions.(link to battery -https://www.jaycar.com.au/lithium-axial-1-2aa-3-6v/p/SB1771)This needs to be waterproof.I'd like to pot the lot in casting resin.I'm planning on a little protective sleeve for the mercury switch in case it was to break in shrinking/moving resin.The battery - can I directly pot it? Should I wrap it first?The circuit planned draws between 0.9-1.7mA @ 3V.Thanks for any suggestions.....Lee

author: coppercone2
author_rank: Super Contributor
timestamp: September 10, 2019, 01:46:56 am
content: I think this is a bad idea. I think you should use a external battery like this and pot the wires inhttps://www.us.schott.com/epackaging/english/products/battery-covers/lid-systems.htmlI have seen things like hermetically sealed lithium thionyl batterieshttps://www.mouser.com/ds/2/118/ltc7p-932904.pdfthen maybe just dab the connection points over the solder with low expansion epoxy.

author: Audioguru again
author_rank: Regular Contributor
timestamp: September 10, 2019, 02:02:06 am
content: About 59 years ago I made neon light chasers that used a 67V carbon-zinc battery. One exploded because the battery normally produced gassing and the potting caused the gas to build up a high pressure.I do not know if the battery you are planning to pot produces gassing.If you add a vent hole then it could easily become plugged by something.

author: Stray Electron
author_rank: Super Contributor
timestamp: September 10, 2019, 02:21:46 am
content: I think that most batteries today are sealed so I don't see why potting them would harm them.

author: techman-001
author_rank: Frequent Contributor
timestamp: September 10, 2019, 02:36:48 am
content: Quote from: Audioguru again on September 10, 2019, 02:02:06 amAgreed, potting batteries can make them explode.I used to repair constant loss speedway bike ("solo's") ignition units that used nickel cadmium batteries and they were always exploding.The potting compound wasn't resin but a VERY tough slightly flexible material.

author: Psi
author_rank: Super Contributor
timestamp: September 10, 2019, 02:39:54 am
content: i would use a soft rubbery 'like' potting compound instead of a hard compound like resin.

author: floobydust
author_rank: Super Contributor
timestamp: September 10, 2019, 03:03:07 am
content: I don't know your temperature, lifetime requirement, pressure rating.Hard casting resin is a problem due to different expansion/contraction than glass, steel, smt parts.I have seen hard epoxies shear off smt parts because the resin shrinks during curing and then expands more due to heat, than the internal parts. I had to use dual-coatings: first dip in silicone rubber and then place that in the hard epoxy. Needed some cush room. It is very labour intensive and messy and costly. You must use "electronic" potting compounds. An O-ring sealed box may be better.There are many lithium primary batteries that are in a gas-tight package.Tadiran lithium thionyl chloride (LiSOCl2) Batterieshas more info.ATtiny85 is not the best for low current, it is not picoPower. Cheap yes but you need to spend more on the battery.

author: Lee697
author_rank: Regular Contributor
timestamp: September 10, 2019, 04:13:08 am
content: Thanks everyone....I do have (just enough) room to make a small 3D printed sub-enclosure to house the battery/chip, then pot around it, so the components would have an air gap around them. The battery is pretty oversized for the application, and for the duty cycle expected will outlast the project - I like it due to its solderable leads mainly. This will live in the Australian outdoors, temperature 5-45degC.A lithium coin cell would probably be plenty of capacity, I might use one instead.Can anyone suggest a good electronics potting compound/supplier?

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 10, 2019, 05:50:38 am
content: Going to chip in here, even though you seem to have decided on a way forward already.Lithium cells expand and contract. In cellphones, per example, this means additional room is foreseen around the battery to allow this. As this pertains mostly to those "bag" type cells, I'm not sure if perhaps the cylindrical incarnations have the margin *inside* of the cell's enclosure, but I'd be concerned if I didn't know for sure.Another thing that would cause concern for me is the fact that the chemistry of the battery is not defined in the DS. Lithium, sure, but what chemistry exactly. Probably not an issue, but I wouldn't include something like that in a product if it was not better defined.

author: Zero999
author_rank: Super Contributor
timestamp: September 10, 2019, 09:35:28 am
content: Here's a link to a resin I've used in the past:https://www.electrolube.com/products/polyurethane-epoxy-resins/er2001/resins_epoxy/How about wrapping it in foam before potting it? It will: cut down on the amount of resin required, provide insulation and allow room for expansion/contraction or the components and resin.You can get non-mercury tilt switches. I'd be very uncomfortable about using a mercury switch in a single use product. It just seems unnecessarily bad for the environment. I get it, single use products are sometimes necessary, but try to limit the use of toxic substances more than usual, unless you have a plan for reuse/recycling it.https://uk.rs-online.com/web/p/products/2357623?cm_mmc=UK-PLA-DS3A-_-google-_-PLA_UK_EN_Automation_And_Control_Gear-_-Industrial_Switches%7CNon-Mercury_Tilt_Switches-_-PRODUCT_GROUP&matchtype=&aud-359023887559:pla-394441777621&s_kwcid=AL!7457!3!243856881545!!!g!394441777621!&gclid=EAIaIQobChMI5Zja_vjF5AIVDLTtCh1CmQurEAYYAyABEgIxH_D_BwE&gclsrc=aw.ds

author: Lee697
author_rank: Regular Contributor
timestamp: September 10, 2019, 09:44:48 am
content: Quote from: Zero999 on September 10, 2019, 09:35:28 amThanks - I was actually pretty surprised when I could still buy mercury switches at Jaycar.... I'll take a look at the alternatives....

author: amyk
author_rank: Super Contributor
timestamp: September 10, 2019, 11:31:00 am
content: Those SRAM+battery ICs that were so common many years ago had a (hard) potted primary lithium battery, so I don't think the battery would be a problem.Agree with the others here about not making more throwaway items...

author: techman-001
author_rank: Frequent Contributor
timestamp: September 10, 2019, 12:36:51 pm
content: Quote from: amyk on September 10, 2019, 11:31:00 amIt depends on the Lithium battery chemistry as I understand it and these days there are a number of different types.Where silicon is used in the anode, it can expand 300 to 400%.(The Truth About Tesla Model 3 Batteries: Part 1)

author: austfox
author_rank: Regular Contributor
timestamp: September 10, 2019, 12:50:10 pm
content: QuoteI was also surprised that Jaycar still sell glass encapsulated mercury switches. Most of the switches I see today are housed in a metal case, and also have a lot smaller footprint.I think I have a couple here I can pop in the post for you?

author: mariush
author_rank: Super Contributor
timestamp: September 10, 2019, 01:29:32 pm
content: Have you considered using AA batteries ?You can get lithium based AA batteries like Energizer Ultimate Lithium that have something like 3500mAh at around 1.4v..1.7v ... see :http://data.energizer.com/pdfs/l91.pdfThey're not cheap at around 2-3$ each but it's way cheaper than your 14$ battery.Regular alkaline AA batteries get close to 2500-2800 mAh and discharge down to around 1..1.2vYou can buy battery holders that solder to the circuit board and then you can pot everything leaving just the battery contacts on the outside. Then insert battery and use hot glue or that selastic / flexible gunk / whatever... something to lock it in the battery holder to prevent vibrations from breaking contact.There are microcontrollers that run from battery but if you need 3v, why not just use a very efficient voltage doubler IC... would be over 95% efficient at a few mA of current.Here's some examples:1. TPS6031x series : 0.9v..1.8v , 2 outputs, 1: 2xVin max 40mA, 2: regulated 3.3v 20mA up to 90% efficiency :http://www.ti.com/lit/ds/symlink/tps60310.pdf2. TPS6030x series : pretty much same thing?http://www.ti.com/lit/ds/symlink/tps60300.pdfThe above are ~ 1.7$ in 10pcs or higher and need only 5 ceramic capacitors to work. This + battery is still way cheaper than 14$There's also very efficient switching regulators.. here's ex<1$ / 10+ pcs TLV61225 - min 0.7v in, fixed 3.3v out, 94% efficiency, needs only 2 ceramic caps and inductor :http://www.ti.com/lit/ds/symlink/tlv61225.pdfThere's also TLV61224,TPS61261 - min 0.8v, fixed 3.3v out or adjustable options, up to 95% efficiency, same min. parts :http://www.ti.com/lit/ds/symlink/tps61260.pdfEven when you add the price of a surface mounted ceramic inductor and a bunch of ceramic capacitors, you're still below 2$.There's loads of microcontrollers from Microchip that use less power and can run at lower voltages (like 1.8v..3.6v for example). There's also Silicon Labs with their BusyBee micros that can work at low voltages and they're quite nice micros.Here's some examples:https://www.digikey.com/short/pftvj0

author: Lee697
author_rank: Regular Contributor
timestamp: September 10, 2019, 08:43:04 pm
content: I only have a fairly small space to use for this, hence the small battery, I've got several packs of various lithium AA/AAA's/9V around, I'd certainly use those if I could. The cost of the battery doesn't worry me, at 1200mAh it should deliver through to its expiry, if I can keep it all dry....I haven't discounted trying a CR2032 or two instead, although capacity would take a hit....I'll take a look at the the regulators you've linked. Cheers.The reason I've chosen the ATtiny85V is I have some here, and find them easy to program, and they're small enough. I only really need the one output, but may add a little battery state LED as well yet.Quote from: mariush on September 10, 2019, 01:29:32 pm

author: mzzj
author_rank: Super Contributor
timestamp: September 10, 2019, 09:23:05 pm
content: Quote from: mariush on September 10, 2019, 01:29:32 pm3.6v primary lithiums can be also found cheaper than 14$IE:https://www.digikey.com/product-detail/en/tadiran-batteries/TL-5101-P/439-1003-ND/512505https://www.digikey.com/product-detail/en/panasonic-bsg/BR-1-2AAE2PN/P630-ND/1118808

author: Lee697
author_rank: Regular Contributor
timestamp: September 10, 2019, 11:07:46 pm
content: Quote from: mzzj on September 10, 2019, 09:23:05 pmI'm sure they can... by the time I factor in USD/AUD conversion and shipping, or buying a bunch of stuff I don't really need right now to get free shipping (I seem to do that a lot!), it becomes a bloody expensive saving....