Batteries in a "small vacuum"

[Toffe]

Hello.

I was planning to add a logger function to my "vacuum chamber" to indicate for leaks over time. My plan is to have a circuit to log pressure using BMP180, and give an indicator if there is leakage over X number of psi after 15 minutes.

My problem is batteries, will they withstand 5-7psi ? I think that is no problem but I want to hear a better opinion from other. Thinking about mt. everest i read is -4psi and I do think people brings their stuff up there with batteries?

Any good ideas? THinking about 9v Lipo or 2x AA batteries or coin cell. Not sure yet on what to use which can power the arduino without beeing too big and clunky.

[Conrad Hoffman]

Almost all batteries are designed to vent excess pressure, so I'm guessing they won't be happy under vacuum. Duracell specifically forbids it.

[Toffe]

Even down to 5-7 psi? So basically you cannot take batteries to everest? No camera, no phone, no watch?

Full vacuum i understand (1psi) but just wanted to make sure on a half vacuum kinda

[IanB]

It depends on the kind of battery.

Alkalines have a mild seal to prevent them drying out, but it is a low pressure seal. Vacuum may cause the vent to open. However, alkaline batteries are disposable, so for a single 15 minute occasion of data recording I wouldn't worry about it. It will be irrelevant.

Some other batteries like NiMH and lithium ion have a much tighter seal. I am sure they would be fine if you used those. NiMH cells in particular are designed to withstand very high internal pressures without venting. So for them a mild vacuum on the outside would be nothing at all.

[jbb]

Non-rechargeable coin cells might be good, too.  But capacities are a little limited.

[beanflying]

Have a serious consideration of sealed lead acid/gel batteries. The downside will be they are a bit bulky and generally don't come in tiny packages but I have used D size 2V cells (6V, 3cells) in the past and boost converters have come a very long way since then for whatever voltage you need.

https://www.master-instruments.com.au/products/55470/0820-0004.html full spec sheet at the bottom of this link.

[Ian.M]

SLA/gel cells typically have an over-pressure vent that's designed to reseal once the excess gas pressure from over-charging is removed.  You generally don't know the pressure differential it will open at and you don't know how leaky it is, so they'd be one of my last choices for vacuum work, unless you are buying specifically vacuum rated cells at a premium price.

Personally, if you cant add hermetic feedthroughs to the chamber to use external power and external data collection, I'd go with IanB's NiMH suggestion.

[beanflying]

One of the reasons for those cells existence was Gates (rubber) initial involvement was that seal. They were branded Gates when I used them last. The out gassing under use is listed in the spec sheet and I know they are certainly used in aircraft so take that as a sort of recommendation for consideration. Unlike a no name brand I think their technical department would give advice one way or another.

NiMh have some shortcomings and I have seen plenty let the smoke out and leak when abused.

[Toffe]

Hey, thanks for all the information. I've used alkaline batteries before to test with, the reason I ask is since I want to produce a PCB for this project so I can have e.g. a logger which monitors pressure until it reaches 7psi, then the led swaps from e.g. yellow to green, after 15 minutes it can start blink green if the pressure has not exceeded 7.5psi, but if it has it should blink red or maybe add a lcd to show pressure. Not sure on how to do this. But should be "pocket friendly" just to toss it into the chamber.

Its not a vaccuum chamber it is a ROV and the vacuum is to test the seal. Currently I use a analogue gauge on the vacuum pump but it is not easy to see 0.5psi difference, I've used the buildt in pressure sensor on my control PCB before and it seems to be accurate enough but then I needed bigger battery to drive it. So hopefully making something small and easy to use would be perfect.

Saw there was 1.3A 1.5V AA NiMh batteries, 4 of them would be enough to power this unit for a long time I think.

[Ian.M]

I wouldn't even consider putting the Arduino and batteries inside the chamber unless it was *IMPOSSIBLE* to add feedthroughs.  Assuming you ground to the chamber, a BMP180 sensor will only need three wires.   At 1/3 atmosphere (5 PSI) partial vacuum, a hermetic seal isn't difficult to achieve - a carefully cleaned bare tinned copper wire through a blob of epoxy in and either side of a small hole in a carefully cleaned metal plate or pipe end fitting will do the job adequately.   You only have to be concerned about epoxy outgassing if you are doing high vacuum work.

However, for testing an existing pressure vessel without available ports, then yes a standalone pocket logger would be useful.   You need to keep power consumption as low as possible so it doesn't heat the remaining air in the chamber and give a false pressure rise, and also log temperature so you can correct for the effect of temperature on the pressure (by Boyle's law).

LCDs at low pressures may be problematic, so there's some experimentation required there.   Blinking a small high efficiency LED in Morse code might be a safer option, or for an opaque pressure vessel, maybe a high intensity audio beeper ora buzzer that will conduct sound to the chamber wall

[Conrad Hoffman]

maybe useful- https://www.ultralifecorporation.com/PrivateDocuments/BR_Battery_Assembly_Guidelines_for_UN_T1-T8_Testing.pdf

[Gyro]

Some pretty exacting and closely specified tests...

Test T6: Impact
Procedure
Simulates an impact. Place a 15.8 mm diameter bar across the sample and then drop a 9.1 kg
mass from a height of 61 cm on to the bar, and then observe for 6 hours.

(My emphasis)

It's a shame they didn't remember to mention the length or composition of the bar (its mass). 

[beanflying]

See our batteries won't catch fire in an Aircraft hold at 40,000' Interesting @Conrad 

Under light use like you are proposing any loss to those figures would be about zero. Avoid using an evilbay option if you do go lithium (obvious I know).

After many years of R/C use I am still to catch a LiPo on fire in cluding a 6S heli pack I wrapped around a steel post at over 100km/hr  $450 of bits later......

[tpowell1830]

Hello.

I was planning to add a logger function to my "vacuum chamber" to indicate for leaks over time. My plan is to have a circuit to log pressure using BMP180, and give an indicator if there is leakage over X number of psi after 15 minutes.

My problem is batteries, will they withstand 5-7psi ? I think that is no problem but I want to hear a better opinion from other. Thinking about mt. everest i read is -4psi and I do think people brings their stuff up there with batteries?

Any good ideas? THinking about 9v Lipo or 2x AA batteries or coin cell. Not sure yet on what to use which can power the arduino without beeing too big and clunky.

Is this remote operated vehicle going into space or at high altitudes?

I am a harness design engineer for the Boeing Starliner and we use batteries all over the place to power everything. Some of the batteries are gel cells and some are LiPo, and others are LiIon. Batteries power the entire ship. The way that we manage batteries under a vacuum is to place them in a metal sealed box and allow access through a sealed connector.

In your case, you could build a sealed box and place your batteries in and put something like sealed binding posts so that you could plug in your device. The batteries will stay at atmosphere, while your DUT is at a lower pressure. Don't use high outgassing seals such as acetic cure silicone rubber. Learn from our mistakes, don't feed insulated wires through a sealed bulkhead using a silicone rubber compound. The wire insulation itself will leak around the conductors. Use a seal that is molded from an epoxy, rubber or synthetic rubber compound that has low outgassing and don't attempt to feed insulated wires through thinking that they will seal. The down side to this is if you haven't properly sealed your box, you may have a bomb, if the batteries short from the vacuum. Always remember, safety third. With vacuum chambers there is always the risk that your chamber could pump down to vacuum because your gage stuck or your equipment malfunctions, so make sure it is vacuum safe to the lowest levels.

Also, to get a more precise scale, use the Torr scale. There is seven hundred and sixty Torr in atmosphere, so you could measure very precisely the change in pressure. Just figure out what Torr range that you need based on your required test pressure.

Hope this helps...