Method for isolating battery and load during storage/transportation time

[juanfermed]

Hi Everyone!

In the past few weeks I have been looking/thinking about a method to isolate a Battery from its load during its transportation and storage. The reason of this is that I have some equipment that comes from another country, with battery, and the battery comes already connected so that we dont have to disassemble the kits to connect it (which I would have to do if it came disconnected) because that would imply to much work. But this also implies that the battery will be discharging because the load would be connected during all this time.

The method should be a one time use only, in the sense that it isolates the battery from the load until some action connects them forever.

Of course, we cannot use a simple ON/OFF switch. This would be a simple solution, but we cannot use it because, once the unit is ready to be installed, it must be ON all the time and the end user should not be able to turn the unit off.  The unit uses solar panels to recharge the battery so it lasts as long as possible. Adding a switch and then removing wuld imply disassembly, cable cost, switch cost that should not be necessary.

The storage time can be up to 3 months, and we dont want to be recharging every battery to get the complete unit ready. It is very time consuming and simply not effective.

We have think about some pull tabs that cheap chinese clocks have, like this one: http://www.att.com/support_media/images/Wireless_Support/Novatel/Everest/KB417943_Battery_SIM/Batt1.jpg, but they are not a solution because: 1) We are using SLA batteries and the conectors are screwed with nut and bolt to the battery and 2) The battery is safely protected and surround by the case so there is no access to it. Anyways something similar  would be a great solution.

We have also think about electronic solutions like this one:
http://e2e.ti.com/blogs_/b/powerhouse/archive/2014/06/05/maximize-shelf-life-with-a-one-time-push-button-switch.aspx
and its valid but some points are 1) A lot of passives and active componentes for a single use, 2) Would increase the Bill of Materials.

So the question is open, it has been a real challenge for me to think about something creative and simple enough. I think an electronic solution might work but since it is going to be single use, it should not be complicated or should be very cheap (<$0.20) or, if it costs more, it has be very easy to operate.

I think I am not forgetting any detail so any suggestions or ideas are very welcome.

[tautech]

I think I am not forgetting any detail so any suggestions or ideas are very welcome.

As your SLA batteries are stud terminal, this "switch" might have to cope with some current?
How much current?

[juanfermed]

The total current drawn from the battery would be between 0.05Ato 10A. I think that not all of this current needs to pass throught this "switch".

 I failed to mention it, but the system has a microcontroller that could turn ON or OFF other pass elements that connect and disconnect from the battery the loads that require high currents. In this way, the current throught this "switch" that isolates the battery from the loads during storage/transportation can be reduce to up to 1.5A.

[sacherjj]

Is there a way to have the battery connected with a sprung terminal?  I'm thinking of the pull out "switches" of plastic that isolate CR2032 type cells from discharge. 

Other than a physical switch (electrical or mechanical) it seems like you are looking for a MOSFET and some way to lock it on.  Maybe it is pulled high by a line that is electrically isolated by a pull tab.  So it is in off state.  Then when you pull out the tab, it pulls high and turns on.  You would have to deal with tens of milliohms loss on the MOSFET for the life of the device.

[ManCave]

You can have a spring contact anywhere on or off your PCB with a pull tab to permanently enable the device.

[juanfermed]

Hi,

@sacherjj  what kind of sprung terminal? Something like this? http://static.rapidonline.com/catalogueimages/Product/S17-0966P01WL.jpg

The MOSFET + pull tab solution is something I will investigate. The pull tab does not need to isolate the battery terminal on the battery right, that sounds good.

@ManCave, this kind of spring contact? http://i00.i.aliimg.com/img/pb/872/454/111/111454872_184.jpg

One thing with the pull tab is that it must have a tip out of the case in order for anyone to pull without disassembling. This unit will be working in places where insects are nearby and can get inside, so small ventilation holes are allowed both larger holes or slots are not desirable. 

[Jeroen3]

How about a small switch (or reed relay) and a relay or contactor? Maybe a mosfet, depends on your safety demands.
Once the switch enables the coil or gate, the relay or fet does not turn off until the battery is removed.
If you're running from huge SLA batteries you sure can spare a few milliamps for a relay. Mosfets use nothing. But beware of voltage spikes!

*Are you allowed to ship including batteries?

[sacherjj]

I'm thinking more of a piece that is soldered on a PCB and is flat metal that provides a spring.  Then you run a piece of non-conductive whatever in between them.  When you pull it out of the way, the gate of the MOSFET is pulled high, instead of through the large pull down resistor.  This locks the MOSFET on.  The spring "switch" would only be on the gate pull up.  Very low current.

However, there is possibility of corrosion adding resistance to that contact and eventually shutting off the battery.

[Niklas]

Passenger cars have a similar problem during transport from the factory and storage before delivery to the customer. About 10 years ago, one manufacturer I worked for replaced a mechanical switch with a software solution. The switch was placed on one of the battery's terminals and had to be removed and shipped back upon arrival. The solution was to lower the standby current of the electronics and to introduce different modes of operation: Factory mode, transport mode etc. In transport mode the microcontroller is put into deep sleep and need some stimuli to wake up.

In transport mode the maximum speed was also limited to approx. 30 km/h to reduce damage to cars and to lower the risk of theft.

[ajb]

For a REALLY one time switch, MOSFET with a very weak pullup on the gate, and the smallest possible fuse pulling the gate down.  Us a small pushbutton (or maybe switch+transistor, depending on the fuse rating) to pop the fuse and allow the pullup to turn on the MOSFET.  Could make testing a bit tricky, though.

[kolbep]

Put  a Normally closed microswitch somewhere inside the device.
drill and tap a 5mm hole in the outer casemof the device.. Get a screw of the right depth. When the screw is all the way screwed in, it is pressing against the microswitch lever, which opens the circuit.

when device arrives at customer, you just remove the one screw on the outside, it releases pressure on the switch, which reverts to the closed state, and starts working.

then you will only have a small 5mm hole left over after device is activated.

you could also use a wingnut headed bolt to allow activation by unscrewing by hand without a screwdriver.

[ManCave]

@ManCave, this kind of spring contact? http://i00.i.aliimg.com/img/pb/872/454/111/111454872_184.jpg

One thing with the pull tab is that it must have a tip out of the case in order for anyone to pull without disassembling. This unit will be working in places where insects are nearby and can get inside, so small ventilation holes are allowed both larger holes or slots are not desirable.

I meant more like this:

http://www.stonemountaintool.com/HTML/PRODUCTS/Hardware/lspring/open.jpg

or this http://www.bosistampingpart.com/products/Beryllium-copper-finger-contact-1820901.html

but single would suffice in your case. You would mount this on your PCB with the contact connecting your circuit and place a pull-out tab underneath. If the tab is non-conductive (i.e. piece of mylar) you can use a strong string through a tiny vent hole to pull it out from underneath the contact and just leave it inside the case.

However, I just had a possibly better idea, but AJB managed to post the same thing first. +1 AJB!

For a REALLY one time switch, MOSFET with a very weak pullup on the gate, and the smallest possible fuse pulling the gate down.  Us a small pushbutton (or maybe switch+transistor, depending on the fuse rating) to pop the fuse and allow the pullup to turn on the MOSFET.

[juanfermed]

How about a small switch (or reed relay) and a relay or contactor? Maybe a mosfet, depends on your safety demands.
Once the switch enables the coil or gate, the relay or fet does not turn off until the battery is removed.
If you're running from huge SLA batteries you sure can spare a few milliamps for a relay. Mosfets use nothing. But beware of voltage spikes!

*Are you allowed to ship including batteries?

Yes, dont ask me why or how, I do not manage that process. But, are all battery chemistry banned from being shipped? or only Li Ion?

@sacherjj, @Mancave  I guess your suggestion and ManCave suggestion are very similar. I like a lot that it is PCB mounted and it seems that they are designed for that purpose. Very interesting one, I would have to deal only with thinking about manufacturing and placing the pull tab (+string if needed) and a space in the case for it to be taken out. The problem that sacherjj mentions is very important I guess: corrosion. I have seen this types of contacts (but I could not come up with the idea  ) placed near batteries and oxide starts building on them. Our service life must be >7 years and the unit is exposed to somewhat harsh environment conditions that can promote corrosion.

@AJB thats a great idea, and really ONE time switch. I would need the lowest rating fuse possible. I was looking at this low rating fuse http://gt.mouser.com/ProductDetail/AVX/F0402G0R05FNTR/?qs=sGAEpiMZZMtxU2g%2f1juGqemH8VKR32Dc1C6vRYh%252b9uc%3d, the datasheet says 50mA but I guess it would require some testing and statistical results. The other thing, and its funny how everything seems to work against us, is that we are using a flexible keypad (no documentation btw  ) and I would guess that the keys cannot handle higher currents. I would actually doubt it can handle 50mA. The other thing is that their cost is $0.84 and the lowest cost one (at least in mouser) is $0.16 in unit and $0.125 in 1000s, but its rated at 3.5A hahaha 

@kolbep, thats also a neat solution and something we have thought about before. The thing is that, as you might understand, end users are really intelligent and creative when trying to hack stuff. That has been our experience. This switch can be pressed again by the user with a stick. Of course we could cover the hole or fill it with something. Anyways its cheap, reliable.

@Niklas, thats a solution that we can partially implement. Some of the electronics inside are not and will not be designed in house, so we have no control on their IDLE state consumption. We can only connect them or disconnect them through a MOSFET for example. This can help, but we have not control over the complete kit.

Another detail is that we are using a wide range of batteries: from 12V 7Ah up to 12V 100Ah. For the smaller ones, we for sure want to save every milliamp, given our storage time.

I will actually buy some components to try out suggestions.

It seemed at first that this "easy" problem would be "easy" to solve...

[aargee]

I think there is a problem with transport and active battery connections. SLA batteries are about the size where authorities get interested in having them disabled during transport, especially air transport.
We have equipment that is air transported, the batteries are removable LiIon packs and have a flight procedure where they are turned around in their frame so there is no electrical connection.
This also solves the problem of the batteries discharging through the connection.

[eliocor]

I used Keystone 591 and Keystone 637 with 591 spring compressed against 637 contact.
Between spring and contact there was a plastic strip tab: when removed the circuit was powered.
http://www.keyelco.com/product-pdf.cfm?p=830
http://www.keyelco.com/product-pdf.cfm?p=836
http://www.keyelco.com/userAssets/file/M60-2p-batterymountinglayout.pdf (page 2)

[ManCave]

@sacherjj, @Mancave  I guess your suggestion and ManCave suggestion are very similar. I like a lot that it is PCB mounted and it seems that they are designed for that purpose. Very interesting one, I would have to deal only with thinking about manufacturing and placing the pull tab (+string if needed) and a space in the case for it to be taken out. The problem that sacherjj mentions is very important I guess: corrosion. I have seen this types of contacts (but I could not come up with the idea  ) placed near batteries and oxide starts building on them. Our service life must be >7 years and the unit is exposed to somewhat harsh environment conditions that can promote corrosion.

Use gold plated PCB and spring.

[miguelvp]

I used Keystone 591 and Keystone 637 with 591 spring compressed against 637 contact.
Between spring and contact there was a plastic strip tab: when removed the circuit was powered.
http://www.keyelco.com/product-pdf.cfm?p=830
http://www.keyelco.com/product-pdf.cfm?p=836
http://www.keyelco.com/userAssets/file/M60-2p-batterymountinglayout.pdf (page 2)

Seen that method many times, you just pull the plastic tab and it's on.