Lithium Ion battery stand-in?

[Dr. Au-fait]

I'm having a battery failure problem much like I imagine is happening with the Boeing 787, except less life-or-death-ish. I'm looking for a less dangerous stand-in replacement that can emulate this battery for a short duration without catching on fire.

History: Some time ago I purchased a device intended for automotive (12VDC) use which contains a 3.7V 800mAh lithium rechargeable battery fitted in the case with the PCB. This battery (thin and flat like a cell phone battery) is not used except to help the system shut down gracefully when the truck is turned off and the 12VDC goes away. That shutdown takes about 30 seconds, and the device draws about 0.75W during that process. The battery is under a "float" charge (embedded charging circuit) during normal operation except for the initial power-up state when it's recharging from the last power down.

Under various conditions the battery has either caught fire or over-heated and expanded so much that it physically destroyed the device. Sadly there's enough data to establish a pattern, having now replaced this thing six times. Hot days: environment in the cab is around 50C (122F). If the device is left running in that heat the battery can fail in bad ways including fire and catastrophic expansion. This happens hours after leaving it running in the hot cab. Cold nights: cab has been sitting overnight in temps of under -25C (-13F) with device powered off, but when powered on (at that cold temp) the battery immediately expands to 3-4 times normal size and destroys the case and PCB both physically and electrically.

I want to try a cap in place of the battery but nothing of the capacity needed can fit in the case. I'm also not sure if it's electrically going to in some way damage the charging circuit. Can you think of any other battery technology that can give me the 30 seconds of runtime while properly charging and not failing with wide temperature swings?

[Zener]

NiMH?

[franklin97355]

This battery (thin and flat like a cell phone battery) is not used except to help the system shut down gracefully when the truck is turned off and the 12VDC goes away.

Could you not just hook to an unswitched point in the battery circuit? Using a switching regulator if necessary.

[Zener]

That is a good idea. As far as a battery, do you want one the same size? If so you should say how big it can be...

[Dr. Au-fait]

This battery (thin and flat like a cell phone battery) is not used except to help the system shut down gracefully when the truck is turned off and the 12VDC goes away.

Could you not just hook to an unswitched point in the battery circuit? Using a switching regulator if necessary.

That would be ideal if knew more about the embedded battery charging circuit, but I'm likely to let the smoke out.

That is a good idea. As far as a battery, do you want one the same size? If so you should say how big it can be...

And here is the problem I'm facing with NiMH. Lack of a good reference for battery dimensions is hampering my ability to find a suitable replacement.The lithium battery is a common LI-40B, of which I've yet to find in NiMH in an equivalent or smaller package.

All good ideas, thanks for the feedback.

[Dr. Au-fait]

I should also point out that the charging circuit may expect something unique in terms of charging characteristics with the lithium vs. NiMH, but that's not an area of expertise for me. Maybe it's not fussy enough to care one way or the other. The 0.1V difference with NiMH probably won't matter much.

[Zener]

I think yours is about 7mm thick and AAA are 10.5 so that won't work. You can't put the battery external? I think the charger would be OK with the NiMH. Of course I can't guarantee it... Might do more research if you decide to go that route.

[Dr. Au-fait]

5.7mm x 30mm x 39mm and it's a tight compartment. External is probably my only reasonable option especially with connector arrangements having to match. Make a dummy battery with the proper contacts and run the wires outside. Not sure what the insurance company will make of that if something happens even with the NiMH, but that is a different matter entirely and likely preventable by keeping it in a fire-proof external enclosure.

I'll be sure to follow up with a photo of the charred remains if it comes to that.