Lithium Backup Battery Pack

[Rick]

I just received this reply from Diamond Tech Support:

The emergency battery pack powers the glare shield flood light and the standby attitude indicator. There are three different battery packs that may be found on the DA40. The old steam gauge aircraft have a battery pack that is a 42V unit which has 26 replaceable AA cells.

When the G1000 came along, from approx 40.345 to 40.671 the battery pack used is a 12 cell 36V lithium pack, D41-2560-93-00.

Later G1000 aircraft 40.672 and up, had a change in the standby attitude indicator, this indicator required less voltage to run, so the battery pack was again revised to a 10 cell 30V lithium unit D60-2560-91-00.

So there you have it - the reason for the change was the new standby AI! That actually makes sense now.

This also answers a question from another thread, having to do with the different versions of the backup AI. It looks like the change was made mid-2006.

[rwtucker]

I haven't removed my battery pack yet (soon, I'm 40.876) but from the size of the pack, I'm wondering if the latest batteries are Lithium-thionyl chloride (Li-SOCl2) rather than the more common chemistry. Either way, the shelf life life of the Lithium-thionyl chloride (they can deliver 90% of original capacity at 10 years and 80% at 20 years) would make them attractive replacements. At 3.6 volts, 8 cells should do the job. I can't tell for sure but my pack looks more like it has eight cells than the 10 required for more the common chemistry.

One source: http://www.saftbatteries.com/Technologi ... fault.aspx

[Rick]

Robert,

Diamond's 2-year requirement for replacing the battery pack has nothing to do with the expiration of the batteries. It is an unfortunate result of Diamond not updating their documentation to match the reality of what is supplied in their aircraft. Back in the day (pre-2004, maybe?) the "emergency battery pack" was a plastic tray full of AA batteries, and listed in Charpet 4 of the AMM (Airworthiness Limitations) was a requirement to "replace ALL cells every 2 years, or after use, or upon expiry date" (whichever comes first). That probably made some sense for AA cells, but certainly not for the Lithium cells used in the battery packs Diamond uses post-2004. However, Diamond has not changed this requirement in Ch 4, so we are still obligated to replace the battery every 2 years.

Below are pictures of the old and new packs from my 2007 - I just replaced mine at my last annual. The old one was good until 2020, and the new one until 2022. You should have the same pack in yours, since you are 2008 and GFC700. Hey, I'll make you a good deal on my 'old' one - still good until 2020!!!

[rwtucker]

Interesting! My expiration date was blacked our (by hand) at the factory. Your comment may have been tongue in cheek but the establishment of an exchange might present a technical loophole. I may build a Lithium-thionyl chloride pack just for the fun of it and see how it comes out.

By the way, Diamond's attempt to shield the panel from the heat generated by a shorted LiIon pack is inadequate. You could have a serious situation at hand if the pack shorted. I inspected their wiring precautions in that area of the panel. It seems adequate for normal situations but probably not in the event of a full-on short. A safer way would be to space these cells a distance representing their diameter with high-temp insulation inserted between the cells. It would require more internal panel space but that panel is not crowded and, in fact, is one of best the designs I have seen. When I visited the factory in London, I was told that they have won awards for wiring design and execution. Kind of interesting, just three people there whistling and wielding soldering irons with rock music playing in the background. All the work appeared to be by hand.

[Rick]

Robert, since you sound like you know a lot about batteries, I'll also post the pictures I took of my 12-cell battery pack (from my 2004). The expiry date was also blacked out by Diamond, as shown in the picture, but I could still read it as 2013. Dissection shows the pack is simply 12 CR123A cells in series.

So... I was only half-kidding about the 'old' battery pack. When I went to buy the new pack to replace mine a couple of months ago, I found several that were nearly expired (one already expired) by Diamond dealers here in the US (I was trying to avoid the "HazMat shipping" charge from the factory in Canada). That got me thinking - if I could leagally put in a 'new' pack that has been sitting on some shelf for 8 years, what is stopping me from replacing my current pack 2 years from now, with the one I just took out, which is actually 7 years NEWER that the one I could have bought from Diamond? Legally, I can't see where this would be a problem, since you are still replacing the pack with a non-expired, non-used pack, "full-strength" pack. Does it matter if you are storing the pack on a shelf or in the airplane?

And I second your comments on the factory tour - AWESOME! Happiest bunch of "factory workers" I have ever seen. Everyone was proud to explain what they were doing, and answer my questions. A great bunch of people!

[rwtucker]

Good ideas. And CR123s is an economical and reasonably safe way to go. A lot of places sell Panasonics (one of the best) for a buck or two each. http://www.batteryjunction.com/pri-cr123a.html For the GC700 systems, only 10 would be required.

I have always worried about any LiIon cells in the aircraft. They have been known to short for no reason. Add to that the variables of high altitude and perhaps a sudden drain in an emergency . . .

Is there a way to STC a battery pack? That way, we could come up with a safer spacing configuration to prevent the adjacent cell thermal runaway problem that can occur with dense packing. Even an air separation of 1/2 inch would make failure less worrisome. I would feel much better if I built my own pack. Doing so for under $20 and having the freshest batteries would be a real bonus.

[Steve]

Is there a way to STC a battery pack? That way, we could come up with a safer spacing configuration to prevent the adjacent cell thermal runaway problem that can occur with dense packing. Even an air separation of 1/2 inch would make failure less worrisome. I would feel much better if I built my own pack. Doing so for under $20 and having the freshest batteries would be a real bonus.

Think Boeing 787 Dreamliner...

Steve

[Rick]

I have already suggested to Diamond that they just build a battery "tray" like they had for the AA cells, except for the 10 (or 12) CR123s, so we could just change out the cells instead of the entire (proprietary, hand-made, very expensive...) pack from Diamond. One (carefully-designed) tray could work for both the 30 and 36v packs. Plus, if they did this, they could (hopefully) space the cells appropriately to discourage thermal runaway. Maybe if you called and offered to design it for them...

[rwtucker]

I'm not the guy to design it for them but I'll add my voice. If the rest of the members do the same, we may get a pack. They may find a battery holder product already on the shelf that they could "incorporate" into their release, much the same as many Diamond components.

[gsontheimer]

The AMM states to replace the battery pack when the voltage is < 36 Volts. Now that there are two packs with a nominal voltage of 30 Volts and 36 Volts, what would be the "open current" voltage of the 30 Volt version? 30 Volts? 32 V? 36 V?