LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[deckofficer]

Originally Posted by FlyingCloud1937

So is that Marketing 101, besides the facts?

L

Yep, a product trying to find a home.

[FlyingCloud1937]

Originally Posted by deckofficer

Yep, a product trying to find a home.

Dude if nothing else I love your passion.

I once had a Carrol Shelby modified A gas, at green valley , North Dallas..hehe.

This is a 2008 photo but looks as if I can remember to be the same/similar car.





It's not the same, we had a 427, this clearly is a Hemi.

lloyd

[deckofficer]

Well this is a big drift. You happen to know I'm a fellow gearhead? We can take it to http://www.cruisersforum.com/forums/...-75902-19.html so as to not clutter this thread.

[FlyingCloud1937]

Originally Posted by deckofficer

Well this is a big drift. You happen to know I'm a fellow gearhead? We can take it to http://www.cruisersforum.com/forums/...-75902-19.html so as to not clutter this thread.

it's a big drift, but it's also the glue that brings us together.

in any event i'll wonder on over.

lloyd

[ebaugh]

Well the preliminary NTSB report on the Boeing battery is out. 48 pages, complete with lots of pictures. See it here:

http://www.ntsb.gov/investigations/2...787_3-7-13.pdf

I have not read every word...but it looks like a metal bolt punctured one of the cells, shorting the battery to ground. If a cell of this chemistry is shorted, the end result is not especially surprising. I did not see what would have caused this yet, did the cell overcharge and bulge into the bolt? Or did the bolt just work it's way through?

My conclusion, metal cases maybe more fireproof, but also are conductive, so I'm not so sure that's the best answer for a battery case, apparently even a well engineered one. At least this one cost enough to be well engineered. LiFePO4 would not generate near this amount of heat, even in a similar worse case scenario based on the chemistry. The data I have indicates the max is maybe 350F for LiFePO4, so wood seems good enough? My wife bakes fish on cedar in the oven at higher temps.

A quick search said a conservative estimate shows you need about 500F, or more to start a wood fire. See:

http://www.doctorfire.com/wood_ign.pdf

Mine are in a wood box, so unless anyone else thinks otherwise, Im still happy.......

[durundal]

LiFePO4 would not generate near this amount of heat, even in a similar worse case scenario based on the chemistry. The data I have indicates the max is maybe 350F for LiFePO4, so wood seems good enough? My wife bakes fish on cedar in the oven at higher temps.

Be careful wither assumptions like that, the temperature will depend on how much heat can be dumped to the outside environment. If you put a shorted battery on a table in free air and it reaches an internal temperature of 350F it will get much hotter if you surrounded all sides with a foot of styrofoam, the heat can't move without a much higher difference in temperature (heat transfer by radiation, conduction and convection are all driven by a difference in temperature). Putting the batteries in a sealed box will reduce free convection compared to no box, and wood is less thermally conductive than metals so any heat generated inside will increase the temperature moreso than a metal case. The temperature the wood would reach is dependent on the specifics of what environment it is in and how many watts you think the battery would produce in the worst case scenario you are planning for, so it might work fine, but a metal box (particularly aluminum for its better thermal conductivity) would do better at reducing peak temperatures given the same geometry.

Andy

[hellosailor]

Where are your reading that as saying a metal bolt punctured anything?

[bill good]

EBAUGH You are the only person to have found the short was due to a metal bolt. Please give more information as there are a lot people looking for the root cause. Yes there are lots of data to go thru but the risk that the two battery events under investigation has at this stage are lessons for all Li-Ion users. Houses made of wood still burn down & boats are still destroyed by fires. The risk appears hopefully is less for DIY boat owners with more resourses than Aerospace companies like Boeing with scientist & engineers looking for answers.

[daddle]

A shorted battery will only reach a few hundred degrees? With the energy available in an LFP the temperature will easily reach whatever temperature is needed to melt away the parts involved in the short. Thousands of degrees in milliseconds if necessary. That's the magic of electricity.

A wooden battery box would be my last choice.

[Opie91]

Originally Posted by ebaugh

My conclusion, metal cases maybe more fireproof, but also are conductive, so I'm not so sure that's the best answer for a battery case, apparently even a well engineered one. At least this one cost enough to be well engineered. LiFePO4 would not generate near this amount of heat, even in a similar worse case scenario based on the chemistry. The data I have indicates the max is maybe 350F for LiFePO4, so wood seems good enough? My wife bakes fish on cedar in the oven at higher temps.

You might want to rethink your conclusion. Airplanes have been flying around with metal battery cases for years.

[ebaugh]

I attached a couple of screen prints from the NTSB file. The one with the scan of the cell is clear evidence the cell was punctured. The other one shows where.

[ebaugh]

The supposition that the ground connection took a bunch of current is the middle of this page.

[ebaugh]

Originally Posted by durundal

Be careful wither assumptions like that, the temperature will depend on how much heat can be dumped to the outside environment. If you put a shorted battery on a table in free air and it reaches an internal temperature of 350F it will get much hotter if you surrounded all sides with a foot of styrofoam, the heat can't move without a much higher difference in temperature (heat transfer by radiation, conduction and convection are all driven by a difference in temperature). Putting the batteries in a sealed box will reduce free convection compared to no box, and wood is less thermally conductive than metals so any heat generated inside will increase the temperature moreso than a metal case. The temperature the wood would reach is dependent on the specifics of what environment it is in and how many watts you think the battery would produce in the worst case scenario you are planning for, so it might work fine, but a metal box (particularly aluminum for its better thermal conductivity) would do better at reducing peak temperatures given the same geometry.

Andy

I understand your points. My wood box does have "vents", that would ventilate some heat outside the box.

Maybe you are better off with no enclosure? The max free air temps are not much higher than that oil temperature?

An issue outside the engine room for sure, but mine are between 2 gensets, in a 20 ft by 15 ft by 5.5 ft engine room.

[ebaugh]

Originally Posted by bill good

EBAUGH You are the only person to have found the short was due to a metal bolt. Please give more information as there are a lot people looking for the root cause. Yes there are lots of data to go thru but the risk that the two battery events under investigation has at this stage are lessons for all Li-Ion users. Houses made of wood still burn down & boats are still destroyed by fires. The risk appears hopefully is less for DIY boat owners with more resourses than Aerospace companies like Boeing with scientist & engineers looking for answers.

I posted what refers to the "bolt" already. Maybe it was something else, but something punctured the cell.

Wood will burn. But I think a poor battery connection, with high resistance, can cause a heat, potentially a fire regardless of the battery chemistry. Probably in excess of the temps shown in the attached document from Los Alamos. There is no free lunch with 15 kW of stored energy. Just not sure the battery type is the biggest risk?

It may be a fallacy that the temps can't ever reach more than any specific point as shown on the attached overcharge graphs. Which is where some of my assertions lie. But short out any battery and heat is generated no matter the chemistry. The good thing, probably no different than lead acid, is the adjacent cells should not pour coal on the fire for LiFePO4. As you can see the Boeing chemistry is the worst shown in the second graph. And in fact at some point does add additional heat to an already bad situation.

[ebaugh]

Originally Posted by Opie91

You might want to rethink your conclusion. Airplanes have been flying around with metal battery cases for years.

You are correct. Even my old Piper had one. I'm still not sure it's a great idea. Part of the design issue for Li of any chemistry is it will produce huge current until empty. Many of the airplane batteries are much smaller capacity (<100 Ah, probably 55 or 60) than a marine Li bank, and are somewhat self limiting based on size. The LA just won't keep going as long in a shorted condition. They just quit. But a 400 (or sometimes much larger) Ah bank calls for different considerations regardless of chemistry.