Best way to manage this LFP house battery

[sensij]

That and also in another thread I was asking karrak details of his data logger and came to realization AC inverter must introduce AC component on the current shunt at double AC frequency, should look similar to the AC component on top of DC coming from common diode rectifier connected to a capacitor. I never worked with LFP batteries myself but I think hooking up scope there and checking how big is that AC component relatively to DC is worth the trouble. In karrak's case his data logger is taking samples at about 1 Hz intervals and if 120Hz AC component is present it would be also present in the measured data but at much lower frequency due to aliasing. Depending on AC 'noise' amplitude it might present a problem or not.

Take a look a @BackwoodsEE's mppt thread for evidence of this. He is plotting the input side of the mppt at 4096 Hz sampling, but if you can see it there, you can be sure it is on the battery side as well.

[Mike90250]

worth a thousand words. These burnt battery boxes were not DIY:

Li-IonFires.png

[nebster]

Mike's examples are different as I believe they show relatively fresh banks and it is hard to tell the exact reasons for those failures. My concern with Mike's cases is the empty battery cabinets- looks like there were no design measures taken to prevent chain reaction.

At the end of the day, certainly, everyone will have a different safety level that allows them to sleep at night. Myself, I'm primarily worried about something so exothermic and rapid that it prevents me from physically escaping a runaway before it becomes deadly. I'm convinced LFP can't achieve this, while conversely I think the cobalt chemistries are all too risky. There are plenty of other combustion mechanisms on board my RV that can also result in the whole thing ending up on fire; I can't fully eliminate all of the risk in any case.

I'm underwhelmed by the LFP combustion evidence that's available so far. I want to see documented examples of thermal events in LFP systems that we can show are reasonably-managed, and then those events need to exhibit the kind of high-speed reactions that differentiate them from traditional electrical fires or propane fires or similar, common occurrences to really justify an aggressive additional layer of physical protection.

That and also in another thread I was asking karrak details of his data logger and came to realization AC inverter must introduce AC component on the current shunt at double AC frequency, should look similar to the AC component on top of DC coming from common diode rectifier connected to a capacitor. I never worked with LFP batteries myself but I think hooking up scope there and checking how big is that AC component relatively to DC is worth the trouble. In karrak's case his data logger is taking samples at about 1 Hz intervals and if 120Hz AC component is present it would be also present in the measured data but at much lower frequency due to aliasing. Depending on AC 'noise' amplitude it might present a problem or not.

You're talking about AC bleeding back into the DC side of the system? I would expect that to be well-filtered, but also I think if it were causing measurement issues we would hear about them (?). My DC SOC estimator seems very, very accurate so far at least at 1/6-scale (AC and DC) loads. I can put my scope on my shunt to see what we see, but practically speaking I don't think there is much of an effect, if I'm understanding what you're saying. I'll put it on the list to test, though... it's easy enough.

[nebster]

worth a thousand words. These burnt battery boxes were not DIY:

It's certainly effective marketing...

[max2k]

At the end of the day, certainly, everyone will have a different safety level that allows them to sleep at night. Myself, I'm primarily worried about something so exothermic and rapid that it prevents me from physically escaping a runaway before it becomes deadly.

OK, that's your choice

I'm convinced LFP can't achieve this, while conversely I think the cobalt chemistries are all too risky. There are plenty of other combustion mechanisms on board my RV that can also result in the whole thing ending up on fire; I can't fully eliminate all of the risk in any case.

there's a difference- other 'dangerous' things on board of your RV probably passed certain certification before you bought them. That would mean they have history of things going wrong and those certifications reflecting this. You creating LFP bank yourself and I simply wish you to avoid learning one or two small details the hard way. I have no doubt you know what you're doing electrically but safety is different kind of expertise often learned off failed designs.

You're talking about AC bleeding back into the DC side of the system? I would expect that to be well-filtered, but also I think if it were causing measurement issues we would hear about them (?). My DC SOC estimator seems very, very accurate so far at least at 1/6-scale (AC and DC) loads. I can put my scope on my shunt to see what we see, but practically speaking I don't think there is much of an effect, if I'm understanding what you're saying. I'll put it on the list to test, though... it's easy enough.

In abstract sense- yes, AC bleeding. What I meant is inverter feeding AC loads would consume energy according to sine wave on the load side. It would cause equivalent unipolar sine wave on DC side in the consumed current as the voltage would remain relatively constant.

Depending how big that AC component on top of DC component of the current going through shunt is it could be ignored for the purposes of coulomb counting or not. If you could hook up scope parallel to the shunt you'd see this right away.