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I have Goal Zero Yetis 1000X & 500x (plus several other smaller power banks) with GZ's Boulder 100 Solar case and some other 100 W solar panels I've used for camping. I mainly use all of this at home for emergency power outages. I live in a big building in a city and cannot attach anything to the building outside, but do hang things from windows or off my balcony. This has all been enough for me in the past.

We recently had a major outage for over 24 hours localized to just a few blocks around me, but I had to continue to try and work from home through out it. My setup was starting to run out of power towards the end so I decided that I'd like to extend it with an extra 12V battery. I looked into the GZ link and expansion setup, but decided against it and purchase a 12 V, 100 Ah LiFePO4 battery to chain manually only when needed.

I've been trying to learn as much as possible about LiFePO4 batteries and have seen a lot about the fire risk dangers. I've read lots of discussion on the voltage to use for "storing" them for months and also not having them fully charged unless you plan to use them right away, etc. I've seen to never go over 75-80% to prolong the battery, but again talks about immediate usage. I'm not sure if what I am doing would be considered storage since it would just be sitting in my house all the time until needed. I can't say when it will be needed for certain until the power goes out.

In a situation like mine, where I won't be using this that often (hopefully anyway,) I just want it to be ready and available if the power goes out, at what level should I keep the battery charged to be safe from fire dangers, etc.? I guess I'm not fully understanding the level of danger of having a fully charged 12 V, 100 Ah LiFePO4 battery at home at room temperature and waiting for it to be needed.

For reference, I intend to chain it exactly like Todd Parker's video with my GZs. So when my power goes out, I'd attach LiFePO4 battery to GZ for additional power.

https://www.youtube.com/watch?v=iWnY5w2UeeI

Chaining lithium-ion batteries to a Goal Zero Yeti: DIY Tank expansion for Battle Born LiFePO4 batteries

I ended up choosing the same battery he uses in the video, though that wasn't why. I looked at lots of others including cheaper ones, but ultimately chose them mainly because they are a local company to me.

ExpertPower 12 V, 100 Ah Lithium-ion LiFePO4 Deep Cycle Rechargeable Battery

I thought maybe you all here could help me.

Can I keep the battery safely fully charged as an standby power source at home?

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The fire risks of a LiFePO4 battery are mainly associated with operating it under abnormal conditions which result in the plating out of metallic Lithium or the formation of metallic whiskers in the battery.

These conditions include over charge, over discharge and fast charging at sub zero temperatures, see the cell data sheet for the full specs. The pre built battery you have bought includes a Battery Management System which should prevent all of these.

It is however still possible to overcharge the battery by continuing to charge the battery at a low rate yet still within the voltage specs of the battery. For this reason you should not float charge the battery.

You can keep the battery fully charged without fire risk but keeping it at 100% charge will adversely affect the cycle life. Put simply with the cathode in a high energy sate and packed with Lithium the material will degrade.

These batteries are best used in cycling applications their big advantage is cost per charge cycle where they outperform lead acid. For a standby application like yours lead acid batteries are more suitable. In this application you are not discharging them on a regular basis, you can float charge lead acid to or at 100% and you can maintain this level of charge without damage.

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Can I keep the battery safely fully charged as an at home standby power source?

Yes

at what level should I keep the battery charged to be safe from fire dangers

Any level you wish: there is no unsafe State of Charge for LFP Li-ion cells. A high SoC level does affect the cells, but not in an unsafe way.

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