I've tried to charge my prism-cell LiFePo4 with smartphone charger using dc-dc buck converter to 3.6 volt, and it works. I suggest set the output voltage to about 3.34 volt so when the battery voltage is 3.34v, it will automatically terminate (at least with in my case).

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**Hi @SamGibson. I'll answer here, as It is too long for comment section:**

My case: I'm using Aukey 54W smartphone charger. Using cheap dc-dc buck (XL6009), gives about 7watt current. I use it to charge my 23Ah prism-cell. So, the charging is <1C in my case == safe.

About my answers, as far as I know, smartphone chargers is just using CC, about ~5 volt with variable current. Cheap dc-dc buck will scale down the volt to 3.34 and the current usually to 2.1A, around 7 watt (using Buck Boost XL6009) or less. It should be quite safe to charge using this method, provided we know dc-dc buck converter output and maximum charging current (we can buy converter with configurable output voltage and current). Nothing wrong will happen when we charge like that, as LFP charger, too, will use CC to charge to around ~95% (~3.34v), and continue with CV to charge to 100%.

Me? I don't want to charge to 100%. So CC is fine. As for terminate charging, when input = output, will terminate automatically in my case. Tried that multiple times.

**A few sources. We can googling for others if the info here is not enough:** 
 1. https://www.powerstream.com/LLLF.htm#:~:text=Because%20an%20overvoltage%20can%20be,batteries%20are%20safely%20force%20charged;
 2. https://battlebornbatteries.com/charging-battleborn-lifepo4-batteries/;
 3. https://www.power-sonic.com/blog/how-to-charge-lithium-iron-phosphate-lifepo4-batteries/#:~:text=Float%20charging%20is%20only%20required,any%20battery%20without%20causing%20damage.