What's the best way to monitor battery SOC when it's being charged?

[sleepless]

Not sure if this is even possible bought thought I'd ask. Thanks.

[Ampster]

It may depend on the chemistry. Some Lithium chemistries have a flat discharge/charge curve. In that case coulomb counting may be the most predictable. Otherwise voltage can be useful although it is more predictable when resting after charging.
Tell us more about the application and the kind of battery you are using and others more experienced in those particulars may be able to better inform you. My useful experience is limited to Lithium chemistry in motive and stationary applications.

[Mike90250]

The gold standard, is to understand what chemistry your battery uses, and in the case of lead acid, monitor the charge current, When it reaches a low % of battery capacity, you can consider it fully charged.
See page 17 for the Trojan advice https://www.trojanbattery.com/pdf/Tr...UsersGuide.pdf

* Phase 3: Constant current finish charge A constant current equal to 1-3% of C20 is applied as the voltage increases. The finish phase ends when the battery is fully charged. Flooded batteries will gas (bubble) toward the end of the finish phase to ensure proper mixing of electrolyte.
* Refer to Table 5 and Figure 6 for system charging parameters and typical voltage and current profiles