How long is your Prius c and mileage? What is the voltage showed when you considered a weak battery? NS40 is rather small, NS60 is a better option. Do check with battery shop expert the differences.
Your information will help us to alert on battery, thanks.

Thanks for the info, I am very forgetful. A battery can last for >2 years is great. I normally will change the battery 1.5 years. Original battery from new car normally do well and lasting but not those replacement. 10~11 voltage is definitely weak, time to change a new one, don't hesitate and delay further.
Battery volt meter installation is not adviseble. There is plug in volt meter for this purpose in the market, which you can consider.

2 years 2 months is worth it, your battery is consider very good performance . Don't squeeze further liao. You find good battery replacement? Let us know.

Since you are so interested in mod the Prius c, have you done the sound insulation? The car rather noisy; road noise, tires noise...etc.

You have very good record, congrats. Mind tell what is the actual cents/km when you pump the petrol over the mileage done? I found that when using diff petrol, the display show different but actual mileage is the same. Some petrol the display show >26km/l and some show 17km/l but actual mileage are the same which is about 10~12cents/km.
thanks.

This is what I worry. I used to pump Shell petrol the meter shows 16~17km/l which actual fc is 11cents/km for distance driving. This time, I pumped Caltex petrol, immediately the meter show 24~25km/l. I am not sure what cause the meter to read differently. Surprisingly the reading is immediately after I left the petrol station.