Many factors affect the price of lithium batteries, mainly the battery capacity, its age and brand, etc.
It has nothing to do with voltage. If you want to make an electric car, you can't use reconditioned batteries
The minimum lithium battery must be 10AH, which costs more than 300 coins
You said that the price is probably that of a lead acid battery.
Even reconditioned lithium batteries are not that cheap
If you have less money, use lead-acid batteries, which cost around 100 yuan. Don't buy one that's too cheap
It is not recommended to use it this way (except in an emergency).
Old batteries mean aging and reduced capacity, which is manifested by the fact that the electricity discharges very quickly when discharging and the maximum voltage is reached in a very short time lors of the load.
Put the new and old batteries together in parallel. When discharging, the old battery will discharge quickly and the voltage will drop accordingly. However, due to the parallel connection, the new battery will charge the old battery and. the overall discharge time will be shortened, which will also affect the lifespan of a new battery.