I've been using a 36V motor that draws ~13A current at peak. I initally used a set of three lead-acid (12V 12A) batteries in series to power the motor. They had sufficient power to easily take a payload of more than 60kg and output ~35km/h at full speed. I have now replaced them with a set of Li-ion batteries (10 x 12.8Ah + 10x 12.8Ah = ~26Ah joined using short copper wire strips ~1cm) however, I am worse off than the lead-acid batteries. They can't even provide enough power. Without the load, battery voltage stays constant at ~4.2 but when I put load on it, battery instantly drops below 3.8 and is unable to provide neither speed nor power.
Can anybody tell what is going on and how can I get the power from the batteries?