How to get A51 to charge in the car through USB

[hswil4]

HI! I just started using the A51 but when I connect it in the car to try charge or keep 100% charge for car journeys whilst using google maps I actually end up loosing charge. 

I set it up so it only charges when connected to the car but this didn't help.  I close all other apps so its only maps but still I loose on charge instead of even just staying the same. Previously on my old one it was a slow charge but it at least stayed the same charge or gained 2/3 % over 1 hour. 

I am using my old cable since the cable for the new phone is too short for the car now but they have the same connector so I don't see this should be an issue.

Thanks in advance