Electric Bikes - 36v 750w new build , intermitent under load

135063have recently compleated new bike build, its a 36v 750w china kit ( the big black one) i used the batteries off my old 250w ebike which i have been riding for around 12 years and was very pleased with. the problem arose when i had to climb a not very steep hill. every 10 or twenty yards it cut out for a few seconds and then came back on again. i put this down to old batteries so got myself a new set of 14amp and fitted them. the voltage shows 40.5v which i thought would be ok but it is still doing it. it is going a little farther than before but am only getting around 2-3 miles befor it starts. if i run at half trottle it seems to help but im stumped as to what it can be.
if there is someone out there who has had the same problem or who knows what it can be id be most greatfull

Endless sphere is a good technical place and they will probably answer this in moments.

the hill lasts more than 3miles? If it's a very steep hill and it lasts more than 3 miles, you might be better off with a gas-powered scooter.

Also, what kind of battery are you using? If it has a BMS, then the BMS could be kicking in prematurely.

no the hill is not three miles thats the distance on average i travel when if i climb a hill or open the throttle for more than 10-20 seconds it cuts out

the batteries are sla so no bms ?.whatever bms is

I think it's your controller. I think your controller has a LVC in it and I think your batteries are sagging so much that you hit the LVC on it.

If you aren't worried about breaking your controller (you might fry it doing this) , put 48v on the bike and then try the same hill or same route and see if that fixes your problem. 48v should put you well above the low voltage cutoff on your controller and then it won't cut out on you. You could fry the controller doing this too, though. Or your other option is to buy some batteries that don't sag as much like some lifepo4 batteries with big capacities. (this is only if I have diagnosed the problem correctly, I could be wrong but that's what it sounds like is happening to me....it's your controller).

I'm almost positive that it's your controller. Put 48v on it and see what happens. Where ever you bought the 36v14ah batteries, buy another 12v14ah battery and hook it up.

should have said befor but have changed controller and tried 48v. but still no joy.
but did get adrenalin rush

have solved problem with a little help from you all. simple answer was- upgrade all the wiring on the battery side to 12g. now runs faster, further and with more torque.

Could be a heat issue as well. See controls and some motors have a built in heat cut off. So check to see if the motor is hot or if the control is getting hot.