Battery - Help?!

Ev4043

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New to ebikes and just got my first build. GT Aggressor with Bafang 750w 52v battery. I'm really confused about the battery performance. Can someone explain this to me like I'm from another planet please?! I don't speak watts and volts very well!

I started my ride the battery display was at 52v and 100%. As I ride, the volts fluctuate up and down depending on my input and what PAS setting I'm using - fair enough, so far I get it.

Then, somewhere in the region of the display reading 47v / 46v, the battery cuts out and the display goes dark. No more juice :( uh oh... I'm 8 miles from home.

I go on my own steam for a bit, then out of curiosity I try turning the controller back on. It comes on and is now reading about 48v, so it's gone up a bit, and will now work again. The power bar reads as 76% remaining.

As soon as it gets near 47/46v again it cuts out. So I know now, I need to be able to either get home in under 6v of use (if starting at 52v), or expect to peddle myself home beyond 47v.

What I don't understand at all is this - the battery itself is still showing 76% power?! The guy who built it said that's not a good indication and I should watch the volts not the power bar, but I don't get why I'm lugging so many cells around that are still 2/3 charged, and of a 52 v battery I actually only have about 6 or 7 volts to play with before there's no more juice.

If I fill my car with petrol I know how much fuel I have in the tank until empty. To me this battery business is kinda like saying I've got a full tank of petrol, but the car will die after I've used only a third of the tank. Or, 100% charge on a mobile phone, but you only get to use 20% of the battery before it dies at 80% remaining.

I'm obviously not thinking about this right.

I have the bike set to 9 modes of PAS. I got under 20 miles using PAS 1 - 3 without any major hill climbs. Wasn't a windy day either. Is this normal? It doesn't seem very good - my neighbor gets 40+ miles on half a charge with her 3 yr old Giant, I was expecting more out of mine tbh.

Any advice? Someone able to educate me on this?!

Thanks.
 

Hoggdoc

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Not all battery packs are created equally. It is strongly recommended to use cells made by Samsung, LG or Sony. Mant battery packs from China use lesser quality cells to keep the price down. The end result is the kinds of performance you are experiencing.

The dip in voltage you are seeing is called SAG as you vary the amount of power (amps) you are pulling from the battery pack. It also depends on the controller setup.

For instance if you have a 52 vdc battery and are pulling 25 amps the wattage the motor sees is 1300 watts. Nearly twice as much as the motor is rated as. That in itself is nit a bad thing unless the motor is of poor quality, your's isn't but can draw a battery very fast.

I have a 48v 17.5 ah battery pack on my bike and at full throttle I can pull 1200 watts from the pack. I rid in very hilly country and am using max power often during my rides. Typically I can get 25-35 miles on a charge. My bike is very heavy at 72 lbs I weigh 195 lbs to give you an idea.

I would surely contact the builder and ask some questions about the source of the cells in the battery pack. Based on the information you have given you should be getting much better range for sure.
 
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