Battery power drops near 25%

[Saracen]

I did a 32 mile round trip to Aberystwyth from home yesterday (LOVED every second)

But I have a question, being Wales there are hills, not massive but some along the B4340.

I believe I have the 36v battery on the 250w B001 Bafang. (works perfect)

Question is, I do believe I could have done the whole 34 mile trip, but last hill I swapped over, didn't know how little I had left to do.

I noted that even with supposed (I know they are not accurate) battery still just in the yellow, no near the red, there appeared to be a lack (not great but enough) of power on the hill.

I thought they maintained maximum power until they hit the red.

Thanks

Fortunately I have 2 bikes so always take the spare battery

 

[sjpt]

The voltage drops as the battery depletes, from around 42v full to around 30v empty, and the power drops off proportionally. In addition, you are likely to hit voltage sag on hills as more current is needed; that can easily be 3v or more.

It's a shame that the battery gets tired at the same time you do; when you hit a hill at the end of a ride when you are nearly home.

 

[saneagle]

I did a 32 mile round trip to Aberystwyth from home yesterday (LOVED every second)

But I have a question, being Wales there are hills, not massive but some along the B4340.

I believe I have the 36v battery on the 250w B001 Bafang. (works perfect)

Question is, I do believe I could have done the whole 34 mile trip, but last hill I swapped over, didn't know how little I had left to do.

I noted that even with supposed (I know they are not accurate) battery still just in the yellow, no near the red, there appeared to be a lack (not great but enough) of power on the hill.

I thought they maintained maximum power until they hit the red.

Thanks

Fortunately I have 2 bikes so always take the spare battery

A 36v battery is 42v when fully charged and 31v when empty. Your controller gives a maximum current, say 15 amps. When you start, you can get 15x42 watts from the battery = 630w. When it's empty, you can get 15x31w = 465w, which is only 3/4 of what you could get when full. Your bike is about 70% efficient, so the output power starts at about 440w and gradually decreases during your journey to 330w.

It's possible to avoid that by putting a buck converter between your battery and controller, but then, you wouldn't able to see how empty your battery is without adding a voltmeter; your controller wouldn't switch off when the battery is empty; and you'd lose about 5% of the battery stored energy during the conversion.

 

[guerney]

I enjoy monitoring voltage sag. Here's the voltage on my DPC18 display when I stopped to let a police car zoom past at some crazy speed, shows 38V:




During the same journey six miles later, as I started climbing this hill (I'm about a tenth of the way up), you can see the voltage sags to 34.3V - pictured before the 25kmh cutoff, after which speed dropped a bit, then back to 25kph etc. It's like that on most hills on my bike, now that I have increased the BBS01B controller's amperage limit to 18A on my brilliant 20" wheeled hill conquering Dahon folding bike conversion.





I live on a hill, therefore during every return journey there is voltage sag. My bike has only ever sputtered to a stop uphill once, when I was on extremely low battery, in which case I simply lowered the assistance level ie continuous current drawn from the battery, so I could keep getting motor assistance (albeit less) to enable me to get uphill home. On our Bafang BBS01Bs, you can define in firmware via programming cable, how much continuous current (100% keep current") each level uses - thus avoiding voltage sagging so low uphill, that the controller cuts power to the motor. My level 9 draws 18A continuously from the battery.





My level 1 overcomes motor resistance, plus a bit - I've only ever used level 1 once, and by that time after 42 miles over mixed terrain, my battery was near pooped anyway, so it gave up the ghost shortly after. Fully depleted after 43 miles.

Sometimes for your longer journeys, you may have no choice but to lower current drawn from the battery, so you can power uphill home.