Acceleration Rate vs. Battery Voltage

During my commute to work, which is mostly on a rail trail, I use the throttle on my Magnum Metro+ to help me get across intersections quickly. I've noticed that the acceleration rate drops with the battery voltage, so I set out yesterday afternoon to measure that drop. Starting with a fully charged battery, I measured the acceleration rate to 15 mph using the throttle only in both directions in an effort to cancel out at least some of the wind effects (it was a little windy. I then biked around for a while, letting the battery voltage drop a volt or so. The acceleration test was repeated at the same location. I continued this pattern until the battery voltage reached 44.4 volts (just above 20% for a 48 volt battery). Here's the results:




As you can see, with a fully charged battery, the time to 15 mph was just over 5.5 seconds. With the battery nearly discharged, to dropped to around 7.25 seconds. This is a dramatic change and is certainly noticeable to the rider. I assume the controller is limiting the amperage to the motor so as the voltage drop, the wattage (or power) drops as well.

Do you experience the same with your ebike? I imagine so unless it has some way to compensate for the reduced battery voltage.
 

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AHicks

Well-Known Member
Not that this is a new concept, but it is a nicely documented way of proving the point. That drop off starting at about 47 volts is usually noticeable to those that have been riding the same bike for a while. Tells me when it's time to charge!
 

Ebiker01

Well-Known Member
yes, that's wby they should make them all 52v. That's 58.6v ? when fully charged. Not sure if 58.6 or 58. ? anyway , that would be about 4volts extra and very usefull.
 

rich c

Well-Known Member
My recumbent tadpole eTrike is that way. My Bosch powered eBikes are smooth as silk and accelerate at an even rate no matter the battery charge level. Same at 20% or 100%. Trike is 48v, Haibikes are 36v.
 

fooferdoggie

Well-Known Member
My recumbent tadpole eTrike is that way. My Bosch powered eBikes are smooth as silk and accelerate at an even rate no matter the battery charge level. Same at 20% or 100%. Trike is 48v, Haibikes are 36v.
yes I see this on my Bosch too. very nice.
 
I’m glad to hear that about the Bosch bikes. The controller must be compensating for the reduced voltage by increasing the amperage to maintain the same wattage to the motor. Anyone else’s bike do that? I need to test a Bosch bike this spring.
 

AHicks

Well-Known Member
I would have to bet that the Bosch systems in question have so little (amperage) draw on the battery that you don't see the kind of voltage sag you might see on systems drawing more power (amperage).

Kinda like using a small light bulb to discharge a battery vs. a hair dryer for instance. The light bulb will stay bright until the battery is nearly totally drained, where the dryer performance will fall off noticeably just prior to the battery becoming drained.

Lithium batteries, in my experience, will all display properties similar to that shown in the chart above (depending on the load).
 

Mike's E-Bikes

Well-Known Member
My recumbent tadpole eTrike is that way. My Bosch powered eBikes are smooth as silk and accelerate at an even rate no matter the battery charge level. Same at 20% or 100%. Trike is 48v, Haibikes are 36v.
How exactly do you know that, as there is no throttle to test the premise with a Bosch mid drive ? Unless you are under full motor power and no human assist involved, you're statement is very subjective if not purely speculative.
 

rich c

Well-Known Member
How exactly do you know that, as there is no throttle to test the premise with a Bosch mid drive ? Unless you are under full motor power and no human assist involved, you're statement is very subjective if not purely speculative.
Purely by feel. My recumbent trike with PAS has a completely different feel than my Bosch. On my trike, the top speed at any PAS level lowers with the same cadence as the battery capacity lowers, not so with the Bosch. Just based on 10,000 miles of riding, no scientific charts or graphs.
 
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Purely by feel. My recumbent trike with PAS has a completely different feel than my Bosch. On my trike, the top speed at any PAS level lowers with the same cadence as the battery capacity lowers, not so with the Bosch. Just based on 10,000 miles of riding, no scientific charts or graphs.
Interesting. My daughter's bike (Faraday) uses a torque sensor and doesn't have a throttle. Getting consistent acceleration results were difficult but achievable. Essentially, we use power measuring pedals with a real time readout to keep power at the same level while accelerating. I need to get a Bosch bike to test and do the same thing, and then we will have some data to sort this out.

My Magnum is noticeably less powerful as the voltage drops so my hunch is that you are correct, the Bosch keeps its max power the same as the battery loses charge. Interesting!
 

fooferdoggie

Well-Known Member
Interesting. My daughter's bike (Faraday) uses a torque sensor and doesn't have a throttle. Getting consistent acceleration results were difficult but achievable. Essentially, we use power measuring pedals with a real time readout to keep power at the same level while accelerating. I need to get a Bosch bike to test and do the same thing, and then we will have some data to sort this out.

My Magnum is noticeably less powerful as the voltage drops so my hunch is that you are correct, the Bosch keeps its max power the same as the battery loses charge. Interesting!
that would be cool. the bosch is so energy efficient that I bet it is running at a lower voltage. then if so it would not losoe power as the voltage drops but who knows? the batteries are the best in the industry and that helps too.
 

Handlebars

Well-Known Member
That drop is the nature of lithium batteries. You will always get more out of the first 50% than the last 50%
My battery now drops voltage very quickly from 100% whereas from 80% it doesn't drop voltage as quickly. I think it's got some weaker cells mixed in there, maybe, that don't charge all the way?
 

AHicks

Well-Known Member
My battery now drops voltage very quickly from 100% whereas from 80% it doesn't drop voltage as quickly. I think it's got some weaker cells mixed in there, maybe, that don't charge all the way?
That could also be indicative of a battery that's not getting a balance charge often enough......
 

Handlebars

Well-Known Member
That could also be indicative of a battery that's not getting a balance charge often enough......
I've recently tried leaving it plugged in for some hours after 100% and I've also tried intermittently stopping the charge and starting again after the battery rests then charging to 100%. Now that the weather is warming up I'm going to unwrap the battery from all the insulation and duct tape I had it in for the winter (taped in place on the bike also with steel bands under layers of duct tape to help make battery theft a lot more more difficult) and I'll be able to use the voltage meter on it to see if it's actually charging all the way. Previously it took a couple of tries of a couple of hours duration, after 100%, to get to full voltage by my meter. Will see what happens. I might as well unwrap the battery today and check that out.

Thank you Mr. Hicks!
 
Yep, this is pretty common with lithium ion batteries.

My bike has PAS levels 1-5, and power levels 1-3. Luckily this means I can compensate when things get low. If I ride outbound at power level 1 or 2 (and say PAS 3) and I want the same effect coming back on the last half of the battery then I can up the power level 1 step and get roughly the same performance.