Thanks, Velo. I appreciate that. Regardless of the messenger I need to make sure I don't pass along bad info.
Doug,
I said this before, your obviously experienced with electric bikes and your input has been appreciated. When working through the variables to determine gearing I needed a reasonably accurate h.p rating. With my gas bike experience I know approximately what gear ratio a given h.p. can push on my bikes. Looks like I was pretty close in this application but did not consider voltage drop off at full throttle. Is there a rule of thumb so to speak for the percent dropoff? Say 5% or 10% in a 52 volt system?
Hi,
I am not sure a 'rule of thumb' is really possible, since there are so many variables:
- Exact cells used in the battery, and their characteristics;
- Series / parallel configuration of your particular battery;
- Age and condition of the battery;
- State of charge of the battery;
- Power usage curve of the particular motor used;
- wiring (too small a gauge anywhere in the path can add to sag);
- specific power application curve of the controller used;
- RPM of the motor at the moment the throttle is opened (greatest sag typically at lowest RPMs);
- load conditions (weight of bike / rider, slope, speed, rolling resistance);
...and probably a few others I have missed.
Working with 'HP' ratings with electric drive systems, although tempting because it is familiar, for all of the above reasons is really not that useful. If you want to use 'one measurement' for comparison purposes, watts is more useful, but needs to be considered / calculated for each of the different components / voltage / amps...
The bottom line, is this is simply how batteries of ALL types work, to varying degrees - voltage drops when load is applied.
Even the AC coming out of the wall in your home or business will sag under heavy load, like when a compressor or large electric motor first starts up - ever notice the lights will dim slightly, temporarily? That is sag.
Lithium cells in general are noted for having greater sag than most other battery chemistries; indeed this was a major obstacle to them being useful for motive applications in the early days of their existence.
At the end of the day, the greatest defence against sag, is more amp hours capacity in the battery, relative to the number of amps being drawn under full load.
All e-bike controllers suitable for use with lithium batteries, should have a programmable low voltage cut-off to protect the battery from discharging too much (this prolongs battery lifespan considerably).
If this voltage cut-off level is set too high, or if the battery amp hours are not enough for the application, OR if the battery is getting old and tired, or nearing the end of its current charge, sag can cause the voltage to dip low enough to activate this cutoff...abruptly cutting power to the motor, usually only under full throttle / high load. Depending on the controller, the 'ignition' might have to be cycled off and on again to restore operation, or it may simply resume, but cut out again under high load.
If you experience the above with a known good battery still holding plenty of charge, and the cut-off voltage is set correctly, the only remedies are a different battery with more amp hours, OR possibly programming the controller to provide less than full output when the throttle is fully turned, OR substituting a less powerful motor that draws less under full load, reducing sag.
I hope this is helpful, if not a direct answer to your question...
Cheers,
Doug