What is voltage sag?

Voltage sag (or voltage dip) is what happens when you apply a load to a source of electricity. If the source is really big and the load is really small then you won’t notice it that much. But if the load is large enough the voltage will drop.

Now your e-bike battery is powerful but not like a the power lines connected to your home… So it’s limited capacity and limited output power grid. So when you apply a large load to your battery the voltage will drop from the initial resting voltage to to a certain level. Now the amount of sag will depend on many factors but can be put into 2 categories:
  1. The motor : size of the motor and controller and amount of power to the wheel you ask to produce.
  2. The battery : IR (internal resistance of the battery), chemistry, capacity (how many amp hours or watt-hours).

Voltage sag is not all bad, it’s normal, some good high quality battery sag more than some cheaper cells it depends on the chemistry.… But it’s also a good indication on how to use your battery and how to apply power!

For example. If you have a 52v Shark Pack fully charged to 100% (58.xV) while in a resting position and you take your BBSHD capable of 1500W and hit the throttle to accelerate, you would see the voltage “sag” or “drop”a few volts down to 55-54ish Volts.. And will continue to drop until you let go of the throttle, where it will raise back up to slightly less than before obviously.

But when the battery is being used and drained, it generates heat and with less capacity and heat you can get a little more sag near the end. And again it’s normal... the battery has less punch when it's tired!

So let’s say if you’re near 46v (resting voltage) and you ram the throttle wide open....well under load that would put it close to 41V and what is at 41V on your BBSHD? The controller set point to cutoff power to prevent over discharging your battery!

Once you hit about 46V on your 52V battery (or 44V on your 48V) you should consider yourself low on battery and start “limping” home using minimal power to reduce the voltage sag to avoid hitting the low voltage protection….Lower the pedal assist mode, lower your speed, let go the throttle and pedal to get the most milage of whats left…This will also let the battery cool down while you ride back home!

Want to see a real test (not some spec sheet).

Well this is an example of a used/abused 30Q mini 52V pack with at least 100cycles on it and I have to admit that I charged it to 100% more than I charged it to 90 and 80% combined.

It was charged to 58.80V and then applied a instant load of 14amp (800W) to it. You see the voltage curve go down in less than a fraction of a second. Down to 56.19V

So 2.6 volt drop on 800W so it would probably mean that I would see more than twice that amount (6-7V drop) on a 1500W BBSHD with the same battery.
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