Autocut adjustment

How to adjust the autocut threshold on the Charge Doctor to stop charging at 80% or 90% of full capacity is a frequently asked question.

First, some essentials about LiIon batteries: charging to 100% leads to maximum voltage over the membrane separating + from – in the battery’s cell. The higher voltage, the more stress to the membrane and that has an impact on the cell’s health and lifespan. By stopping the charging at 90% or even 80% of full charge (the lower %, the better), the battery’s useful mileage can be doubled or even tripled, giving a substantial total cost reduction. Charging at 80% also means 20% less range, something not always possible or pratical on small batteries. But on bigger batteries (eg > 360Wh), it is mostly not a problem. So whenever possible, just do it with the Charge Doctor, since charging 10x at 80% is always better than charging 8x at 100%. It should be ok though to charge to 100% and riding just afterwards since it means the battery has not been submitted to Vmax over a long period. That’s the same reason why storing the LiIon battery should never ever be done at 100%.

The threshold default value is 1.00A, triggering, in most cases, the autocut at 90%. This threshold is an approximate value though since it depends on the charge current but also on the battery’s age. Fortunately, it doesn’t have to be accurate since the first goal is to avoid charging at 100%. Wether the autocut occurs at 80% or 85% doesn’t change the main picture.

However, if the threshold needs to be finetuned, here are the steps to follow, calculations has been made with Wh but can also be in Ah (consult the Charge Doctor’s user manual if needed) :

Step Screen
1. phase 1 : charge up until autocut => display voltage=67.7V, current = “OFF” (0A), charge = 183.2Wh (red). The Wh value of phase 1 depends of the emptiness of the battery at the beginning the of charge session. But it has no importance here since it is not used in calculations

 

2. reset the Wh counter (“0.0”, red)

3. disable the autocut : display = “—A” (or “0.00A” on firmware 2.02 or earlier) => charging resumes

4. phase 2 : charge up to 100% * (connect several hours or even one night for example)
End of phase 2 => voltage =67.7V, current = 0.05A (low enough to be considered as “end of charge”), charge = 30.4 Wh

5. divide phase 2’s charge (30.4Wh) by YOUR batteries nameplate capacity (Firewheel 260Wh in this example)
=> % residual capacity = 30.4/260=12%
=> the autocut occured at  88%
6. if the 88% doesn’t fit your desire, change the threshold (by repowering the CD with the button pressed) and restart at 1.
*Important : don’t forget to charge to 100% every 10 or 20 charges to enable cell-balancing, a vital requirement for LiIon batterie packs. To ensure proper cell-balancing, let the wheel rest one night or even 24h after charging, since riding would cause current outrush and a voltage drop that would cancel the ongoing balancing.

On big batteries (>680Wh), when charging at 2A, current drops very slowly towards the end of charge (after about 80%) so the threshold resolution may be insufficiant to autocut at 90%. To get more resolution and a more precise autocut, switch to autocut-by-voltage mode. The current and voltage thresholds are stored in separate non-volatile memories so their values don’t change when switching between modes.  Voltage mode is less pratical since the threshold value depends on the battery type (12S on hoverboards, 14S on scooters, 15S on Ninebot wheels, 16S on most unicycles and even 20S on the Gotway MS3/1600Wh or on Inmotion…).

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