Limiting Maximum Battery Power Draw¶
Sometimes power needs to be limited to prevent issues with battery sag or overheating. This can occur because of the use of lower C rated batteries, high power motors, high ambient temperatures, uncooled engines or ESCs, or other reasons. This topic discusses ways to limit the maximum power draw from batteries.
Methods to Limit Maximum Power Draw¶
The first and simplest is to restrict MOT_THR_MAX to a lower value. This limits the maximum throttle controlled allowed. Unfortunately, this will not set an exact power limit and might require some trial and error to find the right setting.
A better way is to use a power limiting parameter (shown for first battery monitor):
The BATT_WATT_MAX parameter can be used to limit the total wattage that can be drawn from the battery(s). If battery power exceeds this value then the system will reduce max throttle (MOT_THR_MAX) to lower the power draw. If power demand is reduced, the max throttle will slowly grow back to MOT_THR_MAX. Use 0 (default) to disable this feature.
The power limiting feature uses a low-pass filter to smooth out the power readings and prevent rapid fluctuations in throttle. In some cases this can lead to a delay in power limiter activating or deactivating. The MOT_BAT_WATT_TC parameter can be adjusted to change the time constant of this filter and improve the responsiveness of the power limiter. The default value is 2.
The following table shows the effect of different MOT_BAT_WATT_TC values on the responsiveness of the power limiter. The power ratio is the ratio of actual power draw to the BATT_WATT_MAX limit. The percentage values show how much the max throttle is reduced per second at different power ratios for different time constants.
Power Ratio |
|
|
|
|---|---|---|---|
1.1 1.5 2 5 10 |
reduced 9%/sec reduced 49%/sec reduced 98%/sec reduced 392%/sec reduced 882%/sec |
reduced 4%/sec reduced 24%/sec reduced 49%/sec reduced 198%/sec reduced 445%/sec |
reduced 1%/sec reduced 9%/sec reduced 19%/sec reduced 79%/sec reduced 179%/sec |
For example, if the power draw is 50% above the BATT_WATT_MAX limit (power ratio of 1.5) and MOT_BAT_WATT_TC is set to 2, then the max throttle will be reduced by 24% per second. If the power draw is 10 times above the limit (power ratio of 10) and MOT_BAT_WATT_TC is set to 5, then the max throttle will be reduced by 179% per second.