Battery telemetry alarm voltage setup.

[David Hall 9]

I recently bought a few of the cheap Rx mounted battery telemetry devices for my Flysky system.. The system returns the LiPo pack voltage to the Tx screen and allows alarm points to be set, which sound an alarm on the Tx when reached. (Oddly, it measures my 3s packs and returns the actual value, but will only allow alarm points under 10v to be set....., I just monitor two of the cells... not an issue for here, but just for info...)

It's nice to have an idea of what my pack status is without having to land to measure it, or to use a timer for every flight. Timing my flights works well for my general aerobatic things but is not so great for my F5j style electric gliders, which use the motor only intermittently.

So, I have the monitors set up in a few of my e-gliders. As the battery is used, and well before it would be considered as near flat, the alarms tend to go off as the motor is run, then to return to non-flat voltages when off load. It's a bit annoying and disconcerting to fly during the alarm condition only for it to cease when the motor is shut down, on landing finding that it has 60% capacity left....

I wondered if there is a "rule of thumb" that can be applied to calculate the on-load pack voltage alarm points that would be useful to set up. My guess is that it must be something that is set up for each motor load and battery combination.... Anyone do this?

[MattyB]

This is a fundamental weakness I have noticed in flight pack telemetry that is not easy to address without some quite sophisticated thinking and TX setup. On models without on board current sensors I have simply used delays on the logic switches in my Taranis to address this (i.e. I do not have it report an audio warning until it has been below the target voltage for >5 seconds), plus an old fashioned timer as backup. This does not solve the problem completely though.

What you really need is to a current sensor onboard so you can use a combination of current drawn over time to estimate the mah removed from the pack, then raise alarms on that figure. A Lua script has been written for OpenTX transmitters that does this, but I've not used it as yet:

Even with this though the effect of temperature on your battery should not be understated - at low temps the available capacity drops significantly. This could be mitigated by having alarms sound at different capacity usage points though I suppose (i.e. a first alarm at 50% of labelled capacity used, second at 40%, third at 30%).

Matt

Edited By MattyB on 27/03/2017 18:19:25

[Former Member]

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[David Hall 9]

This radio has no voice alerts or anything near programmable alarms (it has been a great buy, and has worked very well for me on my sports models). It can be set up to provide two separate alarm points based on voltage reached (and an over-voltage alarm).. I plan to measure the voltage drop due to internal impedance on load by running the motor (usually full power or off) and account for this in setting the alarm point voltage.. but stay on the cautious side....