LI-PO protection with Zeener diode.

Hello,

I’m currently making a tricopter, I thought about using the arduinos analog inputs to shut the copter down if the voltage’s gets under the 12V (3v a cell 4 cells) limit.
However I was wondering if an added analog, fail proof circuit with a relay and a zeener diode is possible, or just stupid?

I was thinking something like this: (attached)

Thanks a lot!

You really want a circuit that switches off cleanly on under-voltage with some hysteresis - otherwise the voltage will recover and the circuit will oscillate - there's probably an IC that does this available, try searching for power management and battery management ICs for LiPo cells?

If you're LiPo has a balancer connector on it to access each cell of the pack you could try putting one of these http://www.dsscircuits.com/lipo-fuel-gauge.htmlLiPo fuel gauges on it to monitor the charge.

Hi,

I'm no expert, but I've used a bit LiPo in RC toys (car & helicopter). From what I know a cell of LiPo battery will go up to 4.2V when full charged and you shouldn't go below 3V. Many people stop themselves at 3.1V as a safety measure. And the everage voltage over a full usage (between 4.2v and 3v) will be 3.7v

I'll post another thread to ask about what type of LiPo to use. But if you use a 4S, your voltage when fully charged will be 16.8V. That will create some overheating somewhere, won't it ?

Now my question/suggestion : Regarding the hysteresis issue, if you put a large capacitor in parallel of the battery, can it help suppressing this problem ?

The problem is battery voltage will recover markedly when the load is disconnected, so the protection circuit might disengage immediately, reconnect, and the voltage drops again, so protection circuit reconnects.... This can cycle applying and cutting power in an oscillatory manner which is not particularly nice (and can be really bad in some circumstances).

So the trick is to cut out till the voltage is above a separate higher threshold, and one sufficiently higher to prevent these oscillations.

A simpler strategy is to cut out until the protection circuit is reset - if the power for the relay were powered from the relay contacts this would be automatic.

Using a zener to directly drive a relay would actually have some hysteresis anyhow due to the relay's mechanical hysteresis, but some experimentation might be needed to see how much hysteresis (and I'm a bit worried about the power dissipation in the zener).

Plus driving a relay coil continuously with battery power is kind of a waste of a precious resource, mah capacity, all that is used up by the relay coil is unavalible for the load. If you have to use a relay in some battery powered circuit, at least use a latching relay where you can turn off coil current when not switching states.