Conrod

I have been using LiFe 6.6 volt for my RX batteries and am now looking at trying Lipo 7.4 volt batteries, my question is can I measure voltage for safe operation and what voltage am I looking for before I stop flying.
Thanks
Geoff

tedsander

Well, not easily, like in the NiCad days. Lithium cells (both LiFe and LiPo) are noted for staying very consistent through their discharge, and then falling off the cliff to the point of damage very quickly. So volts may be good at the start of a flight, then fall too far by the middle or end. At least when using them for the propulsion system, you get an indicator via the motor losing performance. With a straight flight pack, you're unlikely to see anything in the air until it is too late. You could get fancy with telemetry to send info about how many milliamps have been used back to the TX...but that gets technical fast, and there are lots of discussions about the pros and cons of how to implement. Easiest is to just start with the chosen pack, and put several flights on it. Recharge and note how many ma's were put back. Divide by the number of flights, to get an average flight usage. Then divide that number into the capacity of the pack, to get a max number of flights possible. From then on, fly a few less flights than that max number, so you are sure you are always keeping a reasonable reserve.

Check out the specs for your receiver/servos for allowable voltages. Older systems for 4.8v NiCads generally tolerate 6.6v LiFe's pretty well (but there are many exceptions). Pushing them to 7.4 may be a bit too much, leading to greatly shortened life of the components. I caution only because your question seemed to indicate familiarity with old NiCad systems, so speculate you may be using old-ish components.

Xpress

Sorry, but that information is not correct for LiPo cells. LiFe are the only cell chemistry that maintains a single voltage (3.3v/cell) for the entire duration of its cycle, and then abruptly discharges at the end of the cycle. LiPo cells are very predictable based off of their voltage, for example we know that 4.20v/cell is fully charged, and 3.7v/cell is considered discharged, and the pack is generally very linear in getting there.

To the OP: There's no issue with using LiPo cells, but you need to ensure your equipment can handle it before you go any further. LiPo cells are 4.20v/cell fully charged, which means a 2 cell LiPo will be 8.4v at its peak. The only way to determine if it's time to stop flying is if the pack is sagging under load below 3.7v/cell. Telemetry equipment can tell you that, you can also use the age old test method to determine the duration of your pack: fly for 5 minutes, land, recharge the pack, and pay attention to the capacity returned to the cells. You can then base your usage time on the pack off of that number.

For example, if you have a 4000mah pack, and you use 500mah in 5 minutes (100mah per minute) then you can figure you can fly for 32 minutes straight before you reach 80% of your batteries capacity, or 3.7v/cell.Generally, it never hurts to recharge well before that. I tend to not let my packs get below ~3.85v/cell, or roughly 50% of its capacity.

Conrod

Thanks for the explanation on the LiPo batteries.
Geoff