| I've done it this way for years and so have most of the other club | members. I've never had a battery fail in the air. Don't need to | worry about the 'curve'. The voltage tells the story.
The voltage tells much of the story, but it's not the whole story.
A key part of having the voltage tell the story that you want is to have the battery be fully charged before flying that day. If you charge the battery only a little bit, the voltage won't tell the right story.
Take a totally dead battery. Zero volts. Charge it for 5 minutes at
1/5C -- it's now about 4% charged. Put it on the ESV, and it'll read like an almost full battery -- for a little while, anyways. Take it up for a flight, and you probably won't ever get to land that plane.
ESVs don't tell the whole story -- you need to fill in some details yourself. But they're pretty good in helping you decide if you should go up one more time or not.
| >A number of club members have started using Hitec Battery Checkers at | >the field and using the readings to decide whether there is enough | >pack capacity to risk another flight.The meter appears to be a | >voltmeter presumably with some electronics to give an expanded display | >around 4v to 5v and a load for the battery.
No electronics needed. It's just a resistor in parallel with the meter to give you a 250 mA or so load. (Without that load, the voltmeter will read around 1.4 volts/cell even for an almost empty pack.)
Actually, you can make a fully functional ESV with a $3 multimeter from Harbor Freight Tools and a 16 ohm resistor for receiver packs and a 32 ohm resistor for transmitter packs. Make sure the resistors are rated for at least about one and two watts respectively -- lower rated ones would be fine for the usual few seconds, but if you ever want to use your ESV to discharge a battery, you'll want the proper resistors or you'll risk burning them up.
| >I am thinking this is a dangerous practice to depend on a single | >reading.
It is, but it's better than nothing.
| >I was always told the discharge curve for NiCd and NiMh was almost | >flat right up until the last few minutes where it drops off | >rapidly.
There's some truth to that, but it's not really _that_ flat. And if you're smart, you charge LONG before get gets to the point where it drops off quickly. If you stop when your battery read 1.2 volts/cell, and you fully charged your battery previously, then you've still got quite a bit of power left in that battery. You could probably make a few more flights, but it's smart to charge now.
| >If this is the case how can these meters be of any use when they have | >no idea where along 'the curve' they are sampling. They could be near | >the just charged end or they could be just a few moments before the | >curve drops away and the battery is flat. The latter being real bad | >news if you used one of these to decide whether to fly or not.
You also sometimes have a little warning before it goes really flat -- the servos start getting sluggish and sometimes jittery. I don't suggest ever letting it get to this point, but I've seen it happen, and seen people notice it and land really quickly, and find the pack at 4.0 volts ... whew!
Personally, I put these --
formatting link
into most of my planes. Mostly for the lost plane alarm functionality, but the low battery alarm is nice too. In a powered plane you won't probably hear it in flight, but you'll hear it while you're refueling and such. (In a glider, you will hear it in flight if the plane is close.) It starts going off when the voltage drops below 4.6 volts -- which generally means that you've got some time left -- maybe 15% or so? -- before it craps out completely. At $15 and 7 grams, it's cheap insurance.