snowboyd

Up till now all my planes and gliders have been motorised with ESC.

I have 2 new slope gliders that I have custom made 4 x AAA 800 mah. At full charge they read 5.5V.

AT WHAT VOLTAGE should I stop to recharge pack.
I tested using 2 channel receiver and after 1 hr 4.8V, at 1.20 3.8 V - all servos still responding ok.

I will also use a similar pack on a 6 channel 60" glider - at what Voltage should I be looking to recharge that pack?

Thanks SB

da Rock

The voltage a pack reads under load (what ESVs were invented to do) is what will tell you when to recharge. A 4-cell NiCd pack's rated voltage will be 4.8 volts.

The way NiCd's discharge, they will show their rated voltage for almost all the time that they are safe to fly. They start out as you've seen, showing voltage above their rating. After a very short time, they will drop down into their rated voltage. They'll then show that up until they're about to go "empty".

Whenever they show a voltage under their rated voltage, they're at least over half empty. When they show more than a tenth or so under that rating, they're very close to empty.

So what most people do is some timing and checking and math.

If you only want to use the ESV readings, then whenever the reading goes under about 4.7 or 4.6, it's time to stop.

But like I said, most people..........

da Rock

You've already done some of the test/recording.

What most people do is figure out the average drain for an average flight and that'll tell them how many flights are in the pack.

There are a couple of ways to do that. The easiest and best is to use a charger that gives informational readouts. You start with a full pack, fly some number of flights, recharge and see how many mAh it took to fill the pack back up, and divide that amount by the number of flights flown. The answer is the average drain for one flight. Divide that into the capacity of the pack and you got the number of flights the pack could have flown.

If you don't have a charger with a readout, it's not so easy or simple.

da Rock

Truth is, if your pack went from 4.8v to 3.8v in that last 2/10 hour, it was maxed out in that last 2/10 of an hour. Most receivers are no longer dependable below 4v.

How many flights in that first hour? That's your limit next time out.

John Walter

My Feedback: (1)

Using 800 mah in one hour is a lot, even on the slope. It suggests that either you were very busy on the sticks or your linkages may be binding. You may want to check your linkages.

BMatthews

Double check for sure but I believe that NiMH cells hit a peak voltage of 1.5 to 1.6v per cell under charge. This quickly drops to about 1.3 to 1.4 once the charge stops. This peak drops down to a nominal 1.25 fairly quickly during the discharge curve with the peak only lasting for like 5 to 10% of the mah discharge. The 1.25 "shelf" lasts for quite some time until it falls off to 1.2 and quickly from there to 1.1 where they are considered fully discharged for the radio's purposes.

In these readings I found they were very similar to NiCd's.

So I think you may be shutting the charge down a little too early. Perhaps try trickle charging them as per the manual and see what the pack voltage is while on and an hour after taking it off the charger.

snowboyd

Thanks for the ideas - great response to the question!

My test was at home constantly moving servos and intermittently leaving them in positions.

Perhaps that would account for the higher drain - as when actually flying with correct trim there would be much less servo usage.

If I wanted to test under load with voltmeter (maybe I could make some test connector) - how I would I correctly do this.

Thanks SB

BMatthews

Testing with a voltmeter and load resistor that draws about 300 to 500 ma is the sort of normal way to test an airborne pack. The common manner of hooking up to it is with a servo harness that plugs into the battery charger socket.

s14slide

could a charger w/ a variable discharge rate be used to gauge it? say for example, you calc a guesstimate of your rx and a pair of servos draw and set the discharge rate for that. then use the discharge time for max flight time. sounds reasonable.

da Rock

You've not mentioned if your custom packs were made with NiCd or NiMH batteries. That'd help.

Also, RCU has a Batteries&Chargers forum, and this topic has been discussed there very, very often.... and covered very, very thoroughly. You might look through it. There might even be a sticky with the info in it there.

If you plan to fly RC with any regularity, it would be a good idea to buy an ESV. They're relatively cheap and make it quite easy to fly without taking the risk of flying out your battery charge. You simply test the charge before every flight. $10 for this one........
http://www2.towerhobbies.com/cgi-bin...?&I=LXNK81&P=7