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Noob Glider Question about NiCad Battery flight times

rockyboy

Skill Collector
Mentor
#1
So I've got a very "noob flyer" question about gliders.

I've got a swap meet rescue 2 meter balsa glider that is using 3 servos (rudder, elevator, ailerons - EMAX 008MA II 12g) a FrSky V8R4-II reciever, and a Futaba 600mah NiCad receiver battery up front. I've never had a NiCad airplane battery before, but I was able to solder up a connector to my charger and with the NiCad .1 amp setting I charged it up for a good 5 hours and it's holding it's charge just fine after a couple days.

So I'm trying to figure out how much flight time I should expect before I need to switch out the battery. The receiver has an operating current of 30mA, and the servos are 200mA each, for a total maximum draw of 630mA.

I found a formula that says take the mAh capacity of the battery divided by the current draw and multiply by 60 to get the minutes of flight time. So plugging in the numbers in a perfect world I should get 57.1 minutes of flight.

But things are never perfect - so what's a good safety margin for a glider like this? Should I pull it down at 45 minutes max? Is there a way to check NiCad capacity (since voltage doesn't seem to be as good of an indicator as it is with LiPo's)?

Thanks!
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#2
So I've got a very "noob flyer" question about gliders.

I've got a swap meet rescue 2 meter balsa glider that is using 3 servos (rudder, elevator, ailerons - EMAX 008MA II 12g) a FrSky V8R4-II reciever, and a Futaba 600mah NiCad receiver battery up front. I've never had a NiCad airplane battery before, but I was able to solder up a connector to my charger and with the NiCad .1 amp setting I charged it up for a good 5 hours and it's holding it's charge just fine after a couple days.

So I'm trying to figure out how much flight time I should expect before I need to switch out the battery. The receiver has an operating current of 30mA, and the servos are 200mA each, for a total maximum draw of 630mA.

I found a formula that says take the mAh capacity of the battery divided by the current draw and multiply by 60 to get the minutes of flight time. So plugging in the numbers in a perfect world I should get 57.1 minutes of flight.

But things are never perfect - so what's a good safety margin for a glider like this? Should I pull it down at 45 minutes max? Is there a way to check NiCad capacity (since voltage doesn't seem to be as good of an indicator as it is with LiPo's)?

Thanks!
I never thought of you posting as a noob for anything! As for the old tech of NiCads firstly unless a high current pack the charging should be at a 10% of "C" rate for upto 14 hours. (Monitor the cell temps to stop the charge if they get too hot - Nicads can also explode if grossly overcharged).

Second the .1 "C" discharge rate would give you just under one hour if the batteries were new and never mistreated otherwise I would derate that by 25% so your estimate of 45 mins is not too far off.

Initially I would not trust the pack for a long flight until I had done a full discharge at the 1 "C" and then a full timed charge cycle. After the full charged cycle I would do a measured and timed .1 "C" discharge, from this you will be able to determine the actual capacity and the health of the battery pack. After the Discharge of course you would again charge the batteries for use and of course when using the batteries use the calculated and measured capacity as the new "C" rating.

Every now and then it would be wise to "BALANCE" the cells by doing the measured discharge and recharge on each individual cell, (this will allow you to detect weak cells and reduce the risk of actually reversing a cell through over discharging.

As for your current endeavour I would seriously contemplate the fitment of a 2S LiPo and a BEC just to be safe in the long term! Making the LiPo and BEC to have the same total weight as the original pack would give MANY hours of flight capability!

Have fun!
 

rockyboy

Skill Collector
Mentor
#3
Everyone is a noob at something, and I'm a noob at a whole lot of things! Life would be pretty boring indeed if there wasn't anything left to learn :D

Are there any clever tricks or DIY tools to make that help with timing the NiCad battery charge/discharge cycle? My charger has a timed charging cycle, but I don't think the timed discharge cycle tracks when the battery is empty, just how long it's going to draw current out of it.

Based on this conversation, my pucker factor is going up a bit, and I'm downgrading these batteries to 20 minutes each until proven better. Really don't want to have a fly-away at the field I'll be flying these from.

I have 3 of these brand new in the box 600mah Futaba receiver packs and it would be a shame to just let them gather dust. Maybe it's safer to put in a 2S LiPo in my glider and see if one of the gassers in my club wants the receiver packs though. As I mentioned, I really don't want to have a dead stick fly away at this field - potential with a long glide slope aircraft to end up somewhere it shouldn't be.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#4
You can still buy the slow chargers for NiCads and most LiPo chargers have the ability to charge a range of different battery types including NiCads.

With the discharge cycle on my Imax B6 I set the discharge current and the capacity is measured by the charger. The charger has its own preset voltage cutoff determined by the number of cells I set it for.

The charge is a little more difficult but a simple trickle charger of 0.1 "C" can be left connected almost indefinitely though it can take 14 hours to fully charge a flat battery pack.

With my charger which has a time limit before it cuts off I just need to set the charge for the required current and if necessary enable the charger a number of times consecutively.

You can charge faster IF the battery is definitely flat and in good condition but normally the max rate should not exceed 0.5 "C" and be applied for not more than 1 hour. This is due to heating of the cells due to their internal resistance.

As I do not know what charger you are using I am unable to give specific directions or charging options. bear in mind that the trickle chargers are still available from Futaba.

Have fun!