I'm now learning to fly on the buddy box at my local field. The other day I got in 4-5 flights when one of the guys came over and hooked his voltmeter to my plane to make sure my rx battery still had enough charge in it to safely fly. That day I had been concentrating most on trying to learn to use the rudder to land straight in the crosswind since my plane was crabbing pretty badly on approach, so I didn't think to ask about safe battery charge/voltages etc. I know my reciever battery is 4.8v 600mah. I to my LHS and got a volt meter b/c I figure this is something I'll need.
I guess my questions are. 1. What is the lower limit for safe flight for my reciever battery and my transmitter battery? 2. I have an old tekin peak charger from when I used to race 1/10th scale electric off-road. If my reciever battery got low at the field would it be safe to hook it up to my rx battery, and if so what type of amps, etc should I use.

Thanks

You should stop flying and recharge when the receiver battery shows less than 4.6 (or maybe 4.4) volts under a 300 mA load. You can charge a 600 mA pack at a 600 mA rate without any problem. But it will take an hour to completeyl charge. If your peak charger has a detection setting that matches your battery chemistry, then you can charge at higher rates (1 amp or higher). You need to know if your receiver is Nicd or Nimh. the field charger should have a setting that tells it to watch for a nicd or nimh peak detection.

He probably hooked up an Extended Scale Voltmeter, not just a voltmeter. The right term is important. There are a couple of them sold in LHSs and they're all better to use than a voltmeter. They're also called ESVs for an obvious reason. They put the battery under load to read it's remaining capacity. Just use a voltmeter that doesn't load the battery and you won't know a thing about the remaining capacity. whatever.........

There are a number of threads on these forums (mostly in the battery forums) that go into detail on your question. But the answer is really somewhat simple.

Whatever the battery says it is........ if the ESV shows LESS, then shut 'er down.

When our batteries are fully charged, they'll all show an ESV reading that's more than the battery rating. Your 4.8 RX batteries will show voltages in the 5s when they're fully charged. And for almost all the time that the battery is safe to use, it'll show it's rating or higher. About the first 10-20% of it's full charge, it'll show higher. About 70-80% of it's use, it'll show it's rating.

As soon as the battery shows LESS than it's rating, it's time to stop.

So as long as you're seeing the rated voltage, you're ok. As soon as you see less, you're not.

What we do to be even safer is.............

When you recharge any battery, remember how much you used it, and see how much it took to recharge to capacity. What capacity was used can be compared to how it was used to give you a clue about using it next time.

If you flew 5 flights and the recharge had to put 550mAh back into the battery, then each flight used 110mAh of the battery's capacity. If the battery was a 600mAh one, then you didn't have another flight in it. You were lucky to quit when you did. If the battery was an 800hAh one, then you had a couple more flights in it.

When you can afford it, a charger/cycler like the Accu-Cycle Elite is worth having. It can do a lot of things that're worth doing, but it's really big value comes from something it does every recharge. It tells you how many mAh it had to put back into the battery. With that info, you can figure how to use that battery next time and still be safe. You can also take that charger to the field and keep all your batteries pumped up all day long. Matter of fact theoretically.... with one..... you would never have to come home from the flying field!!!!!! at least to recharge your batteries...... (now, if I could just figure out how to put big lights in my model so I could fly all night long......

The output voltage of the battery pack isn't the only factor you need to take into account when deciding to fly more flights. Using a 4.8v receiver pack as an example look at the attached diagram. A fully charged pack will actually start at between 5-6 volts and when it's put into use the voltage quickly drops to 4.8 volts. It will stay at 4.8 volts for a long period of time, and at the end of that time it will quickly drop to below safe levels. If just using an extended scale voltmeter you have no idea if the 4.8 volt reading you are getting will provide that voltage for another 30 minutes or only for 3. You need to know the capacity of the pack you are using. This is where a battery cycler mentioned by Darock comes into play. What the cycler will do is discharge the pack at a set level and tell you how long of a period of time it takes to discharge this pack. You can now use this time in conjunction with your ESV at the field to find out if you have enough left in the pack to fly. For example: You test your pack and know that it has a capacity of 30 minutes. You put the ESV on the pack at the field and find that it reads 4.8 volts. You've already flown for 10 minutes. So you now know that you have 20 minutes left to fly, so it's safe to put your bird in the air.

If you don't have a battery cycler you can still get a reading out of your pack. First, make sure you're not anywhere withing 3 miles of a flying field so you don't shoot somebody down. With a full charged pack in the plane sit down in your living room with the plane and your transmitter. Turn on the transmitter and receiver, and start timing. Now continually move the sticks on your radio so that the servos are moving in the plane. Keep this up until the servos quit responding. This will give you a pretty good idea of how long your pack is good for.

Using a battery cycler is a good tool for catching a pack that is going bad. If you have a known pack that tested with a capacity of 30 minutes and it starts testing at lower times, then it's a pretty good indicator that the pack is going bad.

If you want to read up on more info about batteries head on over to [link=http://www.rcbatteryclinic.com]RC Battery Clinic[/link] to read up about batteries. Red is the Battery Guru and you can bank on the information at his site.

Hope this helps

Ken

Great info guys, thanks a bunch. I haven't quite had the cash to spring for a cycler yet, I know its necessary, so its on the list. My Rx pack is 1100mAh, so I figure I should be good for quite a while at the field, I haven't sat down and timed a discharge as you mentioned yet Ken, I may do that.

Thanks,
Graham

I use onboard monitor on my airplane http://www.hobbiesr.com/hca/hca-144/hcap0332.htm
I can tell at a glance if it is safe to fly.
I do use a cycler and as far as I can tell I am using up about 80 to 90 mah per 10 minute flight, flying simple circuits and fingure 8s at half throttle, with 5 standard Futaba BB servos.

I would recommend that after 4-5 flights just put your RX pack on charge and charge it at the field.
Since you have a field charger, no need on testing when you are low. Just recharge.