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[tigertiger]

If I have a 6v pack the voltage will fall with useage.
At what voltage should they be replaced/recharged? as they are about to give out.

Same question for 4.8v as well.

[sheerline]

Hi Tiger, if they won't make the stated voltage after charging they are probably stuffed but wer'e back to the old question of battery capacity and current drain again here. If your batteries were say..  6v 4.5A and your boat draws say.. 2 Amps then you should get around 2 1/4 hrs out of it on a full charge. If your batteries don't make at least 6 Volts off load after a full charge and your run times are  down then its all downhill for your batteries from here on so it's a good idea to sling 'em overboard and treat yourself to a new set.
Chris

[malcolmfrary]

As a rough rule of thumb, if the fully charged battery is not at least 10% over its nominal voltage, it is no longer a happy bunny.  Doing a few charge/discharge cycles MIGHT frighten it back to life, but it is most likely ready for retirement.
The 10% over is true for most types of battery.

[tigertiger]

Sorry guys

I haven't been too clear here.

I often go sailing all day. I also often use standard batteries.

I don't want to have to come home early, or dispose of batteries that don't need changing.
On the other hand I don't want my sail boat to sail off on its own out of control.

I have a small multimeter and can test the voltages. At what voltages should I consider changing my Rx batteries.

I have seen LED Rx battery testers so that must use a preset ordained voltage when a battery is no longer good.
But shipping out to China would cost more than the gizmo, and seeing as I have a multimeter, I would like to use that.

[sheerline]

Hi Tiger, perhaps you mean  disposable alkaline cells in which case my knowledge of end of use state is somewhat thin. I have just measured an alkaline cell removed from a tx which was almost flat and of course off load voltage is still well up at 1.3V but this is a meaningless reading as the cells should be measured whilst current is being drawn from them. I reckon some of the other chaps have more experience of these but if no definitive answer comes up I reckon you should wait until the red light on your tx comes on and consider the cells practically exhausted. With the tx still switched on, try to get your meter probes across one of the cells, measure the voltage and make a note of it. Personally, I would consider the cells exhausted at this point even though they will still go on running for quite a time but they do actually have little capacity left in them.
In the event of no additional info coming back from the other chaps, I hope this helps.

[malcolmfrary]

If a 1.5 volt cell was down to 1.4 volts, I would not want to send a boat out using it.
A meter will just tell you the voltage under the conditions at the time, it cant tell you directly how much actual power is in there.  In the  elder days, the test on a dry cell was to take three readings, V1 was "as is", just the cell and the meter, V2 was under load, V3 was about thirty seconds after the load was disconnected.  If V2 was too low, reject and replace.  If V3 was not within 0.1 volt of V1, replace.
This checked that there was enough voltage to handle the load, and that the cell could recover from a load. 
If going for an extended sail, I would make sure that I was starting with fresh batteries, and had some spares.  I would hope that the RX batteries would outlast the TX battery, so that when the TX started whimpering about low voltage, I could bring the boat in and change both lots of batteries.

[Stavros]

Mark go out and buy a AA batt tester similar to this

http://cgi.ebay.co.uk/Universal-AAA-AA-C-D-PP3-Battery-Tester-for-Batteries_W0QQitemZ320303756848QQcmdZViewItem?hash=item320303756848&_trkparms=72%3A1301%7C39%3A1%7C66%3A2%7C65%3A12%7C240%3A1318&_trksid=p3286.c0.m14


Stavros

[tigertiger]

Thanks guys.

[victorian]

Assuming we are talking about AA Alkaline cells:

Set your multimeter to Amps, not Volts. You must use at least the 1A range, 10A if it has one. Check the cells individually one at a time, not when in the pack. You should get at least 0.5A out of an AA cell into a current meter which is effectively a short circuit. A new cell will read 1 Amp or more in this situation depending on the meter resistance. If in doubt, check a brand new cell and then the one under test.

Only touch the meter on momentarily otherwise you may damage the meter (typically they have a 10A fuse). This is much easier to do with an analogue meter than a digital one.

There are no golden rules for scrapping Alkaline cells because their discharge curve tails off toward infinity, unlike most rechargeables which just collapse almost without warning, especially Nicads. With Alkalines it's usually safe to carry on until you start to get actual range or behaviour problems because you still have some power available to recover the model. Don't quote me on that!!

However, be sure to throw away any cells giving less than about 0.5A because electrolyte leakage is a big enemy in model applications!