Lithium Battery Watch

Footy Class General Discussion (Footy)
1

Since March 17th, the 2009 USA Footy rules just say “Batteries in the hull” and I have switched to using three Lithium batteries which has me wondering if the Volt Watch 2 might useful in checking these 3 packs? The VW2 has a small switch to select a 6 Volt (not useful here) or 4.8 Volt range.

The “Volt Watch 2” has 7 LEDs to indicate the voltage of a battery pack & is available at most hobby stores for about $10.00. I keep one installed in each of my larger boats & find it very helpful. I also have one with a connector that mates directly to all the battery packs.

Here are the VW2 LED indicators/voltage levels from Left to Right when switch is in the 4.8 V scale position:

Green 5.20 Volts (Safe Range)
Green 5.10 Volts (Safe Range)
Green 5.00 Volts (Safe Range)
Green 4.85 Volts (Safe Range)
Yellow 4.76 Volts (Caution Range)
Yellow 4.70 Volts (Caution Range)
Red 4.69 Volts (Low Range)

I have 4 four triple packs of Energizer “Ultimate Lithium” batteries (2 AA packs & 2 AAA packs) with the following voltages:

AA = 5.20 Volts (Used)
AA = 5.44 Volts (New)
AAA = 5.25 volts (New)
AAA = 5.23 Volts (Used a Little)

Sorry I don’t know the number of hours used on each of the above. I will keep monitoring these packs and let you know if it is useful for monitoring Lithium 3 packs and predicting when they will fail. Right now, all the battery packs indicate the “Full Level” Left LED.

In the meantime, if anyone has a 3 pack that just stopped working, measure the voltage and post it here along with the model # of receiver being used.

I am using Spektrum AR5100e with DX6i in Footy “GoldFoot” and the AR6000 with DX6 in Footy “Half @$$”.

Maybe someone can post a photo of a VW2 for all to see as this post is being sent from a Palm Centro phone. Needless to say, I will be using the pack with the highest voltage for the Sheboygan Footy Fest this Sunday.

2

Frank,

This is an interesting problem, to find a criterion for anticipating the demise of these lithium batteries. Running time is an obvious answer, but useful running time will be strongly dependent on wind strength and sail size. There are several possible measurements:

  1. No load voltage (which you are already getting with your device); this might not change much.

  2. Voltage under load; this can also be measured on the boat, in several conditions (wiggling the sails, wiggling the rudder, or stalling the sail servo). The sail servo will draw about an ampere when stalled, and the voltage drops significantly, and keeps dropping. You probably don’t want to do this for any length of time, for fear of burning out the servo.

Test Results

I have been doing some tests on a used 3-cell AAA battery pack, which has sailed in 3 of our weekly races. The races typically last for less than an hour, so it has less than 3 hours of use. A test box was built, which can be plugged in between the battery connectors. It has test points for a voltmeter, and also contains a series resistor of 0.050 ohms to measure current. There is also a capability to insert a dummy load.

So far, the tests have generated some useful data, but no conclusions, other than that the batteries still have some life remaining. The data so far:

After any heavy load, the unloaded voltage always returns to 5.2V eventually.

The stall current of the Bluebird sail servo is about 1,000 ma.

Wiggling the servos (no other load) can draw up to 200 ma

Static load is about 20 ma.

A load of 20 ohms (about 240 ma) will immediately drop the voltage to about 4.8 V, but it will continue dropping, settling at about 4.2V after 3 minutes of load. I kept the load on for a half hour, with no further change in voltage.

A quick stall test immediately drops the voltage to about 4.3 V, but I haven’t waited for it to stabilize.

I will continus testing this battery until it dies, hopefully getting some results which are more useful. Comparison with a new battery will also be made.

With regard to estimated runing time, it could be as little as an hour under worst case conditions of high wind and big sails, but the servo motor would probably burn out before the battery died, so this is unrealistic.

The data on thge Energizer web site is not directly useful, since it doesn’t show no-load voltage. The most directly useful curve shows a 600 ma constant load; perhaps a load of this size could be correlated to the manufacturers curves, but it would have to be held for a length of time (several minutes) to stabilize. However, I believe that these curves are highly pessimistic.

3

Hi Walt

Excellent analysis. I am using these batteries in a Footy which has a switch on the deck. I remove the boat from the water and turn off the power between each race to extend bqttery life. Under these conditions, it is very difficult and time consumming to keep to keep track of usage time.

I think I will design & build a low power CMOS timer to track actual usage in the boat.

Where did you find manufactuer data on these batteries? Link would be nice.

Hopefully we will get some addition help from forum members as none of us wants a boat to fail in the middle of a race.

4

Guys, use rechargeable NiMh cells. Charge them before race day and have no worries.

I’d rather carry a bit more weight than rack up a DNF in a regatta that I traveled to. Also, it is inconsiderate to make other skippers wait while your boat is retrieved for a foreseeable problem like dead Lithium Energizers.

5

Hi Neil

The top three boats at the Sheboygan Footy Fest used Lithium batteries and the trend will continue until something better comes along. Until then, the best solution is to use technology to predict when to switch to new lithium cells. In this thread, we are looking for ideas that will help predict and prevent Lithium failures.

We have had numerous NiMh battery failures on larger boats at our pond in Tanglewood. That is why I put “Voltage Watch” circuits on each of my larger boats to prevent this problem.

6

Frank,

Battery data is at energizer.com. Click on the “technical info” tab. It will lead you to data on all their battery types.

Data on the NiMH types is also available. The AAA NiMH when fully charged has about the same ma-hr capacity as a fresh Lithium, but is heavier, and you need four instead of three. So the lithiums have a definite advantage if you start a race with fresh batteries.

7

Endless, ongoing and extensive discussions here:
http://www.rcgroups.com/forums/showthread.php?t=966437
http://www.rcgroups.com/forums/showthread.php?t=1044988

8

Rusty Nail -

Thank you for pointing us to this information. It relates to lithium photoflash battery packs, which have a different form factor, but should have the same chemistry. The batteries that are tested in this reference also have about the same mah rating as our AAA lithiums.

The reference recommends testing the batteries with a loaded voltmeter and looking for the change in voltage over 30 seconds. Any change above a few millivolts indicates that the battery will soon go bad.

I made some quick tests on 2 battery packs, using the recommended method, with a 20 ohm load.

First I tested a heavily used (but still presumed good) 3-cell pack, and found a voltage change of 190 millivolts. Then I tried a fresh battery pack, which was used only once, for a half hour, in this morning’s races. It had a voltage change of 330 millivolts in the 30-second test. So the voltage change that I am measuring is much greater than the few millivolts suggested as a criterion in the reference. Also, the heavily used battery looks better than the fresh battery.

It appears likely that these AAA lithiums are not as similar to the photoflash batteries as one might expect. I will make more tests as these batteries approach end of life, which may lead to a useful pass-fail test.

Unfortunately, on a Footy we really need a fairly early predictor of failure, since many of us seal our battery hatches with tape, which can’t be replaced reliably on a wet boat.

9

More good comments Walt.

Let’s continue this thread that focuses on a failure prediction solution for AAA Lithiums until we get a good solution.

Frequently I search the forums for an answer and end up reading forever before an answer is found or not found.

I have a microprocessor design & programming background and it seems possible to monitor and accumulate milliamp hours with a micro. Sort of an integration function. If it says 2500 mah capacity, it should be so.

Wish I had time to develop it. I just returned home to Michigan from Florida to an empty rental house and it has top priority as that is a main source of income. Maybe someone will read this and develop it.

10

More information on the photo-flash vs AAA battery types

The photo-flash batteries use a lithium-manganeses oxide chemistry, and have a natural voltage of about 3V per cell, while the AA and AAA types use a lithium-iron disulfide chemistry, and have a natural voltage of about 1.5 volts. So there is indeed a significant difference between the chemistries of the two types of lithium primary batteries, and the criteria developed for the photo-flash batteries should not be expected to work directly.

I have found some stuff in the Energizer brochure that says the no-load voltage of a fresh cell is 1.80V, and a drop to 1.7V indicates that the battery is about to die (this would be 5.1V on a 3-cell pack). However, the measurement is not valid unless it is truly no-load (disconnected from the boat, and any other electronics, like an indicator, that might draw a small current), and has been allowed plenty of time to recuperate from its previous loading. My testing has found that it can take many hours for this recuperation to occur. Therefore this is not something that can be done by looking at an on-board meter between races in a regatta. However, this does not mean that some other criteria will not be developed.