Nimh Mythbusters test

[Lee Smalley]

Hi guys i have read lots of stuff about Nimh and the problems that some people think they have, some people have done testing but they seem to fall down in one area or another one was using old packs and another was using 2amp discharge and then wondering why the voltage sat down so much! so I have decided to sort this out once and for all!!! and so this is what i propose to do.

I have 3 packs all brand new

1 sanyo eneloop 5 cell

1 vapex instant   5 cell

1 JR supplied pack  4 cell

 

1st test charge holding ability

discharge all and then recharge using same charger all charging at 0.5C

hold in the boot of the car for 1 week bring back up to 20 deg c and check capacity at a discharge rate of 600mah

 

i am using 600 as it really  is the max  that the average sport flyer will see

 

2nd test voltage level under load at 0 and then 20 deg c

again the packs will be cycled and allowed to rest for an hour before going into a chamber held at the test temperature whilst being subjected to a constant load of 600mah the voltage of the packs will be logged every minute for 3 hours

 

before you lot ask i operate a calibration lab and have the use of an enviromental chamber to carry out these tests the voltage will be logged with an 5 1/2 digit agilent datalogger, i have limited time i can do these test (as i am at work at the time) but if you would like any other tests carrying out them post them here and i will see what i can do give me a few days to get the discharge boards built and tested and then we will make a start, in the meantime any questions fire away!!

Edited By Lee Smalley on 07/12/2010 14:04:46

[Doug Ireland]

Sorry Lee but I don't see the point in these tests other than for purely academic interest. Most people just recharge battery packs prior to use anyway and are not that interested in their ability to hold a charge over a certain period of time.

[sparks59]

Whilst Doug makes a, no doubt correct, observation of what most people do, I do think your proposed tests have value. At least we will all know the truth about these batteries in certain sets of circumstances.

I'm looking forward to seeing your results Lee.

sparks

[James40]

Go for, it's a great idea. I think the one thing RCM&E sadly lacks is back to back testing of products so the consumer knows what's best for their money.

[Bruce Richards]

I think these test will be very useful. People recharge their batteries before each use because that is what they have always done not because it is necessarily  the best way to do it. More information on the subject will be most useful.I have to say that the Instant Cells in my Dx6i have on ly ever been charged 3 times in the year I have had it and even then they did not take a any where near a full charge each time.

Edited By Bruce Richards - Moderator on 08/12/2010 07:46:48

[r6dan]

I think it is a really good idea go for it,maybe some lipo testing in the future?

[Brian Parker]

Just use Sanyo Eneloops and charge every other full moon.
Seriously, It might prove informative if you could include a budget supermarket ‘4 pack’ for comparison.

[Stephen Grigg]

Sorry to sound nieve Lee are these batteries  RX batteries or power to motor batteries please ?

[Ian Jones]

Do it Lee.

 

When I arrive at the field with a fully charged battery pack  I want to know  that when I land I will stil 95% or more capacity left. If I make two flights and the capacity is down to around 50% then the pack goes in the bin. Hardly scientific but it keeps me out of trouble.

 

The ability of a battery pack to hold it's charge can be the diffrence between a landing & a crash. I check my flight pack at least every other flight so that I can monitor & get to know how a pack is behaving and it would be useful have something to measure against.

 

Then there is storage of course. A pack unused for 6 months - will it be dead or just in need of charge? I've found them quite unpredicable. What should I be expecting?

 

Your results will make intersting reading.

[Allan Bowker]

Any test is worthwhile Lee, even if we think we know what the result will be.

Your work should confirm or prove otherwise.

 

[Biggles' Elder Brother - Moderator]

Experiments are always good Lee - go for it. I'm predicting a set of very positive results.

 

Only one observation, 600mA is still a discharge rate way more than these batterries will see in service in a sports model. I seriuously doubt they would see that rate of discharge even momentarially - let alone for 3 hours!

 

BEB

[Myron Beaumont]

I have several Rx packs that I've had for about   18 months (Instant 2100mAh ones) Three times I've checked their voltage over that period of time which only dropped about 0.2 volts.every 6 months . The second time I checked them ,not knowing what to expect , I recharged for a short while to "Top them up". Although never flown I worry a bit that maybe I'm using the wrong technique ie NiCds ritual having read somewhere that they don't like a top up or that maybe they should be flattened every now and then & then re-charged at 1C .Complicated isn't it ? Queries abound don't they ? Just who do you believe? Carry on the tests Lee !

Myron (trying to get ready to move house )

[Lee Smalley]

Totally agree BEB 600mah is too much really but there are people out there who claim to be pulling AMPS!!! yeah right, i have decided on this as its about in the middle of what people are claiming from 100mAh to 2 amps and its the load using the components i have here to make the constant current dump boards up with

 

i have included the 4 cell pack from JR as this is the nearest i can get to std Nimh packs they do not claim to be LSD cells !!

 

Doug the reason for the test are because people are still repeating the old myths about the poor old Nimh cells and this is based on very old data, and lets face it if the data goes the way i think it will you no longer will have to top up before flying (providing you use LSD cells)

[Lee Smalley]

sorry Mr Gigg they are Rx batts only

[Lee Smalley]

the test equipment i will be using

environmental chamber

agilent voltage logger

[Lee Smalley]

ohh and everthing is calibrated as well i will also be calibrating the current dump boards when i have built them

[Peter Beeney]

   Lee,
          One additional test you might consider including is a pack of Vapex VAP-ACC-AA2900 2900 mAh AA cells. These seem to be available from a number of sources, from Play.com at £10.99, free delivery, or Amazon, £2.99 + £4.99 delivery via Component Shop, or £2.99 delivery via Digital Additions. Thus in this respect they perhaps come in the ‘budget’ bracket, although of course they don’t claim to be low self-discharge cells.
There was a very comprehensive and high performance specification published on these cells, which strangely, or perhaps significantly, now seems to have disappeared. The nearest I could find was at Firesupport, £7, where they advertise a 3C discharge rate plus a one hour charge rate. Also, and I quote, a high shelf life (80% initial Capacity storage). Whatever that means, exactly?
   I did a run on a pack of 4, from idle curiosity. As I’ve been tinkering with batteries for fifty years plus I felt this seemed to be a little bit too good to be true and thus it proved to be. Big time! The capacity proved to be 1800 mAh, and all the high rate tests collapsed the moment I tried to start, i.e. the voltage fell below one volt per cell, 4 volts.
   The capacity test that I did was the standard one that charges to 150% of capacity and then discharges at one tenth C, down to 4V/cell; if it doesn’t pass this it won’t pass anything!
   So I discharged the cells until flat then charged at 0.1C, 290 mA, for 15 hours, left for 24 hours, and then discharged at 300 mA until the pack reached 4 volts. Which it did at 1800 mAh.
   Discharging straightaway after charging made no difference. The 3C discharge rate was in the original spec., in theory it should discharge for 20 minutes at 8.7A down to 4 volts, in my case it was probably less than 20 milliseconds.
   Having said all the above, as I’ve said before in these threads, this would still fly a model as a rx battery, all day, simply because the power demand is so light. It’s just that it’s nowhere near the super battery it claims to be.
   A comparison check would be very useful indeed, as I tend to think this is not a rogue pack, another set of figures would at least make that a little clearer.

   Regarding the low discharge aspect, again, as I’ve said before, I’ve run a little check for more than six years on a standard Ni-MH. This doesn’t seem to lose capacity over time in a very short time, so I don’t have any issues with low shelf life.

   I agree with you, I use all nimhs, or nicads, they are all old, they simply refuse to wear out! But there are faulty cells in some pilots packs occasionally, and I’ve always said a regular check-up does help to catch these before they are able to cause any grief.

   Good Luck with your testing.

   PB         

                                

[Biggles' Elder Brother - Moderator]

Looking at that environmental chamber and the data loggers you really do mean to nail this one - once and for all don't you

 

I assumed you were going to stick them in the fridge at home! (Or even on an external window ledge in this weather!)

 

Go for it man - the applicance of science!

 

BEB

 

PS Glad to see your employer is right behind you on this one Lee

[Stephen Grigg]

Ive a 5 cell pack in my Harmon Rocket thats been chaged only twice this year and the Rocket gets used the most,When I put the battery check on it its never been below 99%

[Ian Jones]

Oops! I found that with one pack - then I changed the settings on the battery checker to 5cell/6volts. Fortunately I realised before any critical loss of capacity occurred.

[Lee Smalley]

For gods sake don't tell the boss ! Peter I am not going to test those cells 2900mAh as they are not used for rx packs I will be keeping the test to proper rx packs pre made I only have a limited time to run the test before my boss finds out I suspect there is a reason why these cells are not used for rx packs

[Stephen Grigg]

Mine is one of those that checks Lipos and everything Ian but Ill check it out.thanks for the direction,Lee ,Behind you

[tom clarke]

hi people.. im doing my first large 3d model and need help with batteries, i have two identical 6v 3300 nimh packs to go into the plane, can i have one as a back up or how do i run both packs safely? (plane is 50cc sbach with 5 savox 20kg digis hence the need for lots of battery power) i know lipo is the way to go but never had a problem with nimh and the plane balances with both packs upfront.

[Peter Beeney]

Tom - I believe you can connect these batteries in a small variety of ways and at the end of the day you would not be able to tell very much difference. The one common area is that they are all generally connected in parallel.

There is a black box connection, which is basically a change over switch, one battery is on standby, whilst the other in in circuit, and should the first fail it’s switched out and the second is is connected in. I have a feeling I’ve seen this as a Multiplex ad, but I could be well wrong. Don’t know the cost, by I’d think relatively expensive.

Then you can get a Power Box, this simply connects the two packs in parallel, with a diode in each leg, this to prevent one battery discharging into the other but you get some extra stuff, like servo signal amplification. Again, I don’t know the cost, but I do know, from doing some previous checks, on something that is very similar, under certain conditions that it was causing a quite severe volt drop across the circuit, the pilot, a Scale man, threw it out and decided he would just use one battery for half of his flying day, then change and use the second battery for the other half.

Or you can simply put a forward biased diode in one leg of each battery, in the receiver side of each switch, and connect them in parallel anyway. Use a Schottky diode, they have a forward volt drop of about 0.2 - 0.4 volt, but as the current goes up the volt drop can become greater, for instance, the MBR745 7.5A Schottky diode has a maximum volt drop of 840 mV, or 0.84 of a volt.

The way I would do it is to just forget all the add-ons, simply add a switch to each battery and connect the two outputs into a Y lead and then into the receiver. Then you can charge each battery independently, you can also test each battery independently by simply switching one at the time; very importantly, as far as I’m concerned anyhow, you can leave everything in situ, there is no disconnecting and reconnecting. … I always try to remember the slightly mis-quoted Occam’s Razor, as in KISS, or Keep It Strictly Simple!

I personally do not think it’s a good idea to fiddle about with the connections of two batteries; if you did inadvertently manage to somehow get a cross connection it could be the equivalent of putting a full short across a 12 volt battery! If we then say the total impedance (resistance) is 0.12 of an ohm, then the the current will be 12 divided by 0.12, that’s 100 amps! In a flash there would be liquid insulation from the wiring dripping all over the place, and nicely alight at the same time…… This had definitely happened!!

If you charge or discharge batteries, two or more, in parallel, the voltage will always go up and down together, it can never vary. I’ve never ever seen any evidence that batteries go short circuit, they always go high resistance. If they did go short circuit I think the model would soon be alight, as above. I’ve seen more significance circuits than those in model airplanes, such as emergency equipment, that don’t have complicated parallel circuits; and where there are two parallel batteries for increased capacity, they never have a diode in circuit, at least not for the short circuit theory reason anyway.

However, these circuits do have their batteries checked on a regular basis, and the voltage will be constantly monitored, so with that in mind I would make sure that I cast an eye over the capacity regularly, once or twice a year, and I’d also install an on-board voltage monitor, the GWS one is very good. But I’d need to check that it was switched to the correct scale first, recently a club member threw a perfectly good battery away, his monitor was switched to the wrong cell count, and he blamed the battery! Others have also nearly done the same thing. Always check your instruments first. In fact, I’d say that it might be a good idea to get a volt meter and kick this stuff around a bit, try and get familiar with it; you can never really have too much information.

With a total of 6.6Ah capacity I guess you won’t have worry too much about a shortage of watts to drive the radio - at least for the first couple of days flying…

As always, this would be my personal choice, it’s not meant to be instructions on how it should be done. Like many other situations, it’s very flexible and can be done a number of ways, and until something goes wrong it never really makes any difference at all.

Good Luck!

PB

[tom clarke]

thanks peter,will try and get the wiring sorted out as i hope to get this plane out for the weekend so if needs be i may just fly on one 6v 3300mah battery then do check and charge between flights but long term i want the peace of mind that extra power will give.