NiMH Batteries

[RobbieB]

John,

Despite NiMH batteries being considered old hat by many (but certainly not me) they are still a very good choice for an onboard power supply - and preferably dual at that.

Charging NiMH's is always best done at 0.1C as it will produce the maximum overall battery life that is possible. The higher the charge rate, and many NiMH's will take considerably higher rates, will cut down their useful life considerably.

New packs will always benefit from a few charge/discharge cycles before going into service as they will not reach their full capacity before they've gone through a few. Be careful not to discharge below 1V per cell, 5V for a five cell pack as below this there is the risk of one or more of the cells going into un-recoverable deep discharge or become reversed polarised with regard to the other cells in the pack. This cut-off voltage is easily set on modern chargers.

[terry white]

Hi lads, I still use NiMH in all of my models.However I personally liked the Ni-cads even better.
I appreciate any one who has had trouble in the past changing to the latest thing to come out of Japan,but I have never had any trouble with the Nickle stuff in the past,and the new generation of low self discharge cells make them even better for our purpose.
I do however look after them well.I run duel battery banks of 5 cells right into the receiver for redundancy, each has a diode in line to stop back feeding. I have a battery checker coupled to the receiver to give a glancing check whilst the bats are under the exact load of usage.
I check for black wire syndrome quite often,certainly at the start of each flying season.I have found some in the past and I am now extremely wary of this phenomenon.
So I would say go ahead John and use the NiMHs.Take Robbie's charging advise and my B.W.S experience and go fly.

Regards Terry

[RobbieB]

John,

Yes, in parallel but isolated from one another either with a commercial battery backing device or the much simpler and cheaper two diode system, the schematic for which I have attached.

Be aware, using the diode system a 5 cell pack is mandatory as there is a small voltage drop across the diodes dependent upon the load.

battery_backer.pdf

(45.39 KiB) Downloaded 185 times

[FrankS]

The only thing I'd add is to watch the high capacity AA size cells, I have some 5 cell 2500 mah packs that are fine at 200ma, but ask them to give 1 amp and the voltage drops dramatically. So I stick with Sub C Nimh cells in 1/4 scale gliders.

[terry white]

Hi John,
For those new to Black wire syndrome:-

'Black wire syndrome' is inspected on the black power cable (negative) very close to the battery. If necessary,cutting a very small nick into the outer insulation covering down to the copper conductor inside the cable.
The copper should be clean and bright like one would expect inside a new cable.However if the dreaded B.W.S. has visited, the copper will be dull, then later in time, black and weak. Once inspected at this point there is no need to go further along the cable. Further inspections later can be carried out at this point.

If you find B.W.S don't fly the model again until you have renewed the cable.

As the B.W. becomes worse the ability for it to pass current becomes less, until it wont pass current at all and the copper conductor will just fall apart in your hand.This can happen over a surprising short time, certainly a winter lay up, so a regular check is a good idea.
Do not leave a battery connected to the receiver over a winter lay up as the dreaded B.W.S can travel through the copper conductor into the receiver and do it a lot of no good. Best to take the battery out of the model and store it in a warm dry environment at a storage charge.
There has been a lot of discussions on exactly what causes B.W.S and I am not sure what the latest thoughts are, but what I do know is storing a model with the battery connected in a damp environment seems to attract it. Perhaps Peter B can help here?

Manufacturers of servo's and extension leads etc, say that their mechanical crimp of the terminals ends are far better then a soldered joint because of it being more flexible at the immediate joint position, and I have never had any trouble here it must be said.

However I would always suggest that good quality electrical components be used which are well checked out with a multi meter while building the model and fitting out. I have never crimped the ends into a termination plug preferring to join mid cable where mechanical damage is less likely to occur and heat shrinking covering over the joint, so I am not sure on the quality one can achieve with self crimping to advise.

But if at all in doubt with any electronic component or your ability, bin it. It is not worth loosing a good model with all that work you invested in over some small inexpensive component. Hope this helps

Has anybody else had B.W.S. I believe we would all like to hear your story. Regards Terry.

[jimbo]

I also use NiMH extensively. Also newer li ion. I won't touch lipos unless i have no choice, after experiencing a lipo fire first hand. I have had b.w.s but never on a new battery. I think it must be caused by the battery breaking up over timer and the chemicals starting to travel. If you change the battery once you start to lose holding mah under charging you should be OK.
Terry, where do you source your diodes from? For say an alpina size model with two 5 cell such c packs? I Assume you solder the diodes in line with the cable?
Cheers,

[Simon WS]

Terry I thought BWS was only a problem on Nicads, but without direct experience I am relying on the dreaded T'internet!...

Si

[RobbieB]

Interesting point Simon makes regarding nicads.

I have suffered BWS on a couple of occasions in the past but many, many years ago and both times on nicads but never since.

Thing is, three things have changed since then. I am now using NiMH cells, packs are never left (or charged) in models, and both models and packs are now stored in warm, dry conditions.

As a further point, during many years involvement with low voltage data and signal transmission cables, BWS was not uncommon in negative rail voltage cables but only ever where those cable junctions were exposed to high humidity conditions, warm or cold - every time located some distance from the power source - which were not battery packs.

[terry white]

Hi Simon,
As Robbie said you have brought up an interesting point. Of all the times I have suffered or known friends to suffer B.W.S I can now only remember it being on Ni Cads. Unless it is a chemical thing as Jimbo suggests I can not see why this should be. Perhaps if we new without a doubt why it happens in the first place we would have the answer. Any takers? Peter? Chris?

Jimbo, P.M. me your address and I will send you some Diodes. Yes in-line and heat shrink over the whole thing, best supported to the fus rather then left flapping in the breeze.

Terry.

[Roger-Spragg]

BWS is an interesting subject. I don't think it has anything to do with NiMH batteries. I've experienced BWS in yachts. We get it with Nicad and NiMH and lead acid for that matter, but not with Lipo's. The big difference is that we tend to leave Nicad/NiMH in our models, switching the red lead but leaving the black wiring connected to the battery. Our lipo's are always disconnected. In boats we place sacrificial zinc anodes to protect other metal work from electrolysis. I suspect that we may have a similar process occuring here. The black lead in our models will always be in our normal (damp and mildly acidic/corrosive) atmosphere. We don't see BWS in coils of disconnected wire. Bottom line - if you are worried about BWS and want to use NiMH batteries, remove them like you would a Lipo.

Hope this throws some light on the subject

Roger