Nickel based batteries

[Tim Mackey]

Following another posting on another thread,I have put my moderators hat on and allowed myself to sort of cross post and elaborate a little further on NiMH and Nicd cells, as there are often questions from beginners about using these packs. I have also made this thread a sticky. Feel free however to contribute within the thread.

Like for Like size / weight comparisons, NiMH have much higher capacity than Nicd at least 20 - 30 % in most cases. ALL nickel based batteries should be trickle charged at about 1/10C for the first few cycles to "form" them properly, equalizing cells and compensating for possible inbalance due to differing self discharge rates and possible gravitation during storage. All nixx batteries actually charge better at fast charge rates ( 1C, or above if cells are capable of 1C+) - typically achieving 90% efficiency compared to slow charging ( overnight or approx 1/10C) where around 70% is the norm with a full charge taking more like 14 - 15 hours minimum. "Burp" charging improves things even further, but most simple chargers do not deploy this method, relying instead on "delta peak detection" or NDV ( Negative Delta Voltage ) where the cell exhibits a very slight drop in voltage as it peaks. This is where the difference between hydride and cadmium cells can cause the problems mentioned in a a Spektrum tech bulletin released recently. The typical NDV for cadmium is around 30mv - and most chargers will spot this easily and correctly terminate the charge or switch to trickle mode. However - hydride cells exhibit a much lower NDV...typically around 15 mv or even lower, sometimes only 7 or 8 mv. This very tiny NDV is very hard for the charger to spot, meaning some ( many actually ! ) chargers just carry on pumping in charge, causing the cells to overheat and possibly be destroyed. NiMH cells are particularly vulnerable to excessive heat, leading to poor lifespan and compounding the problem of NDV detection even further. Because they do not tolerate overcharging the charger MUST switch to a very low trickle rate of around .05C and chargers designed for Nicd will not be set that low with the recommended trickle charge for nickel-cadmium being anywhere between 0.05C right up to .1C - thats why you should NOT use a Nicd charger for NiMH batteries. To avoid the dangers of missing the NDV and overcharging your batteries, many chargers have their NDV detection circuitry set to very sensitive, and this can often cause the charger to "false detect" so stopping the charge too early. Result? a partially charged battery which you think is full.

Further result? Well you dont need a further example do you

So.... if using NiMH ( and most of us do these days thanks to the green brigade ) ensure you use a good properly designed charger and preferably use a loaded EVM / indicator regularly to check the charge state of your packs. Simply sticking a DVM across the pack when its not actually connected to anything is pretty much a waste of time due to the flat discharge curves of nixx cells.

[Former Member]

[This posting has been removed]

[Tim Mackey]

I'm sure I don't know what you mean Eric

Ah the beauty of moderator privileges  - permanent post editing facilities

[Myron Beaumont]

Timbo  If I replace my nicads in my Tx's & Rx,s with Nimhs can I leave them on the 60/70/ mAh nicad charger twice as longish without any fear of overcharging them.Sorry about the question but having read so many conflicting statements over 'lecktrickkery I'd like a straight answer from the horses mouth so to speak

Grumpy

[Tim Mackey]

Well you can charge them on that charger, but 60 m/a is too high a current to leave them on for too long - EG: trickle, so best to calculate the time required to charge ( cell capacity / 60m/a + about 10% extra time ) then take them off - nicads dont mind being left on trickle ....but NimH dont like it and can be overcooked.

[Myron Beaumont]

Thanks Timbo  Only obvious problem is How flat should they be (ie voltage off load) before giving them the required amount of input easily calculated plus 10 % is it for luck .Seems like an inexact science thing to me .Would you recommend I go straight for a lipo set up for which I have a balance charger etc.It would appear to be the latest future  anyway .You know I like things right

[Myron Beaumont]

PS  Bet all watching "overlord"

[Tim Mackey]

Ideally they should be at around 1.0V per cell before being charged. The "add 10% or so " time addition is down to losses /efficiencies - it will always require more actual output from the charger than the battery accepts as input in order to "fill the pack".

[Owen Hailey]

Hi Timbo.

I have found that NIMS are very prone to going flat when not in use so all my TX and Rx are now running on LIPS so I do not have to worry about lack of volts, and every time I finish flying I can bring them up to full charge with out any guess work.

peak power Owen.

[seecomber]

        Newcomerrs and beginners at model clubs are still advised by older members to `give the batteries a full charge after every flying session' Assuming the trainee has three sessions of ten minutes each doing circuits and has NiH batteries in both Tx and Rx of 2400MA each is it really desireable to recharge them fully? Having read the above with particular reference to overcharging I wonder if this advice does not date from the days of 600 ma. nicads in both units and is quite simply `out of date' However one hesitates to cotradict experienced flyers who we assume follow their own advice without apparent ill effects to their radio  equipment.Most chargers supplied do not automatically detect the state of charge and are disconnected afer a pre determined time which cannot be calculated with partially discharged batteries.

[Tim Mackey]

Well I think the last paragraph of your post is the key here...if the charger has auto / peak detect algorithms, then charging a partially discharged pack is Ok...if not, then discharge it fully  first

[Tim Mackey]

Your welcome Ron...always nice to know this type of stuff is appreciated

[Myron Beaumont]

Yes  Thanks Timbo  I think what I'll do when I change up to nimh 's is keep  meters in line with the charger s( 70 mA ) .Then keep an eye on voltage as well. With data obtained I can plot out graphs  Time vs current flow vs voltage. T'will take out some of the guess work - especially if I put a load on the batteries at intervals ( say maybe every half hour) to get a "proper " voltage reading ( after of course letting them settle down for 5 minutes) in  the hope of building up a picture of "whats what "!

Any comments welcome Timbo or anyone ( & yes I do have an old clockwork alarm clock to wake up 'er indoors when she does the required night watch ! )

[Tim Mackey]

Crikeys...never thought I would ever here anyone stating that moving to NiMh was changing up!

If I were you...I would just get a half decent charger that is capable of doing all modern chemistry - if you look out in the next RCME special ( due October I think ... ) I have an article in there all about chargers - and why you need one

[Myron Beaumont]

YEE HAH

[Erfolg]

Hmmmmmm

I, like so many other modellers, just keep an eye on the voltage displayed or the needle on the meter.

I then charge for 4-9 hours using the charger supplied with the equipment (Futaba). I have never worried about overcharging. This has been based on a booklet supplied by Ever Ready, which gave a fully reverisible electro-chemical process, at low rates of charging and discharging a battery. I have been quite happy, never worring about peak detection, as i believed that is the mode that my transmitter and charger operated.

My main concern has been,"black wire syndrome" which all my receiver packs seemed to suffer from in the past.

My Futaba Gold, which must be +20 years old still has the original Nicadds in it, and spent about 18 years not in use, in a unheated garage. This killed my (much loved) Sanwa set. Now that I only fly electric, using Lipos only, "black wire" has not crossed my mind at all in the receiver side of the package.

I am now thinking should I worry, or keep going as I have always done 

Erfolg

[Myron Beaumont]

 Erfolg Dont worry .The only thing I know about black wire syndrome is that it's something others have encountered for some reason unbeknown to me        Bit like typhoid !

[Chris Dowsett]

seecomber wrote (see)

        Newcomerrs and beginners at model clubs are still advised by older members to `give the batteries a full charge after every flying session' .... is it really desireable to recharge them fully?

Depends on the make of cell and the rate of charge.  The only way you can tell is to check the specification on the web and assume C/10 charge rate.

 The GP200AFHR, for example, will stand a C/10 charge rate for 1 year. 

[Chris Channon]

Hi all, as regards the " a charger designed for ni-cd should not be used for nimh"

Confused now as the charger supplied with my Futabs Skysport 6 is a 70 x 70 mah output, fitted with ni-cd batteries.

My latest radio a 6ex 2.4 has nimh batteries but supplied with exactly the same charger as supplied with the Skysport set !!!

It must work ok or Futaba would not supply it ?

 

Regards as ever ( but confused )

Chris.

[Myron Beaumont]

Chris

You do mean 70 mA output don't you ?.Although I don't know for sure of course  I would think your packs (Tx & Rx) have 700 mAh capacity . As I mentioned before ,when I went all "Vapexised" (2100 mAh  ) I bought the Fusion £9.99 wall charger with delta peak cut- off on the Rx side of it .Works a treat ! Good value I reckon .

Not like me to be complimentary - Must change my pain killers !(that's better)

[Malcolm Fisher]

I'm using some seven cell NiMH batteries in my two electrically powered planes. I want to know what type of Lithium based battery would make a suitable replacement.

For one model, I think a new ESC would be required as I don't think that the LVC can be adjusted from the setting for the NiMH battery.

Both are "can" motors and one ESC can easily be changed for LiPo operation so that one at least is no real problem. I just need to know whether to use a two or three cell pack to replace the NiMH.

 

Malcolm

[PatMc]

I used 2s lipo as an alternative to 8 nimhs in a vintage model with a geared 480. Never had any problems but I never ran it down to LVC as the duration with lipos was so much longer. I used a bigger prop with the lipos to get the same power with the lower voltage.

I've just upgraded the model to a brushless with 3s lipo - first flight with the new power will be when we have a decent bit of weather.

Edited By PatMc on 13/08/2011 00:44:53

[Malcolm Fisher]

Thanks for that Pat,

What capacity lipo did you use?

One of my NiMH batteries is 3800 mAh and I've only once run that down to LVC as a test to get an idea how long it would last at WOT - not that I fly it that way as a rule.

The others are much smaller in a park flyer type Cub, and these always hit the LVC when I fly it.

Malcolm

 

Edited By Malcolm Fisher on 13/08/2011 19:54:43

[PatMc]

Hi Malcolm, with the 480 brushed motor I was using 2s 2100 rated at 20c. I would have used lower capacity but happened to have them to hand.

 

 

 

 

 

Edited By PatMc on 13/08/2011 20:50:34

[PatMc]

For some reason I was unable to complete the above post & couldn't edit it either.

 

I was going to add - that the 2s lipo battery weighs 120g whereas the 1050mAH 8cell battery it replaced weighed 180g. The model's AUW was just over 1Kg.