Problem charging 6V 2300mAh NiMH receiver battery for the first time!

[JohnnyB]

 

Hi everyone,

I have just bought a new Overlander 6V 2300mAh receiver battery. On the label it says - "Standard charge 250mA for 10hrs'.

OK, I thought, I'll just use my old Futaba Tx/Rx wall charger just as I used to do with my old 4.8V receiver batteries. This states an output of only 4.8V and 70mA.

I connected it all up and indeed the little red light came on to indicate charging. Then when I went back to check after an hour, the red light had dimmed. I checked the connection and no difference. So I left it and checked again after about 90mins. By now the light had almost disappeared! This is a new battery so it shouldn't be fully charged after such a relatively short charging period surely.

So, after pondering for a while I decided to use my faithful Graupner Ultramat 16 charger that does have a NiMh charging program. I decided to use the manual programme as I do not want the delta cut off to cut in. I set the charge rate to 0.3A and plan to leave for about 10hrs. I will keep checking the battery for temperature (feeling by hand).

The battery has now been charging for 25mins and it has 115mA(?) put in as indicated but the voltage is indicated as 7.17V(!) Isn't this voltage already getting too high considering it's supposed to be on charge for 10hrs ..... !!! The battery is cold to the touch.

I must say I have very little faith in this battery at present, especially as I intend to use it in my model on Thursday. How can I be absolutely confident that once in the air this battery will not let me down, especially after a few flights? I do have a voltage checker showing red, amber and green when system is switched on in my model. Would it be safer to use a 4.8V NiMh 1200mAh battery that I can charge on my Futaba wall charger for the required time. My model has 4 servo's and my radio is 2.4 Spektrum.

Also, I have read somewhere that new NiMh batteries should be 'cycled' after being charged for the first time.

John.

Edited By JohnnyB on 17/05/2016 10:57:15

[Masher]

I think all is well JB

Nominal voltage is 1.25V per cell and off load this will be higher so 7.17V is as expected at this stage.

For belt and braces checking of new packs (having had failures in the past) I fully charge them and then connect up an old car sidelight bulb which, in my case draws a couple of amps. I can visually check that (a) the pack can provide a decent current and (b) that it has about the right duration - depends on your bulb & battery combination but I also check with DVM too.

Finally, if all is well, don't forget to re-charge!

[Hangar 9 nut]

Johnny a fully charged 6v battery should peak at about 8v .3 is a good charge rate for the first charge !

[Cuban8]

All is as expected from your description. The Futaba charger is designed for a four cell pack, so its not surprising the check led was dimming after a short time and only charging at a very low rate.

Stick with the Ultramat at 300mA and all will be well. I'm not familiar with your Graupner charger, but if it has a discharge facility, use it to cycle the pack occasionally to check for capacity/weak cells. Just my preference, but avoid fast charging if at all possible.

[JohnnyB]

Hi Everyone,

Thank you Masher, Hangar9 and Cuban8 for your valued replies. I really am fully appreciative of your experience and advice.

After just over 3hrs charging at 0.3A the charging programme stopped having only charged to 773mA. ( it has a capacity of 2300mA's) Would this be because of the delta peak intervening. Surely the battery will not be fully charged? Would you suggest I begin another charge attempt at the same rate of 0.3mA and see if it continues to charge or would it be better to cycle the battery down and start again?

What is the best way to check that a NiMh battery is fully charged? (or not!!!!!).

John.

[Peter Beeney]

JB,

I’m sure your battery will ultimately be absolutely fine, but if it were mine I’d consider that it was imperative that I test discharged it first. I’m sure your Ultramat will do this ok. This has been an ongoing problem for many years now, I’ve posted about it in various threads in the past and I’ve also written factual letters to a variety of suppliers and the BMFA; needless to say, I’m still waiting for an answer… Although, again as I’ve said before, an answer would prove that they’d actually received it and then in the event of an incident they might be asked some awkward questions.

From my deductions over the years I think that when these new packs hang around for a while they get a bit lethargic and the internal resistance goes up a bit; this then becomes a bit of a hurdle for the charger to overcome. Particularly the Futaba wall-socket type. It’s definitely resulted in packs receiving little or no charge at all, thus an instant flat battery and very soon after a splintered model! I know this for at least one fact, it happened to me, I was test flying an immaculate brand new Flair Magnatilla for a club member, he’d assured me at least three times that the battery had been on charge all night, and when after about five minutes flying it was lying in ruins he told me it was a brand new pack!

When it comes to power supplies I now never ever assume anything…

Discharge your pack, at about 1 amp, say, to save time, ignore this capacity reading because it might be quite low, recharge, discharge and repeat until you are getting at least close to 2300Ah; it will get there. It’s possible to find a duff new park, I’ve seen about 3 in a lifetimes flying, but it is rare.

I always used homemade nickel packs, (although now LiFe) for years, 5 cell Uniross 800mAh, for lightness. I charged them using a Schulze charger on auto setting; this took a half an hour from flat to full, the charge rate would start slowly, increase to up to several amps and then taper off. The pack never even got warm. They lasted for years doing it like this.

For safety reasons probably the most positive recommendation in our club is the use of an on-board battery voltage monitor. I’ve checked a number of these and they are surprisingly accurate. Modern kit is easy to make cheap and precise! The use of these has stopped two certain crashes in just our small gathering, once particularly when someone spotted a glowing red led deep in the cockpit of a large Pitts just as the pilot was starting the engine!!

Good maintenance and regular checking will ensure trouble free power supplies. Without power everything else is just useless…

Once your pack gets in the swing of things the wall-socket will charge it, albeit slowly. On 5 cells the output will probably be less than 50mA, I’ve tried it, so a standard charge, 150% of capacity, will be 3450 divided by 50 = 69 hours. You could leave it on indefinitely. The voltage comes up straightaway, which is why the led dims and never try to check the state of a nickel pack by simply reading just the open circuit voltage. It could be a disaster…

Good Luck.

PS Addendum   JB,  One way to check the pack is fully charged is to discharge it and make sure you put virtually 2300mAh back in. But the on board monitor does run it a very close second…    

Your charger will discharge a NiMh down to 1 volt per cell - 5 volts. Don’t worry about any individual cells, the charging regime sorts all that out. For nickel, that is.   

PB

 

Edited By Peter Beeney on 17/05/2016 14:09:12

Edited By Peter Beeney on 17/05/2016 14:15:42

Edited By Peter Beeney on 17/05/2016 14:21:38

[JohnnyB]

Thank you Peter, that's a great reply.

What individual cell voltage do you suggest I discharge down to before recharging again?

John.

[Peter Beeney]

John,

Don’t worry about running these cell down to flat. They are stored for longer periods in a fully discharged state, often with a short across the terminals to stop any local action. But as I’m sure you can appreciate they might then need a good kick-start to get going again. Generally speaking, if a nickel cell is good it’s very difficult indeed to break it; and if it’s bad it’s very unlikely, (impossible) that you’d be able to repair it…

PB

[Denis Watkins]

Hi JohnnyB, your battery is fine. Just follow the advice and charge it at 1/10th C, that is 250 - 300 mAh, as advised. Can I simplify the problem in ballpark substitution. Your wall charger is a man with one cubic foot of air capacity lungs trying to blow up a 35 cubic foot of air balloon. Although the man may partially inflate the balloon, it will never inflate to full pressure. Your wall charger is fine for 600 - 700 mAh cells, but that charger will make no impression on cells your size.

[Frank Skilbeck]

Posted by JohnnyB on 17/05/2016 14:00:15:

After just over 3hrs charging at 0.3A the charging programme stopped having only charged to 773mA. ( it has a capacity of 2300mA's) Would this be because of the delta peak intervening.

John.

Check your charger doesn't have safety timer cut out, one of mine does and the default is 180mins.

[John Privett]

Posted by JohnnyB on 17/05/2016 14:00:15:

After just over 3hrs charging at 0.3A the charging programme stopped having only charged to 773mA. ( it has a capacity of 2300mA's) Would this be because of the delta peak intervening. Surely the battery will not be fully charged? Would you suggest I begin another charge attempt at the same rate of 0.3mA and see if it continues to charge or would it be better to cycle the battery down and start again?

 

Many chargers have a safety cutout after a certain time - 2 hours on my Quattro B6. I believe the Ultramat 16 has one too, and this is probably why it stopped charging after 3 hours. You may be able to change the cutout time, or disable the timer.

[EDIT]  Looks like Frank types faster then I do!

Edited By John Privett on 17/05/2016 19:12:33

[PatMc]

If this is an LSD battery it should already be nearly fully charged when purchased.

[John Lee]

The Ultramat 16 has a 180 min safety timer by default, it can be adjusted or switched off - see page 29 of the manual. Info for discharging or cycling NiMh is on page 34 where Graupner recommends: 'The set final discharge voltage should be around 1.0...1.2v per cell, in order to avoid the pack becoming deep discharged. Deep discharging a pack runs the risk of reversing individual cells.....'

[onetenor]

I have 2 chargers for Nimh /Nicads, Set no of cells push the button and hey presto. One is a hitec with the word Hitec as tge heat sink Barely gets warm. Charged batts unused for 8 yrs

[JohnnyB]

Hi everyone,

Thank you all so much for your replies. Reading them through helps to give me the confidence I need with this new battery. Lipo's are not a problem as I can connect to a battery doctor or similar and see the percentage capacity,individual cell voltage, whether the cells are balanced and even select 'balance' if they are not etc, etc!

It's a different story with this NiMh battery though. I can see how much capacity has gone in and on the first occasion from new after 3hrs at a charge rate of 0.3A it was 773mA. Then I discharged down to 1.2v per cell, the lowest option it would allow in the programme, and then recharged. On this occasion after 3hrs it charged to an indicated capacity of 808mA. The battery capacity is 2300mA!!!

Another factor I haven't mentioned so far is the temperature of the battery. On both occasions, after the 3hr charge sessions the battery was quite warm/hot. If this is the case after just a 3hr session with only an indicated 808mA capacity, how hot will it get to get the full 2300mA charge?

The reason for all this frustration with charging a NiMH battery is that I now have an i/c model whereas until now I have only flown electric. I need the RELIABLE, and have confidence in, this NiMH battery before committing to a take-off. At the moment I just don't have that confidence in the battery at all. I wanted a large capacity battery to last confidently for a good number of flights. Now it's looking as though I might just go with a 4.8v 1200mA NiMH battery but then I run the risk of 'brown outs' as I'm still training. At least I can charge this 4.8v battery overnight without cutting out after 3hrs and the battery doesn't get hot, when using the simple wall charger.

Thank you guy's who pointed out the safety cut out period on the charger. I hadn't realised that, but as John Lee points out, it is in the operating manual!!

John.

[PatMc]

John, the input mAH is not a measure of the battery capacity. You would need to discharge it at approximately the normal in service current down to about 4.5v (under load) to get a rough measure of the capacity - it will never be a definitive figure.
IMO your battery was partialy charged when purchased, in fact probably nearly fully charged if it was an LSD type.

[Frank Skilbeck]

Once you've done the forming charge on a Nimh battery, i.e. the 300ma charge, which would probably never trigger the peak detect, you can then fast charge in future, I typically use around 40-50% of the C rating (say 1A for your battery) which would mean it would charge up with in the safety timer limit.

At 1/10 (10%) C you will not damage the Nimh battery, so I would discharge it down to 4.5v give it three x 3 hr charges at 300 mah and then do a discharge/recharge at say 0.7A discharge and 1A recharge and check the capacity.

BTW I've stopped using AA Nimh batteries and gone to 2S LiFe batteries which are about the same voltage as a 5 cell Nimh on those models.

[simpson j]

Hi my battery's ar all 2500 my charger is a fusion tx & rx all 5 cell Nimhs from Leeds model shop £9.99 spot on