NiMH charge rates

[Geoff Daunt]

Hi,

Just decided to change the Rx Battery in my 1/4 scale piper cub.

The old battery was not holding its charge.

The new one is an Eneloop,4.8, 2000MaH (although the battery was bought about 9 mnths ago)

Not knowing what was left in the battery l put on Discharge @ 0.5A to 4.00 V.

It took out about 1300A.

So...onto charge at 0.2A overnight.

In the morning it was at 1950.....and still had not terminated it's charge so l left it on and got on with some gardening.

When l remembered to go back and check it was lunchtime, and it was still going and showing an input of 3000MaH.

I pulled the plug. !!!!!

Testing the V with a meter shows 5.4V. The battery was not hot just very luke warm.

Can this be correct to have put in so much and will it have damaged the battery?

My charger is a FUSION Elysium LX 60B Pro, and has not given any past problems.

[PatMc]

Geoff, the charge rate you used was almost certainly too low for the charger to recognise the delta peak. I very much doubt that such a low rate would damage the battery but it's not advisable to regularly "trickle charge" Nimhs. Probably best in future to charge at around 1A & keep an check that it terminates within a reasonable time.

 

Here's a link to the Sanyo data sheets that shows they can be charged at 2A but the wire & plug on the model may not be capable of taking this current.

Edited By PatMc on 14/11/2011 15:24:27

[Martin Harris - Moderator]

It won't have done it any good but it depends how hot it was before you noticed how much it had taken. This type of incident on charge has happened to me - as has the more common false peaking (with a variety of chargers) and illustrates why I am very wary of NIMH batteries. Others will assure you that they are wonderful and have never had the slightest problem...

 

Also, be aware that there have been reported incidents of mechanical failure of the wiring on some Eneloop packs recently.

 

Check out the recent A123 cell and battery related threads for more thoughts.

 

Have you considered a dual battery arrangement - a 1/4 scale Cub may be a relatively staid beast but it would still be a bit of a lump to fall on anyone's head from a battery failure when the fail-safe wouldn't be active...mine weighs around 20 lbs with an ASP 160 twin whirling the 18 x 6 meat cleaver at the front - although I haven't skimped on the scale details so yours may be somewhat lighter.

Edited By Martin Harris on 14/11/2011 15:22:14

[Peter Beeney]

 

 

Geoff, - Quite by chance I’d say you’d actually got this about right. This might be classed as trickle charging, although strictly speaking trickle charging would be at a much lower rate. When you charge nickel cells any overcharging, i.e. any charge over and above a full capacity charge is turned into heat. It is generally considered that any charge rate at or below 0.1C, or C/10, is low enough to dissipate any heat as it is generated, thus if you you overcharge it still further no great harm will be done. This is the the reason your pack only felt warm as opposed to hot.

One very typical rule of thumb rate relating to slow charging NiMHs is to charge at 0.1C for 150% of the capacity. So you might notice the instructions on a 2000 mAh pack, charge at 200 mA for 14 - 16 hours.

So it would seen as though that’s exactly what you’ve done. 2000 by 150% = 3000 mAh. Or, of course, 200 by 15 hours = 3000 mAh, or from overnight to the following lunchtime. Had you carried on for a while it would not have got any hotter, and they can withstand a degree of overcharging. If they are charged at a higher rate the heat then cannot be dissipated fast enough which soon leads to overheating problems.
I’m not familiar with your charger, and it’s very possible that it might not be able to detect the slight (if any!) ‘delta peak’ when charging at this low rate. Or it may not have quite got there, although I suspect that’s unlikely.

I’ve just checked a Vapex low self-discharge 2100 5 cell pack, the instructions say ‘Standard charge 210 mAh 14 hours. Max charge 1000 mA 2.5 hours’. How many people have a charger that can deliver a rate of 210 mA? I suspect 200 mA is the nearest. Tx and rx batteries have been charged by plug in wall socket 0.1C constant current type chargers for many years without suffering any ill effects.

So, my view would be is that if you are charging that pack at 200 mA’s then you don’t have to be to concerned, stop when they start to feel warm. When I was flying powered gliders I used to charge 7 cell flight packs straight off a 12 volt car battery and just take them off when they started to get hot.

Whenever I’m checking out any cells I always give a full charge at 0.1C for 15 hours, quite often I’m playing with faulty packs and they can sometimes seriously confuse fast charging regimes, so at least I know that I’ve put the correct amount of charge in, even if sometimes I get hardly anything out.

Just my view, which is all of little consequence anyway, and as ever, it’s not intended to be any advice or instructions.

PB

[Codename-John]

on my charger my eneloop packs charge at 6.13V and when fully charged read 5.8V on a futaba br-2000 battery checker, ive had one of futaba`s own rx pack the 600mAh ones take 1300 mA when its been cycled down to empty and charged back up, wasnt warm or anything and always been fine

 

 

[Geoff Daunt]

Hi All,

Many thanks for the replies........which has helped me understand a bit more.

Normally l would charge at about 1/2 rate, but l read on one of the blogs from our Administrator that it is prudent to charge "new" batteries at 1/10 rate for the first 2/3 charges to get all the cells working at thier best..

Next charge l will keep ane eye on input and terminate at 2000

Geoff.

[Martin Harris - Moderator]

Geoff,

 

Contrary to my earlier post, when I re-read it in the light of Peter's post and realised I hadn't digested it properly, you don't appear to have put your cells at any risk by charging them as you did and as he says, you hit the numbers spot on!.

 

What might happen, though, is that another pack may actually terminate early should a peak be detected and not not end up fully formed. The theory behind the forming charge is to ensure all the cells reach a fully charged condition so that they should discharge to the same levels in use. Then, when delta peak charging, the cells should all reach full charge together.

 

The danger is of an early trip on one cell leading to progressive imbalance on susequent charges. if you simply terminate at 2000 mAH then if one or more cells are above the nominal capacity, not all cells may have reached full charge.

 

I built a battery charger from a Maplin kit many years ago which I set up to select from a range of charge rates with a 14 hour timer which I use for forming charges of new packs. Using a dumb charger such as this or a (preferably) higher rated wall charger to charge at C/10 or a little less for a proportionately extended time forces the charge to go on long enough to ensure all the cells are fully charged.

Edited By Martin Harris on 15/11/2011 12:49:18

[Geoff Daunt]

Thanks Martin,

Ok so when l next charge, again at 200Mah, when do l pull the plug , if still goes upto say 3000 and Delta peak has not kicked in to term. the charge.? As l said erarlier when l terminated the last one the battery was only mldly warm and not at all hot, but when would it have stopped if at all.!!!!

If l understand the earlier information then the extra input does not go into the battery but is converted into heat.!!,after it has reached peak capacity. Am l correct in this understanding.

Regards, Geoff.

[Ben B]

Does you charger have a capacity limit function (most of them do and they often use the same software for different chargers).

 

Yes, if you charge too slowly you won't get a nice "peak" to your voltage curve so the charge will never terminate (unless you have the capacity limit function on) but the extra current will just turn into heat. Not bad for the cells just a strange way to heat your house

 

Personally I'd just charge it faster so it gets a nice peak and the charger cuts off the charge. Something like C/2 should do!

 

but some people like to do a "forming charge" on a new pack to get maximum capacity. But that generally invovles 16-24 hours on a C/10 rate which sounds like what you did Personally I just cycle them on my charger and see the capacity. If I get a good capacity reading I don't bother mucking around with them.

[Peter Beeney]

Yes indeed, in truth I suspect the 50% overcharge rule stems from the earlier days, of nicads and low rate chargers. Also related to the way in which large capacity backup lead acid batteries used to operate. We first had 500 mAh cells, with a 50 mA output charger, I checked the output on some of these and they were quite accurate, then they increased to 700 mAh cells, with a 70 mA output charger. These were not so good, possibly the same charger, who knows, the output was still only 50 mA or thereabouts, sometimes. So you would have to charge for 14 hours just to put 700 mAh’s in. I only do this 0.1C charge when testing cells, if I stick rigidly to my spec. I get an accurate result on the condition of the pack.

I would experiment a bit. Discharge the pack and charge it at 1 amp. Monitor the charge amount returned back, it should be at least plus 2000 mAh and make sure that the charge terminates with the proper detect. Then discharge, at say 500 mA, down to an end voltage of 1V/cell, very important this, if you had a duff cell in there you could still get a fairly good milliamp reading but the voltage would be well down, and note the amount discharged. If ok, as Ben says, just continue to use! I personally never bother to cycle packs; I do that when I fly ‘em! I might consider that cycling just wears them out a little bit more. Make a point of always charging just after flying, or just before, so that it doesn’t get forgotten. An on board monitor is good, too, I like the GWS version, it has a nice bright amber led in the line up, very eye catching.

Then check the capacity in the same way from time to time, such as when you think about it but at least once a year. If anything looks suspicious, give it a good checkout, you can’t go wrong giving it a standard 0.1C 14 hour charge and test discharge.

Keep on Charging………

PB

[PatMc]

Posted by Geoff Daunt on 15/11/2011 11:17:36:but l read on one of the blogs from our Administrator that it is prudent to charge "new" batteries at 1/10 rate for the first 2/3 charges to get all the cells working at thier best.

This advice isn't really applicable to Eneloop & similar type cells. They come ready charged & in a balanced state so there's no need for a so called "forming" charge.

[FlyinBrian]

Posted by Geoff Daunt on 15/11/2011 13:39:24:

Thanks Martin,

Ok so when l next charge, again at 200Mah, when do l pull the plug , if still goes upto say 3000 and Delta peak has not kicked in to term. the charge.? As l said erarlier when l terminated the last one the battery was only mldly warm and not at all hot, but when would it have stopped if at all.!!!!

If l understand the earlier information then the extra input does not go into the battery but is converted into heat.!!,after it has reached peak capacity. Am l correct in this understanding.

Regards, Geoff.

 

How about plugging your charger into a timer which will stop the charge after whatever period you set. In the example above set the timer for 11 hours (assuming you know the pack is virtually empty to begin with.)