Nimh discharge/charging question

[Hogster]

Hi Guys, another newbie question.

 

I have a new Overlander RC 6s charger and new 4.8v 2100maH battery. I set up a discharge/charge program with a discharge rate of 1.0A to a min Voltage of 1.0v/cell and a charge rate of 0.5A. After completion the Charger displayed Dchg1 1216mAH and Chg1 0953mAH.

 

Now this is confusing me on 2 counts. 1) it appears that there is more charge taken out than put in. I was expecting them to be almost the same. 2) why is the battery rated at 2100mAh when you can only discharge safely to 1216mAH?

 

Have I missed out something crucial? I must say the manual that came with the charger was not a lot of help and the online manual from Overlander wasn`t much better.

 

Grateful for all advise as usual

[Peter Beeney]

Hogster, With respect, and only if you wish, I can try and take you through this a couple of steps at the time. However, I’m just off flying for the rest of the day now, though, so if this does appeal to you and you’d like to have a little go, in the meantime: -
Take your battery through a couple more charge/discharge cycles, ignore any figures for now, particularly the discharge, just try and make sure that a reasonable charge is put in.

I’ll see what info I can find on your charger, and I’ll get back here later.

Is this worth a try? We can cover a few principles as we go along.

PB

[John Payne]

Peter, I think that Hogster has highlighted something that has confused me in the past with regard to the mAh figures that are shown after discharge/recharge of a receiver battery, especially Nimh batteries. Any info would be useful to me as well.
Cheers, John.

[Hogster]

Yes yes yes Peter. I studied electrickery at college 30 odd years ago so I`m up for a bit of revision. The charger is currently taking the battery through another 2 cycles. It will be interesting to see how different the end result is, if at all.

[Ben B]

Has the charger got a time-out function on it? I couldn't work out why my charger was giving wierd charge/discharge readings until I found that because I was charging slowly the time-out feature (default 120 minutes) was kicking in and stopping the charge / discharge....

Edited By Ben B on 08/06/2011 12:52:56

[Martin Harris - Moderator]

Horrible things, NiMHs...

 

Finicky delta cut offs - better if you're charging at somewhere around the C rate but still not very reliable.

 

They can die with little warning so the smart advice is to ditch them after a couple of seasons even if they seem to be working well!

 

Modern high capacity ones seem to be mechanically frail.

 

Eneloops are reputed to be better but I'm still a little wary.

Edited By Martin Harris on 08/06/2011 13:17:20

[YakMad]

I bought an Overlander 5000mah sub C 4.8 volt pack and had to give it a 4 charge/discharge cycle before it would hold a full charge, this seems to be a common trait with NiMHs that have not been used for a while. I use alot of 2000mah ENELOOPS in smaller models and so far they are performing as advertised, I just wish they would launch a higher capacity ENELOOP battery.

[Lee Smalley]

eneloops and vapex instants have been fine for me for 5 years now Martin your experience is not the norm and is certainly not mine

[Tim Mackey]

I hate em too.

Incidentally, did you do a "forming charge" on the new pack Hogster, this should be done on every new NiMh pack to ensure they get balanced properly before using the normal NDV charge system.

Another thing to check is whether your charger is correctly detecting the small drop off Voltage correctly, or is maybe peaking too early?

[Martin Harris - Moderator]

A (fairly old) Vapex pack let me down (big style!) a few weeks ago but I did say that I've heard that Eneloops may be OK - I've got a set in my transmitter but no experience of airborne packs.

 

My main reservations are over conventional NiMHs - especially 2300 AAs and similar.

[Lee Smalley]

Incidentally, did you do a "forming charge" on the new pack Hogster, this should be done on every new NiMh pack to ensure they get balanced properly before using the normal NDV charge system.

 

 

never done such a thing and never had a problem i have over 20 packs of Nimh and they have never once given me a cause for concern, although they are all newer generation

 

Why would you hate them ?

[Hogster]

Posted by Ben B on 08/06/2011 12:52:26:
Has the charger got a time-out function on it? I couldn't work out why my charger was giving wierd charge/discharge readings until I found that because I was charging slowly the time-out feature (default 120 minutes) was kicking in and stopping the charge / discharge....

Edited By Ben B on 08/06/2011 12:52:56

Yes it does Ben. The manual tells you how to work out the correct time to set. But I cant for the life of me find any info in the manual that tells you how to actually set it!

[Hogster]

Well I now have a "breakdown" error. It appears the unit is faulty. That is very dissapointing. But possibly the source of my problem.

[Peter Beeney]

Starting at Hogster’s O/P, and running through the test procedure, there are some rather obvious points, but I can only say please bear with me for a while. Also, of course, this is entirely my own way of tinkering with this, there are quite a number of different thoughts on this subject. I’ve now got the charger manual on the desktop, but as you say, there seems to be a lot of words but not much practical advise on actual values.

To determine the quality of our battery pack we need to do a test discharge and compare that result with the quantity that is printed on the label. As we can only take out what we’ve put in we need to fully charge first; and because no system is perfect we will inevitably get out slightly less than we put in. So to this end, and because nickel cells will tolerate quite a degree of over-charging we can put in an excess of charge, provided this is kept at a low level, C multiplied by one tenth or less, that’s 0.1, where C is the Capacity of the pack in question.

So in our example, C = 2100 mAh. A reasonable degree of over-charge might be considered somewhere between 40% to 60%, I generally go down the middle road and use 50%. Therefore 2100 + 1050 = 3150 mAh. This is our Capacity plus Over-charge. Now we say 2100 by 0.1 = 210 mAh, this is the charge rate. If we now charge our pack at this rate we will supply 2100 mA in 10 hours. Charging for another 5 hours adds another 50% so if we charge at 210 mA for 15 hours our charging total supply will be 3150 mAh. Once the pack reaches it’s fully charged point, the over-charge is transformed into heat but at this low level it can be safely dissipated. It’s only when the rate goes up a bunch and the heat cannot escape quickly enough that the problems begin. Which can sometimes become a bit noticeable. In the old days the instructions on say a 500 mAh rx nicad pack would say something like ‘Charge at 50mA for 14 - 16 hours! Exactly as the above. 16 hours is often quoted and this the standard I used when I was working.

Now we can see that ideally we need to charge at 210 mA for 15 hours, but you may be able to charge at the nearest round figure, 200 milliamps, or 0.2A, so that would be near enough. It’s a flexible arrangement. So the first thing is to try and sort that out on your charger, which looks to me as though it might be possible.

Discharging - This is just the reverse of charging, of course. In theory, this should be done at the same rate but that gets a bit protracted, so you can speed it up a bit. If you discharged at 500 mA that should take around 4 and a quarter hours. Down to 4 volts that should be 2100 mAh of course. A good result would be very close to this, 5% down, 1995 mAh, is perhaps acceptable, but sometimes the packs are well short, and then you have to decide whether to return it as faulty or just put a new label on it.

In a nutshell - Discharge the pack completely, then charge as above. Leave for 24 hours, then discharge. If the pack fails this test, if done as per, it’s unlikely that it will ever pass any other.

I’ve lifted the paragraph below from the manual, is it possible this is where you can adjust for Manual or Automatic? Also set the Safety Timer, but I’m not entirely sure this would become operative at a low charge rate.

Once the required operational screen has been reached, the user can adjust the parameters to suit his requirements. This process is generally by pressing button B which causes the parameters to flash and then allowing the value to change by pressing the Plus or Minus buttons. Once a particular parameter has been set, further presses on B will move through the remaining parameters allowing each to be adjusted in turn. When the final value has been set a last press of B will complete the process. At this stage the battery may be connected to the unit and the button B pressed to start the procedure which will then continue automatically until complete. The unit signals completion by a series of audio beeps.

This has probably caused more confusion than it’s cured, so if you have any questions at all please ask. I wouldn’t guarantee to be able to answer but I can try.

 

To briefly sum this procedure up, it ignores any delta peak type charge termination's but it does ensure the battery is fully charged in the first instance. It also doesn’t consider any higher rate discharges but for a standard rx I personally don’t consider that to be too much of a problem. Essentially the only real important aspect of a cell is the level of it’s internal resistance, if it has full capacity it’s internal resistance will be low, that’s good, if the capacity is down the resistance will be high, that’s very bad!

PB

[Martin Whybrow]

Peter's made some very good points. However, there's an important thing to consider when looking at cell capacity, it's almost always calculated at a C/20 discharge rate, in your case that would be 105mA; discharging the pack at 1A will give a much lower measured capacity than it is rated at.

[Hogster]

Thanks for all your suggestions and especially to Peter for taking the time for your post.

 

I have set the unit to a manual charge of 200mAH. I`ll let that run as you suggest. Then I`ll do as Martyn suggest and discharge at 100mAH and see what the results are.

 

I can still find no way of accessing the safety timer setting. Scrolling through by pressing B just allows to set a program for different battery types to Charge/Discharge and at what rate. Maybe the safety timer is automatic. It seems you should also be able to change the default delta peak V but I cnt access that either. Do Overlander have a technical help line?

[Ben B]

The timer cut-off (and capacity cut-off) are under User Set Program on the main screen. The main chip in this charger is the same as most chargers (certainly the imax b6). The overlander manual isn't very thorough compared to say the imax one

which might suit you better. The programming flow chart of 13/32 might be of help programming the unit.

[Pete B]

This seems to be a Turnigy Accucell 6 charger in a different packaging. Try this to access the safety timer setting:

 

Scroll through button A to reach User Set Program.

 

Press button B - it will display a screen showing the min cell value for Li batts.

 

Press the Plus button and you will scroll through various parameters until you reach the Safety timer.

 

Press button B twice - the time value will flash. Adjust in 10 min intervals by using the +/- buttons.

 

Pete

 

[Hogster]

You guys are geniuses. Was it me being thick? Would I have been able to get to it using the manual?

Its currently set at 120 mins. So unless I charge at 0.5C or more thats going to be to short. Is that right?

[Ben B]

Yup, that's too short. 120 mins (2 hours) at 0.5A will give you 1Ahr. Your battery obviously needs more than double that if you're going to charge at 0.5A.

[Peter Beeney]

Martin @ 23: 06: 47, Yes, this is a factor to consider, but from experience, I’ve found that at the lower rates of discharge this actually has very little effect. I did consider this a long time ago but after fiddling about with it for a while I decided that to all intents and purposes I could ignore it. I think it’s a bit more pronounced on lead-acid cells anyway, and even then it’s only really when you start to belt them a bit. Indeed, it’s a long time since I was back there!
Regarding modelling batteries, I seemed to have found that a set of rubbish cells are still rubbish, however gently you discharge them, whereas a good pack will still deliver the Full Monty under virtually any circumstances. Plus the fact I like to think that I’m aware of the capacity when it’s in use, testing at a higher rate may help to verify this. Some manufacturers data might be useful, I’m sure it is obtainable but I don’t seem to find it. I believe that test discharges are done in a temperature of 25 degrees C, too, the best possible conditions. Hopefully when Hogster has got his charger sorted, he will then come back with some definitive, (good!) results.

PB

[Hogster]

I`m doing a slow discharge at 0.1A. This in theory should complete in 21 hours. So by 9.00am tomorrow I`ll have a good idea how things are going. BTW I have just discovered the safety timer also activates when discharging. I have turned it off for now.

 

Thanks for your help. I`ll report back tomorrow with my results.

[Hogster]

Well the results are in. To recap I dishcharged a 4.8v 2100mAh battery to a 1.0 V/Cell from a full charge. And the process terminated after 1249 minutes with a capacity of 2099mAh. Which looks pretty impressive to me .

 

I shall now charge the pack at 0.5C which should take 4.2 hours. So I`ll set the safety timer to 5 hours. I leave the delta peak Voltage at 5mV.

 

Fingers crossed its making sense now. Thanks again for all your help guys.

Edited By Hogster on 10/06/2011 08:36:18

[Hogster]

Well its looking good. I stopped it charging after 275mins as the charge was at 2200mAh and the voltage at 6.09. I`m surprised the charger didnt stop automatically. Just shows the importance of using a safety timer. I set mine at 300mins, I think that would have been ok to prevent any permanent damage, dont you?

 

Anyway the results seem to prove I have a decent battery and I now know how to use the charger. Thx again.

[Martin Harris - Moderator]

Posted by Lee Smalley on 08/06/2011 17:28:37:

 

Why would you hate them ?

A possible example?