NiMH charging questions

[alltracturbo]

I bought an Energizer charger that came with 4 rechargeable NiMH AA's like 4 or 5 years ago.

Charger: Energizer CHCAR1
-Charges AA's at 1A, 1.4v/cell
Batteries: 2500mAh NiMH AA's (came with charger)

I used the batteries when I bought them, but then stopped using them for about 2 years. I only gave them a charge once every 5 or 6 months when they were not being used. Now I wan't to use them again and they still work, but as expected don't hold a charge as long as they use to.

I have a battery cycler that I use to use for RC car and plane batteries.

Hobbico Accu-cycle (HCAP0260) - Charges at 25, 50, or 125 mA and discharges at 250 or 500 mA. Discharges to 1.05v/cell.

I charged the batteries on the Energizer charger and then discharged them with the accu-cycle and they only showed 1750mAh.


1 - Is it possible to put some more life back into the batteries by charging and discharging them multiple times?

2 - The batteries are warm when they are finished on the energizer charger and I wanted to see if they would take more of a charge, so I let them cool and then put them on the accu-cycle charging at 125 mA. Do you think this is charging them farther and is it safe?

3 - Can I do damage to the batteries without them getting hot? The accu-cycle has a 15 hour timer and I think that is all, no temperature or voltage safeties. I wanted to see if they would take anymore of a charge if I continued to charge them at the 125mA. They get a little warm, but probably less than half as warm as when they are on the energizer charger. Was wondering if this slow of a charge would make the pressure rise in the cell without producing much heat. Don't need any batteries venting in my bedroom and I don't want to overcharge and do more damage to them since they still work.

 

[Mr Happy]

Re: Couple NiMH charging questions

Batteries: Energizer 2500mAh NiMH AA's (came with charger)

Uh oh...

Hobbico Accu-cycle (HCAP0260) - Charges at 25, 50, or 125 mA and discharges at 250 or 500 mA. Discharges to 1.05v/cell.

Nice. Hold onto it. :thumbsup:

I charged the batteries on the Energizer charger and then discharged them with the accu-cycle and they only showed 1750mAh.

That's not bad for Energizer 2500's

By all means you can charge batteries for ever at 125 mA without risk of damage. A long slow charge is known as a conditioning charge and is a good way to breath life back into batteries.

However, you need to know that Energizer 2500 NiMH cells are sssh crap cells. There is a limit to how much life you will be able to breath into them. To be honest, if you have a need for NiMH cells you will be better off buying some new good ones like eneloops.

 

[T24]

1 - Is it possible to put some more life back into the batteries by charging and discharging them multiple times?

Maybe. Its worth a try.

2 - The batteries are warm when they are finished on the energizer charger and I wanted to see if they would take more of a charge, so I let them cool and then put them on the accu-cycle charging at 125 mA. Do you think this is charging them farther and is it safe?

Thats normal, that a NiMh get a bit warm during charging.
Farther charging is a bad idea. Overcharging NiMh is no good

Overall:

I would try 3 Cycles (charging 0,1C 11h / 1h rest / 0,5C discharging / 1h rest) and then check capacity again.

 

[alltracturbo]

Re: Couple NiMH charging questions

Thanks for the quick responses.

However, you need to know that Energizer 2500 NiMH cells are sssh crap cells. There is a limit to how much life you will be able to breath into them. To be honest, if you have a need for NiMH cells you will be better off buying some new good ones like eneloops.

Maybe thats why there were so cheap. I think I paid $15 shipped on ebay for the charger and batteries together and it came with a wall plug and car adapter.

They worked great for what I needed them for. I originally bought them for my camera (Panasonic DMC-LZ2). I wen't on a roadtrip cross country and was taking a lot of pictures. I was getting 400 pictures on 2 AA's. Then when I sold the camera I had no use for the batteries and they sat unused.

I don't have a high demand use for NiMH AA's right now, so i'm just going to try and get these working as good as I can. I just got an xbox 360 and the wireless controller uses AA's. I don't wan't to spend the money to keep replacing alkaline batteries.

I would try 3 Cycles (charging 0,1C 11h / 1h rest / 0,5C discharging / 1h rest) and then check capacity again.

I'm working on it, but it's taking a while b/c I don't have much of an option for charging and discharging rates. The energizer charger will only charge at 1A (0.4C). I have been using that b/c the accu-cycle was meant for smaller batteries and will only charge at 25mA(0.01C), 50mA(0.02C) or 125mA(0.05C). Even on the fastest 125mA it will take over 20 hours to charge and I would have to turn it back on b/c it has a 15 hour timer.

The only option I have to discharge is the accu-cycle. It has an option for 250 or 500mA, but last time I tried it on 500mA, it only took out 119mA before it automatically started charging. I'm not sure if there is something wrong with the charger or the batteries are messed up from sitting around not being used for so long and they can't put out over 1.05v for that long on the 500mA draw.

 

[Mr Happy]

Your options are a bit limited.

Some of the problems that may be experienced by old and unused batteries are reduced capacity and high internal resistance. The high internal resistance is what prevents them from maintaining a decent voltage under discharge, and this is likely what you are seeing with the accu-cycle.

The best cure for these problems is what is known as a conditioning cycle, which means to charge at 0.1C for 16 hours, then do a slow discharge at about 0.2C down to 1 V, then repeat until no more improvement is seen. Your accu-cycle would do this OK for older batteries of about 1200 mAh, but unfortunately it doesn't have the right setting for newer 2000+ mAh cells. However, you could put previously charged cells in it and give them 15 hours at 125 mA. That would be a sort of compromise that would come closer to a conditioning charge.

To discharge each cell fully, you could connect it to a low voltage bulb and discharge it for a few hours until the bulb goes out. Make sure to do this with individual cells and don't try to put cells in series or you will damage them. Once the cell is discharged, charge it up on the Energizer charger and repeat.

However, there is another problem often seen with Energizer 2500 cells, which is that they may cease to hold a charge when off the charger. In some cases they will self-discharge to empty within days. If it happens this cannot be repaired and it seriously reduces the usefulness of the cells.

 

[alltracturbo]

Yeah, the accu-cycle says right on the front that it only does up to 1875mAh on the 125mA setting. I'm guessing thats b/c of the 15 hour timer and then it goes into trickle.

Do you think there might be any advantage to charging at 125mA from fully discharged. It would take a long time, but if there is any benefit, I don't mind waiting since I don't need the batteries for anything right now. Do you think 30 hours at 125mA would be long enough for a conditioning charge?



When you say to connect each cell to a light bulb to fully discharge it, what is the advantage of this? Is it faster than using the accu-cycle at 250mA? Will it bring the cell lower than 1.05v, which is where the cycler stops.

Was wondering about the voltage b/c I was reading this earlier from energizer about over discharging and doing damage to the cell. (PDF file, Pg. 14-16)

http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf

 

[Mr Happy]

Do you think there might be any advantage to charging at 125mA from fully discharged. It would take a long time, but if there is any benefit, I don't mind waiting since I don't need the batteries for anything right now. Do you think 30 hours at 125mA would be long enough for a conditioning charge?

My gut reaction is to do a 15 hour charge at 125 mA starting with a fully charged cell. Part of the purpose of the conditioning charge is to circulate and mix up the contents of the cell. This circulation only happens once the cell is fully charged (the oxygen recombination reaction).

When you say to connect each cell to a light bulb to fully discharge it, what is the advantage of this? Is it faster than using the accu-cycle at 250mA? Will it bring the cell lower than 1.05v, which is where the cycler stops.

Was wondering about the voltage b/c I was reading this earlier from energizer about over discharging and doing damage to the cell. (PDF file, Pg. 14-16)

http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf

Here is a where a bit of folklore and mystery comes in. The theory is that over long storage cells tend to grow "large crystals", and these crystals cannot release their energy very fast leading to a high internal resistance. So the voltage drops down to 1.0 V under discharge, but there is still a lot of stored energy inside the cell. To break up these crystals and free all the chemicals you have to discharge the cell really slowly for a long time without stopping at 1.0 V. Connecting a bulb to the cell will do this.

You don't have to worry about damage if you only do this once in a while and as long as you don't store the cell with a bulb attached. Taking a single cell down to a voltage below 1.0 V for a few hours and then charging it again will be fine. No harm will happen.

But as I warned above, don't do this with two or more cells in series. If you do that one of the cells might get reverse polarized, and that really is bad.

 

[Mr Happy]

If you want to tell whether a cell is fully discharged, drain it down to the 1.05 V cut-off point or wherever, then remove it and let it sit for a few hours. If the cell is fully drained the voltage should settle out at 1.1 to 1.2 V. If the voltage rebounds to a higher value like 1.25 V that means there is still some charge left in the cell.

 

[alltracturbo]

I just finished another discharge and this time it only showed 1649mAh. I charged it with the energizer at 1A untill it was done and then put it on the cycler to charge at 125mA for about 2 hours.

I'm not sure why the last discharge showed around 1750, but it may be b/c I tried to do the discharge at 500mA that time and then it automatically started charging after it only took a little out. Then I did the rest of the discharge at 125mA. Maybe it was charging for longer than I thought. The 1649 is probably more accurate.


I didn't get to check the voltage right after being discharged, but I just checked about 15 minutes off of discharge and there at:

1. 1.237
2. 1.236
3. 1.231
4. 1.231

Next discharge i'll get a reading if I can right when they stop.

The problem is that when they are done discharging, they automatically start charging right away. I have to keep an eye on it when there almost done. They were probably on charge at 125mA for about 5 minutes before I took them off, so the voltages above won't be correct.

I'll try what you said about charging them and then condition at 125mA for 15 hours. Then i'll test them again.

Thanks for the help.

 

[Mr Happy]

I didn't get to check the voltage right after being discharged, but I just checked about 15 minutes off of discharge and there at:

1. 1.237
2. 1.236
3. 1.231
4. 1.231

Next discharge i'll get a reading if I can right when they stop.

Actually you want to give them a couple of hours before you measure them. The discharge takes the voltage down to 1.05 V under load and then stops. If you remove the battery at this point the voltage will rebound, slowly climbing back up to a higher resting value. It is this resting value that tells the story about the state of charge. For instance a number like 1.237 V tells that the cell did not finish discharging. It still contains what may be 100 mAh or 200 mAh of remaining charge. To discharge that remaining capacity and thus fully exercise the cell you will need to use something like the bulb trick.

 

[alltracturbo]

I'll check that this time if I can get to them right when they get done discharging. I didn't wan't to wait and check that set b/c when they were done discharging I didn't get to them right away and they automatically started charging again.

I'll keep an eye on this set when they get to around 1600 mAh discharged and then i'll take them off right when their done before they start charging again. Then i'll take some voltage readings over the next couple hours and see what they do.



What kind of light bulb would I need to discharge the individual cells farther. I can get one of those small bulbs that goes in a dome light in a car, but I think there 12 volt. Will that work? Also have pleanty of 12v brake lights and blinkers.

I'm sure I can find an led, but I don't know how long that would take to drain the battery since they're really efficient.

Also, how far should they be discharged. According to that Energizer .pdf I posted earlier, it starts to reverse one electrode below 0.9v. Will that only happen if I am discharging more than one cell in a series? Is it safe to go below 0.9v when only discharging one cell?

Also, is it safe to connect multiple batteries in a parallel circuit and then discharge them together so that I don't have to connect multiple light bulbs and have to check multiple voltages? If it is safe that way, will they drain equally?

 

[tylernt]

I don't think it's ever safe to go below 0.9v, series, parallel, or single cell.

Discharging to 0.9v in parallel should be fine though. In fact it would even be preferable, as the more charged cells will discharge more quickly until all cells perfectly match each other in voltage. If you do not have an independent channel charger, balancing the cells like this will be beneficial as they will be then charged more equally.

You should be able to use a 12v bulb but the resistance will be different and the cells might discharge too quickly too slowly. If you measure the resistance of your bulb you can calculate how many mA it will flow at 0.9v and at 1.45v to make sure they are both within safe values.

Be aware that the bulb's resistance may change a little as it gets hot... not sure if it will be enough to matter.

I have a thread on here somewhere about making a battery discharger. I didn't want to babysit the process so I proposed building a device that would stop discharge automatically before 0.9v. It would have used a pair of transistors to shut off the discharge at a certain voltage level. However, I was unable to find real-world transistors that would work like my simulation so I gave it up. Another option may be to use a regular (0.7v) diode and a schottky diode (0.3v) in series which would hopefully stop the discharge at 1.0v... not quite as low as you might want but at least it would be failsafe.

 

[alltracturbo]

I don't think I need to make something to discharge since I have the cycler that will discharge to 1.05v. If I need to go lower than that it will just be until I get the cells back to where they should be and then I hopefully won't need to do it again. Just to do that, I don't mind using a bulb and watching the voltage.

Do you know the formula to check how much current will flow through the bulb after finding the resistance? Does this sound right?

http://en.wikipedia.org/wiki/Electrical_resistance

The resistance of a resistive object determines the amount of current through the object for a given potential difference across the object, in accordance with Ohms law:

where
R is the resistance of the object, measured in ohms, equivalent to J·s/C2
V is the potential difference across the object, measured in volts
I is the current through the object, measured in amperes

And what voltage will I put into the formula? Is it 12v or is it what I am putting into it from the battery?

What would be a good mAh range? If it draws too much, couldn't I just connect the batteries in parallel so it will distribute the load between each cell evenly?

Also, you talk about balencing the cells while discharging. Won't that only happen if I have something to stop discharging at a certain voltage. If I just connect them all to a bulb directly, will they balance or discharge at the same rate from each cell and not balance.

 

[tylernt]

And what voltage will I put into the formula? Is it 12v or is it what I am putting into it from the battery?

Yes that is the correct formula. Use battery voltage (0.9v-1.45v), not bulb rating.

What would be a good mAh range?

I'll leave this to someone else, not sure. My guess is smaller is better but will take more time.

If it draws too much, couldn't I just connect the batteries in parallel so it will distribute the load between each cell evenly?

Yes.

If I just connect them all to a bulb directly, will they balance or discharge at the same rate from each cell

At first, the cell with the higher voltage will contribute more current than the weaker cell. After a time, they will balance out and contribute (nearly) equally).

 

[Magic Matt]

I don't wish to hijack the thread, but this has me thinking...

To discharge NiMH cells ready for 'recovery' they need to follow the following steps:
1 - Discharge to 0.9V at a rate of about 0.1C
2 - Leave for approx 2 hours
3 - Check voltage, and if it's over 1.2V then go back to step 1, repeating until the voltage doesn't spring back to over 1.2V.
4 - Slow-charge the cells at 0.1C for 11 hours.
5 - Rest for 2 hours
6 - Discharge at 0.5C to 0.9V and check the capacity (which should be "Time to discharge in hours * current in mA * 2 = capacity in mAh" by my reckoning)
7 - If the capacity has gone up, go back to step 1, otherwise stop because the cells are not improving.

Do this with the cells in parallel to balance them (I'm assuming they should be charged in parallel as well as discharged in parallel, or should they be charged individually?).

 

[Mr Happy]

A few notes:

To discharge NiMH cells ready for 'recovery' they need to follow the following steps:
1 - Discharge to 0.9V at a rate of about 0.1C
2 - Leave for approx 2 hours
3 - Check voltage, and if it's over 1.2V then go back to step 1, repeating until the voltage doesn't spring back to over 1.2V.
4 - Slow-charge the cells at 0.1C for 11 hours.

This should be for about 16 hours.

5 - Rest for 2 hours
6 - Discharge at 0.5C to 0.9V and check the capacity (which should be "Time to discharge in hours * current in mA * 2 = capacity in mAh" by my reckoning)

Just time to discharge in hours * current in mA = mA * hours = mAh. There is no factor of 2 in there.

7 - If the capacity has gone up, go back to step 1, otherwise stop because the cells are not improving.

Do this with the cells in parallel to balance them (I'm assuming they should be charged in parallel as well as discharged in parallel, or should they be charged individually?).

You can discharge NiMH cells in parallel, but never charge them in parallel. The current can get unbalanced and you can end up with a melt down.

 

[Magic Matt]

The C9000 seems to hit all the criteria except resting time (unless I can't find it on the options) and discharging in parallel.

A possible project for the future then... my approach would be to use a microprocessor and do it that way rather than a transistor (I know it may sound daft to you, but I'm very comfortable with programming systems, and a bit lost when it comes to basic electronics!)

 

[vali]

The C9000 seems to hit all the criteria except resting time (unless I can't find it on the options) and discharging in parallel.

It dont discharge in parallel, but in indivivual slots.

AFAIK there is no option to change the resting time from 1 to 2 hours

 

[alltracturbo]

First discharge showed 1760mAh. Dont know if this was accurate.
Second discharge showed 1649mAh
And now I just finished the 3rd discharge and it is showing 1624mAh.

This time I did a full charge at 1A (only option)
Then charged at 125mA for 15 hours (fastest option other than 1A)
Then discharged at 250mA (only option)

A problem I noticed is that they are supposed to be discharged to 1.05v. When the batteries were almost done, I waited and the second it finished I pulled the batteries off and checked the voltages.

1- 1.220
2- 1.220
3- 1.215
4- 1.215

Looks like I am going to need something to drain them farther. I'll look around for a light bulb that will work. What would be a safe mAh range to drain the batteries through a light bulb? Should I try and stay at or below 0.1c (250mA)?

Taking a single cell down to a voltage below 1.0 V for a few hours and then charging it again will be fine. No harm will happen.

But as I warned above, don't do this with two or more cells in series. If you do that one of the cells might get reverse polarized, and that really is bad.

Do the cells get reverse polarized specifically b/c they have another cell connected to it at that low of a voltage? Would discharging a single cell below 0.9v reverse the polarity?

 

[Mr Happy]

A problem I noticed is that they are supposed to be discharged to 1.05v. When the batteries were almost done, I waited and the second it finished I pulled the batteries off and checked the voltages.

1- 1.220
2- 1.220
3- 1.215
4- 1.215

This is not a problem, this is about right. They are supposed to recover to about 1.2 V when removed from the discharging circuit.

As for the capacity measurements, you are learning that cells can only be recovered so far, and then no further. Once the capacity stops increasing that is the limit. They have improved as much as they can.

For reverse polarization that is when one charged cell pushes current through a less charged cell. Thus it can only happen with two or more cells in series.