A look at NiMh Parallel Charging

SilverFox

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I am sure that most people on CPF understand the importance of keeping Li-Ion cells in balance during charging (and use). To keep Li-Ion cells in balance, you need to charge them independently or in parallel.

Nickel chemistry is different. The best way to keep cells in balance is to charge them independently, however that may not be an option if your cells are put together in a battery pack. Then you need to charge them in series and balance them with a low current (0.1C) top off charge at the end of the normal charge.

What happens if you parallel charge NiMh cells?

There are several chargers available that require you to charge cells in pairs. Two that immediately come to mind are the CCrane Quick Charger EDIT: The CCrane will charge a single cell, but if more than one cell is charged, they are charged in parallel. ENDEDIT and the RipVan100 Lightning Pack 4000N (also available from Amondotech as the Titanium V4000). EDIT: It appears that the 4000N is actually a series charger. ENDEDIT People using these chargers have good things to say about them and, if your cells are reasonably balanced before charging, they do a very good job of charging.

On the other hand, if your cells are not balanced, they don’t do very well, unless you leave them on the charger to balance through the top off and/or trickle charge.

It is often thought that you can equalize Nickel chemistry cells simply by hooking them up in parallel. This works for Li-Ion cells, but not for NiMh or NiCd cells. You will find that the voltage will tend to wards equalization, but very little capacity will be added to the discharged cell from the charged cell. Tests have shown that, at the extreme, a fully charged pack connected to a fully discharged pack will see less than a 10% transfer of capacity in 24 hours. With single cells, a fully discharged cell will often have a resting voltage of 1.15 – 1.20 volts, and a fully charged cells comes in at around 1.42 volts. It takes more than a 0.27 volt differential to charge a NiMh or NiCd cell. NiMh cells can not be equalized by simply hooking them up in parallel, unless you leave them hooked up for several days or perhaps even months.

I took two AAA cells and discharged them. To create an imbalance condition, I took one of the cells and charged it to 20% full. The first graph shows that this imbalanced condition was not corrected by charging in parallel.

ParallelChargingUnbalancedCells.gif


I used the Lightning Pack 4000N for these tests, but the results would be the same for any parallel charger. EDIT: The 4000N appears to actually be a series charger. ENDEDIT The cells were pulled about 1 hour after they had finished charging.

Next, I took the same cells after the discharge test, charged one of them to 20%, then put them into the parallel charger, but this time I ran a discharge/charge cycle to see if that would help. The next graph shows that there was very little change in the imbalance condition. This indicates that you can’t balance cells in parallel by adding a discharge cycle.

ParallelDischargeChargeUnbalancedCe.gif


The final graph shows that if both cells start off in an equally discharged condition, they pretty much stay in balance while being charged in parallel. I added the discharge curves for the same cells after they had been charged on an independent channel charger (Vanson BC-1HU).

ParallelChargingBalancedCells.gif


It is interesting to note that the independent channel charger brought the cells in closer balance than the parallel charger did, but the parallel charger was able to put a little more capacity into the cells.

So, what does this all mean?

If you have a charger that requires that you charge in pairs, you need to remember to leave the cells on the charger overnight to balance them, and check the voltage of the cells after charging to make sure they are equally charged. If you are using the cells in pairs, there is a good chance that they are reasonably equally discharged, and a few hours on top off or trickle charge will balance them. However, if you are using single cells in your applications and charging them in pairs, you may end up with balance issues.

If you study the manufacturers guidelines on charging NiMh cells, they tell you to call them if you plan to parallel charge. When you call them, they will tell you that if you must parallel charge, you need to monitor the voltage of each cell independently. Looking back at the graphs you will see that the under performing cell is also starting off at a lower voltage. They will go on to explain that the only way to balance cells while parallel charging is to maintain an extended top off charge. During this extended top off charge, one of the cells will be overcharged. Problems arise when you incorporate this extended top off charge into the charging algorithm and end up charging balanced cells. In this case, both cells will be overcharged. After a detailed discussion explaining all of this, they will go on to encourage you to charge in series or independently.

Does this mean that we should throw out our chargers that charge in pairs?

No.

It simply means that you need to be aware of the limitations of parallel charging. EDIT: After observing that the 4000N is a series charger, I would add that you also need to be aware of the limitations of series charging as well. ENDEDIT If you can upgrade to an independent channel charger, that’s great. If not, just remember to let the cells stay on the charger for a little extra amount of time and check each cells voltage when it comes off the charger. You may also consider letting the cells settle down after the charge is complete, then recharging your cells. One of the cells may end up a little overcharged, but you will also bring the lagging cell up to full charge.

In multi cell applications, cell imbalance robs you of performance, and can ruin your cells through over discharge. By keeping your cells in balance you will enjoy better runtimes and increased cycle life.

EDIT to add:

Since this testing was done using the 4000N for charging, and since the 4000N is supposed to charge in series, I was surprised that the cells did not end up balanced after letting them stay in the charger for an hour past the end of the charge.

I decided to check this again.

I took the same cells, charged cell 3 20%, hooked it up in series with cell 4 which was fully discharged, and charged them with the Schulze isl 6-330d/RS charger. The charging rate was 800 mA, which I think is close to what the 4000N uses. At the end of the charge, I topped the cells off using the 0.1C rate for 30 minutes.

I then measured the voltage of each cell, and they were the same. I let the cells sit overnight, then measured the voltage again, and once again they were the same.

Here is the discharge graph.

SeriesChargingUnbalancedCells.gif


As you can see, these cells ended up well balanced in spite of the short 30 minute top off.

I am not sure what is going on with the 4000N, but it seems to need more than 1 hour to bring cells back into balance.
ENDEDIT


Tom
 
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VidPro

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now add the part about that all going to heck when you use different charging style.

like parellel charging when a Vdrop is termination, and no topping occurs.
or
when dumb slow charging.
then
Rate of charge. (rate of charge, sets the voltage differential to each cell)

like with a slow dumb charge, one cells can Vdrop, before both cells are completed, curcuit total voltage drops, because of the Vdropped one.
OR
with a fast smart charge , it can take one or both cells to vdrop before vdrop is spotted. one could be tortured before the computer sees it correct.

i think (myself) that with ni-mhy its better to not parellel, and just use cells of the correct capacity for the application. Put D cells in D flashlight, end of science. put 4x AA cells in parellel in D container, start minor issues.

with li-ion parallel works good , as long as they were pretty close before tossing them together.
but then a lot of that, can be related to the TYPE of charging that both of them are using.

EX: say you applied li-ion charge characteristcs to ni-mhy battery.
and terminated charge at 1.4v always, using a constant voltage type termination.
the battery just like li-ion would never reach pressure , never be 100% charged, and yet parelleing with that TYPE of charging parameter would work, as long as the cells were similar.

but ni-mhy charging doesnt work that way, we torture them to max, to get max out of them.

Rate of charge ex:
blast the cells with a high rate of charge, like a 2V voltage potential, and each cell is being hit with a voltage differential that encourages each to accept the same charge, reguardless of the charge voltage of the other cell.
 
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LuxLuthor

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Tom, very interesting results which I would not have expected with parallel charging of NiMH. What about a similar study of charging 4 NiMH cells in series? If you already did it, a link and/or the "punch line" would be great.

How do you get those graphs when testing with some of those chargers?
 

SilverFox

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Hello VidPro,

You are correct. I did leave out some of the details of why parallel charging doesn't work. When you have two cells in parallel and they are not balanced, the first cell to reach full charge can depress the voltage total while it is being overcharged. The charger looking for a -dV of a value for both cell will continue to charge until both cells are over charged.

A timed parallel charge, if it is charging at 0.1C or lower, may work OK, but at higher charge rates, it will overcharge the cell that starts at a higher state of charge.

Things work out IF the cells are balanced, but can quickly go bad if not.

Tom
 

paulr

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Do those pair chargers actually charge the cells in parallel?! I'd have thought they charged the pairs of cells in series.
 

VidPro

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but mine were balanced :-( and the vdrop hit the one set, in a perellel series configuration, and it was terrible :) (sniff ) (sniff) poor battery, poor me, i spent money for them :)
 

SilverFox

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Hello LuxLuthor,

The results for serial charging are very similar to the results for independent charging, if you apply a balancing charge at the end of the main charge. The RC people match the cells in their packs, match the pack by discharging each cell to 0.9 volts, then assume that the cells will be balanced at the end of their fast charge. If a pack gets out of balance, a cell within the pack can get hot, vent, and may explode if things really get out of hand.

Balancing a pack is not necessary with every charge, but it should be done periodically. When you notice changes in the packs performance, the first thing to do is to balance the pack.

These same principles also apply to Li-Ion packs. If a Li-Ion pack is used within its capabilities and not over discharged, the cells will stay in very close balance. However, if you push the limits, things go out of balance quickly. Most people are now balancing during charging, just to make sure there are no surprises with their Li-Ion packs.

As I said before, if all your cells are in balance, everything works great. If things get out of balance, performance suffers.

Tom
 

SilverFox

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Hello Paulr,

That is a good question. Most of the time the chargers that charge in pairs do not give enough information in their specifications to tell if it is charging in series or in parallel. I believe both the CCrane and the 4000N are charging in parallel. EDIT: It appears that the 4000N is actually a series charger. ENDEDIT I don't know about others.

Tom
 
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Brody

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Good work, as always, Tom! I use my Lightning Pack 4000N quite often, but then, most of the devices I use my batteries in are using them in either pairs or 4 at a time. I guess I will just make sure to balance the cells with my Maha c808m charger every few months. I usually leave the cells in the charger for around 12 hours and I have had no problems with them. I make it a habit to never mix brands, and to always keep cells from the same package together, as much as possible.
 

jusko

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Hi, Tom,

The 4000N is a 2-channel charger. Each channel I think charge 2 cells in series. So if you put 2 cells in one of the two channels, they are just being charged in series. But I am not sure whether the 2 channels are in series, parallel or independent.

The CCrane/Saitek is a one channel charger and all 4 slots are in parallel.

Am I missing something ?

Best Regards
jusko
 
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65535

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For packs the thing I have done is discharge them individually, it's what a lot of RC racers do, a discharger that takes the cells down to about .1 volts and then lets them jump up.
 

SilverFox

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Hello Jusko,

Oops, I think you are right. The 4000N does seem to be a series charger.

Now I need to figure out how long it takes to balance the cells out. I let it trickle for an hour, and that did not do the job.

This thread came about because of repeated inquiries of what is the cheapest charger available. I kept looking at low cost chargers and many of them charged in pairs. The specifications were none existent, so I went with the worst case that they charged in parallel.

I am surprised that the 4000N did not do a better job of balancing, but I can see that more testing is needed...

Tom
 

SilverFox

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Hello 65535,

Over discharging can damage cells. The higher capacity cells seem to be more fragile and I would not recommend taking them to such a low voltage. There are a new generation of discharging trays that limit the discharge to 0.7 volts. I am not sure there is any advantage in going below that.

Also, keep in mind that in a perfect world, starting with equally discharge cells should end up with a balanced pack at the end of the charge. However, as cells age, their impedance profile can change as well. This results in an imbalance in the pack at full charge, and at full discharge. Re-peaking, in addition to warming the pack up for peak performance, also can serve to balance the pack at full charge. If peak performance is not required, equalizing is usually done by an extended trickle or toping charge.

Tom
 

wptski

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Tom:

On a parallel cell charger, what voltage drop are they looking for with -DeltaV? I know that a series charger it's twice the value of the drop for one cell.
 

SilverFox

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Hello Bill,

There isn't much information available on how to set up for parallel charging. I would think you would set it up for a single cell and hope the cells are balanced.

Tom
 

SilverFox

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Update:

I took the same cells, charged cell 3 20% and charged this cell with a fully discharged cell 4 on the Schulze isl 6-330d/RS. When the charge was completed, I did a top off of 0.1C for 30 minutes.

I checked the voltage of the cells after the charge, and they were the same. I let the cells sit overnight and checked them just before the discharge test, and they remained the same.

The cells ended up nearly perfectly balanced.

The graph has been added to the first post.

Tom
 

europium

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Presently, I frequently use AA NiMH in pairs (eg., in a digital camera), so I am keenly interested in what procedures to follow when charging in order to get balanced cells.

I have the 4000N, which I got after reading the many positive recommendations here on CPF, so I am very curious as to its actual charging behavior. I believe that after the 4000N indicates charging is finished (when the LED flashes green slowly), that the cells are only about 90% charged.

What would be a good amount of time to leave a pair of cells in the 4000N charger in order to balance them? In this example, assume the cells have been used together in the same device and so have a similar state of discharge prior to charging. Is it true that the trickle charge on this charger stops after 24 hours and, if so, would this be wise, to leave the cells in the charger that long?

Also, since the 4000N has a refresh function which discharges cells (fully?) before charging, is it more likely to produce a balanced pair of cells using this method? Supposed instead that the cells started out at different levels of discharge before using the refresh function. Might this not possibly ruin a cell, since this would result in the discharge in series of mismatched cells?

I also have an Energizer 15 min. charger. Since it has independent channels, and assuming I take the cells out of the charger just after the fan goes off, am I more likely to get balanced cells using it? Even if the cells were not discharged to the same degree before charging?

Assume for all questions that I am using a pair of cells with the same capacity made by the same manufacturer (most likely also taken from the same package).

Thanks, Tom, for all the research you do and for sharing your results with us. :bow:

Eu
 
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