SilverFox
Flashaholic
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.
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.
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).
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.
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
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.
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.
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).
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.
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
Last edited:
(sniff ) (sniff) poor battery, poor me, i spent money for them 
