Cell Break In and Balance Charging

for NiMH and NiCad packs in a nutshell.


What exactly is the balance charge you are talking about and how do I do it? Is it the charge/10 rate for 16 hours that the digipulse will do? By the way, thanks very much for going out of your way and finding that information, I really appreciate it. Thanks again for the help and take care.

Will, USAF.edu

Yes. All cells have a daily self discharge rate. Each individual cell will have a slightly different rate. Generally for NiCads this is .5% per day, for NiMH cells its 1% per day. Note, as the cells age this rate tends to increase so it changes over time. This is one of the big reasons it becomes too risky to use TX and RX packs beyond the 3rd year. Be aware also that these rates are only approximate; they will be slightly different for each individual cell in your pack. The cells in a new pack will be some weeks or a few months out from the manufacturer. (With some suppliers they can be over a year old! Yikes!) So, they've in a sense, been in storage and had time to change levels from each other. Also, a pack you own that has been in storage (model last flew last season or few months ago for example) will have a different level of charge in each cell.

When you charge it, some fill up before others. When charging at 1/10th Capacity, (C/10) (100mah in the case of a 1000mah pack/cell) the cells that fill up first won't be damaged while the remaining cells are filled. Also bear in mind you need to put in about 140% of an empty cell’s capacity to fill it up. There is some energy losses during slow charging. So it takes 1400mili amps total (100mah charge rate x 14 hours) output to fill up a 1000mah hole in a battery pack.

To visualize this cell un-balance condition, lets use our example 4 cell pack, think of 4 tall drinking glasses all side by side in a row. The first glass is 1/2 full, the second one is 1/3 full, the 3rd one is 1/8 full, the 4th one is 3/4 full. We start to drip water into the glasses at an equal rate of one drop every 5 seconds into each glass. The 4th one fills up first, then a little later the 1st one fills, then the 2nd one, then the 3rd cell fills. The pack of glasses are now all full. However, the 4th cell was 3/4 full at the start of the charge (water dripping in the glass) so it was overflowing for a long time while the charger one by one topped off the other 3 cells. If our rate of charge (speed of the dripping water) is faster than 10% of the capacity of the cell then damage will occur on the first cell to fill. It will be overfilling faster than it can accept, this causes it to overheat internally and contributes to gassing. So, in effect, we’ve done damage to our “best” cell.

A 1000mah cell can take an overcharge rate of 100mah without a problem. It will warm up when full but the heat being generated is at a rate the cell can dissipate into the atmosphere. However, if the over charge rate is greater than 100mah (higher than 10% of our 1000mah capacity) a problem is caused. One good way to damage a battery is to fast charge an old pack that's been in storage for a long time, or a new pack. It's always best to slow charge these kinds of NiMH and NiCad packs at 10% for 14-16 hours to avoid this problem and equalize (or balance) all the cells.

A pack that is in regular use gets some overcharge all the time. You can’t know exactly the moment it will become full while slow charging. For example: You fly a day, charge all night, probably some time during the night the pack has refilled it’s “hole”. It probably Isn’t empty when you put it in charge unless you crashed and the battery was dead ;-) . We’ll presume you brought the model home in-tact. When our partially discharged battery fills, the charger (wall wart you got with system is presumed or Digipulse or DDVC or whatever kind of slow charger) doesn’t shut off, it just keeps on charging. A cell that is behind a few Mili-Amps is caught up during this time.

Even when fast charging a pack that’s in regular use, there is a little bit of an overcharge at the end of the charge cycle. (Excepting Sirius and Ace fast chargers which are peak predictors an go into trickle to accomplish same effect.) A peak detect charger is looking for a drop off in voltage to find the “Full Point” of the pack. It’s voltage is dropping because it’s heating up, it’s heating up (where you paying attention earlier?) because it’s full and still under charge.

With all the above in mind, here is what you do:
Find the capacity of your battery pack. Lets say it’s 1650mah.
Multiply the Capacity X .10 to arrive at 10% of this number. The answer here is 165mah.
Set your charger to charge at 165mah.
Come back 14 to 16 hours later and the battery is ready to use.
What if I can’t set a charge rate of 165mah?

Another way to skin the same cat is to multiply your capacity (1650) by 1.4
This gives us the 140% we need to put in to fill up a pack. The answer is 2310mah.
Take the answer (2310mah) and divide by the output of your charger, answer is “Hours” to full charge.
If your using a Digipulse (max setting is 150mah) the answer is 2310/150= 15.4 hours.
If your using a 100mah wall wart the answer is 2310/100=23.1 hours.
If your using a 50mah wall wart the answer is 2310/50=46.2 hours.

Warning! If your answer is less than 14 hours then your charging higher than 10% of capacity, the pack will be damaged!

That is balance charging for NiMH and NiCad battery packs in a nutshell.

Dave Thacker