Confused about safe NiMH charging rates.

When I first got into NiMH batteries, I bought a Maha C9000 charger, and I've always charged at .5C (except during a break-in of course), because the standard advice on here and in the manual is to charge at .5C. I've got all Eneloops.

I've been in the market for a less expensive, more compact travel charger. I'm not finding too many chargers that charge at .5C/1000ma... for example, the Sony Cycle BCG34HRE charges at about 1000ma if you have two AAs on it, but only 640ma if you have four. The Eneloop charger MQN06 has a 7-hour charge cycle for four AAs, which I assume means it's .14C! Even their two position "quick charger" MDR02 has a 4-hour charge cycle, which means it's only .25C.

From what I've read here, the issue with charging at a slow rate is that the charger may not detect the delta-V - or the battery may not produce it, so the charger could fail to terminate and overcharge the cell. I know that my C9000 has other safety precautions, such as a thermal cutoff and I think a timer cutoff, but I doubt that a lot of these less expensive ones do.

So what's the deal here? Is charging at a slower rate really all that bad? Why the inconsistency with these lower-end chargers? Is there a safety issue? Or am I just confused?

I only have the C-9000 and a BC-700, but MaHa states that a charge rate of .3-1.0C is fine, as anything lower might not trip the thermal protection and actually over charge the batteries.

Some feel that a slower charge rate is better for the batteries, extending their life, but I don't know if going .5C vs. .8C matters too much?

I just charged some batteries on both chargers and went with .5C, as I wasn't in any hurry.

Chris

The most dangerous chargers are those charging from .1C up to .3 or .4C or so. Up to 0.1C chargers (probably dumb) will overcharge the cell but the damage will be minimal (similar to what the break-in do in MC-9000). I'm sure however such frequent dumb charging will eventually kill the cell after a while if they are not fully discharged. Chargers at 0.5C and beyond can detect much easier the delta V signal (it is stronger) therefore much unlikely to have a missed termination. Rates of .1C - .4C may not allow a proper delta V detection (depends on the cell quality and condition) and if missed the signal, the cells are under heavy stress dealing with the higher current, the most sure way to kill the cells rapidly. This situation is much worse than a dumb timed charger really!

So basically, these cheapo chargers that charge at <.5C aren't necessarily dangerous (to health and safety), just to the life of the cells.

Hello Awyeah,

The "safe" charge rate for NiMh cells depends upon the method the charger uses for charge termination.

The most common method looks for a voltage drop and terminates on that. It is also an inexpensive method that keeps the cost of the charger low.

New cells will usually give a strong enough termination signal to stop the charge at about any charge rate. The problem comes when the cell ages a little and the internal resistance of the cell goes up. Overall, charging in the 0.5C - 1.0C range gives a strong -dV termination signal over the life of the cell.

NiMh cells used for emergency lighting are left on trickle charge at 0.1C 24 hours a day. In a year they will lose about 20% of their initial capacity and should be replaced.

These cells have a vent to prevent explosions. These vents are not perfect. When the cell is charged, internal gas forms. When the pressure of this gas gets too high, the vent releases it. If you charge at a high rate of charge, you run the risk of having the cell vent. When it vents, it loses some of the electrolyte and the capacity is reduced. At high charge rates this happens quickly, but it can also happen at low charge rates over extended periods of time.

I have a set of 9 Eneloop cells in a battery pack. I was balancing the cells at 0.1C and let it go on for a longer period of time. I noticed that one of the cells was just beginning to vent. I don't recommend using low self discharge cells in emergency lighting that has 24 hour trickle charging.

When NiMh cells were first introduced the chargers were left over from NiCd cells. They basically set a timer and charged at around 0.1C for 12 - 16 hours, then shut off. The problem with this is that NiMh cells self discharge. This lead to supplying a trickle charge. The most inexpensive chargers simply supplied a trickle charge all the time. The idea is that this would keep the cells ready to go. The problem is that eventually they dried out. The next generation promoted fast charging. These chargers charged a cell in as little as 5 hours. Later this was improved again to complete the charge in as little as 3 hours. Unfortunately, these charge rates resulted in numerous missed terminations and a lot of cells were ruined.

Over the years a few good chargers have emerged. One charger used temperature to terminate the charge. It worked great until a ray of sunlight hit the charger causing a premature termination. Also, fluctuations in room temperature could cause problems like having the charger too close to an air vent for the furnace. That charger is no longer available. One of the most sophisticated chargers, as far as charge termination goes, is the 15 minute chargers. Unfortunately, 4C charging is hard on cells, even with good termination.

With the wide variety of cell capacities, it is difficult to find a one size fits all charger that is inexpensive. If money is not a concern a programmable charger is good, but you have to have enough of an idea of charging to be able to use it properly.

Most people expect NiMh cells to perform at least as good as alkaline cells. They may review some information about the cells, but usually ignore the charging process. When given a choice between a $5 charger and a $100 charger, they will purchase the $5 charger. The $5 charger will work for awhile, but it is not optimum for cell cycle life. This results in cells having to be replaced more frequently.

On the other hand if you understand the principles of charging, you can use a $5 charger with very good success. It is all about understanding the limitations and adjusting for them.

Charge termination methods include a change in voltage, a change in temperature, peak temperature, peak voltage, maximum time, and prescreening the cell prior to charging for high internal resistance. In addition there are advanced algorithms that combine some of these with excellent results. The more sophisticated the charger, usually the higher the price.

Most batteries and cells don't die... They are tortured to near death, then murdered. The hope is that the information on CPF can help people learn how to treat their cells better and get a whole lot of better performance from them.

Tom

Thanks for all the clarification! In another thread I recently started, asking about <$30 travel chargers that are smaller than my C9000, the suggestions were good. I was just confused about some of the charging rates they had. Clearly I'm limiting my options - the ones I've come across so far charge at .25C-.5C dpending on the number of cells in the charger, so I'll just proceed with caution.

I figured that once I pick a travel charger, I'll obviously only want to use it when my C9000 isn't available. Even if I end up reducing the cycle count on my Eneloops, I doubt I'll even notice.