Re: Appropriate (dis)Charge Rate for NiMH Batteries
Boy, the debate over the 'correct/appropriate/best' charge rates is still raging strong I see.
Tom (Silverfox), myself, and others have debated/discussed this many times over the years, especially early on when I too had just found this forum.
I have noticed throughout the discussions that many people seem to latch onto certain pieces of information or read a piece of literature and suddenly take it as the 'gospel' of how its supposed to be done. Even the battery manufacturers can't 100% agree on the rates. The best they can come up with is a .1C charge rate and a .2 c discharge rate to determine capacity. But even they CANNOT agree on the BEST charge/discharge rates.....because there IS NO 'BEST' RATE!
Tom and others seem to love to quote the 'recommended' rate is '.5C to 1C'. Others seem to latch onto the recommended rate from the battery manufacturers is 200mA or some fairly low rate. Guess what, BOTH are true from certain perspectives.
The REAL TRUTH......There is NO SINGLE BEST RATE!!!!
Mostly the debate goes between the 'slower charging rate' being the 'best/better' and the other end......the 'faster charging rate' is the 'best/better'.
Each rate has it's advantages and disadvantages.....but more importantly I think its better to understand where some of this is coming from.
Let's start with the '.5C to 1C' rate is the recommended/best rate. A battery company recommendation, in particular PowerEx. Yeah, others recommend it, but trace it back funny how it started being more strongly recommended slightly after the MH-C9000 appeared.
This really came to life when the MH-C9000 came out because MAHA couldn't get the charger to terminate reliably at the lower rates.
The first versions of the MH-C9000 truly terminated on -deltaV as their 'primary rate', but the batteries were coming off the charger way 'too' HOT. I guarantee this... I had one of the early revision 'F' chargers. Everyone wondered how come my Enloops after break-ins were about 100mA higher than theirs. Then we found out about the change between revision 'F' and revision 'G and above' chargers. They then lowered the maxV voltage so that it terminated at 1.47v. Now Enloops almost ALWAYS terminate because they hit the maxV instead of -delta V. So now, maxV of 1.47V is really the primary termination method that most batteries encounter when using the MH-C9000 charger. Yeah the other methods are there, but the battery will probably never use/hit them.
As for the battery manufacturers recommendations of 200mA or really low rates.
There are a few reasons for this.
One reason is its an older hold over from the days when there were not smart chargers and 'fast' charge rates. You basically timed the charge and did it at a really low rate so the battery didn't die right away or explode.
Another reason, it is fairly hard to damage a modern 2000 mA battery charging it at 200mA and below. I didn't say it couldn't be done, just that its not easy. The battery can take that level of a charge for days and still work fine. I have torture tested Enloops charging them at 200mA for 7 days straight...yes 168 hours....guess what. They still work fine. I did notice a 3-5 mohm increase in the internal resistance though.
Here is one of the best reasons that almost everyone seems to overlook....
ALL NEW NiMH Batteries can miss their termination charge on the first few charge cycles....NO MATTER WHAT RATE YOU USE!!!!!
It take 1-5 charges on a battery before the end of charge signal (-deltaV) becomes as strong as its going to get. On some new batteries, it can take 1 or 2 cycles before you even 'see' and end of charge signal! Yes, generally speaking a larger charge rate will produce a larger end of charge signal...but not always on the first couple of charges.
That is why you see brand new batteries charged at 1A still miss their termination charge. It is also why batteries charged at ANY rate can miss their end of signal charge the first couple of cycles. I see so many people blame the charger or a low charge rate for a missed termination on a brand new battery....probably wasn't either. It was the new battery itself. Run it through a few cycles and it will terminate just fine I bet on the lower rate...that is assuming you don't have a charger that is always unreliable at the lower rates.
So...battery manufacturers play it safe and go the 'charge it at a really low rate' safe method......1. Harder to overcharge/overheat the batteries and cause a problem. 2. Don't have to worry about missing a termination charge signal. 3. Generally always get a good, complete charge.
Now with all that said, back to what is the 'best' rate. Again it varies depending on ALOT of factors; personal preference, technical factors.
There are trade-offs with each method.
Through the years and with many discussions here with Tom and others, I have finally fallen into the following patterns.
I generally try to charge them at the highest rate I can that DOES NOT cause them to get hot. Warm is ok HOT is NOT.
With a brand new AA cell, that can be .500mA, 1A or even 2A. The point is are they getting HOT. HEAT KILLS A BATTERIES LIFE (AMOUNT OF CYCLES).
Now with that said, the higher the rate, the more heat generated, so the more cycles lost. It seems to be a bell curve, where it helps to a certain point, then after a certain point, it starts killing cycle life off the battery. So again, its a trade-off.
I have found that MOST NiMH batteries, as they age, develop a higher and higher internal resistance. This in turn causes the batteries to started heating up when charged at the higher rates. So, as they age a bit and develop a higher internal resistance, I lower the rate so again they are NOT getting HOT when charged.
Some might say well why not then always just use a low charge rate? Well, larger rates do produce a larger end of charge signal. So a cheaper charger or a charger that 'needs' this to work reliably would have a problem with the lower rates. It is possible to cook a battery charging it at 200mA if you leave it on a charger for WEEKS.
Another good, often overlooked reason that I have noticed,....while I won't go as far as calling it a memory effect. I have notice that if you continually discharge or charge a battery at the low rates, it will get to where it does not like charging/discharging at the higher rates. Generally you will find it has developed a higher internal resistance. It will perform poorly under high discharge rates and heat up under high charge rates. So here you see that a low charge rate can also cause the cycle life to decrease. You generally cannot 'fix' this type of problem once it occurs.
I generally charge them at the highest rate I can when new (.5A, 1A, or 2A) where they only get warm. I will mix in a few slow (200mA or .5A) charge/discharge cycles ever now and then on a battery that always charged/discharged at a high rate (1A or 2A). Like a high output flashlight. I also do the opposite. I throw in a high rate charge/discharge cycle (1A charge/.5A discharge) on batteries that are continually used in a slow discharge rate application. Like a travel clock).
Also as the battery gets older and develops a higher internal resistance, I lower the charge rate so that they are not getting HOT when charged. Usually you will also start to see a decrease in capacity. One the capacity gets to about 70-80% of the original, I dispose of/recycle the battery
Now even though I bashed the MH-C9000 ...... I have 2 of them and find them to be one of if not the best NiMh AA/AAA Analyzer charger on the marker right now. Yes, it has some features I do not like, but so do all the other charger/analyzers out there right now. I personally own 1 - CBA II, 2 - BC900's , 2 - MH-C9000's, BT-C2000, and a BT-C3100 on the way. Like them all, for various reasons. Can't beat the CBA II for discharge testing.... But I like the MH-C9000's the best right now for newer batteries, charging, and capacity testing. I like the BC900's for older batteries, cycling older batteries, and CAN"T BEAT ITS DISPLAY METHOD...all batteries at once!
