Correct technique of DIS/CHG NiMH and Li Ion

[kalkwasser]

Hi everyone,

I am have some NiMH and Li Ion which I use to run some hotwires/HID.
And wondering what is the optimum technique to discharge and charge them. Maybe this topic has been well covered and mentioned. And I have also read them brieftly.

Just want to sumarize if I have done the correct thing.

1) For NiMH batteries: 7.2V 2000mH (Discharge rate. 0.8C, charge rate 1C)

This is what I do. Since I have read that unlike Li Ion. NiMH batteries prefer to be fully drain before charging them. Hence, I have this battery pack discharge to 6V (1V per cell) before charging them. Usually, it's value is about 8.5V when my charger beep to indicate chargeing complete.

2) For Li Ion batteries: 11.1V 2600MH (3x17670)

For this Li Ion, there is no discharging before charging as I again read that its not necessary to do that for Li Ion. Hence, I "top" them up whenever I think they are "weak". Maybe 20-30 mins runtime on flashlights. Or should I measure it before I charge them again? Setting is 1C charging.


My questions:

1) Is it alright to have a higher discharge rate than a charging rate for both NiMH and Li Ion?

2) And how to determine what charging rate for different capacity of batteries? Etc, a 3.7V 800mH and 3.7V 2600mH?

3) Why there are some Li Ion at 3.6V and some at 3.7V.
Can I use 3.7V setting to charge the 3.6V?


Thank you in advance.
Kalk

 

[SilverFox]

Hello Kalk,

First of all, let's take a look at terminology... Capacity is given in milli amp hours (mAh) or amp hours (Ah). Charging and discharging current is given in milli amps (mA) or amps (A).

Next, NiMh packs don't need to be fully discharged every time. They do benefit from a full discharge around every 20 - 25 charge/discharge cycles, but you don't need to do it every time. Use your pack for awhile, then just charge it back up. Every once in a while, do a full discharge (as you have indicated down to 1.0 volts per cell).

I should add that you need to do a 0.1C charge for 16 hours to get the NiMh cells in your pack in balance. Once everything is in balance, you can use your 1C charge rate. Depending on how hard you are driving the pack, you will periodically have to repeat this 0.1C 16 hour charge to maintain balance. If you drain your pack in 5 - 10 minutes, you will need to re-balance every 5 - 10 cycles. If you drain the pack in 15 - 30 minutes, you only need to re-balance every 10 - 15 cycles. For all other discharge rates, you can do it every 20 - 25 cycles in conjunction with your full discharge.

Now, on to your questions...

1. Most consumer NiMh cells are rated for a maximum discharge rate of 2C. Some can be pushed further, but if you look at the battery data sheets, you will usually find the maximum discharge rate recommended. There are cells that are capable of high discharge rates, so if you need that you must buy the cells that are good for that.

Li-Ion cells are similar. 2C is usually the maximum discharge rate recommended.

2. I assume you are referring to Li-Ion cells with the 3.7 volt reference. Actually both NiMh and Li-Ion cells have a maximum recommended charge of 1C. The ideal charge rate is more like 0.7C. With Li-Ion cells, you can go lower, but with NiMh cells you may run into problems below 0.5C. The exception is the 0.1C 16 hour charge.

3. There are different chemical mixes used in Li-Ion cells. Originally Li-Ion cells were all 3.6 volts and charged to 4.1 volts. Advancements have pushed the nominal voltage up to 3.7 volts and these cells are charged to 4.2 volts. The only consequence with charging a "true" 3.6 volt cell to 4.2 volts (using the 3.7 volt setting) is that you may get reduced cycle life.

Tom