Rule for discharge rate of Nimh?

[SuRgE]

Please forgive such a newbie question. Can someone give me the "C" rule for rate of discharge and amp handling capability of Nimh cells?

I read in another thread that Lithium ion cells can only handle 2C discharge and that is why higher powered lights use Nimh cells. I assume cells such as the 1700 elites have a discharge of greater than 2C and that are better than 3300mah C li-on cells when it comes to bulbs 50 watts and up?

Just curious so that when I decide on putting together another torch; I use the right type of battery to achieve the desired Volts and discharged amps.

Thanks all

 

[mdocod]

with NIMH you can pretty much do whatever you want and still be safe. They are very safe so even heavily abusing a NIMH cell doesn't USUALLY lead to any personal injury or property damage.

As for the "rule."
There is no specific rule, the rule is laid out per cell by the manufacture. Some cells, like the Elite 1700s you mention, can tolerate over 20 amps, which means OVER 10C discharge rates no problem. It can not only deliver this current, but hold voltage up around 1.1V while doing so... pretty amazing..

Take a cheapo random consumer grade cell, and you'll find that many of them will deliver 5+ amps, but some of them will loose so much voltage to internal resistance that they are not good performers at it.... working a cell hard like that that isn't designed for it will wear it out faster. Seems like the first problems to develop on consumer grade cells pushed hard is faster self discharge, followed by reduced capacity.... Cells like the Sanyo Eneloops seem to be the exception to this rule, as they are "consumer" grade cells that seem to handle 5-10 amps very respectably.

 

[Mr Happy]

To add just a tiny note of caution -- "safe" is a relative word here. NiMH cells can release a lot of energy in a short period of time. If you accidentally shorted a battery pack, things could get very hot, very quickly. Hot enough to burn you or cause a fire.

That nice 100 watt bulb contains a bit of metal glowing white hot. Metal doesn't necessarily have to be inside a glass envelope to glow white hot like that!

Treat high capacity cells with respect, whatever kind they are.

 

[worldedit]

The Problem is the inner resistance of the cell. At low current, this doesnt matter. But at high current you produce a Voltage drop over that resistance.
U=R*I
Since this Voltage drops inside the cell you measure less Voltage on the cell. This results in a power waste of:
P=R*I^2
You can see that for examble double current draw will heat up the cells 4 times faster.
Solution: buy cells with low resistance like eneloops or cool them (less efficient)

 

[SuRgE]

So basically, I really shouldn' t be using cells such as the C lion protected 3300mah, protected 18650s, or 17670s to power bulbs that draw 6+ amps. Stick with high quality NIMHs such as the Elite 1700s or better for bulbs such as the 50-100 watt Oscrams.

I can always depend on you all; Thanks again,

 

[SilverFox]

Hello SuRgE,

Quality cells are generally good for 2C, but sometimes they go up to 3C.

This is really an area that you need to refer to the manufacturers specification sheet, or do some testing on your own. One of the reasons I put so much effort into cell testing is to understand how hard you can push different brands of cells. As you may have discovered, a lot of the "off brands" do not make their data sheets available.

The next "issue" has to do with the size of the cell. Sub C cells seem to have the right shape for very high drain capabilities. AAA cells seem to have the worst shape.

Each manufacturer has the ability to produce cells capable of higher currents than their normal cells. I believe they are motivated by money as far as high current cell development goes. This seems to be a specialty area.

With all of that said, a formula for maximum continuous current draw surfaces from time to time. I am not sure if it applies to all cell sizes, but it seems to work pretty well for Sub C cells.

Max continuous current Amps from a cell = weight in grams * 14400/(mAh*mOhm))

So, if we look at a GP 3900 mAh cell with 0.006 ohms of resistance that weighs 63 grams, we come up with a maximum discharge current of around 38 amps. Interestingly, GP gives 30 amps as a maximum current for this cell, so in this case the formula seems to work.

It tends to get quite optimistic with cells of very low internal resistance. For example looking at the GP 3700 cells, we have an internal resistance of 0.003 ohms and a weight of 64.5 grams. This gives a maximum current of around 83 amps. GP only rates this cell at 30 amps, but it may be possible to push it a bit harder due to the reduced internal resistance.

Please note that I took the information I used for these calculations from the data sheet. I would normally do my own measurements and use that information when trying to figure out if a cell will work in an application.

If nothing else, the formula is fun to play with...

Tom