Max Temp for NIMH?

[BP-Revo]

Just like title says...

I just got my ICE charger in and I'm giving a glance over the manual, and it says "dont overheat NIMH packs" but it never gives a ball park temp...

[Procharged5.0]

NiMH Charging (borrowed from powerstream)

Basics

The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse this gets.

The minus delta V bump that is indicative of end-of-charge is much less pronounced in NiMH than NiCad, and it is very temperature dependent. To make matters worse, new NiMH batteries can exhibit bumps in the curve early in the cycle, particularly when cold. Also, NiMH are sensitive to damage on overcharge when the charge rate is over C/10. Since the delta V bump is not always easy to see, slight overcharge is probable. For this reason PowerStream does not recommend using minus delta V as a termination criterion for nickel metal hydride batteries.

As the battery reaches end-of-charge oxygen starts to form at the electrodes, and be recombined at the catalyst. This new chemical reaction creates heat, which can be easily measured with a thermistor.. This is the safest way to detect end-of-charge during a fast charge.

Overnight Charging

The cheapest way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mAH battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours. Examples of this kind of charger are shown at http://www.powerstream.com/NiMHWM.htm. This charger uses a microprocessor to report the state of charge via an LED as well as performing the timing function.

Faster Charging

Using a timer it is possible to charge at C/3.33 for 5 hours. This is a little risky, since the battery should be fully discharged before charging. If the battery still has 90% of its capacity when the timer starts you would have a good chance of venting the battery. One way to ensure this doesn't happen is to have the charger automatically discharge the battery to 1 volt per cell, then turn the charger on for 5 hours. The advantage of this method is to eliminate any chance of battery memory. PowerStream does not currently have such a charger, but the microprocessor board used in the C/10 charger http://www.powerstream.com/NiMHWM.htm could easily be modified to do the discharge. A power dissipating package would be needed in order to dissipate the energy from a partially charged battery in a reasonable amount of time. Another example of a 3 hour charger is http://wwww.powerstream.com/9vnmh.htm . This is a very inexpensive microprocessor based 9 volt "transistor radio" battery charger that that drops to low current when the battery voltage indicates a full charge.

Fastest Charging

If a temperature monitor is used NiMH batteries can be charged at rates up to 1C (in other words 100% of the battery capacity in amp-hours for 1.5 hours). The PowerStream battery charge controller shown in http://www.powerstream.com/product3.htm does this, as does the battery management board shown in http://www.powerstream.com/product5.htm.

This board also has the ability to sense voltage and current for more sophisticated algorithms.

When terminating on temperature rise the dT/dt value should be set at 1 to 2 degrees C per minute.

Trickle Charging

In a standby mode you might want to keep a nickel metal hydride battery topped up without damaging the battery. This can be done safely at a current of between 0.03 C and .05 C. The voltage required for this is dependent on temperature, so be sure to regulate the current in the charger.

dT/dt versus -dV/dt

These two termination methods work well for NiCads, and are both applied to NiMH as well. dT/dt measures the temperature rise at the end of charge. After the battery is fully charged it starts new chemical reactions in order to absorb the unneeded current. In nickel hydroxide style batteries this consists in generating and recombining oxygen. This process heats the battery. The sudden increase in temperature rise can be used to terminate the charge.

Another effect of the oxygen generation/recombination cycle is to depress the voltage of the battery slightly. If you can detect this voltage depression you can use this signal to terminate the charge. Of course, -dV/dt is the easiest because it doesn't require a temperature sensor. The best method for NiMH is the dT/dt method. There are two main reasons. With the NiMH battery the voltage depression is smaller, and harder to detect than with the NiCad battery. This almost always ensures an overcharge, which will limit the total number of charge/discharge cycles before battery failure. Second, a new NiMH battery has false peaks early in the charge cycle, and so the charger will terminate too soon.

At PowerStream we have experimented with a number of -dV/dt algorithms, and have not found one that works well for new and old, hot and cold, fully discharged and partly discharged conditions. For this reason we recommend dT/dt. We don't recommend trying to charge a NiMH battery with a NiCad charger unless it uses dT/dt.

[BP-Revo]

Wow...thanks Procharged...that was alot of information

Does anyone know what mode I should use? The Ice has like 4 different charge modes... I assume linear would be the best?

[glassdoctor]

I didn't read the big post above, hee hee... but here's a quick thought...

Anything under 140* is acceptable imo... but depends on the brand etc. Some are more durable. I recall when I first got some GP3300s there was some battery experts that said 140's were not only ok but that's what they expected for a fast charge peak temp.

Personally I think that's too hot but apparently it's not supposed to hurt them. I usually see somewhere in the 120's when peaked for most of my packs. That's where I like them. It's very warm... maybe even hot... but not HOT HOT. :)

[MetalMan]

120 deg. F will feel like burning to most people, and is about the point where you are just able to pass the 5-second test.

[BP-Revo]

Ok, well on my previous, more-automated charger, the packs would peak at a temp were I could just hold them long as I wanted. Any warmer, and I wouldn't (and I have a low temp tolerance).

I'll give a quick look at the manual and see if the temp is only for backup, and I'll set it at like 120 or so. Should be about right.

What about charge mode? Linear yes?

Any general info on what mode/way to charge will help...

I've never been able to choose so many variables before...my old charger was just set cell type, amps, cell count, safe timer, start...

[glassdoctor]

20 degrees over body temp isn't that bad, you wussy. lol...

[BP-Revo]

lol...wussy?

Buddy, even though we are internally 98.6 degrees, we are more like 87 externally. This is coming from an AP Bio student who has an A without even reading the chapters...So it comes out to like 30+ degrees above...and thats a pretty big difference.

When you are in the hot tub, most modern heaters limit temp at 104. That is pretty darn hot. It burns for the first minute or so when you step in...