NiMH Smart Charging - need to know this...

[WarrenI]

This is for everyone out here who are really into charging NiMHs using smart chargers. I ran into a an issue with my Quest Q2 charger as it did not seem to detect a full charge status from cells that were on the outside ports. The charger kept being in a quick charge mode, which will overcharge the cells. The middle two ports were not having problems. I was charging the cells with the door of the charger open to cool the cells. Well this problem did not happen every time, where sometimes it was OK...

Well, I tried charging the cells with the door closed and the problem did not happen. What I suspected, was that my Q2 had a thermal shutoff instead of the negative delta voltage sensing. The side ports was having the heat of the cells being vented to the side without a cell...

I dropped an e-mail to the Quest people and guess what! They were very responsive and did confirm that the Q2 is a negative delta voltage sensing unit, but what I was experiencing was being caused by the cells being allowed to cool. Their tech support mentioned that the door of the charger should remain closed during charging. The cells need to heat up to a certain level, then the chemical reaction will cause a moderate internal drop in resistance, and trigger the negative delta voltage state...

This is especially true if the charger need several hours to charge a cell. The heat generated may not be too great, so it need to retain the heat to cause the effect. The Ray-o-Vac 1 hour charger may not require the door, since it uses much more current, it will generate much more internal heat to cause the effect. Whatever heat escapes may not make a big difference...

Thanks to Harding Energy (the Quest people) for this informative look at NiMH smart charging...

 

[Saaby]

Ah but not all chargers are this way. The Maha rapid charger that many many people love, including myself, specifically specified to charge NiMh cells with the door OPEN. NiCds don't seem to be a problem though. It uses the voltage drop without temp sensing so aparently it overcharges the cells a bit but NiMhs can take a lot more abuse than people think they can so I'm not worried.

 

[lemlux]

Different NiMH's and NiMH groupings may show different Delta Peak Voltage performance.

That's why I really like my MRC SuperBrain 912. It displays charging voltage. It is presettable to charging voltages of 0.5 V to 4.5 V in 0.5 V steps. It is programmable to switch into a short 100 mA trickle charge mode after a programmable delta peak of from 0.005 V to 0.05 V. It charges anywhere from 1 to 8 cells at a time (except 2), All this for $46.99 made it a good addition to my charger collection.

 

[WarrenI]

Saaby; As the people at Quest mentioned, if the charger is a much more rapid as in the Ray-o-Vac and the Maha rapid version, the door may not be need as the cells will produce much more internal heat quickly to cause the negative delta voltage effect. In fact looking at the Maha charger, it charges two cells at once. It would only need the effect to be caused on one cell to trigger a full charge condition. My bet is that you middle located cell is the one who does this triggering the most. As for the Ray-o-Vac, it is like the Quest Q2 as it is as single cell monitored charger. The difference is that it charges the cell a lot harder and the increased internal heat causes this effect, regardless that it is an open door charger. The Q2 is not a thermal shutdown charger, but the heat effect is directly related to the negative delta voltage.

 

[JollyRoger]

Interesting....I've been doing a lot of research on chargers lately, and decided that the Quest Q2 was the best value (at the $21 I found it). I am willing to pay around $30 for a much superior one, but is there?

I really like the Rayovac 3-in-1 charger (wrote about it in another thread). It is a slower charger (overnight) but is smart. It can charge 4 different cells, AA or AAA nimh, nicad, or Renewal alkaline. Pulses current at around 200mA when charging. Then switches to trickle mode. I believe this might detect a full charge in the same manner as the Quest charger, since it has a cover, and when I kept it open and tested a fully charged battery in the charger...it didn't stop charging it.
At $9.99, it's a STEAL!

 

[**DONOTDELETE**]

I'm not sure about the delta stuff but, I can tell you the Maha777 won't stop charging 8 Maha nimh C cells...gets only slightly warm...
'Superbrain' sounds interesting though..

 

[JollyRoger]

Lemlux, where can I find the MRC SuperBrain 912?

Sounds interesting....

 

[lemlux]

JollyRoger:
http://www2.towerhobbies.com/cgi-bin/wti0001p.pgm?Q=1&I=LXAVN1&P=7

Have fun

 

[Saaby]

No the reason (They claim) for the open door on the MAHA is (Wait. This is kind of what you said) that the batteries can get too hot with it closed. Melt the plastic

no I don't think they get THAT hot but it's better for the cells. Won't ruin them if you charge door closed though I suppose.

 

[JollyRoger]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by WarrenI:
....Their tech support mentioned that the door of the charger should remain closed during charging. The cells need to heat up to a certain level, then the chemical reaction will cause a moderate internal drop in resistance, and trigger the negative delta voltage state...

Thanks to Harding Energy (the Quest people) for this informative look at NiMH smart charging...<HR></BLOCKQUOTE>

If this is indeed true, then individual battery compartments would be better...or more isolated compartments at the very least....since this charger can charge a mix of different batteries....and individual compartments would be better for charging one or two batteries at a time.

 

[WarrenI]

Good questions all the way around! It's interesting to note that there is a direct relation to the spike of heat produced as to detecting a full charge of the cell by monitoring for the negative delta voltage effect. It's kind of like running out of coolant in your car. Not only will your engine overheat, but the oil pressure will drop due to thermal breakdown. I guess this is the cause and effect. Might be wise to keep our doors covered when charging, due to the issue of overcharging NiMHs are the leading cause of cell failures...