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Old Apr 21, 2003, 10:15 AM
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Nimh Batteries Peak Charge Too Hot!

I'm using a new Orbit V6.2 and the MRC Super Brain 959 to charge my transmitter packs 8 cell AA NiMh 1800 mAh Dynacharge batteries. I use the Radio Shack 8 cell battery holder with the 9 volt type connector. The pack fits perfect in my new Hitec flash 5x. I'm using a Tamyia connector and 18 guage wire soldered onto a Radio Shack 9 volt female connector to perform the charge. I charge them at 2 amps on the Super Brain and C (1.8 amps) on the Orbit using Reflex charge as this is the only way to get them to charge properly on the Orbit. When the peak charge is completed the batteries are very warm and too hot to handle. The batteries work and I have charger them probably 20 times this way without the batteries venting or splitting the side. I would think the shrink wrap would melt if there was shrink around the battery pack which there isn't. Has anyone else experienced this and should I be worried?? Is the Radio Shack holder providing too much resistance for the chargers? I also have a Radio Shack NiMh 1 hour charger that charges 4 NiMh batteries at once and they get too hot to handle also but they work.

Keep 'em up.

Ed
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Old Apr 21, 2003, 11:18 AM
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It's never a good thing to allow batteries to get that hot in charge or discharge. It will most definitely degrade their lifespan and performance and can quit just when you need them the most.
Murphy says so.

mw
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Old Apr 21, 2003, 01:16 PM
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Thanks for your post Mark. Can you recommend something I can try to charge the batteries without them getting hot?

Ed
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Old Apr 21, 2003, 03:11 PM
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Sorry, Ed. I don't have any experience with those chargers. I use a Simprop Intelli-something (like a SuperNova) on manual to charge my nimh packs. I don't trust automatic on my charger either. Had a couple packs get too hot for my tastes.

mw
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Old Apr 21, 2003, 03:41 PM
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Hmm, these "Dynacharge" cells... Are they high-discharge cells? I suspect that they are bargain-basement, no-name, consumer-grade AA cells, meant for digital cameras and kids' toys. They're not designed to discharge at high rates, and as such are not designed to be charged at high rates. In all likelihood, a C/2 charge is probably the highest they'll take without getting too hot.
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Old Apr 21, 2003, 08:27 PM
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Mkirswch,
I think that they were manufactured by Duracell. I peak charged the cells at 1C, are you suggesting they be peaked charged at 1/2 C or 900mAh? I hadn't thought of that. You can find out more about the Dynacharge NiMh cells at the following thread:
http://www.rcgroups.com/forums/showt...=%2Bdynacharge

Tks,

Ed
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Old Apr 23, 2003, 10:39 AM
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The springs on those cheap battery holders will get HOT if you force that much current through them!
I've melted the plastic on one of those before...
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Old Apr 23, 2003, 12:12 PM
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Tks simingx,

That makes sense. Probably too much resistance. Now I will have to learn how to build battery packs.

ED
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Old Apr 23, 2003, 12:33 PM
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Are the batteries getting hot on both chargers? iirc the MRC will only charge up to 7 cells, not 8. The other thing is exactly how "hot" is hot; iirc from Panasonic's tech app publication, the max case temp for a fully charged NiMh at fast charge is 55C/131F (AA cells and up) - that is hot-potato hot. (of course you don't want it to reach 130F; I usually have it set in the lower 120's if I have temp monitoring).
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Old Apr 23, 2003, 12:33 PM
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I've melted those plastic cases when running motors to batteries encased inside them. But that never happened while charging, maybe due to the high resistance not allowing the charger to push much more current into it.
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Old Apr 23, 2003, 02:19 PM
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Quote:
Originally posted by Peter Khor
Are the batteries getting hot on both chargers? iirc the MRC will only charge up to 7 cells, not 8.
Actually, the MRC 959 is suppose to be good up to 8 cells according to my owners manual.

ecweaver

When you used your 959, what was the delta peak set at? If you left it in default mode, it will peak using 40 mV; way too high for a Nimh battery - lower it to 10 mV. Also, lower your charge current to 1 amp. I've tested some of the Dynacharge AA Nimh and found that their capacity are nowhere near 1800 mAh; closer to 1500 mAh actually. They are definitely not meant for high charge/discharge rates either, but when used as you're using them, in a transmitter, they will be fine. I'm using them in my Flash 4X (converted to a 5X) using the same Radio Shack battery holder; works great. I also use the same charger that came with the Flash to charge them; works great.
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Old Apr 23, 2003, 09:07 PM
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Peter,
The batteries get hot on both chargers. Interesting you mention Panosonic AA peaking at 130+ F degrees. That is what I think the temp gets to. Where did you get the info the MRC superbrain charges only 7 cells NiMh?

Babblefish,
I agree that overnight charging using the Hitec charger will work. I haven't tried using the Dynacharge cells in the Radio Shack holder and using the Hitec charger that plugs into the Tx to charge the pack. Is this what you are doing or did you change connectors? There is another thread on RC Universe that says Panasonic suggests NiCads can delta peak at 15-20 mV per cell drop and NiMh can delta peak at 5-10 mV per cell drop. That would put the mV at 40-80 mV for an 8 cell NiMh pack. Others are more conservatinve and say 3mV per cell which would be 24mV for 8 cells. The two charges on the Orbit V6.2 using the Radio Shack holder and the 8 cells Dynacharge 1800 mHa batteries showed 1846 and 1899 mHa charge using Reflex mode at C 1.8 Amps. Delta peak was set to minimum NiMh setting on the Orbit. I don't know what mV that equates to on Delta Peak. Check out the following thread on MRC Delta Peak at:
http://www.rcuniverse.com/showthread...&highlight=mrc
Tks for the info. I will try 1 Amp at 20mV using the MRC Superbrain and see how hot the cells get.

Ed
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Old Apr 24, 2003, 01:08 AM
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I haven't verified it yet, but I think the delta peak setting for the 959 is per cell. One thing for sure, if I let my 959 use it's default setting, my batteries come out way too hot.
As far as charging Nimh in my transmitter, I just replaced the factory Nicd pack with the Radio Shack AA battery holder and plugged the factory charger into the transmitter. Works great and nothing overheats. The transmitter indicates a voltage of about 12.5 volts right after unplugging the charger.
Glad you had better luck with your Dynacharge AA capacity. I couldn't get more than 1500 mAh or so using my Triton charger. The batteries finished their charge warm, not hot so I'm pretty sure they peaked correctly. Maybe I need to cycle them a few more times...
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Old Apr 24, 2003, 01:51 AM
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Ed,

The Panasonic tech sheet is here wrt to temp - also great info on NiMh charging in general (including Delta Peack voltage, etc):

http://www.panasonic.com/industrial/...rgeMethods.pdf

As for charging 8 cell, I don't think the 959 has a voltage boost circuit, so if you're using it off a 12V battery, your max cell count is 7. On A/C, it'll probably do 8 cell I guess (as spec'ed by MRC).

I used to charge a TX pack in a RS holder at no more than 1Amp or so (even with 1600/1800 NiMh) and had no problems, these days that's the max current I charge a TX thru the socket (best deal is to remove the pack for quick charging).
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