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Old Oct 19, 2010, 11:06 PM
So many UMs, so little money.
Nashville
Joined Jun 2008
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Question
Imax B6 isn't discharging correctly

Hi all,

I have an Imax B6 that is not discharging correctly. The charging is fine and seems to be accurate. But when I set it to discharging, it stops very early.

Specifically, I set the cutoff to 5v for a 5 cell NiMh. The charger shows it cuts off at 4.98v or 5.01v usually, but when I go back and check the pack its actually at 6.3v or so.

Anyone have any thoughts on this? I just got it and I was hoping to use it to cycle my new batteries, but that isn't looking too good at this point.

Any help is much appreciated!

Brandon
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Old Oct 19, 2010, 11:10 PM
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dmcquinn's Avatar
Chesterfield (St.Louis) MO
Joined Jan 2004
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The discharging is under some load and when it reaches 5 volts, it shuts off. But the pack recovers to 6.3 volts under no load. So I think what you are experiencing is normal. Are you getting reasonable capacity? In other words, if you cycle a 600 mAh pack, are you seeing a discharge around 550 - 600 mAh?
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Old Oct 19, 2010, 11:14 PM
So many UMs, so little money.
Nashville
Joined Jun 2008
381 Posts
I can understand a bit of load, but 1.3v? I would think there wouldn't be that much drop, but maybe with these batteries being such high capacity (2200mAh), the internal resistance is causing it to be off by that much.

And its only taking about half of the capacity out... usually around 990 to 1300 mAhs.

Thanks for the reply.

Brandon
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Old Oct 20, 2010, 12:52 AM
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United States, WA, Seattle
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dmcquinn is correct. There will be rebound from the voltage sage during discharge. You can DC a NiXX to damn near dead, leave it alone for awhile and it'll tell you it's still 1.2v/cell...but it's an empty promise. If you watch it near the end you can see it. Or just DC it and then do it again and watch. It'll only take a couple minutes to tank again. There is essentially no capacity left.

Don't be surprised if hi-cap NiMh capacity is disappointing. They get those specs from VERY VERY VERY slow discharges (and they lie a lot IMO). I have NEVER gotten the kind of mA claimed on hi-cap NiMh.

Slow charge the pack at 1/10C (220) for ~ 15 hours to make sure it is full. Then DC at 0.2A for a LONG time. You'll see better results more like they get in lab conditions, maybe 80% of claimed has been my experience.
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Old Oct 20, 2010, 09:22 AM
So many UMs, so little money.
Nashville
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Ok. Wow... I didn't think there would be that much drop in voltage.

So with this in mind, I probably just need to charge and discharge very slowly so the voltage doesn't drop too much, right?

Many thanks for your replies, guys.

Brandon
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Old Oct 20, 2010, 11:15 AM
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Here's a lot more info on the subject so we don't have to re-write the book:

R/C Batteries Tech Tips & NiMH / NiCAD FAQ

Technical Tips for NiCd and NiMH Battery Pack Users

Some of my older NiMh RX batteries will kick out almost immediately on a 1A DC but go 800mA at 0.2A DC. They really can't even handle 0.5A DC. Luckily that's all the draw the system needs or I'd have to tear them out (ugh!)
Some batteries are just plain crappy and old batteries get crappy. NiMh go to hell faster than Nicad and do so faster and easier. NiMh ONLY advantage as far as I can see is better capacity. The new Sony Eneloops seem to be a notable exception to that.
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Old Oct 20, 2010, 11:31 AM
So many UMs, so little money.
Nashville
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Interesting. Many thanks for the replies and links, guys!

I'll start a slow cycle when I get home and go from there.

Brandon
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Old Oct 20, 2010, 12:10 PM
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Once you start digging into NiXX technology lipos begin to look easy.
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Old Oct 20, 2010, 01:29 PM
So many UMs, so little money.
Nashville
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I would definitely have to agree. I'm actually regretting my NiMh purchases a little... but at least I can just stick them on the trickle charger for general charging.

Brandon
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Old Oct 20, 2010, 01:44 PM
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What are you using this for? RX pack?
I used to think peak chargers were superior and slow chargers were cheap junk. I now understand they are different tools with different applications, both useful.
Figuring out the delta-V peak cut-off in peak chargers is an art in itself with almost no guidance available and seems to be very pack dependent. In general that charger is set too low, at least my similar clone Accucel-6 is.
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Old Oct 20, 2010, 02:11 PM
So many UMs, so little money.
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Yep. Two Rx packs and a Tx pack.

Now I know, though, so I feel a lot better. I was thinking I had just wasted my money! Turns out that I'm just ignorant... or was.

Brandon
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Old Oct 20, 2010, 05:50 PM
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For TX I've gone to Sony Eneloops. GREAT batteries, truly a cut above even if they are "only" 2000mA. I had a 2500 NiMh TX pack and the eneloops are way better.
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Old Oct 20, 2010, 05:50 PM
So many UMs, so little money.
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Sweet. I'll have to check them out.

Brandon
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Old Oct 22, 2010, 10:07 PM
So many UMs, so little money.
Nashville
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After turning my timer off and reducing my charge/discharge rates, this thing is working like a champ.

Thanks for your help!

Brandon
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