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Old 11-21-2009, 01:46 AM   #1
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Default Are my battrey packs messed up? 6 cell reading 8 volts charged, 7.5 discharged

Ok, so my nimh batts SEEMED fine for the longest time, but today when I tried to charge one of them, it triggered the peak detection in like 5 minutes. So I was kind of curious and stuck the battery in my truck. It also registered something like 8 volts, despite it being a 6 cell pack. It ran like a fully charged pack for maybe 5 minutes.

I threw it back on the charger and this time it seemed to charge normally, but it still read 8 volts. I charged my other pack and it was also reading 8 volts. I then discharged both packs by driving the truck til it would barely move on its own power, and then used an automotive 1157 bulb to discharge them until the bulb wouldn't even glow. Brought out the multimeter again and this time the pack read about 7.5 volts? wtf?

So I guess what I would like to know is if anyone has measured the voltage of their 6 cell packs before and after discharging them, and what kind of voltage did they get? Because I find it very odd that the 7.2v battery would be 'dead' and still read 7.5 volts.
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Old 11-21-2009, 07:55 AM   #2
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Quote:
Originally Posted by ZiG-87 View Post
Ok, so my nimh batts SEEMED fine for the longest time.............
like anything, battery packs don't last forever.
and not knowing what kind of packs you have, how old the packs are, how much they've been run, or what kind of charger you're using, or how you've cared for them (or not) -- there's really no telling what's up with your packs.

but, usually, it comes down to "user-error"
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Old 11-21-2009, 09:27 AM   #3
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When NiMH pacs peak quickly if the pac is not hot it has either false peaked or it was pretty much charged to begin with. Typical NiMh cell fresh off charger can easily have 1.35-1.38v/cell making a 6 cell pac read 8.1 - 8.3v. My last set of matched SMC sub Cs were 1.497v or 8.9v if assembled in a 6 cell pac.

NiMh recovers voltage very quickly, as soon as you take the load off, the voltage climbs rapidly. If you want a meaningful discharge voltage, measure voltage before you pull the load off. When I set mine on a discharge tray thats set for 0.9v/cell, it reads 5.4v for a 6 cell pac when they're done.

Try charging again, if you have a peak delay set it for 10min. Pack shold get hot if its charging correctly. If not then try another charger, I've not seen every cell in a pack go bad at the same time, yours may have, but I woulndn't believe it til I saw same results using another charger.
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Old 11-21-2009, 01:14 PM   #4
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Ok, thank you very much. Good to hear that 1.2v cells can actually read higher once they have peaked.. as for discharged voltage, I'll have to go set mine up again on the discharger and see what it reads. I guess what happened was I did have a false peak, because the first sign to me that something was wrong (other than the time) was the fact that the pack was not warm at all.


Just for reference, and for justanotherdude's sake, They are racers-edge 3300mah packs. I have been using them since mid summer, with maybe a charge a week, roughly. Maybe more. I have done my best to take proper care of them, discharging them when not in use, etc. I fully expected that I might screw 'em up at some point, but I've done my best to prevent that. My charger is an onyx 200. No peak delay, unfortunately.
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Old 11-21-2009, 01:48 PM   #5
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I looked on Racer's Edge and those appear to be stick pacs. There is just so much you can do as far as discharging stick pacs. With the cells connected like they are, there is no way to insure each cell is getting to the same voltage like with a discharge tray and a side-by-side pac. With a side-by-side pack and a discharge tray, you discharge each cell individually down to your selected discharge voltage. When you remove from the tray and charge, you know you are starting each cell at same voltage. This is what leads to better performance. This is why racers use side-by-side packs. Plus when a cell does go bad, it can be easily replaced.

All you know when discharging a stick pack is that at some point the total of the cells reached the pack discharge value. You can damage a stick pac like this by overdischarging and actually reversing a cell. When this happens, that cell usually turns into a bad cell and pack performance falls way off. I would only discharge them til I see the lights dim noticeably and then pull them off, I don't think going lower helps the pack and it carries risks that may lead to a damaged pack.
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Old 11-21-2009, 09:01 PM   #6
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Yeah, they are stick packs. And I don't feel terribly inclined to cut them part and try to match the cells and all that. Not on a $25 battery. I just cycled each pack once and they seem to be ok now, but I guess only time will tell.

A lipo setup just made the top of my christmas list though.
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Old 11-22-2009, 06:08 AM   #7
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most stick packs are cells that didnt come from the cream of the crop. so higher charge voltages are common (goin to 10v or 11.0v under charge load back in the old days). ur method of discharge is a bit deceiving to you. idk what charger your using, but if available use ur discharge function to take the packs down and see what the voltage is as ur doin so. u were measurin the packs at rest so i can be a bit deceptive.

issue occur in nimh when cells rear 0 or near zero (and seemingly need a 'bump' to 'wake up') which is BAD or when a cell goes far higher on voltage vs the others (say 1.7v or higher, i've seen em go to 3v underload) that equals VERY BAD. either case means chuck the cell.

sport packs can and will go higher 10v, but honestly good matched nimh cell really shouldnt go past 9.5v under charge load (near completion of charge) before start thinkin bout replacement.

general rule we had back then was save ur killer packs for big races, near new batteries that would peak as close to 8v and possible, hopefully no more than 8.5v. club racing, no problem packs that would peak from 8.8v to 9.2v. once batts went past about 9.3v, they were just practice units or we'd hand em off to a kid.

lotta folks (includin myself when i was a kid) didnt understand that a pack that peaks at a lower voltage is superior to that of a pack that goes out to 10, 10.5, or even 11v under load.

nimh still want to have a cutoff on ur discharge. .9v per cell. not .5 or even .7. .9 is the tool.

granted now with lipo thats all gone. love not havin to think bout it. can relax on raceday. as u can see i'm postin on here instead of preppin my batts for today.

onyx 230 charger (70bux) and 1 lipo (65bux for a good 1) is the whole day right there. no more 'battery cases' and 'discharge/equalizing equip'.

nice isnt it?

R
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