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Need NiMH Battery Help - 2/21/2008 11:20:41 PM   
Bigmax96


 

Posts: 15
Joined: 2/19/2008
From: Portage, MI, USA
Status: offline 		I recently bought a used Losi Mini-T that came with 3 batteries, a Team Orion 7.2V 1200 mAh, another 7.2V 1200 mAh, and a Losi 6V 1100 mAh. I also got a Duratrax Piranah Digital Peak Charger.

The car has the stock ESC and a Reedy Mini-Max mod motor. The issue I'm having is that both of the 7.2V 1200 mAh batteries run very slow and the steering is slow under power when compared to the Losi 6V 1100 mAh battery. When I got the car both of the 7.2V batteries seemed dead so I charged them at 1.0 A (they're both rated for 1.2A rapid charge). They both gave the same result of slow speed and slow steering under power. I then used the "tail light bulb" method to discharge both batteries until the light went out and then re-charged with the charger set on 1200mAh, Auto, & 5mv sensitivity. When checking the charger it ranged from 0.0 - 0.1A for the charge rate and after the first pack was finished it read over 8V. The other pack hasn't finished charging yet so I haven't tried it, but the Team Orion pack is still giving the same result. Any suggestions? Are these batteries hosed or is there something I can do to bring them back to life?

Thanks,

Chad
       Post #: 1

RE: Need NiMH Battery Help - 2/21/2008 11:33:14 PM   
guver


 

Posts: 8975
Joined: 3/7/2003
From: LaGrange, OH, USA
Status: offline 		Hi, welcome to RCU.

My first guess is that each of the 7.2 packs have 1-2 bad/weak cells.

You can probably tell by monitoring the voltage (during the bulb discharge) and will probably read 1-2 volts less than the 6 volt battery. If it's a single tail light then the packs should hold at least 1 volt/cell during the discharge.

To spot the bad/weak cell feel them all during or right after the discharge and/or charge. It will be the hot cell that's bad.

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RE: Need NiMH Battery Help - 2/21/2008 11:57:55 PM   
Bigmax96


 

Posts: 15
Joined: 2/19/2008
From: Portage, MI, USA
Status: offline 		Thanks for the reply guver!

Here's a litte more info... The other pack just finished up charging and here's the voltage readings from them when hooked up to my tail light bulb (1157 bulb hooked up to only one of the elements):

Team Orion 7.2V 1200mAh: 8.0V and 7.4V when hooked up to the light
Boost 7.2V 1200mAh : 8.8V and 7.9V when hooked up to the light (this is the one fresh off the charger)
Losi 6V 1100mAh: 6.5V and 6.3V when hooked up to the light (I have also ran this battery probably 4-6 minutes in the car out on the street, mostly full throttle)

Does this help at all? Neither of the 7.2V packs read less than the 6.0V pack, but their voltage drop under load is significantly more. Any more suggestions? Should I just hook it up to the light and see if one of the cells gets warm?

Thanks,

Chad

(in reply to guver)
       Post #: 3

RE: Need NiMH Battery Help - 2/22/2008 12:21:09 AM   
guver


 

Posts: 8975
Joined: 3/7/2003
From: LaGrange, OH, USA
Status: offline 		Hmm, perhaps I am wrong about the bad cell. Or perhaps the bulb isn't enough of a load.

Could all 3 packs be full charged and left to cool and then do the test again?
Are they AA or 2/3A packs?

Hooking up to the light and checking for hot cell may work good too, and if voltage can be monitored during the test might help spot the weak cell. It may not show up until late in the discharge test.

The mini-T ISN't grossly overgeared is it?

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RE: Need NiMH Battery Help - 2/22/2008 3:18:14 AM   
Bigmax96


 

Posts: 15
Joined: 2/19/2008
From: Portage, MI, USA
Status: offline 		Gearing shouldn't be a problem... it's 15/60 and I think stock was 14/60.

I just got home and the Boost battery (the one fresh off the charger) has been sitting on my desk. Plugged it into the car and it will spin the tires (although not as fast or as much accelleration as the 6V pack) but when I put it on the ground it acts like it's dead and the steering almost doesn't work when giving it throttle. I checked the voltage on this pack and it's 8.4V.

When I did the test before all of the packs were cold (even the one just done charging). And they are all 2/3 cells and not AA.

I can charge all of the packs up again but I don't know if it will make much difference since the two in question, one has about 500' of drive time and the other has about 12 inches. If I charge them back up should I drain them first with the light? And rapid or slow charge?

(in reply to guver)
       Post #: 5

RE: Need NiMH Battery Help - 2/22/2008 12:37:48 PM   
guver


 

Posts: 8975
Joined: 3/7/2003
From: LaGrange, OH, USA
Status: offline
quote:

ORIGINAL: Bigmax96

one has about 500' of drive time and the other has about 12 inches. If I charge them back up should I drain them first with the light? And rapid or slow charge?


This indicates a battery that just isn't charged. Maybe there's a charger problem on the two 7.2 batts. I thihk the best thing is to do a slow charge @100 ma for 15 hrs, then retest or reuse.


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RE: Need NiMH Battery Help - 2/22/2008 11:06:45 PM   
Bigmax96


 

Posts: 15
Joined: 2/19/2008
From: Portage, MI, USA
Status: offline 		Hey guver, here's some more info after some testing...

I finally got some solder so that I could hook up my light right. When I hooked it up before it was on the "tail" element instead of the "stop" element and wasn't pulling much current. I soldered leads onto both elements and hooked up the two 7.2V batteries and here's the results:

Orion battery - 8.0V no load and 5.6V w/ load

Boost Battery - 8.2V no load and 4.9V w/ load

So is this telling me I have a bad cell (or two) in these packs? If so... should I try to find the bad cells by using the "stick pin" method to measure the voltage of each cell under load or hook it up and see if one of the cells gets hot?

Thanks,

Chad

(in reply to guver)
       Post #: 7

RE: Need NiMH Battery Help - 2/22/2008 11:15:49 PM   
guver


 

Posts: 8975
Joined: 3/7/2003
From: LaGrange, OH, USA
Status: offline 		Yep, you got it. The stick pin method is best and will show up imediately when hooked up to that 2 amp load. Both filaments with that voltage is around 2 amps and those 2/3 A batts should carry way over 1 volt per cell.

Orion has 1+ bad one and the boost has 2 + bad ones.


There's a slim chance that it is "cells out of balance" rather than bad cells, but I doubt it. Here's an interesting thread about cells balance if you can wade thru the muck and read the graphs. Post #6 might5 be interesting.

http://www.rcuniverse.com/forum/m_7011783/anchors_7071116/mpage_1/key_mec/anchor/tm.htm#7071116

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RE: Need NiMH Battery Help - 2/23/2008 8:24:47 PM   
Bigmax96


 

Posts: 15
Joined: 2/19/2008
From: Portage, MI, USA
Status: offline 		Well, here's an update after more testing...

The Orion pack had one cell that dropped down to .65V when under load. All the rest were over 1.0V.

The Boost pack had 4 cells that dropped down to .6V - .7V when under load.

I think we may have found the problem... LOL I took the two good cells out of the Boost pack and replaced the bad cell in the Orion with the better of the two, discharged with the light, and now I'm waiting for it to charge.

Thanks again for all the help guver!

Chad

(in reply to guver)
       Post #: 9

RE: Need NiMH Battery Help - 2/23/2008 8:32:44 PM   
guver


 

Posts: 8975
Joined: 3/7/2003
From: LaGrange, OH, USA
Status: offline 		Hmm, that should work.

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       Post #: 10

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