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Author Topic: Li-Po batteries can't hold voltage problem  (Read 1577 times)

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Offline AdminTopic starter

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Li-Po batteries can't hold voltage problem
« on: June 19, 2009, 04:15:37 PM »
I have two 7.2V 2.4Ah Li-Po batteries that both have this problem. They have passed down a few channels in the Navy so I know little about them, but they look like unused custom batteries (I was also told they were good batteries). No datasheets. Upon visual inspection, looks like 2-cell, in series. I was told that it has built-in balancing electronics (but he didn't sound 100% sure). I don't know charging/discharging rates.

I charge them individually using a Dynamite Vision Peak Ultra Battery Charger (on the Li-Po setting). I've tried both 0.5A and 0.65A charging rates. I don't remember exact times, but both charged for about ~1.5 hours, then after waiting a day, I charged them again for another ~30 min.

Both the charger and my multimeter confirm the battery finishes charging at around 8.2V. But here is where it gets strange . . .

Attaching a simple LED/resistor combination, the LED lights up for a second, then quickly fades away. My multimeter reads ~1.4V with that plugged in. Unplugging the LED, voltage jumps right back to 8.2V. This is 100% repeatable.

Attempting to run a servo results in a ~0.4V reading. Unplugging it, back to 8.2V.

Either the charger isn't working, both the batteries are faulty, or I'm doing something really dumb . . . The charger works fine for NiMH batteries (on the NiMH setting, of course).

Ideas?

Offline Soeren

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Re: Li-Po batteries can't hold voltage problem
« Reply #1 on: June 20, 2009, 09:31:53 AM »
Hi,

Off the top of my head...
Many Lithiums have 3 (or 4) connections where the third has got a resistor to positive.
That might do something like that, since your problem is inner resistance (although I don't see why the LED should light initially then, unless a cap were in the circuit)

For correct charge, you should use at least C/2 (1.2A) or C, but I don't think the lower current explains it.
The end voltage of 4V1/cell sounds reasonable.
Charging Lithiums is not done with peak detection, but I'm sure your charger does it correctly if it has a setting for lithium.

Do they contain "smart" circuits/gas gauges? They might then need to be reset.

Perhaps the batteries were discarded for that reason.
Regards,
Søren

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Please remember...
Engineering is based on numbers - not adjectives

Offline Razor Concepts

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Re: Li-Po batteries can't hold voltage problem
« Reply #2 on: June 20, 2009, 11:09:16 AM »
Lipos are rated for 7.4 volts nominal and 8.4 volts on a full charge - the sightly lower 8.2 may indicate the lipos are old and are probably worn out.

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Re: Li-Po batteries can't hold voltage problem
« Reply #3 on: June 20, 2009, 11:21:07 AM »
When I got the Li-Po batteries, they had these plastic sticker tabs on each red/black wire (no 3rd wire). It was as if they were packaged from a factory and never used. Not sure how long they were sitting around . . .

Quote
Do they contain "smart" circuits/gas gauges? They might then need to be reset.
No idea . . . I'd have to break them open to find out. Any ideas on how to reset them?

Quote
For correct charge, you should use at least C/2 (1.2A)
I'll try charging at 1.2A on Monday and see what happens.

Offline Soeren

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Re: Li-Po batteries can't hold voltage problem
« Reply #4 on: June 20, 2009, 06:12:38 PM »
Hi,

If you have to break one open, hopefully a part number will lead to a datasheet that can help you further.
I'd say that there's a good chance that they are too old, but 5..10 of full cycles may help as well - but they probably have to sit on discharge for quite a while - don't load the voltage down too much though.

Lithiums is best stored at around half charge and should never get too low or they'll get hurt - did you measure the voltage when you got them (before charging them?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline TrickyNekro

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Re: Li-Po batteries can't hold voltage problem
« Reply #5 on: June 21, 2009, 02:09:06 PM »
Try 1A..... If the are used for handheld devices.... most of the batteries around 800mAh to 1200mAh.....
So near 1C charging should be fine.....


What are these things made for........??? I mean type of device.... A power hungry on or... any simple device...
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

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Re: Li-Po batteries can't hold voltage problem
« Reply #6 on: June 26, 2009, 12:38:04 PM »
I tried charging them again today. Set the charger for 1.1 A, the charger ran for one minute, then refused to charge any more. It declared 8.3V. Was as if the batteries were already fully charged . . .

I think I'm just going to ditch them and try some others :-\

 


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