Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: thesa1nt01 on July 03, 2009, 08:12:17 PM

Title: Lithium-ion Iron Phosphate Batteries discharging irregularly
Post by: thesa1nt01 on July 03, 2009, 08:12:17 PM

So, here's a thing, this happened to me today:

(http://lh6.ggpht.com/_b1-391vCRLM/Sk66WUo6DfI/AAAAAAAAAKQ/66pNlPs0weI/s800/Battery%20Problem%201.jpg)

I have 4 Lithium-Ion Iron Phosphate 26650's from A123systems wired in series-parallel and the bottom "half" of the cells went dead and the top "half" didn't.

Anyone have any ideas?

(the wire in between is for the charging circuit)(the batteries were not charging when this happened but they were wired up to my microcontroller and servos)

These voltages were measured while still in the circuit and the "0 volts" was actually more like 0.8 volts.

Ideas would be helpful.

Title: Re: Lithium-ion Iron Phosphate Batteries discharging irregularly
Post by: sonictj on July 03, 2009, 08:16:51 PM

I'm not sure but I thought one of my friends told me that if you over discharge a lithium battery the cells will flat out die.  Were you being careful not to pull too much energy from the batteries?  The voltage should never drop below 3V per cell.

Title: Re: Lithium-ion Iron Phosphate Batteries discharging irregularly
Post by: thesa1nt01 on July 03, 2009, 08:39:19 PM

At the time the robot was on but nothing except the Axon was actually doing anything. The servos were connected but just sitting there not moving. I can't imagine what could have got them like that. They haven't been used to do any "work" yet.

Title: Re: Lithium-ion Iron Phosphate Batteries discharging irregularly
Post by: Razor Concepts on July 03, 2009, 08:52:15 PM

Well the batteries should be fine, a123s are really tough so something like this won't kill it completely.

Title: Re: Lithium-ion Iron Phosphate Batteries discharging irregularly
Post by: thesa1nt01 on July 03, 2009, 09:34:27 PM

But any ideas as to what may have caused that?

Title: Re: Lithium-ion Iron Phosphate Batteries discharging irregularly
Post by: thesa1nt01 on July 04, 2009, 11:27:51 AM

UPDATE:

Just before I was about to rip everything apart this afternoon I checked the batteries out with my multimeter again and:

The overall voltage has risen from 3 v to 4.2 v

The middle reading is at 2.5 v now like one would expect it to be.

The charger recognizes all of the cells and is now charging.

I guess the batteries just needed a rest?

Any thoughts?