MidniteTweeker
100 W
- Joined
- May 16, 2008
- Messages
- 121
I acquired a LiFePO4 battery pack back in 2008 as part of the group buy that Joshua facilitated. I realize that this is a controversial subject and I'm not looking to stir anything up. I guess I must have been one of the lucky ones because I experienced no problems with the pack until now. I don't recall the amp-hour capacity of the pack but it has (24) screw terminal 38120 Headway cells arranged in a 12S2P configuration. I believe the Cycle Analyst said 42 volts or so fresh off the charger back then.
The pack was used during the summer for 2 years. A combination of pedaling and motoring yielded a range of about 30 miles. At that point there was a noticeable power loss, but the BMS never shut down. I don't recall for certain but my guess is pack voltage was down to 32-33 volts by then.
The next couple years the pack wasn't used but it was recharged a couple times a year. Lately I've blown the dust off and begun riding again. Fresh off the charger the Analyst said 41 volts and things were looking good. After a couple easy rides with minimal motor usage the voltage was around 38-39. Unfortunately I don't recall the a-h usage. Then a couple days ago during light usage pulling around 100 watts the power cut out completely. This had happened before and turned out to be a thermal overload cutout on the motor controller, usually on a hot day, and caused by an improper component in the controller. Inconvenient, but tolerable. After a 5 minute cool down things would return to normal. But not this time. Power never came back on. The pack had been running strong just before this.
I checked all the wiring and connectors for broken wires, corrosion or loose connections and found none. A couple days later I hit the switch and Hello! The power was back and showing 38-39 volts. But at the first touch of the throttle it cut out again in less than a second. This time I dug into the battery looking for loose screw connections and found none. The metal straps between terminals were bright and shiny. The fine copper wires running from the straps back to the BMS were enclosed in woven spaghetti tubing and secured to the straps with some white goop as a strain relief. No broken wires were observed, but the goop was haphazardly applied and I'll redo it with acid-free RTV when I reassemble the pack.
The pack was recharged and measured 41 volts. Then the connecting straps were removed and individual cell voltages measured. The 24 cells range from 3.25 to 3.60. Because every 2 cells were paralleled the voltage of each pair is nearly identical. I almost expected to find a dead cell but didn't. The median is around 3.37. I've been fooling around with RC planes and LiPo batteries the last couple years, and this degree of voltage spread between individual cells of a battery pack would not be acceptable. My RC balance charger typically balances to within .1 volts. Not sure whether a spread of nearly .4 volts in this kind of pack is normal or not.
Basically I know nothing about the BMS that came with the pack. I don't know the low voltage cutoff, or even whether it's a balance charger. Now I'm wondering if one or more of the lower cells are sagging under load and causing the BMS to shut off the pack. Should some cells be replaced? If I used 3.30 volts as a lower limit to determine how many, 6 would qualify. I could probably delay replacement by charging at more frequent intervals, but wouldn't the usable amp-hours continue to diminish until the pack was basically useless?
I'm hoping for suggestions and ideas. I thought LiFePO4 was supposed to have more life than this. The pack probably has no more than 50 charges on it. Apparently a new cell has around 3.6 volts. Is there a way to revive these, perhaps by cycling individually with my RC charger? Could the problem be something else?
Thanks,
MT
The pack was used during the summer for 2 years. A combination of pedaling and motoring yielded a range of about 30 miles. At that point there was a noticeable power loss, but the BMS never shut down. I don't recall for certain but my guess is pack voltage was down to 32-33 volts by then.
The next couple years the pack wasn't used but it was recharged a couple times a year. Lately I've blown the dust off and begun riding again. Fresh off the charger the Analyst said 41 volts and things were looking good. After a couple easy rides with minimal motor usage the voltage was around 38-39. Unfortunately I don't recall the a-h usage. Then a couple days ago during light usage pulling around 100 watts the power cut out completely. This had happened before and turned out to be a thermal overload cutout on the motor controller, usually on a hot day, and caused by an improper component in the controller. Inconvenient, but tolerable. After a 5 minute cool down things would return to normal. But not this time. Power never came back on. The pack had been running strong just before this.
I checked all the wiring and connectors for broken wires, corrosion or loose connections and found none. A couple days later I hit the switch and Hello! The power was back and showing 38-39 volts. But at the first touch of the throttle it cut out again in less than a second. This time I dug into the battery looking for loose screw connections and found none. The metal straps between terminals were bright and shiny. The fine copper wires running from the straps back to the BMS were enclosed in woven spaghetti tubing and secured to the straps with some white goop as a strain relief. No broken wires were observed, but the goop was haphazardly applied and I'll redo it with acid-free RTV when I reassemble the pack.
The pack was recharged and measured 41 volts. Then the connecting straps were removed and individual cell voltages measured. The 24 cells range from 3.25 to 3.60. Because every 2 cells were paralleled the voltage of each pair is nearly identical. I almost expected to find a dead cell but didn't. The median is around 3.37. I've been fooling around with RC planes and LiPo batteries the last couple years, and this degree of voltage spread between individual cells of a battery pack would not be acceptable. My RC balance charger typically balances to within .1 volts. Not sure whether a spread of nearly .4 volts in this kind of pack is normal or not.
Basically I know nothing about the BMS that came with the pack. I don't know the low voltage cutoff, or even whether it's a balance charger. Now I'm wondering if one or more of the lower cells are sagging under load and causing the BMS to shut off the pack. Should some cells be replaced? If I used 3.30 volts as a lower limit to determine how many, 6 would qualify. I could probably delay replacement by charging at more frequent intervals, but wouldn't the usable amp-hours continue to diminish until the pack was basically useless?
I'm hoping for suggestions and ideas. I thought LiFePO4 was supposed to have more life than this. The pack probably has no more than 50 charges on it. Apparently a new cell has around 3.6 volts. Is there a way to revive these, perhaps by cycling individually with my RC charger? Could the problem be something else?
Thanks,
MT