Recent build LiFePo4 concern

[shizack2000]

I recently built a front hub motor trike using a Schwinn Meridian, a no-name 1000w motor, and a 48v 20ah lifepo4 battery with "built-in" BMS. The battery looks just like this one https://www.ebay.com/itm/48V35Ah-Te...a=1&pg=2332490&_trksid=p2332490.c100930.m5375 but it wasn't listed as a Ternary and it is 20ah instead of 35. Otherwise identical.
I had put about 40-45 miles on it (under 20mph, mild hills, paved roads and trails except a hundred yards or so of mildly bumpy grass to and from the shed) and the 3-led indicator just started flickering the green under power. Yellow and red were solid. Within a half mile, LVC kicked in.
After a full charge all has been well for about 20 more miles. Fresh off the charger it measures just over 59v and even now it was at 55-point-something resting volts. After LVC it was 36 or 39v (can't remember which).
It seems to be performing fine but it does concern me.
I'm fairly knowledgeable concerning rechargeable batteries(lipo, sla, nimh) but lifepo4s are a new animal to me.
Is something amiss or do I just need to get used to this newer tech?

 

[harryS]

Most of us ebikers don't use LiFEPO4 batteries. You might read up on the differences between LIFEPO4 cells and the round 18650 cells more common in ebike packs. The operating voltages are different,, 3.2V vs 3.6V for nominal, and 3.7V vs 4.2V for max. Both state 2.5V minimum. SInce they are so different, what you might read as typical voltages reported on most ebikes don't apply to LIFEPO4. Your numbers threw me, until I did some calculating.

If your pack is 16 cells in series, then my math says its max voltage would be around 59V, and its min around 38V, just as you said. A comparable 18650 pack is 13 cells in series with a max of 54.6V and a min of 32.5V, but we never get to see that minimum. Ebike controllers usually have their own LVC, and it is set around 40V, because running an 18650 pack down to 2.5V/cell shortens life. I believe you may not want to run your pack down to 39V. I'd quit around 44-45 volts

Finally, your charger is not compatible with a 48V 18650 pack, since its max voltage is 59V. Don't mix chargers if you ever get a regular 48V pack. That would be quite risky. Although batteries have overvoltage protection, sometimes it doesn't work.

Nice thing about LiFEPO4 is that it's reputed to be quite safe, less likely to burn. Not so with 18650.

 

[indianajo]

I keep warning people not to buy batteries off ebay, you didn't listen.
It sounds as if you have 1 or 2 stacks of your 14-16 stack battery actually connected. So you have 1 or 2 AH battery. Sun-ebike of Baldwin city CA sold me a warrenty return, I surmise after finding the build date on the battery, 18 months before I bought it.
To prove it, charge to full. Connect a 5 A or 10 A load. I used 10 ohms at 550 watts; two 5 ohm 225 watt logs series connected. Voltage collapsed to 7 under that load. Spec on a bike battery should be 50 A load. A 20 AH battery should carry a 10 A load at over minimum voltage for 2 hours. Do this outside on concrete; if you start a fire you don't want it in your house/garage/shed. Wear safety glasses.
I didn't get my money back because it took me longer than 31 days to haul the battery to town where my resistors & instruments are to do to the test. I suggest, if it has been less than 31 days, you request your money back. I cut the shell off my ebay battery and proved my theory by probing the BMS with a meter. 1 stack connected.
I got a good, but expensive battery from lunabikes.com ebikeling.com has a good reputation. Prof fate said he got a good battery off amazon from unitpackpower . I'd prefer a LifePo4 due to the flamability of LiIon, but that makes 3 vendors selling trash LiFePo4 they called a battery. My other trash battery was from btrbattery on amazon.