TimeUSB 140Ah LiFePO4 tested capacity and concerns

[Flyview]

Hi everyone!

I just bought two of these TimeUSB 140Ah LiFePO4 batteries to replace my two 10 year old 100Ah LiFePO4 with no BMS (no protections).

Has anyone on here got the same battery? They use pouch cells and are about the size of a regular 100Ah with prismatic cells. I did my best to test the capacity with a (cheap) shunt and battery monitor I've had for years. My first test came out at ~172Ah for the first battery and ~180Ah for the second. I went and bought a hall sensor DC clamp meter and "calibrated" my little battery monitor at 50A and ran the test again at about 50A discharge rate, with the batteries both hooked up in parallel this time, and got about 325Ah or ~162Ah each. Is it possible that they are that much over rated capacity? I will be getting a Victron battery monitor soon and run a hopefully more accurate test.

One concern I have is that the new batteries don't seem to have any "absorption" time and actually turn off when they are fully charged. As soon as they hit 14.5-14.6V they stop accepting charge. Then something weird happens: the batteries will provide some voltage but not what you would expect. If you put on a small load, the voltage drops pretty quickly from 13.6V to something like 13.1V and eventually the battery will come back online and go back up to 13.7V and start discharging normally. I don't have much experience with batteries that have a BMS. Is this normal? In practice this isn't the best behavior because when the batteries are in their "off" state after fully charging, the solar charge controller is powering everything directly and my fridge makes the voltage jump all over the place from 13.8V to 15V as it tries to maintain a 14.4V absorption voltage. Should I turn off absorption altogether? Even with absorption off there will be a gap in time from when the batteries turn off from being fully charged until they start supplying their 13.6V under float conditions. Thoughts?

 

[sunshine_eggo]

I expect a quality battery to be 5-10% over rated. This gives the manufacturer some wiggle room on end of life by having more "ah" to lose before the 70-80% rated capacity is hit.

Charging to 14.5-14.6V requires cells to be in near perfect balance. This is rare for new batteries. They were made at least 90 days ago, and they had to ship at < 30% charge to meet hazmat requirements.

The odd voltage behavior you observe is typical of some batteries. When the BMS hits over-voltage protection, some report a false low voltage and then revert to actual when a load > XA is applied.

Recommend you set absorption to 13.8V and float to 13.5V. If you can set the absorption period, limit it to 2 hours. After awhile, the cells will eventually balance out allowing faster charges to higher voltages, or you can leave it at 13.8 and potentially experience improved cycle life.

Since you're pulling > rated capacity (subject to confirmation with Victron shunt), it's a pretty minor imbalance.

 

[Flyview]

Thanks for the advice! So it was one of the inner cell voltages getting too high at the 14V+ voltage causing the shut down, but if the cells were better balanced it would remain on into the higher charging voltage. That's good to hear. Assuming the cells are perfectly balanced, would leaving them charging at say 14.4V never trigger a cell over voltage protection? I suppose if you don't care too much about the speed of charge, absorption at 13.8V would still fill up the cells?

What is a BMS doing in order to balance the cells, and why does it take so much time? I figured it was always balancing them, so I don't understand why they would be out of balance just by sitting for 3 months.

Sorry for all the questions! I'm very interested in understand how things work.

 

[sunshine_eggo]

Thanks for the advice! So it was one of the inner cell voltages getting too high at the 14V+ voltage causing the shut down, but if the cells were better balanced it would remain on into the higher charging voltage. That's good to hear. Assuming the cells are perfectly balanced, would leaving them charging at say 14.4V never trigger a cell over voltage protection?

Yes, but you never want to hold them at 14.4V indefinitely. It will degrade the cells.

I suppose if you don't care too much about the speed of charge, absorption at 13.8V would still fill up the cells?

I got to 99.7% @ 13.6V, but it took a long time:

LFP full charge @ 3.4V/cell

Thought I'd try a little something on a cheap 12V I've been testing... Charge to 20A @ 13.6V (3.4V/cell), 2A tail current. When testing at cell level and dealing with testers that don't have separate sense leads, lead resistance plays a big role, and I would typically observe cells get charged...

diysolarforum.com

What is a BMS doing in order to balance the cells, and why does it take so much time?

Most BMS have very small resistors that burn off excess charge (passive balancing). The current tends to be very low, like 0.035 - 0.070A, so it can take a very long time to bring them into balance. This typically only happens when cells rise above about 3.40V (some may require higher) where the voltage curve is accelerating upwards. The time batteries actually spend in this condition can be very limited.

More expensive BMS use active balancing where charge is MOVED from higher voltage cells to lower voltage cells.

I figured it was always balancing them, so I don't understand why they would be out of balance just by sitting for 3 months.

In the operating range (about 3.0-3.4V/cell), the voltage curve vs. SoC is very flat, so there tends to be very little cell voltage difference except when near full charge or near empty. Example:



Balancing in the mid range can actually be counter-productive as all cells might have slightly different voltage to SoC relationships, and it could make the balance at the top worse.

In your application, balance only matters at 100% charge. That's how you maximize your capacity.