Discharging LiFePo4

[elewis33]

Is there any benefit to fully discharging LiFePo4 batteries? I just did my first DIY power pack with a 250wH LiFePo4 battery and know that these *can* be discharged "100%", but what does that really mean? At this point I'm trusting my little battery monitor when it says that it's below 12 volts that it's fully discharged. Is that right? And again, is there any reason that you should or shouldn't fully discharge these things?

 

[John Frum]

Always best to check the doco for your specific battery but generally 2.5 volts per cell is considered 100% discharged.
I won't take mine below 3.0 volts per cell though.

 

[Forbisher]

 

[solarsimon]

typically unwise to fully discharge. One cell will have discharged more than its neighbour, so once it's empty & you keep going, it's getting ruined. That's why a BMS is required, in many situations. I'm thinking 15% should be the min, but let some experts say what the sweet point is

 

[JoeHam]

Discharging 100% will definitely shorten the life of your batteries.

Voltage is a poor indicator of SOC for LiFePO4 as you can see in the chart below:

https://offgridham.com/wp-content/uploads/2016/03/lifepo4.jpg

If you use the 12v cutoff you should be safe since that is 3v per cell and there’s not much energy to be had below that level.

 

[elewis33]

I'm gonna try to revive this thread since the topic is similar/related. Now that I've had my little battery pack for a while https://diysolarforum.com/threads/diy-solar-power-pack.8847/ I'm seeing that it seems to self-discharge and shut down after a couple weeks of no use. Even if the battery is charged to 100% (yes, I have a proper shunt/monitor installed) it discharges to who knows what and the battery monitor shuts off.

The battery is a Lion Energy 25Ah LiFePo4 and I was under the impression that there'd be no need to have any kind of disconnect switch, to protect against full discharge (maybe I made that up in my own head ). Am I mistaken? Should I add a disconnect switch?

 

[John Frum]

I'm gonna try to revive this thread since the topic is similar/related. Now that I've had my little battery pack for a while https://diysolarforum.com/threads/diy-solar-power-pack.8847/ I'm seeing that it seems to self-discharge and shut down after a couple weeks of no use. Even if the battery is charged to 100% (yes, I have a proper shunt/monitor installed) it discharges to who knows what and the battery monitor shuts off.

The battery is a Lion Energy 25Ah LiFePo4 and I was under the impression that there'd be no need to have any kind of disconnect switch, to protect against full discharge (maybe I made that up in my own head ). Am I mistaken? Should I add a disconnect switch?

Does the battery monitor show any current going out of the battery?
If it does then a battery disconnect can help.
If there is no current flow out of the battery then its self discharge and a disconnect switch won't help.
The battery monitor itself uses a tiny amount of juice.
Find out how much your battery monitor uses and see if that explains things.

 

[elewis33]

Does the battery monitor show any current going out of the battery?
If it does then a battery disconnect can help.
If there is no current flow out of the battery then its self discharge and a disconnect switch won't help.
The battery monitor itself uses a tiny amount of juice.
Find out how much your battery monitor uses and see if that explains things.

The monitor does not show any discharge. This is how it looks today after recharging fully a couple days ago and after no use. This monitor has a charge/discharge indicator and it's not displaying discharge, with no load connected.

 

[John Frum]

The monitor does not show any discharge. This is how it looks today after recharging fully a couple days ago and after no use. This monitor has a charge/discharge indicator and it's not displaying discharge, with no load connected.

Ok then either have a load that is bypassing the monitor
and/or a battery self discharge problem.
To bi-sect the problem disconnect the poistive battery cable(assuming you only have one) and let the battery sit to see if it discharges.

 

[elewis33]

Ok then either have a load that is bypassing the monitor
and/or a battery self discharge problem.
To bi-sect the problem disconnect the poistive battery cable(assuming you only have one) and let the battery sit to see if it discharges.

I'll do that, thanks. I'm also going to let it discharge and check the voltage after it shuts down the monitor. This could be as simple as a monitor setup setting. But I'm thinking the battery is fully discharged. More testing to come.

 

[John Frum]

I'll do that, thanks. I'm also going to let it discharge and check the voltage after it shuts down the monitor. This could be as simple as a monitor setup setting. But I'm thinking the battery is fully discharged. More testing to come.

Do you mean bms or battery monitor?
They are different things.

 

[elewis33]

Do you mean bms or battery monitor?
They are different things.

Battery monitor, which I installed myself. The battery has it's own built-in BMS so I never get to touch that part of it.