Can you float charge lithium if you stay below 100% SOC?

[smily03]

Totally random question that I haven't been able to find an answer to with my Google-fu.

I know that you can't continuously charge lithium batteries if you keep them at 100% SOC, because of the risks of over-charging, dendrite formation, uncontrolled disassembly, etc.

But, I know that you don't have to charge to 100% SOC, and it's recommended not to for the cycle life of the battery. So, does that mean if, for example, you only charge your cells to 4v, since they'd never reach termination voltage, you could "float" charge them continually, without risking damage to the cells?

Here's my test rig -- I've got a CV/CC regulator set at 1A and 12v output. It's hooked up to six "salvaged from my old laptop batteries" Samsung ICR18650-26A cells in a 2P3S configuration, bottom balanced, with a protection board (just in case.)

Would it be safe to leave the buck/boost always connected to the cell pack, providing a constant potential source of current? Or would that cause risks, even though I'm not charging to 100% SOC?

[Mike90250]

Set your Absorb to the max voltage you want to charge to, and set Float to 0.2V below that, they won't discharge much, and any loads will be picked up by the Float charger (within limits)

Anyway, I think thats how it works. Since I don't have Li batteries, I've not done a lot of research, but I think thats pretty much it

[jflorey2]

But, I know that you don't have to charge to 100% SOC, and it's recommended not to for the cycle life of the battery. So, does that mean if, for example, you only charge your cells to 4v, since they'd never reach termination voltage, you could "float" charge them continually, without risking damage to the cells?

Here's my test rig -- I've got a CV/CC regulator set at 1A and 12v output. It's hooked up to six "salvaged from my old laptop batteries" Samsung ICR18650-26A cells in a 2P3S configuration, bottom balanced, with a protection board (just in case.)

OK, so you are charging lithium _ion_ batteries (as opposed to LiFePO4.)

In that case, your two worst enemies are temperature and voltage. And high SOC's go along with high voltages. You can mitigate this by charging to lower termination voltages - 4.05 volts per cell, for example, will give you about 80% SOC. If you can live with lower than that, do it. The lower the voltage the longer the batteries will last. In addition, disconnecting after termination, to allow the cell voltages to droop, will also help.

[karrak]

If you float the batteries at less than 4.2V/cell (~100%SOC), the battery is balanced and all the cells have roughly the same capacity and you have a protection board I can't see there is a safety issue. By the way what sort of "protection board" are you using?

I would check the voltage of the individual cells in the battery when it has been on float for some time to make sure the battery is still balanced at the float voltage you are using.

I agree with jflorey2 about running the float voltage around 4.0V per cell. I haven't noticed much "droop" in voltage with my lipo ebike batteries after disconnecting the charger if the charge current has reduced to around zero. I charged my lipo ebike battery a few days ago and the cell voltage has only drooped ~0.020V even with a small continuous power draw from the BMS. I would think your LCO battery will be similar. I haven't come across much information about what effect holding the voltage of the battery with an external power supply at the same voltage as the resting voltage of the battery at a particular SOC has on the lifespan of the battery.

Below are some graphs of how storage SOC and temperature affect the lifespan of lithium ion batteries. As you can see the type and quality of the lithium ion battery makes quite a difference. Unfortunately they did not test the LCO cells that you are using.

If you are going to be floating the battery for a considerable time without using it it would be good to only charge it to less than 50%SOC






Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
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