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LiPo Battery on Holding/Float Charge

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imk:
Hello,
Are there any issues keeping a LiPo battery on a holding charge of 4v?
Basically if i connect a modest load of 40ma or so to a LiPo and then connect the output of a Buck Convertor set at 4v to the LiPo plus the Load are there any issues.
The situation is that i want to run something 24/7 but in case of power failure the device will still work from LiPo power.

I know there are many Lead Acid battery emergency light systems that work like this but how do LiPo's tolerate it?
imk

Nauris:

--- Quote from: imk on June 28, 2021, 02:47:31 pm ---Hello,
Are there any issues keeping a LiPo battery on a holding charge of 4v?
Basically if i connect a modest load of 40ma or so to a LiPo and then connect the output of a Buck Convertor set at 4v to the LiPo plus the Load are there any issues.
The situation is that i want to run something 24/7 but in case of power failure the device will still work from LiPo power.

I know there are many Lead Acid battery emergency light systems that work like this but how do LiPo's tolerate it?
imk

--- End quote ---

I don't think there is any problem with that, maybe 4 V is a bit too high, you should check it does not draw any current after few week at that voltage, even microamperes are cause for concern.

SiliconWizard:

--- Quote from: evb149 on June 28, 2021, 04:01:39 pm ---Typically these kinds of cells don't "like" being kept for long times at either low state of charge or high state of charge.
As I recall there's also some longevity / functionality compromises related to both frequent charging as well as irregular charging.

--- End quote ---

Yep. Well, it would largely depend on how you implement this though. Doing this with a lead-acid battery is much simpler if you want something safe and durable.

What you should usually not do with LiPo is constantly trickle-charge them. Never. You should at least use a safe charging circuit. Barely monitoring the cell's voltage, delivering a charging current if it falls below a given threshold (you mentioned 4V) is a pretty bad idea.

A better approach is to set a much lower threshold, and "isolate" the battery from the load while external power is on. Basically, you'd let the battery self-discharge and only recharge it if the cell's voltage drops below 3.7V, or even lower. Downside is, in the worst case (power loss while the battery is at the threshold), you won't have the battery's full capacity. But that will limit the number of charge cycles to a decent number.

In any case, absolutely avoid constantly charging LiPo batteries with a small current. If said current is low enough relative to the battery's capacity, it may end up degrading even if the cell's voltage has not exceeded the max rated voltage (usually 4.2V). I have actually done something like this years ago with my first LiPo-based backup system, and I was using a proper charger IC though! But with a very low charging current. That would result in the circuit getting in charging cycles way too often, and the battery would end up inflating. A lot. It didn't explode, but that's still scary to see.

thm_w:

--- Quote from: SiliconWizard on June 28, 2021, 07:24:10 pm ---In any case, absolutely avoid constantly charging LiPo batteries with a small current. If said current is low enough relative to the battery's capacity, it may end up degrading even if the cell's voltage has not exceeded the max rated voltage (usually 4.2V). I have actually done something like this years ago with my first LiPo-based backup system, and I was using a proper charger IC though! But with a very low charging current. That would result in the circuit getting in charging cycles way too often, and the battery would end up inflating. A lot. It didn't explode, but that's still scary to see.

--- End quote ---

A large number of phones and laptops do this though. They don't have enough surge capacity from the AC power supply so they pull from the battery, then top it up shortly after.
Not saying its ideal for the life of the battery, but I don't see how its as disastrous as you claim.

edit: https://www.researchgate.net/publication/308854873_Cycle_life_evaluation_of_lithium_cells_subjected_to_micro-cycles

--- Quote ---After about 6ยท10^4 micro-cycles, the capacity is substantially  unchanged  respect  to  the  initial  value,  nearly  of  12.0 Ah, as shown in TABLE IV.

Indeed,  it  is  possible  to  conclude  that  this  kind  of  stress  confirms to have a very low impact on the cell life, mainly in comparison  to  full-depth  cycles,  although  performed  at  lower  current  rates.  Extrapolating  data,  it  is  possible  to  assume  that  the  end  of  life  could  be  effectively  estimated  in  hundreds  of  thousands  micro-cycles,  according  to  the  initial  hypothesis.  Obviously,  since  no  guarantee  exists  of  a  linear  behavior  of  the  cell,  the  test  is  actually  in  progress  to  get  more  accurate  evaluation of the cell life, under the same considered stress.
--- End quote ---

Somewhere from 3.8 to 3.9V should be good, ~50% charge, will vary slightly depending on the cell type. If you want lifespan of the cell only.

amyk:
It's fine. The cell won't draw any current except its tiny self-discharge, and 4V is somewhere around 80% for a 4.2V cell. What kills lions quickly is keeping them at 4.2V for extended periods of time, and more so if they're also kept warm.

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