3.7v LIPO - What is considered low battery

[gregcope]

Hi All,

When monitoring a LIPO 3.7v battery what do people consider "low volts" aka needs attention / charging?

Sparkfun have this example graph.  3.5v seems to be the start of the droop, 3.4v you are well on your way down to 3.3v where I assume beyond that the Moteino is likely to misbehave.  3.0v and its dead.  I am assuming 20C use cases (ish).



Does anyone have a view other than say 3.4v and your 90% done?

[WhiteHare]

Is it 20C in short bursts or something?  Because otherwise it looks like your battery is dead  in 3 minutes or less at 20C.  In that case you're "low" before you even get started as far as issuing a warning is concerned. 

[gregcope]

Sorry - missed that.

I think the discharge rate is high - but from what I could see other charts are the same at slower discharge rates.

My question is about what voltage I should assume it is short, rather than how long it would last :-)

[TomWS]

Greg, did you ever get the answer you were looking for?  I didn't quite understand your question, but I'm not sure you got your answer.

Tom

[Felix]

Based on my own load tests of LiPos (1500mAh and 2000mAh) it's pretty much considered empty at around 3.3V. It will still work with low power nodes I guess but by that voltage it discharges rapidly.
For Moteinos that have a regulator, I would recharge around 3.55V. The bulk charge of a LiPo is above 3.5V anyway. Below that it starts to decline quite fast.

[gregcope]

Tom - not until Felix's.

Basically I was wanting to know when 0% full is, and when to recharge based on a voltage threshold.

Thanks Felix.

I think my pseudo logic is;

# assume charger is not on, switch off it is is AND if charge time is longer than 1hr
if ( isChargerOn() && chargerONTime-Now() > 3600000 ) { chargerOff{} }

# checkBat Vols, if low, put charger on
if checkBattVolts() < 3.55 { chargerOn(); chargerOnTime == Now() }

[TomWS]

Tom - not until Felix's.

Basically I was wanting to know when 0% full is, and when to recharge based on a voltage threshold.

Thanks Felix.

I think my pseudo logic is;

# assume charger is not on, switch off it is is AND if charge time is longer than 1hr
if ( isChargerOn() && chargerONTime-Now() > 3600000 ) { chargerOff{} }

# checkBat Vols, if low, put charger on
if checkBattVolts() < 3.55 { chargerOn(); chargerOnTime == Now() }

Ah, ok.  IIRC, in your project you want to control WHEN you charge and, from this thread, the question is, When it is TIME to charge (ie, power availability) at what battery voltage DO I charge?   If this is close, let me know, but I will point out that the battery should be loaded with a 'normal' load when making this determination.  Measuring with no load will definitely give you the wrong information.   In my gateways I throw on a 100mA load when I test the LiPo.   

I think Felix is right, 3.3V is precariously close to the cliff.  I'd try to charge before the voltage gets below 3.6V.  Below that you need a lot of current to refresh the battery (or a cloud might block the sun and it's too late).

I'd charge whenever power is available just to keep it topped off - the charging current will be low if the battery is close to fully charged.  The MCP7383x does a good job of safely charging.

I think you threw the conversation a-kilter when you talked about 'xC', this has little to do with what I think your interest is.

C, as it pertains to batteries is the current the battery will supply for an hour (ie, mAH rating of the battery).  C is typically used when discussing maximum discharge and charge rate specifications of the battery (you can safely discharge up to 2C (eg 4A for a 2000mAH battery) in most LiPos). 
It can also be used to normalize discharge curves for different capacity batteries.  In the curve you cited, I think the disharge rate was 6C.  This would be the same curve for a 100mAH and an 1000mAH.  The only difference being the actual amount of current you're drawing from each of the batteries.

Tom

[WhiteHare]

There's one LiPo religion  which roughly says that if you  recharge a LiPo when you're about 20% between full and empty and stop charging when your still 20% from the usual cutoff for full, then your battery will last a lot longer than if you always take it to the full extremes. An opposing school of thought says that because battery life is a function of the number of charge cycles, you actually want to take it to both extremes (within the prescribed cutoffs that is) so as to minimize the number of cycles.   I don't know if it makes a difference or if one is right and the other wrong.  The best advice is generally to follow manufacturer's directions, provided they can be found.

[TomWS]

There's one LiPo religion  which roughly says that if you  recharge a LiPo when you're about 20% between full and empty and stop charging when your still 20% from the usual cutoff for full, then your battery will last a lot longer than if you always take it to the full extremes. An opposing school of thought says that because battery life is a function of the number of charge cycles, you actually want to take it to both extremes (within the prescribed cutoffs that is) so as to minimize the number of cycles.   I don't know if it makes a difference or if one is right and the other wrong.  The best advice is generally to follow manufacturer's directions, provided they can be found.

Without getting into 'religion', the schools of 'thought' WRT maximizing years of battery life would be better off thinking about maximizing reliability of the device they're powering.   With that focus, keeping the battery well charged (even if it shortens the battery's life by a theoretical x%) seems to be a reasonable plan. 

You can tell 'wear' of the battery (if the load is relatively constant) by watching the duration between cycles.  As the battery become weaker, the cycle time gets significantly shorter and you can determine when its time to replace the battery based on your risk factors.

Tom

[Felix]

Just another random thought I had, before I lose it, is to check what the voltage is on a cell/smart phone is when it shuts off due to low battery. I can try to remember to check mine, maybe others can do this exercise and we can get some ideas of what the phones are doing. Extrapolated, this figure is in millions billions of devices that have to run for as long as possible, for good reason.