LiPo battery charge times

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Gambalunga
 
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LiPo battery charge times

Post by Gambalunga »

I am using both Feather M0 Express and M4 Express (not in the same device ☺ ) with 7 segment backpacks and Adalogger FeatherWing for RTC and SD card. My question relates specifically to those but I imagine that all the Feathers use the same charging circuit. The battery I am using is a 2000 mAh. As a matter of interest I am getting around 60 hours from a charged battery to run it down to 3.6 v.

I can not find any reference to the output of the charging circuit and I am wondering just how long it takes to fully charge a battery from about 3.5 v when USB power is attached. I guess I could write a routine to check it and log the result but it occurred to me that the answer may be known.

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adafruit_support_bill
 
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Re: LiPo battery charge times

Post by adafruit_support_bill »

The max charge rate on the Feathers is 100mA. Note that a LiPo charge cycle includes a pre-conditioning phase and a constant-voltage top-off phase. So the overall charge time is typically somewhat longer than C/100mA.

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Gambalunga
 
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Re: LiPo battery charge times

Post by Gambalunga »

Thanks.
By a very rough calculation, and no doubt as long as the battery is in good condition, it should be able to re-charge 10 hours a battery use in about 3 to 4 hours connected to the USB power.

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adafruit_support_bill
 
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Re: LiPo battery charge times

Post by adafruit_support_bill »

Also note that the load (i.e. the Feather board and any attached circuitry) is connected in parallel to the battery. So any current drawn by the load is going to offset the charge current.

Based on your 60 hour typical run-times with a 2000mAh cell, your average load current is about 33mA.

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blakebr
 
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Re: LiPo battery charge times

Post by blakebr »

Pete,

I know LiPo cells are more efficient than the old NiCad cells I worked with. The rule of thumb was 140% of the amount of a charge you want to put into a rechargeable Cell. The fudge factor was to compensate for recharge inefficiency and heat loss. So if the cell was down by 1 Amp Hour, it would take 7 hours at 0.2 Amp aka 1.4 Amp Hour to recharge. Your charger should have the ability to monitor the state of charge and drop to a trickle charge when the cell reaches 80% of a full charge.

LiPo cells love to live in the 20% to 80% of charge range. Apple iPhones have an intelligent charge mode where it will charge up to 80% then wait until some number of hours before the usual time you take your phone off charge to top-off the charge. That way the iPhone sits at full charge while under charge for a minimum amount of time.

Bruce

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adafruit_support_bill
 
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Re: LiPo battery charge times

Post by adafruit_support_bill »

Your charger should have the ability to monitor the state of charge and drop to a trickle charge when the cell reaches 80% of a full charge.
The LiPo charge controller on the Feather switches to a constant voltage 'trickle' charge mode when the measured cell voltage reaches 4.2v. Although 4.2v is the 'full charge' voltage, the cell is actually only about 80% of full charge at this point. If you stopped charging at this point, the voltage would start to slowly decline.

Charging a cell is like filling a shallow tray with molasses from one corner. You need to slow down when the molasses reaches the lip of the tray near the fill point so it doesn't overflow. It can take a while for the liquid to level out across the whole tray to fill the far corners.

The constant voltage trickle charge keeps the voltage at the cell terminals at 4.2v until the charge current goes to zero. At that point, the charge has evened out across the whole cell and the cell is fully charged.

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blakebr
 
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Re: LiPo battery charge times

Post by blakebr »

Bill,

Great example!

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