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Author Topic: Charging LiFeP04 batteries below freezing  (Read 242 times)

KD6ZXG

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Charging LiFeP04 batteries below freezing
« on: October 08, 2021, 11:35:28 AM »

We recently purchased a LiFeP04 battery for our mountain top repeater site.  I found out that this battery should not be charged below freezing.  I am having trouble finding a solar controller that can be programed to stop charging at a specific temperature. I know that the Epic PWRgate can, but it is limited to 30 volts coming from the solar panels and our solar panels can generate up to 40 volts.  I had looked at several temperature controllers on Amazon that can be placed between our existing solar controller and the battery that can be programed to stop the charging at freezing.  However, most look pretty light duty with fair reviews and I need something that will handle at least 20 amps.   

Any suggestions on a good quality 12 volt temperature controller switch or a solar controller that can be programmed to turn off at at specific temperature?
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W9IQ

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Re: Charging LiFeP04 batteries below freezing
« Reply #1 on: October 08, 2021, 12:42:13 PM »

Due to the current draw of the repeater and the self heating of the battery due to its internal resistance, you may find that simply insulating the battery will keep it above freezing.

At the same time, you can put a 10 watt resistor in the insulated box that comes on with the charger if the temperature drops too low. The amount of heating needed will depend upon the ambient air temperature and the quality of the insulation.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

K5LXP

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Re: Charging LiFeP04 batteries below freezing
« Reply #2 on: October 08, 2021, 03:54:48 PM »

Lithium batteries do not self heat like lead acid, and even with lead acid it takes some significant currents which aren't going to happen at a solar repeater site.  You will need to provide some heat which at a time of minimum solar insolation will be a hit to the power budget.  The more insulated of an enclosure the less power it will need.  Doesn't need to be 75F, just something north of freezing.  As a data point, some batteries have their own thermal management which simplifies the design, just use an insulated box.  Supplying the additional power will be the hard part.

Mark K5LXP
Albuquerque, NM
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W9IQ

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Re: Charging LiFeP04 batteries below freezing
« Reply #3 on: October 08, 2021, 07:31:15 PM »

Mark,

When we do this with data acquisition systems, a couple of amps from the load provides 0.5 watts of internal heat in the cell. Unless the ambient temperature is extremely low, insulating the battery keeps its core above freezing. But in this application, it does come down to the quiescent current draw.

Good point regarding the battery management system. The last chip we used in a single cell LiPo product implemented that feature through its RTD sensor.

- Glenn W9IQ
« Last Edit: October 08, 2021, 07:35:29 PM by W9IQ »
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.
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