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lithium battery voltage drop with buck converter


ropalm

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I recently build a portable battery backup system to just run my pc when load shedding strikes, I got a 800w pure sinewave inverter with 4 buck converters (link below). I'm using 4 brand new 36v 4,4ah  hoverboard batteries (I had them laying around that's why I used them). 
Upon finishing the build I started to notice that when powering them on the battery only last for 20 min under a 115w load  but when I turn the buck converters off the battery jumps from 10% to 70% , if my calculations are correct the battery (4* 36v 4,4 ah in parallel) should last me 5 hours or less (633wh/115w=5,5hours).  the buck converters are connected in parallel and buck the voltage from 36v to 12v.
please let me know If I'm missing something. or what could be done to eliminate the voltage drop.

https://botshop.co.za/products/300w-step-down-power-module-9a-buck

Edited by ropalm
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On 2022/12/31 at 10:15 AM, ropalm said:

I recently build a portable battery backup system to just run my pc when load shedding strikes, I got a 800w pure sinewave inverter with 4 buck converters (link below). I'm using 4 brand new 36v 4,4ah  hoverboard batteries (I had them laying around that's why I used them). 
Upon finishing the build I started to notice that when powering them on the battery only last for 20 min under a 115w load  but when I turn the buck converters off the battery jumps from 10% to 70% , if my calculations are correct the battery (4* 36v 4,4 ah in parallel) should last me 5 hours or less (633wh/115w=5,5hours).  the buck converters are connected in parallel and buck the voltage from 36v to 12v.
please let me know If I'm missing something. or what could be done to eliminate the voltage drop.

https://botshop.co.za/products/300w-step-down-power-module-9a-buck

Firstly some of these buck converters have a very low efficiency and although you are using only 115W the batteries might be supplying 200W. Having said that the batteries should last over 2.25 hours if they are not just marked 4.4Ah but can actually deliver it. I will start with charging them full. Then connect 3x12V 21W car rear indicator bulbs in series that will draw just below 2A and time how long does the voltage stay say above 32V for each battery. The time taken will give a good indication of their capacity. You should get just below 2 hours for each battery.

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