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Old 10-13-2021, 11:03 AM   #25
Ten Point
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Join Date: Jun 2013
Location: San Antonio

Originally Posted by Mike D View Post
Does your BMS not have a shunt with it?

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Iíve been thinking more and more about this. As stated earlier, Iím still learning about lithium batteries and I just got reminded that the voltage reading is not a good indicator of the battery level in a lithium battery.

The voltage on the battery had dropped down to 13.9 volts with the lights and fans running. The next day I checked and there was no voltage coming out of the battery! The BMS did its job and shut the battery off to prevent it from being drained too low. So I connected up a little 12 volt power supply and immediately it woke up the BMS. The battery voltage showed 11.9 volts.

So, just like a lithium powered cordless drill, the voltage of the battery stays high for a MUCH longer time than what a wet cell or AGM battery shows, then it quickly drops as you get closer to using the full capacity of the battery. With a cordless drill, you have full power and your drill doesnít even slow down until right before the battery dies. Then with almost no warning it shuts off. This battery is performing in the exact same way. So itís next to impossible to determine the battery level via battery voltage.

Because of this, and it was something that I needed to experience myself to be reminded, I ordered a battery monitor with a shunt to better keep track of the battery level.
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