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7. State of Charge
State of charge is somewhat difficult to determine on LiFePO4 batteries by voltage alone
due to the very stable and much higher operating voltage throughout much of the range of
use from 100% down to 20%. AllSpark batteries required 14.6V (3.65V per cell) for 100%
charge, though you will see a slight increase in overall cycle life if you limit charging to as
low as 13.9V (3.475V per cell). This is a user preference. Charging to 13.9V will obviously
give you a smaller capacity range to work with over about 90% less.
A fully charged battery will reach 14.6V at the end of the bulk and absorption charge stages
and be held at that voltage until the termination current is reached (usually between 3-5% or
battery capacity), then the charger will enter the float stage ranging from 13.5 - 13.8V for
most brands. Most chargers will not re-enter bulk charging again until the battery voltage
drops below 13.3V. A fully charged battery at rest (open circuit) will sit between 13.3 –
13.4V. During normal operation the battery state of charge from 100% to 50% will only move
a small amount of around 0.2 – 0.3V. As you can see, due to the very stable voltage curve,
it is hard to determine an accurate state of charge from voltage alone. As a general rule
(subject change due to very high current use from large power inverters) AllSpark batteries
will reach 20% state of charge at approx. 12.5V. This is the recommended minimum voltage
to maximise life of the battery. 0% will be reached at 10.5V. Low voltage protection mode is
activated at 10.0V and the BMS will disconnect all load output.
To accurately measure state of charge and allowing for regular monitoring of your battery
health we recommend using a quality battery monitor with a current shunt to measure
current (or amps) in and out of the battery. Please contact us for recommendations on
reliable and accurate brands.
We have started to introduce Bluetooth monitoring into our range for monitoring state of
charge from smart phones and tablets. This is available in our 100ah battery with 200A
continuous output (500A peak) and will be added to other models during 2020. Available for
Android and Apple devices early 2020.
Page 9 of 14
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