DIY Manual

30 | P a g e 10-543-1 REV A
DIY Manual
Proximity – This means physically locating the batteries near associated solar system
components, such as the charge controller and inverter, to minimize the length of DC cabling
between components.
BATTERY CAPACITY
Battery capacity refers to the amount of stored energy in your battery bank. In other words,
how much energy is available to run your AC or DC loads and for what length of time. The unit
of measure for battery capacity (energy storage) is amp-hours.
Amp-hour capacity is denoted with values such as a 5-, 10-, 20-, and even a 100-hour amp-hour
rating. When shopping and comparing deep cycle batteries for your solar system, use the 20-
hour rating, as this is the most common rating, ensuring you an apple-to-apple comparison.
Charge and discharge rates of a battery are called C-rates. A battery with a C-rate of 1C means
that a fully charged battery rated at 1Ah should provide 1A for one hour. 20-hour batteries
have a C/20 rate. For a 370 Ah battery, for example, the discharge rate will be 18.5A (370 x
0.05, which is 1/20) for 20 hours (18.5A x 20 hours = 370Ah). However, as the load increases,
the realized capacity decreases.
State of Charge & Depth of Discharge
State of Charge (SOC) refers to “How full is the battery charged?”
Depth of Discharge (DOD) refers to “How low is the battery drained?”
The table above shows SOC/DOD percentages and the associated bank voltages of a bank of
flooded or sealed batteries at rest – not charging or discharging (inverting). Lithium batteries
will have a different SOC/DOD scale relative to battery voltage; consult the Lithium battery
manufacturer. If your solar system were designed to support a 50% DOD, then it is helpful to
know that 12.1V (in a 12V bank) is the equivalent voltage of a bank that is half full, for example.
The per cell voltage of an average flooded lead-acid battery is a nominal 2V per cell, ranging
from 1.75V for a fully depleted battery (SOC = 0%) to 2.45V for a fully charged battery under
charge. A nominal 6V battery will have 3 cells (2V x 3 cells = 6V battery).
1. Know Your Voltages
a. Always measure battery voltage when at rest.
SOC
DOD
12V Bank
24V Bank
48V Bank
10%
90%
11.5V
23V
46V
50%
50%
12.1V
24.2V
48.4V
100%
0%
12.75V
25.5
51V