In energy storage, UPS (Uninterruptible Power Supply), or electric vehicle applications, we often connect batteries in parallel to increase the total capacity, thereby achieving longer runtime or backup time. A common question is: How long does it take to fully charge the batteries after the capacity is increased? This article will break down the calculation process in detail using a specific example.
1. Scenario Recap
Assume we have a UPS system with external batteries, connected to two battery packs in parallel. Each battery pack is rated at
48V 50AH
. We use a
charging cable with an output current of 10A
to charge the entire system. So, theoretically, how many hours are needed to fully charge these two completely depleted battery packs?
2. Core Principle Analysis
To calculate the charging time, we need to understand a few key points:
Characteristics of Parallel Connection:
When batteries are connected i
n parallel,
their v
oltage remains unchanged,
while their c
apacity (AH) adds up.
Single battery pack voltage: 48V
Single battery pack capacity: 50AH
Total voltage after parallel connection: Still 4
8V
T
otal capacity after parallel connection: 50AH + 50AH = 1
00AH
Theoretical Basis for Charging Time:
C
harging is essentially the process of transferring electrical energy into the battery. The most basic calculation formula (under ideal conditions) is:
C
a
rging Time (hours) = Total Battery Capacity (AH) / Charging Current (A)
3.
Calculation Process
Now
, let's substitute the known parameters into the formula:
Tot
al Battery Capacity =
100 AH
Cha
rging Current
=
10 A
The
oretical Charging Time =
100 AH / 10 A = 10
hours
Thi
s means that under ideal conditions, using a 10A current to charge a 100AH battery pack from empty to full would take approximately 10 hours.
4.
Important Considerations: The Gap Between Theory and Reality
The
10-hour figure mentioned above is an ideal value. In practical applications, the charging time might be longer. Here are several factors that must be considered:
Cha
rging Efficiency (The Most Critical Factor):
No
c
harging process is 100% efficient. Some electrical energy is lost as heat within the charger, the cables, and the battery itself. Typically, the charging efficiency for lead-acid batteries is about 85%-90%, while lithium batteries are more efficient, often over 95%.
Exam
ple: If
the charging efficiency is 90%, then the actual required charging time = 10 hours / 90% ≈ 11.1
hours.
Cha
r
ging Strategy:
Mos
t
intelligent chargers (or the charging management module built into the UPS) do not charge at the maximum current throughout the entire process. They usually adopt a "Constant Current (CC) followed by Constant Voltage (CV)" mode.
Const
ant Current Phase: Char
ges rapidly at a stable 10A current until the battery voltage reaches a set value (e.g., 54.6V for a 48V lithium battery). This phase accounts for most of the capacity (about 70%-80%).
Const
ant Voltage Phase: The
voltage remains constant, and the charging current gradually decreases until full. This slow "trickle charge" phase is designed to protect the battery and ensure it is fully charged.
Ther
fore, even ignoring efficiency loss, the complete charging cycle from 0% to 100% might be slightly longer than the theoretical calculation.
Batte
ry Status and Environment:
The
b
attery's age, low ambient temperature, etc., can all affect its ability to accept a charge and its actual capacity.