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Lithium Series, Parallel and Series

and Parallel Connections

TECHNICAL GUIDE

discoverbattery.comdiscoverbattery.com

Darwin Sauer is the CEO and founder of

Discover Battery, and CEO and Chairman

of the Board of Discover MIXTECH

Manufactur

ing Co. Ltd. He is a visionary,

innovator and entrepreneur with over 35

years of experience in the industry, and the

driving force behind Discover’s MIXTECH

lineup of batteries and the acquisition of the

MIXTECH plant in Korea.

ABOUT THE AUTHOR

Introduction

Lithium battery banks using batteries with built-in Battery

Management Systems (BMS) are created by connecting two or

more batteries together to support a single application. Connecting

multiple lithium batteries into a string of batteries allows us to

build a battery bank with the potential to operate at an increased

voltage, or with increased capacity and runtime, or both.

To Series, Parallel, or Series and Parallel lithium batteries with a

BMS you must ﬁrst understand what a “true” BMS is, what it

does, and what challenges the BMS in your battery may present

to series, parallel, or series and parallel use.

Battery 1S

Battery 2S

Battery 2P

Battery 1P

Battery 4SP

Battery 3SP

Battery 2SP

Battery 1SP

Series

Connection

Increases Voltage &

Total Energy

Parallel

Connection

Increases Capacity &

Total Energy

Series & Parallel

Connection

Increases Capacity, Voltage,

& Total Energy

Summary of content (18 pages)

PAGE 1
TECHNICAL GUIDE Lithium Series, Parallel and Series and Parallel Connections Introduction ABOUT THE AUTHOR Lithium battery banks using batteries with built-in Battery Management Systems (BMS) are created by connecting two or more batteries together to support a single application. Connecting multiple lithium batteries into a string of batteries allows us to build a battery bank with the potential to operate at an increased voltage, or with increased capacity and runtime, or both.
PAGE 2
TECHNICAL GUIDE TABLE OF CONTENTS 1. What is a BMS, and why do you need a BMS in your lithium battery? 3 2. How to connect lithium batteries in series 4 2.1 Series Example 1: 12V nominal lithium iron phosphate batteries connected in series to create a 48V bank 4 2.2 Series Example 2: 12V nominal lithium iron phosphate batteries connected in series in a 36V bank 5 2.3 Series Example 3: 24V nominal batteries connected in series in a 48V nominal bank 5 3.
PAGE 3
1. What is a BMS? Why do you need a BMS in your lithium battery? The primary function of a BMS is to ensure that each cell in the battery remains within its safe operating limits, and to take appropriate action to prevent the battery and its cell modules being used outside of their designed voltage, current, and temperature limits.
PAGE 4
• A BMS will use either a SSR (made of mosfets), or a mechanical relay. Both SSR and mechanical relays have pros and cons, and both of them have their own voltage and current limitations. • With a SSR, mosfets are connected in parallel on the PCB board and the heat sink. Mosfets are like conductors so the more you have in parallel the more current the BMS can handle.
PAGE 5
• Discover’s 12V LiFePO4 batteries have a nominal voltage rating of 12.8Vn and the BMS will protect at the maximum operating voltage of 14.6V. • A bank of 4 x 12Vn LiFePO4 batteries connected in series will have a nominal voltage of 51.2Vn and a maximum operating voltage of 58.4V. • Each battery in the string has the potential to see the total battery bank voltage across its relays during switching (operation) • The BMS in each 12.
PAGE 6
• A bank of 2 x 25.6Vn batteries connected in series will have a nominal voltage of 51.2Vn and a max. operating voltage of 58.4V • Each battery in the string has the potential to see the total battery bank voltage across its relays during switching (operation) • The BMS in each 25.6Vn battery in the series string must be capable of switching at least the max. operating voltage of 58.4V. • When charging, periodically each battery’s balancing circuits must be allowed to operate for cell balancing purposes.
PAGE 7
NOTE “Volts x Amps = Watts”: One 12.8V-100AH lithium battery has 1280 Watts of stored energy potential. Two 12.8V-100AH lithium batteries connected in series becomes a 25.6V-100AH battery bank with 2560 watts of stored energy potential to 100% DOD. Connecting batteries in Series increases the battery bank voltage and total stored energy.
PAGE 8
NOTE Installers should carefully consider the use of BUS bars for permanent connections of charges and loads and/or easy disconnect cable ends for rapid interchanging between loads and chargers. Even though it is possible, the recommended depth of discharge (DOD) for high-quality deep-cycle lithium batteries is not 100%. Most responsible manufacturers will recommend between 80% and 90% depending upon application.
PAGE 9
• Make sure the combined continuous current ratings of the paralleled BMS can support the maximum load and charge currents • Understand that if one battery fails, the potential currents will then be shared across fewer BMS in the parallel string • When charging, periodically each battery’s balancing circuits must be allowed to operate for cell balancing purposes.
PAGE 10
The parallel connection shown in Figure 3 DOES NOT increase your battery bank voltage. Parallel connections increase the total capacity and the total stored energy potential of the finished bank. It also increases the maximum potential continuous charge and discharge currents by a multiple of up to 2x. (Good system design dictates this be derated by no less than 10%). If each 12.
PAGE 11
If you need even more capacity you will need to connect more batteries in parallel. Figure 4 DOES NOT increase your battery bank voltage. This parallel connection increases the total capacity and the total stored energy potential of the finished bank. It also increases the maximum potential continuous charge and discharge current of each single battery by up to 4x. If each 12.
PAGE 12
The examples in Figures 3 and 4 used 12.8V nominal lithium batteries. If you use lithium batteries with different individual nominal voltages 24V(25.6V), 36V(37.4V), 48V(51.2V), the method of connecting POS. (+) to POS. (+) and NEG. (-) to NEG. (-) as you progress through the parallel string will be the same. Plus, it is a far better option to parallel higher voltage lithium batteries than to series or series and parallel lower voltage lithium battery models.
PAGE 13
paralleled onto positive and negative BUS bars creating a 51.2V 480AH battery bank capable of supporting up to 480 amps of continuous current. This type of battery bank provides excellent redundancy should a battery in a string or a total string fail as the battery bank will still be able to sustain the electrical system voltage although with reduced capacity. NOTE “Volts x Amps = Watts”: One 51.2Vn-30AH battery has 1536 Watts of stored energy potential and a maximum continuous current rating of 30 amps.
PAGE 14
Most Discover Lithium PRO series batteries are designed with dual terminal options making it easy to make low resistance inter-battery connections without applying more than one cable to each terminal.
PAGE 15
When training our customer service groups on battery failure diagnostics, failure mode analysis and customer support, some of the questions they are trained to ask are: 1. How is the battery installed? Is it in parallel? Is it in series? Is it in a series and parallel installation? 2. If installed in parallel, which battery in the string has failed? 3. Are all of the batteries in the string of the same type (AGM, GEL, Flooded, Lithium), the same capacity and the same age? a.
PAGE 16
BAD parallel charging design: Because of uneven resistance levels Battery 4 in the BAD configuration in figure 6, is going charge faster and discharge slower than Battery 3 which in turn will charge faster and discharge slower than Battery 2 which in turn will charge faster and discharge slower than Battery 1.The differences in internal resistance from Battery 4 to Battery 1 can generate a 50% difference in the amount of charge and load currents experienced.
PAGE 17
NOTE When charging in parallel always allow Discover designed Lithium batteries to be left charging at their absorption/balancing voltage for extended periods. Doing so Allows the batteries to balance and will help ensure that the total battery bank will remain in balance. Lithium batteries are easy to charge and discharge.
PAGE 18
Figure 8 is a diagram of a series connected string of 2 x 12.8V 100AH batteries connected in parallel with another series connected string of 2 x 12.8V 100AH batteries to create a 25.6V 200 AH battery bank with 5120W of stored energy. NOTE “Volts x Amps = Watts”: One 12.8V x 100AH = 1280W.Two 12.8V x 100AH batteries connected in series = 25.6V 100AH with 2560W of stored energy.