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The MiniGuide:
Mobile Robots batteries
and types of charging
systems
(ft. Wireless Charging)
Mobile Robot Charging Guide
How do mobile robot
batteries get charged?
The optimal type of battery is determined by the battery charging systems
selected. In reality, battery type and charging technology are closely related.
Typically, mobile robot batteries are charged using one of
the following methods:
Opportunity Charging
In-Line charging
Off-Line Charging
Manual battery swapping
Automatic battery replacement
The best performance
is achieved if the
battery and the
charger are well
matched and designed
as a whole unique
system
Mobile robots can operate 24 hours a day, seven days a week but the charging
strategy and the relative type of battery must be flawless or you will face
disastrous consequences.
What types of batteries
do mobile robots use?
Sealed GEL Batteries
AGM Pure-Lead
Lithium Batteries
Flooded Lead Acid
What is the best solution?
You will not love this answer. It depends on your own project. There are numerous
factors involved, including cost, recharge time, robot type, etc. But do not worry, this
guide will help you to find “your” best solution.
Mobile Robot Charging Guide
Before discovering the types of batteries and their specifications, let’s
have a short recap about the opportunity charging and the battery
swap solutions.
What is opportunity
charging?
Opportunity charging means that mobile robots go to defined charging stations
and they charge while waiting for a new mission.
Robots charge whenever they are idle. Doing this way, batteries are partially
charged during the working hours.
If the system is properly designed, with the opportunity charging system,
mobile robots could work continuously without changing the battery.
What is battery swap?
Well, it simple. The robot works with its battery until it is necessary to
substitute it with a fully charged one.
Of course, you need to have a pair of
batteries for each vehicle.
In this case, the battery exchange can
be done manually (by an operator) or
automatically with a dedicated
exchanging machine.
Mobile Robot Charging Guide
What are the key factors for
choosing a type of battery?
The first question is: Do you wish to perform opportunity charging? If yes, you
should choose Lithium or Pure-Lead batteries. If not, you can consider other types
of battery.
Specification
GEL
PURE-LEAD
LITHIUM
Suitable for Online
Charging
Depth of Discharge
(DOD)
Suitable for High
Current Fast Charging
Battery Life
Gas emission
Need of maintenance
Battery Price
Don’t worry… in the following lines, we’ll understand better the above-mentioned
specifications
Types of batteries for Mobile Robots
Types Of Batteries used for mobile
robots
GEL and AGM batteries
AGM and GEL batteries are widely used in the AGV industry. They are Sealed
Lead-Acid (SLA) or valve-regulated lead-acid battery (VRLA battery). Check this
link with more detailed technical info.
Regardless the specific differences between AGM and GEL batteries (they are
more or less the same), both types present the following attributes:
They are sealed and non-spillable batteries.
They do not need maintenance, so they are maintenance-free.
They are "Deep cycle" batteries. It means that the battery can be discharged
down up to 80% (so when it only remains 20% capacity in the battery).
As we will see in the following chapters, in order to grant the best lifetime, GEL
batteries should be discharged DOD 40%-50%
They have low self-discharge rate.
They present low gas emissions or not emissions at all. They can be used and
recharged in limited ventilation areas.
They are robust and can be transported safely without any special handling
care.
Types of batteries for Mobile Robots
GEL and AGM life and application
GEL batteries are
designed to be fully
recharged once they
reach a depth of
discharge of around
40%
The graph shows the battery life in Number Of Recharging Cycles depending on the
Depth of Discharge.
As you can see, the battery life is near 1200 cycles for a 40% DOD. It means that you
can recharge 1200 times your GEL from 60% to 100%.
For example, if your company works 2 shifts, 5 days a week and you must recharge
the battery around 10 times/week, your GEL battery will last around 3 years.
AGM/GEL batteries allow recharging with current around 25-30% (0,3C) of their
capacity. If you have a 100Ah battery, you can charge it at around 25 Ah, which is
very low if you need a fast opportunity charging.
GEL batteries are suitable for projects working two or three shifts but, in this case,
you will need a second battery to perform battery swap.
If you wish to keep a good battery life and perform opportunity charging, you must
stop the robot in the charging station for long time.
Types of batteries for Mobile Robots
Lithium batteries for Mobile Robots
There are numerous advantages to using lithium-ion batteries for mobile robots'
applications.
Lithium is more efficient, has a much higher energy density, and has a longer life
cycle.
Lithium batteries also require far less maintenance than lead acid batteries.
Advantages of Lithium batteries for
mobile robots
Long cycle lifetime
For a given depth of discharge (DOD), Lithium batteries grant more recharging
cycles so more life.
So, if you always discharge the robot battery to 50% of its capacity, SLA
(GEL/AGM) has around 1500-1800 recharging cycles lifetime. With the same 50%
DOD, the Lithium battery lasts for 5000 cycles.
Mobile Robot Charging Guide
Deep Depth of Discharge (DOD)
Lithium batteries can handle Depth of Discharge (DOD) of 90% maintaining
excellent battery life (still close to 2500 cycles).
It means that in a 100 Ah battery, you can use 90 Ah before you need to recharge.
GEL batteries for example, must be recharged when reaching maximum 40%
DOD. So, your 100 Ah battery should be recharged when you still have 60Ah
available in order to keep a decent battery life. In fact, you’re almost wasting the
60% of the battery.
With Lithium batteries, your robot can perform more missions instead of “losing”
time for recharging.
Wish to know more about lithium batteries for mobile robots?
Discover Wiferion’s
etaSTORE Lithium iron
phosphate- & Lithiumtitanate Batteries (LTO) for
AGV, AMR & mobile robot
Click Here
Mobile Robot Charging Guide
High Efficiency
Lithium batteries efficiency is near 95% while in Lead batteries such AGM or GEL,
it is close to 80-85%.
Roughly, it means that if you charge 1 kW, a lithium battery losses around 50
watts (you really get 950 watts).
Lead battery instead, loses 15-20%, it means that you will need more time to fully
charge your AGM/GEL battery.
Lithium batteries charge rate is faster
While charging, you can pump more Amperes/hour without compromising
battery life, so again, you can charge battery much faster. Typically, the charging
factor is around 1C
Thus, your 100Ah battery can be charged at 100 Ah with Lithium compared to the
25Ah of GEL (huge difference),
In AGM/GEL batteries you needed around 4-5 hours to charge from 60% to
100%. With lithium you only need around 1 hour.
On average, with lithium batteries you should only need 10% of robot cycle
time, it means that your vehicles are available more time for performing
missions and transporting materials.
Higher Energy Density
More energy available in a given volume. In
other words, the same power and less
weight or the same weight and more power.
We can consider that the energy density in
Lithium batteries is three times higher than
in the lead batteries.
Mobile Robot Charging Guide
Charge rate
The charge rate ( C-rate) describes the possible charging current for a battery in
relation to its capacity​
Example:
If a battery has 100Ah capacity and a max. charge current of 50A the Crate would be 0.5C
A battery with a C-rate of 2 can be charged in 30 min.
The C-rate therefore has a direct influence on the charge time of a battery​
The C-rate is mainly defined by the battery cells chemistry and quality​
Types of batteries for Mobile Robots
Disadvantages of Lithium batteries
for mobile robots
Until now, we have only seen “good” things. But, what are the main
disadvantages.
Lithium batteries require protection
In Lithium batteries, the energy current must be kept into defined safety levels.
For this reason, it is mandatory to have protection against overcharging.
Lithium batteries require complex Battery Management System (BMS) to
monitor and control the state of charge, voltage, current, and temperature of the
cells in the battery pack.
Another negative aspect related to the point above is that Lithium batteries has
restrictions for transportation, especially by air transportation.
Lithium batteries cost is still more expensive
The main concern today about Lithium batteries is the price.
It is true that Lithium comes with many technical advantages (they last more
time, the robots can be used better, they can store more energy, etc.).
The problem is that the initial investment is still higher than compared with other
types of batteries. We all know that available money for investment is always
limited.
Typically, a lithium battery costs 4-5 times more than an AGM/GEL battery.
We’ll see the economic aspect in the following points.
Types of batteries for Mobile Robots
Pure Pb Thin Plate AGM Batteries
The pure-lead batteries are a good option to consider if your mobile robot project
requires opportunity charging.
The pure-lead batteries are a specific type of advanced AGM batteries having
pure-lead thin plates rather than standard lead-calcium plates.
Advantages of Pure Pb batteries for
mobile robots
They have excellence battery life, near 1200 cycles with 60% depth of
discharge.
They support short fast recharging without compromising battery life.
They support high current recharging up to 50% of battery capacity
compared with 30% admitted by GEL batteries (0,5C instead of 0,3C).
If you have a 150A/h capacity pure-lead battery, you can recharge it at
75A/h. If it were GEL, you could only refuel at approximately 30 A/h. So
pure lead charging is nearly three times faster.
They are sealed batteries with very low gas emissions.
They are maintenance free.
Less expensive than Lithium
Types of batteries for Mobile Robots
Type of Battery Cost Analysis for
mobile robots
Specification
GEL
PURE-LEAD
LITHIUM
Available Capacity
40%
60%
95%
Suitable Charging Rate
0,3C
0,5C
1C
€/kW
180€
300€
800€
Suitable for Online Charging
24V/100Ah
Battery Example
Battery Cost
430€
720€
1950€
Available Capacity
40Ah
60 Ah
95 Ah
Adjusted cost of kW
450€/kW
600€/kW
840€/kW
Adjusted cost of battery
1080€
1440€
2016€
Indicative battery life in charging
cycles
2000
3000
5000
Let's try to figure out what type of battery is best for a mobile robot system.
The first consideration is whether you want to swap out your batteries. In that
case, you should choose a GEL battery, which typically provides one shift of
operation and can be charged in the following shift. However, keep in mind that
you will require a backup battery.
If you want to use opportunity charging, you must first understand some other
factors, such as mobile robot system saturation.
Before delving into the most cost-effective solution, let's first understand the
various types of charging strategies.
Battery Charging Systems
What is the best charging
system?
You already know : It
depends.
Apologize, you may have expected a direct answer, but there are several factors
to consider when selecting the best charging solution.
The main factors are:
Type of battery and its cost (we’ve seen in the previous chapters).
Mobile robot system saturation
Available time for charging
Human operator saturation and cost
Let’s recap the main types of charging.
Opportunity Charging
Manual battery swapping
Automatic battery replacement
Opportunity Charging
Opportunity charging
It is great! The Mobile Robot stops and charges the battery whenever is required
and whenever it has “free” time.
There are several factors to be highlighted.
Opportunity requires the vehicle to stay stopped.
If the vehicle is not performing a mission, it is wasting time.
Theoretically, mobile robots should work 100% of their time.
How long does mobile robot
opportunity charging should last?
Simply put, the time required to compensate the vehicle consumption. It seems
easy, but it isn’t.
For example, if your vehicle consumes 70% of its battery in a shift and you only work
one shift. Well, in this case you do not have to worry. You just charge your vehicle
during the extra shift and the vehicle will be ready the after.
But if your vehicle works two or three shifts?
You must be sure that you have enough time during the working cycle to perform
the charging.
On average, it is typical to consider:
Near 50 min/shift available for charging with Lithium batteries
Near 2,5 hr/shift available for charging with Pure-Lead AGM batteries
Opportunity Charging
Nominal AGV availability in
24/7 continuous operation
Robot Average current
consumption in A/h
Charge current in A/h
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
5
10
15
20
25
30
35
40
45
50
55
60
71%
56%
45%
38%
33%
29%
26%
24%
22%
20%
19%
83%
71%
63%
56%
50%
45%
42%
38%
36%
33%
31%
88%
79%
71%
65%
60%
56%
52%
48%
45%
43%
41%
91%
83%
77%
71%
67%
63%
59%
56%
53%
50%
48%
93%
86%
81%
76%
71%
68%
64%
61%
58%
56%
53%
94%
88%
83%
79%
75%
71%
68%
65%
63%
60%
58%
95%
90%
85%
81%
78%
74%
71%
69%
66%
64%
61%
95%
91%
87%
83%
80%
77%
74%
71%
69%
67%
65%
96%
92%
88%
85%
82%
79%
76%
74%
71%
69%
67%
96%
93%
89%
86%
83%
81%
78%
76%
74%
71%
69%
96%
93%
90%
87%
85%
82%
80%
77%
75%
73%
71%
97%
94%
91%
88%
86%
83%
81%
79%
77%
75%
73%
17%
16%
15%
14%
14%
13%
12%
12%
11%
11%
10%
10%
9%
9%
9%
8%
8%
8%
8%
29%
28%
26%
25%
24%
23%
22%
21%
20%
19%
19%
18%
17%
17%
16%
16%
15%
15%
14%
38%
37%
35%
33%
32%
31%
29%
28%
27%
26%
25%
25%
24%
23%
22%
22%
21%
21%
20%
45%
43%
42%
40%
38%
37%
36%
34%
33%
32%
31%
30%
29%
29%
28%
27%
26%
26%
25%
51%
49%
47%
45%
44%
42%
41%
40%
38%
37%
36%
35%
34%
33%
32%
32%
31%
30%
29%
56%
54%
52%
50%
48%
47%
45%
44%
43%
42%
41%
39%
38%
38%
37%
36%
35%
34%
33%
59%
57%
56%
54%
52%
51%
49%
48%
47%
45%
44%
43%
42%
41%
40%
39%
38%
38%
37%
63%
61%
59%
57%
56%
54%
53%
51%
50%
49%
48%
47%
45%
44%
43%
43%
42%
41%
40%
65%
63%
62%
60%
58%
57%
56%
54%
53%
52%
51%
49%
48%
47%
46%
45%
45%
44%
43%
68%
66%
64%
63%
61%
60%
58%
57%
56%
54%
53%
52%
51%
50%
49%
48%
47%
46%
45%
70%
68%
66%
65%
63%
62%
60%
59%
58%
57%
56%
54%
53%
52%
51%
50%
50%
49%
48%
71%
70%
68%
67%
65%
64%
63%
61%
60%
59%
58%
57%
56%
55%
54%
53%
52%
51%
50%
In 24/7 operation, AGV availability does only depend on ratio between
charging current and average current consumption.
If your mobile robot average consumption is 30 Ah
and you charge at 60 A/h → Your robot availability
is 67%
Opportunity Charging
Types of Opportunity charging
Mobile robots perform opportunity charging with the classical “contact
charging” electrodes or with the latest innovative “inductive contactless
charging”.
So, what is the distinction between them? The point is fairly self-evident:
There is a physical connection between the charging poles in "contact"
charging.
Actuator to move up and down
the contact poles
Robot onboard
contact copper poles
In wireless charging instead,
the power is transferred
between two coils without any
physical contact (we’ll see
later on).
Stationary contact
copper poles
Opportunity Wireless Charging
Types of Opportunity charging
Wireless Charging
Fast, efficient, clean, flexible... these reasons are leading wireless inductive
charging to conquer the mobile robot industry.
Wireless charging is a trending topic in the mobile robot industry.
More and more manufacturers are incorporating this technology into their robots.
AGV Network has have recently run a poll
LinkedIn and 42% of the agv&amr industry
leaders consider that contactless charging
will be one of the most interesting and
powerful technological developments in
the next years.
Check this Youtube video by Wiferion with a nice general overview about
wireless charging technology
Click the picture to watch the video:
Wiferion Product Overview - Wireless
charging for industrial electric vehicles
Opportunity Wireless Charging
1
How does
Wireless
Charging for
mobile robots
work?
Wall box - Charger
2
Coil System
3
Charging unit
4
Battery
Basically, there are two main groups:
Stationary (on floor or wall): A power transmitter connected to the
grid
1•
Power supply unit (charger)
2a
•
Active Coil (stationary charging pad)
Mobile (on the robot): A power receiver connected to the battery.
2b•
Passive Coil (receiver pad)
3•
Charging unit (to convert AC into DC suitable for the mobile robot batteries)
4•
Battery
Opportunity Wireless Charging
How does Wireless Charging
for mobile robots work?
There is a charger that converts the electricity power into several thousand hertz
and uses the coil in the stationary charging pad to create an alternating field.
This field induces a high-frequency alternating current in the coil in the receiver
pad. The charging unit then uses this current to charge the batteries.
Typically, inductive solutions for mobile robots are 24 V or 48 V, able to supply
from 0.5 kW up to 32 kW.
Wiferion’s etaLink 3000 and etaLink 12000 charging systems are able to deliver
from 3kW to 12kW. Thanks to their design, the chargers take into account the
intelligence and autonomy of the new generation of robotics systems and they
can be easily and independently installed and provide the vehicle with all
relevant data.
Opportunity Wireless Charging
What are the benefits of wireless
charging compared to contact
charging?
The main benefits of contactless charging technology are:
• High Efficiency 93%-95%
👎
Full Power Of High-Energy Streams Immediately After Start
👎
• No Wear And Tear Or Maintenance As There Are No Contacts Involved
👎
• High Positioning Tolerance and Possibility to have Omnidirectional
Charging
👎
• One single wireless charging system can supply power to different
vehicles and batteries
👎
• Intelligent data transfer during wireless charging
👎
Opportunity Wireless Charging
Full Power Of High-Energy Streams
Immediately After Start
With the wireless systems, the battery starts to charge faster compared with
contact charging solutions. The robot does not wait the movement and contact of
the poles
No Wear And Tear Or Maintenance As There Are
No Contacts Involved
Simple mechanical solution avoiding
springs or linear actuators needed in
the traditional contact opportunity
charging
High Mobile Robot Positioning Tolerance And
Possibility to have Omnidirectional Charging
The contact charging poles must be in “contact”, so the mobile robot positioning
will depend on the shape and dimension of these poles (on board and on floor or
wall).
This feature is very interesting for Autonomous Mobile Robots (AMR), which do
not follow a given path and can vary their trajectory depending on surrounding
environment.
Opportunity Wireless Charging
Interoperabiilty - One single wireless charging
system can supply power to different vehicles
and batteries
One wireless charging system can provide different Voltages for different
battery types.
This means you only need one charging system to charge different vehicles, for
example an AGVs with a 48 V lithium-ion battery and an industrial manned truck
with a 24 V lead-acid battery.
Intelligent data transfer during wireless
charging
Some suppliers offer CAN connection allowing to transfer data during the
charging process.
All the relevant battery and vehicle data can be transferred into a cloud or directly
to the mobile robot management system.
Opportunity Wireless Charging
Intelligent data transfer during wireless
charging
Wireless charging can be integrated with CAN comunication allowing to transfer
data during the charging process.
All the relevant battery and vehicle data can be transferred into a cloud or directly
to the battery and mobile robot management system.
Shortened development times for battery and charging system integration into
your vehicle
Automated data analysis for trouble-free vehicle operation
Live information about vehicle condition
Easy handling via the dashboard view
Digital services such as predictive maintenance applications
Developing optimal battery charging strategies without being an energy expert
Long battery life through individual battery maintenance concepts
Learn more
about
etaHUB
Opportunity Contact Charging
How does Contact Opportunity
Charging work?
The concept is similar to that described for wireless charging, but instead of having
touchless power transfer, energy is provided via contact poles.
The are two advantages compared to wireless transfer:
👎
The equipment needed for contact charging is less expensive.
👎
With the same charger, you can transfer more power to the battery.
On the other hand, there are some disadvantages like wear and maintenance needed
for contact poles and the actuator included in the vehicle. For more differences,
check the wireless advantages indicated in the wireless section.
Battery Swap Charging
How does battery swap work?
As its name suggests, in this case, the
exhausted battery is exchanged by a
fresh one when reached a defined battery
level (that depends on the type of
battery).
There are some disadvantages.
👎 A second backup battery is needed - Of course, you need a second battery
for substitution
👎 Someone must be available to perform the battery exchange. Even if the
exchange time is short, the operator should always be present in the recharging
area to avoid keeping the robot waiting for too long.
👎 Or, as an alternative, there’s an automatic battery exchange machine
without operator, that uses to be very expensive
While of course, we also have different advantages:
👍 Cheaper batteries (tipically GEL or lead-acid batteries)
👍 Robots can work all the time. The mobile robot availability is close to 100%.
Battery swap needs less than 2-3 minutes your system could need a smaller
number of vehicles.
👍 Mobile Robots are less expensive because they do not include the
automatic charging module
👍There’s no need of accurate energy balance calculation.
Conclusions
What is the best charging
system?
We now understand how do batteries and charging solutions affect a mobile robot
project. Let us now discuss some additional considerations for the best charging
solution.
Number of mobile robots and their saturation
Let’s imagine that we have a mobile robot system with 5 vehicles. This system is
saturated at 65% without considering the time needed for recharging. It means
that during the 65% of their time, robots perform some missions..
If they never stop, they should have 35% of their time for recharging.
In order to give you an idea, for opportunity charging, we can consider that we
need around:
Pure-Lead (AGM) batteries require near 30% of the robot working time
Lithium batteries require near 10-15% of the robot working time
Lithium + wireless charging require near 8-10% of the robot cycle time
In this case we could choose all the different options for opportunity charging.
But what happens if we have a system saturated at 85% ? In this case we would
only have 15% time for charging.
Conclusions
We would be obliged to use Lithium batteries with opportunity charging or battery
swap.
Otherwise, we could add an additional vehicle. If we are talking about 20K€ robot,
it could make sense, but if we are talking about a 150K€ robot, it would be more
convenient to improve the charging capacity.
Regarding mobile robots, where we need them to be as available as possible, the
best feature of lithium batteries is their ability to be fast from 1C to 6C, allowing
robots to complete more missions.
Total Cost of Ownership vs.
Initial investment
Lithium
GEL
24V – 22AH (0,5 kW)
24V – 50Ah (1,2 kW)
2500 cycles with 80% DOD
Each cycle delivers 0,4kW
1200 cycles with 40% DOD
Each cycle delivers 0,48kW
Module cost day 1: 550€
Module cost day 1: 220€
Total delivered AH over Lifetime:
Total delivered AH over Lifetime:
2500 * 22Ah*0,8 = 44000Ah
1200 * 50Ah*0,4 = 24000 Ah
→ AH per 1€ over Lifetime: 109Ah
→ AH per 1€ over Lifetime: 80Ah
+36%
For mobile robot applications needing online charging, Lithium
batteries offer huge technical advantages and lower total cost of
ownership (TOC) compared to other types of batteries.
Conclusions
Summary
The two main charging solutions are battery swap and online charging.
Online charging requires an accurate energy balance to ensure that the
robots have enough time to recharge autonomously.
The best type of battery for online charging is Lithium. Lithium offers these
benefits for mobile robots
Fast charging → more time to perform missions → less vehicles
Lithium charging requires near 15% of robot cycle time
Long Life batteries even with high charging rates
Total cost of ownership lower than other battery technologies
Wireless charging technology is the best option for online charging.
High Efficiency 93%-95%
No Wear And Tear Or Maintenance As There Are No Contacts Involved
Interoperabilty: One single wireless charging system can supply
power to different vehicles and batteries
GEL batteries are the most convenient option whenever is possible to
perform battery swap giving almost 100% robot availability or for one
working shift projects.
Ask your supplier to provide an accurate energy balance analysis
AGV Network in
collaboration with
WIFERION
Wiferion GmbH
Munzinger Straße 1, 79111 Freiburg
+49 (0) 7611 542 67 0
Wiferion North America Inc
150 North Michigan Avenue, 35th Floor 60601 Chicago, Illinois
+1 (312) 494 2176
www.wiferion.com
CREDITS
KMC Srls (agvnetwork.com)
info@agvnetwork.com