How to get the most out of your batteries in an electric APU?
Prepared by: Anthony Van Grol, VP of Engineering, CTO, Crosspoint Solutions Group LLC;
John Semeniuk, VP, NorthStar Battery Co.
Introduction
A lot has changed in the truck anti-idling market from the first days of diesel Auxiliary Power
Units, (APU’s) in the mid 80’s to the variable speed battery APU’s available today. The market
has been split into two segments, diesel APU with a generator/ 120V electric air conditioning
(AC) or belt driven AC and battery based AC, with a 120V AC or 12/24V AC or variable speed
AC. When looking for an anti-idling solution one must determine the user’s needs and how the
truck will be used.
There are many benefits in using a battery based AC like reduced idle time, low maintenance,
fewer moving parts, simpler install and reduced fuel usage as compared to a diesel APU. A key
aspect of operating a battery based AC is battery management. Proper management of the
batteries increases battery life and ultimately the return on investment (ROI).
Battery management consists of how you manage the power going into and out of the battery.
This includes charge algorithm, depth of discharge, rate of discharge and battery state of charge
(SOC). The Climacab battery based AC uses a power management module to charge the
batteries, manage the flow of energy to the rest of the system and manage the SOCof the
batteries. By managing the batteries we can extend the cycle life and energy retention of the
batteries.
The PMM functions can be separated into three areas of operation, bi-directional battery
charging, boost converter, and power management. The PMM utilizes a solid state buck boost
topology to provide a three stage IUU charger (see figure 1). This allows us to control the
charge current (I) and voltage (V) independently of the alternator. The charger follows the
battery manufactures recommended charge algorithm to ensure we put back into the battery
110% of the energy removed during AC operation. The battery charger can also provide a jump
start feature called “start assist” tm, which charges the start battery if there is not enough energy
to start the truck.
Bulk charge
Absorption charge
Float charge
14.7V
Voltage
13.6V
8.0A
Current
Figure 1 IUU Charge Algorithm
The boost converter converts the 12V dc of the battery to 350V dc to power the variable speed
compressor. By using a 350V compressor, we are able to draw less current to drive the AC
system. This allows the Climacab to use less energy to run the AC. The AC system matches the
load in the cab with the work of the AC system. In maintain mode, the system draws very little
current providing for extended run times
The power management function of the PMM tracks the current flow into and out of the
batteries, calculating the SOC, managing operation of the system to ensure we stop operation
at 80% depth of discharge. This ensures we do not damage the batteries by excessive
discharge. The charge/ discharge rates are controlled and meet the battery manufactures
recommend practices. The PMM monitors the start battery voltages and only draws available
alternator current based on battery voltage. As the start battery voltage rises the PMM will draw
more current as it is available. This ensures that the alternator is not overloaded. Systems
without this capability will parallel the two battery banks which can cause excessive load on the
alternator causing premature failure. The SOC graph below (figure2) depicts SOC values based
on various discharge rates as a battery voltage. This power management ensures the system is
performing at peak efficiency. This reduces the depth of discharge and maintains cycle life. By
utilizing this solid state technology, we are able to have a longer warranty on the batteries. By
utilizing this solid state technology, ClimaCab is the only battery operated APU that is granted a
industy leading battery warranty, by NorthStar Battery company. No other battery manufacture,
offers a similar warranty on an electric APU product.
Figure 2 Battery SOC discharge graph The battery based air conditioners can use various battery chemistries such as flooded, AGM,
pure AGM and Gel. All of these battery types will work, but they have their issues. Most battery
based systems use AGM due to their low maintenance, deep discharge and longer cycle life. A
typical AGM battery has a life of 400 cycles of 80% depth of discharge if managed correctly.
Understanding AGM Technologies
The choice of batteries today has grown, historically you would choose between a flooded
starting battery for the engine or a deep cycle battery for the APU. Today, however the choices
have increased with the advancements made in battery technologies. The buzz word in the
heavy duty market today is Absorbed Glass Material (AGM), which comes in two basic styles
Pure Lead Technology and Conventional AGM.
Conventional AGM Technology
Aso known as Calcium AGM which has a very similar plate manufacturing process to
conventional flooded batteries used throughout the trucking industry for many years. When
manufacturing conventional AGM batteries calcium is alloyed with the lead in order to produce
battery grids that give the plate added strength so that they can be handled in the manufacturing
process without fear of damaging them. The downside of conventional AGM is that the plates
will grow over time as a normal effect of life, space on the top and sides inside the cell must be
left empty to allow for the grid growth. This means less active material can be used as
compared to Pure Lead batteries The benefit is conventional AGM is easier and less expensive
to produce than its counterpart the Pure lead AGM product providing customers with a lower
cost to purchase and with a 150 – 200 cycle life at 80% depth of discharge provide a much
greater life expectancy over traditional flooded batteries.
Pure Lead AGM Technology – The manufacturing of Pure Lead battery plates is much more
difficult requiring automated robotic manufacturing processes. The plates are inserted into the
cell at a much higher compression (approximately 38% compression) requiring stronger, more
rigid battery cases contributing a higher base material cost. A benefit of the added compression
is greater vibration resistance virtually eliminating battery failures caused by vibration. The
greatest advantage of Pure Lead is grid growth is virtually eliminated allowing more active
material to go into the battery, unlike conventional AGM there is no space required for grid
growth. More active material in the battery equates to higher CCA and reserve capacity ratings
being achieved within the same case size. The higher compression and purity of the plate also
provide for much higher charge acceptance rates which allows the battery to be charged more
quickly, a very important attribute with APU’s.
Technology Summary
Conventional AGM has lower available CCA and Discharge capacities, shorter life
expectancies and lower vibration resistance however a lower cost to purchase. Pure Lead AGM
has more active material leading to higher CCA and reserve capacities, higher vibration
resistance and a longer life expectancy. 400 cycles at 80% depth of discharge is very important
to consider when making your battery decision and is further explained below in choosing the
right battery for the job.
The Right Battery for the Job
The greatest contributors to the overall life expectancy of your APU batteries will be temperature
and management of the charge / discharge cycle. The chart below (Figure 3) illustrates the
difference in life expectancies of Pure Lead AGM based on varying discharge levels,
temperature and or both.
Charge Acceptance
Conventional and Pure lead AGM batteries have a greater charge acceptance over flooded
batteries and require higher Charge Voltages to ensure a proper charge occurs after every
discharge cycle. AGM batteries in a deep cycling environment should be charged at a rate of
14.6 – 14.8 volts to ensure a proper charge. Charging a battery at lower charge voltages will
extend the time required to charge the battery resulting in batteries that are not fully charged
during the driving cycle and thus will not perform to the required level during the discharge cycle
of the APU. Many battery base APU companies rely on charging the batteries by the alternator
voltage and current and just connect the two battery banks together. A typical alternator voltage
varies (13 to 14.2 Vdc) with load with a maximum voltage of 14.2V. This battery voltage is
insufficient to charge the batteries in a typical truck drive cycle. By utilizing the dc to dc charger
provided in the Climacab PMM, you can ensure proper charge voltage and current, which will
allows the system to charge 110% of the energy back into the battery and have 100% state of
charge or capacity available. A byproduct of insufficient charging is sulfation of the battery
plates (crystallization of the electrolyte) which reduces battery capacity and cycle life.
Number of Cycles based on Depth of Discharge
Depth of Discharge
Temperature in Celsius
20°
25°
30°
35°
40°
45°
50°
100%
300
300
278
255
233
210
188
90%
355
355
328
302
275
248
222
80%
428
428
396
364
332
299
267
70%
529
529
489
450
410
370
331
60%
676
676
625
574
524
473
422
50%
903
903
835
768
700
632
564
40%
1,288
1,288
1,191
1,095
998
901
805
30%
2,035
2,035
1,882
1,729
1,577
1,424
1,272
20%
3,877
3,877
3,586
3,295
3,005
2,714
2,423
10%
11,671
11,671
10,796
9,921
9,045
8,170
7,295
8%
17,326
17,326
16,026
14,727
13,427
12,128
10,829
5%
35,137
35,137
32,501
29,866
27,231
24,596
21,960
2.5%
105,778
105,778
97,845
89,912
81,978
74,045
66,111
Figure 3 Depth of Discharge Chart
The difference in life expectancy of a battery operating in a constant environment of less than
75°F allows it to be discharged to 80% with 428 cycles, but if the temperature is increased to
100°F the battery can only be discharged to 60% to achieve the same life expectancy as
referenced in the chart above. (See Figure 3)
Temperature
Although you have very little control over the ambient operating temperature outside,
understanding the effect it has on the overall life expectancy will allow you to better budget how
long the battery will perform as required based on the trucks operating area. A life expectancy
calculator is available to assist in making assumption on life.
Reserve Capacity
How does reserve capacity affect the APU performance? If you compare a 180 minute reserve
capacity battery versus a 220 minute battery and discharge both batteries at 25 amps for 110
minutes the following is what you can expect.
Reserve
Capacity
25 amps Discharge
Duration
180 min
Depth of
Discharge
Number of Cycles
Expected
61%
676
50%
903
110 min
220 min
Figure 4 Reserve Capacity
Conclusion
By understanding your idle off system requirements, having the right battery technology and
utilizing the proper charging methods with the ClimaCab PMM charging technology, you will
extend the cycle life of the batteries and maximize the return on investment.