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Lead Acid Batteries:

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A consolidation of notes from Kitty Rodden's presentation to the Peninsula Chapter of the Electric Auto Association, by David Coale

What battery should I use in my EV?

This is the age old question; what battery should I put in my EV? There is no standard answer to this, it depends on what you want out of your EV. The old rule of "thumb" if you will, is the more lead the greater the range and the poorer the performance due to the increase in weight. It is also true that the higher the voltage the better the performance (acceleration and top end speed). This is of course also dependent on the type of driving and on the controller. Since these are fixed for any given EV, these generalizations are not too far off. Two designs illustrate this:

Long range: 96 to 120 volt system using 6 volt deep cycle batteries. This will give you lots of amp hours and weigh 976 to 1420 bls. This type of battery pack should last two to three years depending on the type of driving.

High performance: 96 to 144 and up using 12 volt batteries. If you use a starting battery your car will be light (400 to 700 bls.) and quick and your batteries will last about 3 to 6 months or less. If you use deep cycle batteries (12V) your batteries can last as long as one and a half to two years but the car may be heavier depending on the battery.

So what is the range in each case? Isn't that what we want to know; how far will my EV go? Answering this will help you decide what battery you will use.

To find out how far a particular battery pack will take us, we need to know how much energy is in the pack and how much energy the EV uses per mile.

Range = Energy in pack/(energy used per mile)

The energy in the battery pack (wired in series) is the amp hour rating times the pack voltage. The amp hour rating is how may amps a battery can supply over a given time. Most batteries are measured over a 20 hour period. This is a standard that is used to compare batteries with, and can be found in the specs. on most batteries. The 20 amp hour rating has to be adjusted for EV use. The faster one draws current from a battery the less capacity there will be. This is due to the chemistry of the battery and the internal resistance. Therefore the capacity of a battery at the 20 hour rate is more then the capacity at the 1 hour (EV) rate. The following is a table from Trojan Battery Company showing the conversion factor for finding the X hour rate given the 20 hour rate:

Conversion of 20 hour rate to X hr rate.

   X hr  Conversion 
   rate  factor 

     1   .57 
     2   .67 
     3   .74 
     4   .77 
     5   .82 
     6   .84 
     7   .86
     8   .87 
     9   .89 
    10   .91 
    20  1.00 
Note: these values will be a little different for each battery.

One can see from the table above that as you drive faster the range will decrease due to reduced available capacity. This does not take into account wind resistance which also increases with speed.

Now we need to know the energy or watt hours per mile that your EV gets. The table below lists some common EVs and their watt hours per mile rating.

Impact = 130 W-hr/mi (AC drive very good aerodynamics)
Metro = 160 W-hr/mi (PM drive good aerodynamics)
Metro = 200 W-hr/mi (DC drive good aerodynamics)
Truck = 350 W-hr/mi (DC drive poor aerodynamics)

This table was developed using several cars for each category traveling at highway speeds (60 mph). The numbers reflect the efficiency of the cars listed. With the following information the range equation for a smaller type of EV on the freeway would be:

         20 amp/hr rating X .57 X pack voltage
Range =  -------------------------------------
         200 watts hours per mile
From this equation we can make a table using some commonly used batteries.
Bat.    20 AH   Wt. pack   pack  range  Cycle  miles  miles X cycles 
type     rate  Lbs. volts   Wt.  miles  life   /Lbs.  /Lbs. 

T-105    217   61     96   976   59.4   754   .06086   45.89 \ 
                     120  1220   74.2                         | 
T-125    235   66     96  1056   64.3   650   .06089   39.58   >  6 volt
                     120  1320   80.4                         |  Batteries 
T-145    244   71     96  1136   66.8   625   .05880   36.75  | 
                     120  1420   83.4                        / 

T-875    165   63     96   756   45.1   540   .05965   32.21 \  8 volt 
                     120   945   56.4                         > Batteries 
                     144  1134   67.7                        /

27TM     105   52     96   416   28.7   210   .06906   14.50 \ 
27TMH    117   60     96   480   32.0   358   .06669   23.88  | 
                     120   600   40 0                         | 
                     144   720   48.0                         | 
30XHS    130   66     96   528   35.6   325   .06742   21.91  | 
                     120   660   44.5                          >  12 volt 
                     144   792   53.4                         |  Batteries 
5SHP     165   86     96   688   45.1   560   .06555   36.74  | 
                     120   860   56.4                         | 
                     144  1032   67.7                        / 
As you can see there are no clear cut answers. The miles per pound are better with the 12 volt batteries but the cycle life is not as good; and the converse is true. The last column is an attempt to find a single number to compare the batteries with. This does not show price however, and this will be a factor in any EV conversion.

The above table is for comparisons only and may not reflect your actual range. The information shown here is by way of Kitty Rodden formally of Trojan Battery Company. I do not have any affiliation with Trojan Battery Co. This is just the information I have at hand. Please send questions and comments to David Coale.

David Coale                            ___o\____
(650) 493-4503                  =)----/()_____()\

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