So, I bought a truck. I hope to chronicle here its conversion from drippy, oily, noisy old truck into a fresh, sweet smelling (with addition of tree air freshener), quiet, electric vehicle (that will one day be PV powered).
While we’re waiting for me to get my act together with a couple of other EV posts, I’ll be laying out descriptions of various components and why I’m choosing which (mostly because I’m cheap).
This first post deals with why I am choosing Lead Acid batteries (because I’m cheap).
Bring on the money pit
I recently bought a used Toyota Truck. A 1986 4spd with 170k, it’s not in terrible condition. $950 got me a truck that needs some TLC, but otherwise it’s pretty solid.
I settled on a truck because it gives me more options for carrying lead acid batteries. A lot of garage conversions use lead acid batteries. They’re cheap compared to lithium, and pretty easy to come by considering industrial uses, solar battery back ups, and of course golf carts.
Lead acid batteries will probably tolerate more abuse than a lithium pack. Lead acids don’t typically require a battery management system, Lithiums most certainly do. (Not only is the chemistry expensive, so is its management system.)
It is recommended, though, that some sort of battery balancing system is implemented (more on that later), usually consisting of a bypass circuit, so certain cells in the pack can be bypassed after they’re charged and others can be “balanced” up to a nominal cell voltage, before the charger thinks the pack is charged and ceases. It certainly will lengthen the life of the pack.
So why a truck instead of a car? Put simply, you can get more range using a truck, because it can safely carry the batteries needed to do so.
My truck has a GVWR of 4,400 lbs, 2500 of which is the truck itself (?) That leaves me with about 1,500lbs for a payload, in this case, lead. The calculations I’ve been doing estimate the battery pack needed for my range will weigh around 1,200lbs leaving about 300lbs for payload not to exceed the 4,400 lb GVWR. (The GAWR, gross axle weight rating for the rear is 2,540 and front is 2,050. I’m still confused as to what the weight of the truck is, but hey, I’m pretty sure it’ll handle 1,200lbs payload)
You can read a bunch more about lead acid batteries here and who makes the best ones:
Notice what is at the top of the 6v battery list? It’s the Trojan T-105 battery, ubiquitous in golf carts everywhere.
Also, I might be using abbreviations here so I’ll define them all here: ah=Amp Hour, battery capacity is rated in ah. V= volts, yeah probably didn’t need that one. Wh = Watt hour and Kwh = Kilowatthour.
Why use 6 volt batteries ?
Well, to understand completely why, you’d probably have to solicit the opinion of a battery tech or someone who has built a couple of EVs.
A really good explanation is here: 2 setups, one with 8v and one with 6v: http://evdl.org/archive/#nabble-td2967658%7Ca2967686
(An 8v/12v pack might be easier to kill, basically, if you don’t limit your controller or driving habits accordingly.)
To move a vehicle with electricity takes differing amounts of voltage and current depending on a lot of factors, but the main one is weight. To move a larger vehicle, it’s a general rule that you will need higher voltage in a DC system, because, although you can get controllers that will pump 2000 amps, the usual range is 400-750amps and that is only for a few seconds usually.
So if Volts x Amps = Wattage (power) then higher voltage will keep our amperage down to a level we can buy a controller for (and that won’t exceed the battery discharge rate, which is usually 3C, Capacity x 3, in a simplistic example this would be 660 amps for the T-105).
For a car the size of a VW Bug, most lead acid conversions will be around 72-96v but that doesn’t mean you can’t go higher, again the only limitations are weight and space.
For something like a 1986 Toyota Truck or Chevy S-10, it is highly recommended that you don’t use less than 120v and most lead conversions go as high as 132 or 144v, some even as high as 180v.
Now what about range ?
How I have been calculating range is estimating the trucks efficiency, which is a stab in the dark, but I know a lot of converted trucks are between 350-450 watt hours per mile (reading amps x volts while driving, not while charging which will produce a figure taking into account power consumption before the battery pack).
Now, a Trojan T-105 battery is 6v and 220 amp hours for a watt-hour rating of 6×220 = 1320wh (This is the battery’s 20 hour rate, meaning if we put a load in it that discharged it at 11 amps, it would last about 20 hours before going dead. (11a x 6v x 20 hours = 1320wh))
KNOW THIS, when we are dabbling with the science of battery chemistries, there are certain physical properties which must be considered;
1. With lead acid batteries, the slower you discharge them, the more capacity you will be able to exploit. This is Peukert’s Law.
2.Fully discharging a battery will shorten its life. You have to assume your range is dependent on *not* fully depleting your battery.
Hence, this 220 amp hour figure is a little misleading, in our application, we will never see a battery’s full potential, for the above 2 reasons.
This is why we consider a batteries DOD or depth of discharge, opposite to its SOC, stage of charge (a battery with 80% DOD has 20% SOC).
For an EV with a fairly expensive set of batteries, you’ll want to build in the fact that you really don’t want to pass 75% DOD.
My pack will look like this: 20 x 6v = 120vdc. 20 batteries in series gives me 120volts, 220ah capacity. So 120v x 220ah = 26.4 kilowatt hours
Now, that 26.4kwh is a completely false number when we think about the 2 rules above.
So, I’m going to fudge a bit of math here to attempt to end up on the safe side of a battery estimate. I assume I’m never going to see more than 180ah of capacity from my pack because of Peukert’s Law in my application.
Then I will take into account that I want to only discharge my pack 75%.
120v x 180ah = 21.6kwh x .75 = 16,200wh
That is a full 10.2kwh lower. Thanks physics.
Ideally you’d want to be closer to a 50% DOD and depending on the real world requirements, efficiency and how fast I drive I might get close to that.
Ok, now we can estimate range.
Assuming a 450wh / mile EV efficiency; 16.2kwh / 450 = 40.5 miles
I actually need 28 miles one way. So far so good. (charge at home, then charge at work for the journey back)
Next post will be about Controllers or Motors/Adaptor plates. Stay tuned.