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Discussion Starter · #1 ·
Hello, I have a couple of basic, easy to answer questions regarding car batteries.

for short range conversions, meaning 50 or so miles at most, are Lead acid batteries still a viable choice?

How many kwh is a good amount to get this range? Given the car has a standard weight of around 2000 lbs?

I plan to build this vehicle in Europe, and looking at prices in the UK on ebay, EV batteries are very expensive, around 1000 usd for 5kwh or so. Is there somewhere else I should be looking, or are those prices pretty standard?

Thanks!
 

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For 50 miles you probably need 5-10kwh depending on the efficiency and speeds.

Yes batteries are expensive by themselves but cheap in EV cars. I bought a wrecked LEAF for $3400 that has 17kwh of usable capacity, hard to beat that and I get a motor and a bunch of other parts I can sell.
 

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Discussion Starter · #3 ·
For 50 miles you probably need 5-10kwh depending on the efficiency and speeds.

Yes batteries are expensive by themselves but cheap in EV cars. I bought a wrecked LEAF for $3400 that has 17kwh of usable capacity, hard to beat that and I get a motor and a bunch of other parts I can sell.
If I were to get 1 or 2 12v wet truck batteries (total 4-6 Kwh), would they be able to supply enough voltage to get a 9 inch motor to move a vehicle at a reasonable rate, even if its for less than 10 miles? As a test before looking into/buying full EV batteries?

I want to build an EV, but actually test it before investing into a full EV power pack.

Im still in the learning process, so I appreciate your patience. Thanks!
 

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If I were to get 1 or 2 12v wet truck batteries (total 4-6 Kwh), would they be able to supply enough voltage to get a 9 inch motor to move a vehicle at a reasonable rate, even if its for less than 10 miles? As a test before looking into/buying full EV batteries?

I want to build an EV, but actually test it before investing into a full EV power pack.

Im still in the learning process, so I appreciate your patience. Thanks!
Yes, absolutely. You can use cheap old lead acid batteries for testing. That is ideal because you can probably collect some old car batteries for free. I put out a request to friends and family for old car batteries just for this exact purpose and was able to collect almost 20 of them for free!

for a 9 inch motor I'm guess you need between 100 and 200 volts. 12 old car batteries will give you 100 volts for testing.
 

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Discussion Starter · #5 ·
Yes, absolutely. You can use cheap old lead acid batteries for testing. That is ideal because you can probably collect some old car batteries for free. I put out a request to friends and family for old car batteries just for this exact purpose and was able to collect almost 20 of them for free!

for a 9 inch motor I'm guess you need between 100 and 200 volts. 12 old car batteries will give you 100 volts for testing.
Whats the least amount of batteries I could use to get enough current to power the motor? Unfortunately, in the place im planning to assemble this car (not-US), surplus car batteries arent easy to find. Would 2-3 auto truck batteries be sufficient when used with a step up converter?
 

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Whats the least amount of batteries I could use to get enough current to power the motor?
Depends on what you mean by "power the motor" or "reasonable rate." What you are suggesting is basically like "driving" the car with the starter motor. In this case, you dont have to worry about the motor burning up, but the battery will give out before long.

If I was to make an educated guess, I would say that a brand new 12v 100ah starter battery might get you a mile at about 5mph. It will likely only do a couple trips before it is toast.

As far as lead acids go, they will certainly move a car; but 50 miles is not going to be easy. If you could live with 20 or 30 miles, It might work. You will be replacing them so often though that paying 200$/kwh for lithiums will be money well spent. If you crunch the numbers, that is actually probably cheaper when compared to the true capacity of a lead-acid bank (due to the peukert effect and the fact that you should not exceed 50% DOD on FLAs if you want any sort of longevity).
 

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Whats the least amount of batteries I could use to get enough current to power the motor? Unfortunately, in the place im planning to assemble this car (not-US), surplus car batteries arent easy to find. Would 2-3 auto truck batteries be sufficient when used with a step up converter?
I don't know, how could I know? What motor? What car?
 

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If you crunch the numbers, that is actually probably cheaper when compared to the true capacity of a lead-acid bank
I did the math a couple years ago, and concluded there is no reason for anyone to use lead acid for any EV. You cannot win. You actually can't even win vs. gasoline, before you save as much as you would've spent on gas, you have to replace your batteries.
 

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Whats the least amount of batteries I could use to get enough current to power the motor? Unfortunately, in the place im planning to assemble this car (not-US), surplus car batteries arent easy to find. Would 2-3 auto truck batteries be sufficient when used with a step up converter?
Current isn't the only factor with a battery pack. There are three basic parameters that impact the motor:

1. Voltage. The voltage determines how fast the motor can spin. Low voltage battery packs will not spin the motor fast, so the EV will drive very slowly.
2. Current. The current determines how much torque/power the motor will generate. While small battery packs can offer significant current, in general the more cells a pack has in parallel, the more current it can offer.
3. Capacity. The capacity in kWh determines how much energy the pack can hold which impacts how far you can travel. Since the kWh is determined by the voltage and amp-hours of the battery pack, the bigger the pack and the higher the voltage, the farther you can go with it.

So, let's see where 2-3 auto truck batteries gets you:

1. Voltage: 24-36V. Depending on the gearing, this will get you speeds of 15-20 MPH.
2. Current: Lead acid will offer significant current. So, there will be enough power to move an EV.
3. Capacity. Low voltage and a limited number of batteries will get you a very small capacity. At best you may get 4 or 5 miles out of the battery before it's out of energy.

So, you really need to decide what's important. If you need both distance and speed, there's really no way around the problem. You need a bigger, higher voltage, battery pack.

ga2500ev
 

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Discussion Starter · #10 ·
Current isn't the only factor with a battery pack. There are three basic parameters that impact the motor:

1. Voltage. The voltage determines how fast the motor can spin. Low voltage battery packs will not spin the motor fast, so the EV will drive very slowly.
2. Current. The current determines how much torque/power the motor will generate. While small battery packs can offer significant current, in general the more cells a pack has in parallel, the more current it can offer.
3. Capacity. The capacity in kWh determines how much energy the pack can hold which impacts how far you can travel. Since the kWh is determined by the voltage and amp-hours of the battery pack, the bigger the pack and the higher the voltage, the farther you can go with it.
I see, it seems lead acid is not a viable solution.
If voltage determines the speed/rpm of the motor, and current determines torque, at higher speeds can I use a converter to get higher voltage and lower current from my battery? In the same way manual cars have lower torque but higher speed in their higher gears? Is that the job of the BMS, maybe the controller?

I've been looking at cells recently, i've found 3.6v, 42AH cells for 25 usd. If I were to put these together into a cell, for an EV w/ maximal acceleration, I should focus on having high current, rather than voltage?

Is there a converter I could use to get higher current/torque at the low end, and more voltage/less current at high end? Is this the BMS task?
 

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If voltage determines the speed/rpm of the motor, and current determines torque, at higher speeds can I use a converter to get higher voltage and lower current from my battery?
Yes, and Toyota actually does that in their original-style hybrids, doubling the battery voltage in a converter stage before the motor controller. In an EV-sized battery this doesn't make sense; it makes more sense to just configure the battery for the right voltage.

In the same way manual cars have lower torque but higher speed in their higher gears?
...
Is there a converter I could use to get higher current/torque at the low end, and more voltage/less current at high end?
A converter can exchange between current and voltage, just as gears exchange between torque and speed. But since controllers inherently convert battery voltage to lower voltage and higher current as required, there's no reason to convert voltage differently at different motor or road speeds, like shifting a transmission to a different gear ratio.

Is that the job of the BMS, maybe the controller?
...
Is this the BMS task?
The BMS is the battery management system. It just monitors the battery (including voltages at each cell level), usually enforces some limits by telling the charger and motor controller how much power can be used, and typically balances cell charge levels by selectively discharging the higher-voltage cells slightly. The power from the battery doesn't go through the BMS at all.

I've been looking at cells recently, i've found 3.6v, 42AH cells for 25 usd. If I were to put these together into a cell, for an EV w/ maximal acceleration, I should focus on having high current, rather than voltage?
When you combine cells, you are building a battery, or if the battery is split into several parts, they're called modules. The usual approach is to combine the cells in the way that delivers the highest required voltage (enough for the motor at the highest desired speed, but still within the controller's voltage limit). The controller will divide voltage and multiply current as required... within the controller's current limit.
 

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I should focus on having high current, rather than voltage?
Generally you tackle battery building in this order:

1. How fast do you need to go? This sets the voltage of the pack. As @brian_ pointed out, Motor controllers typically regulate the pack voltage to a lower voltage, so the pack needs to be designed to turn the motor at the maximum speed you need it to go.

2. How far do you need to go? Your range is computed by taking the energy in the pack (in kWh) an multiplying it by the efficiency of the vehicle (in miles/kWh). So for example if you pack has 10 kWh of energy and you have an efficiency of 3.5 miles/kWh then you have a range on 35 miles for that pack. You can improve the range by making the battery bigger, improving the efficiency, or a combination of both.

3. How fast do you need to accelerate? This is the torque which is motivated by the amount of current the battery can deliver to the motor. Generally making the battery bigger by paralleling cells facilitates adding more current to the mix, improving the acceleration.

You've only answered one of these three questions so far by indicating a range of 50 miles or less. You still need to set a top speed. Let's say 60 MPH/100 kph. For the sake of argument doing that computation with the speed that the motor turns, the final gear ratio, and the size of the tires computes out to 96V to generate that top speed (I'll offer to compute the actual numbers given the specific motor, final gear ration, and the actual size of the tires). To get 96V with your 3.6V 42 AH cells you'd need 27 of them. Cost is $700 at $25/cell. Total capacity is 3.6V*24*42 Ah = 4.082 kWh.

You can start to see the numbers now in the example. With 4 kWh such a pack will get you 12-15 miles of range. The lowest current limit of the motor, controller, and pack would determine the maximum current that can be delivered.
Each $700 investment into the batteries adds 12-15 miles more range and improves the batteries ability to generate more current.

But no one but you knows what parameters actually work for you. Answer the three (or at least the first 2) of the questions here along with your maximum budget, and then maybe we can figure out a design that can meet those parameters.

ga2500ev
 

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Discussion Starter · #13 ·
for speed, 60mph is more than sufficient, as I plan to drive this in a large city, never on the highway. 50 mph is a good figure to calculate for as well.

Range, 50 miles is also enough, but I can also add to this by adding parallel cells later, right?

Acceleration is my main desire, and I want this to be as fast as possible, below 5 seconds, but if possible around 3.5 0-60. I plan on using a 9 inch forklift motor, but If that motor isnt sufficient, Ill opt for something else.

Ideally I want to spend around 1000 usd on the battery pack, and around 700 for the motor. Thank you!
 

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Discussion Starter · #14 ·
Also, upon doing further research on my own, Ive found a way to have access to 3.7v, 1000mAh 18650 cells for good prices. Are these a good option for EV batteries, given all the individual cells are tested/not faulty? My understanding is that 18650 cells, and all Lithium ion batteries have to be kept in a certain temperature range, but are there any other obstacles i have to consider?


Thank you!
 

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for speed, 60mph is more than sufficient, as I plan to drive this in a large city, never on the highway. 50 mph is a good figure to calculate for as well.

Range, 50 miles is also enough, but I can also add to this by adding parallel cells later, right?

Acceleration is my main desire, and I want this to be as fast as possible, below 5 seconds, but if possible around 3.5 0-60. I plan on using a 9 inch forklift motor, but If that motor isnt sufficient, Ill opt for something else.

Ideally I want to spend around 1000 usd on the battery pack, and around 700 for the motor. Thank you!
What you want isn't doable at that price point. You can see it from the previous example with the cells you offered. Your 42 Ah cells have a capacity of 150 Wh apiece. You need 7 of them to get a shade over 1kWh of capacity. This will cost you $175/kWh, which honestly isn't a bad price for DIY.

A reasonable efficiency is 3.5 miles/kWh. In order to get 50 miles you'll need 14-15 kWh for your battery. At $175 a kWh, 14 kWh will cost about $2500.

You will have to spend more money to get a battery pack large enough to do what you want. Or you will have to find much much cheaper cells. BTW the 18650's will cost both more money and more headache.

ga2500ev
 

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Also, upon doing further research on my own, Ive found a way to have access to 3.7v, 1000mAh 18650 cells for good prices. Are these a good option for EV batteries, given all the individual cells are tested/not faulty?
1000 mA capacity is very low for an 18650-sized cell in an EV battery. You would need to build your battery with cells having about three to four times that capacity per cell to match a Tesla battery of the same size. I suggest using those 1000mAh 18650 cells in flashlights, and getting something more suitable for an EV.
 

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Discussion Starter · #17 ·
1000 mA capacity is very low for an 18650-sized cell in an EV battery. You would need to build your battery with cells having about three to four times that capacity per cell to match a Tesla battery of the same size. I suggest using those 1000mAh 18650 cells in flashlights, and getting something more suitable for an EV.
The issue is, in the part of the world where I'm planning to build this EV, EV batteries are borderline Impossible to find, and definitely impossible to find at any reasonable price point. Upon further research, I've found that I can get my hands on tested 2-2.2 AH 18650s for around a dollar each. Even if the power density is less than ideal, these seem to be the best option for me in terms of price and availability.

Is 2000 mAh enough for each cell to build a Pack that's usable in an EV, especially if I can fabricate a cooling/temp control system?

Would 27 cells in series, in 10-20 parallel stacks be a good way to start? Could I use that battery for testing, and eventually add more 18650 cells/packs as time went on? I could start with more cells if that would be better.

I want to build an EV more than I want to stick to my budget, so if 1000 usd isn't enough for a battery pack, I could spend 2000 or so.

Is the 99 volts this setup would generate (if my math is correct) be sufficient to reach the 60 mph im hoping for? Would the current be sufficient?

I apologize if I come across as stubborn, I really appreciate the help I'm receiving here, This is still all very new to me.
Thank you!
 

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Discussion Starter · #18 ·
What you want isn't doable at that price point. You can see it from the previous example with the cells you offered. Your 42 Ah cells have a capacity of 150 Wh apiece. You need 7 of them to get a shade over 1kWh of capacity. This will cost you $175/kWh, which honestly isn't a bad price for DIY.

A reasonable efficiency is 3.5 miles/kWh. In order to get 50 miles you'll need 14-15 kWh for your battery. At $175 a kWh, 14 kWh will cost about $2500.

You will have to spend more money to get a battery pack large enough to do what you want. Or you will have to find much much cheaper cells. BTW the 18650's will cost both more money and more headache.

ga2500ev
I understand. I want to build an EV more than I want to stick to my budget, so If I need to spend more, I will.

The 18650s I can get my hands on are are roughly 1usd a piece, with a 2-2.2 mAh capacity, So roughly 135 are needed per kWh, 135usd/kwh.

Could I later add packs to the 18650 battery pack, to make a battery larger as time goes on?

Would 99 volts be sufficient to reach 60mph in a 9 inch forklift motor, similar to the netgain Warp 9?

In what way are the 18650s a headache, besides individual cell testing and temperature control?

Thank you !
 

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I understand. I want to build an EV more than I want to stick to my budget, so If I need to spend more, I will.

The 18650s I can get my hands on are are roughly 1usd a piece, with a 2-2.2 mAh capacity, So roughly 135 are needed per kWh, 135usd/kwh.

Could I later add packs to the 18650 battery pack, to make a battery larger as time goes on?

Would 99 volts be sufficient to reach 60mph in a 9 inch forklift motor, similar to the netgain Warp 9?

In what way are the 18650s a headache, besides individual cell testing and temperature control?

Thank you !
You would spend a lot to time and money buying the holders to put them together not to mention the bus bars. Most people spot weld to them
using a nickle strip, which has a much higher resistance than copper. This ends up with heat next to the cells at the very time when you don't want heat, that is
at full throttle. While it's certainly possible, it's really not that safe either. Cooling is only needed if you are pushing the cells hard. If you are staying within the
design limits of the cells, they don't get very hot and air cooling is ok. However, the design limits are usually pretty low and many 18650's can get into thermal
run-away. Been there, done that. see bbitnerblogs.com/e-miata. Were I to do it over, I'd buy something like this from e-bay

Probably overkill for what you need. Maybe you could sell the batteries you don't need..

unless you already have the motor, a Leaf motor with shipping is likely to be as cheap when the cost of a controller is factored in..
Bill
 
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