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  #21  
Old 05-23-2012, 07:40 PM
Dustin_mud Dustin_mud is offline
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Default Re: Electric crawler

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Originally Posted by ruckus View Post
roughly $1.5/kilowatt hour/ battery. So a 20ah batt costs $30. Of course, at 3.2v each you would need 38 to get 120v. So that is $1100 for 120v 20ah. Now multiply. You want 100ah? $5500 etc..
I'm just a simple gear nut, you just talked over my head. Sorry I am trying to learn, but as of right now I'm not sure what amount of V or Ah I want or need.
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  #22  
Old 05-23-2012, 11:43 PM
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Default Re: Electric crawler

Just so you know, I am currently building a FJ-55 with full 2x6 rocker sliders and 2x4 quarter panel guards. 36" Swampers and airbag lift. So I know exactly what you are after. Step 1 was cut cut cut..

It's pretty simple. 1 amp hour will give you 1 amp for an hour. So a 40ah cell will give you 40 amps for an hour. Then it is dead. Pay attention cause this is most important. AMPS X VOLTS = WATTS. This is most useful because watts is like hp. Each lithium battery is 3.2 volts. That means a 40ah batt puts out about 120watts for an hour. 700 watts is about 1 hp. so 120 is nothing.

I gave you the chart of motor torque at 120v. To get 120v you need 38 lithium batts (3.2v each). It doesn't matter whether they are 10ah or 200ah, you still need 38 to get to 120v. Most lithium batts are good for 3-5C. C is the current rating. A 40ah battery is 40ah at 1C. If that batt is rated at 10C max pulse current, then the max it can put out is 400 amps. If you have a 1000 amp controller, then this is too small a batt. 100ah @ 10C is 1000amps. this is really hard on the batts. But for a 200ah batt 1000amps is only 5c. That is much nicer. So you want big batts. Problem is big batts take up a lot of space and cost money.

So the other way is to go high voltage. if you want a given amount of power (say 80 hp) that is about 60,000 watts. Divide that by 120v and you need 500 amps. but if the system is 240v you only need 250amps. Same amount of power. Thats because AMPS X VOLTS = WATTS

So if you want to be nice to your batts and not go over 3C then if you need 60kw at 120v you will need 38 160-180ah batts. But if you go to 240v then you need 75 80-100ah batts. Same amount of power, different configuration.

AC motors (which have regen or downhill resistance) tend to like higher voltages. DC motors (which have no downhill resistance) tend to like lower voltages. So you will pick a motor first, then spec batteries to power that motor.

Later...
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  #23  
Old 05-24-2012, 01:18 AM
Dustin_mud Dustin_mud is offline
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Default Re: Electric crawler

Thank you again for great info
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  #24  
Old 05-24-2012, 06:44 AM
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Default Re: Electric crawler

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Originally Posted by Dustin_mud View Post
Well I would like to test all of this before dropping 6k in just batts, if I knew this is going to work great then no problem.
You can also test with small lithium pack.
With high discharge rate cells, you can build a small and lightweight pack for less than 1000$. The range (or the time between charge) will be also small but they can weight less than 100 lbs.

I think a used forklift motor is the best way for you because they can be find from free to 500$ and output more than 300 lbs-ft of torque from 0 rpm with a proper controller. I talk about 11'' forklift motor.
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Old 05-24-2012, 07:11 AM
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Default Re: Electric crawler

I don't know a lot about rockcrawling, but it looks to me like a high torque motor is not really required, as referenced by the samurai 80hp engine. I'd look into one of the HPEV AC motors. you'd get a sweet smooth motor, Regenerative Braking, lower voltages, and probably plenty of power if you're geared down enough. 120v of 200ah cells may do it? I don't think you need high C rates, you need kWh.
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Old 05-24-2012, 04:26 PM
Dustin_mud Dustin_mud is offline
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Default Re: Electric crawler

I like the thought of high volts and half the amps, so I could run smaller batts and shave some fat.
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  #27  
Old 05-24-2012, 05:11 PM
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Default Re: Electric crawler

KWh's = Volts x amp hours, so many small batteries or fewer large batteries weigh the same in the end for the same amount of KWh's, which equals range. No fat shaving
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  #28  
Old 05-24-2012, 08:49 PM
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Default Re: Electric crawler

Ok if I look into sealed lead batt how do I do the math for those, same way? Is cranking amps just the amps you plug into the formula? Sorry for all the newbie stupid questions, but trust me theres a ton more. Lol
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  #29  
Old 05-25-2012, 02:15 AM
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Lightbulb Re: Electric crawler

I think the CCA of a battery is just about absolute maximum that can be sustained for 5-10 seconds, and is about 5 to 10 times the nominal A-H rating. So a 90 A-H battery might have a CCA rating of 600 amps, but that should be only for occasional use. Generally about 1/2 the A-H rating is about the maximum you want to sustain for any period of time, so figure 45 amps. And even that may only last about an hour. You can get close to the full A-H capacity at about 1/10 C, so maybe 9 amps for 10 hours. Lithium and NiMH can do better.

It seems that the "Green Dawg" or "Rock Dawg" uses just a 10HP electric motor which can give short bursts up to 40HP for 5 seconds, so for that you would need 30kW for 5/3600 hr or 167 W-h. You can get an honest 50 A-h and 600 W-h from a single battery, but you must figure the peak power. 30kW is 2500 amps. But you don't really need that much HP if you are going slow, so you might need to start with the total torque you need. You want to be able to essentially climb a vertical wall so you need a total force equal to the weight of the crawler and passenger. Let's say 1000 pounds. With 24" tires that is 1000 lb-ft. 60 RPM will give you a speed of about (24*3)/5280 miles/minute or 0.8 MPH. The power needed is 1000*60/5252 = 11.4 HP. Half that on a 100% slope (45 degrees).

So if you can be satisfied with about 1 MPH on a 100% slope then a 5 HP motor will do. That's about 3.8 kW and you want to limit battery current to about 100 amps, so you need 38 volts, or three batteries in series. If your non-climbing travel is fairly level, and no more than 10-15 MPH, you can probably do that using an average of 2-3 HP. Worst case, the batteries should give you 1800 W-H which is enough for about an hour, or 10-15 miles before charging, except for what you use when climbing.

It may be a good idea to use hydraulic motors in all four wheels, as the "Green Dawg" seems to have. That will give you 4WD without CV joints or U-joints. You might be able to scrounge some hydraulic stuff from garden tractors. Hydraulic drive gives you very precise control and excellent hill-holding ability.

Just some ideas, but I think you could put something together for under $2000 if you can do the work and get parts from a junkyard. You might be able to use a three-phase motor and a VF control, and power it from a 3000W 240V inverter, although you'd probably need to wire the batteries in parallel to get 12VDC at 250A. Otherwise, you can use twenty smaller batteries to get 240 VDC at 15A, or 30 batteries to get 360 VDC at 10A.

Good luck! Seems like a pretty cool sport.

Last edited by PStechPaul; 05-25-2012 at 02:23 AM. Reason: Add CCA and A-H discussion
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  #30  
Old 05-25-2012, 06:06 AM
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Default Re: Electric crawler

Here's a real world data point that might be useful. My 6 wheeler weighs around 700lbs, with a trailer loaded with around 400lbs of stone or wood, total 1100lbs, I almost stall out going up about a 45 degree hill. I'm running a 400 amp controller at 48V, so about 19KW, or 25HP, and I'm geared 16:1 overall and unloaded top speed is around 20mph.
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