Eectric go kart

[audioguru]

audioguru:

how do i do the 58V rail. can you be more detail.

Your unloaded rail is about only 51V because you have only four 12V batteries.
With such a high current of 200A to 400A then the rail will drop immediately to about 32V.

 

[ituaku]

how do u calculate that?

 

[audioguru]

The starter motor in a big car draws 200A to 400A. The 12V battery voltage drops to about 9V when it powers the starter motor.

You have four batteries that are much smaller than a car battery.

 

[Hero999]

A couple of posts ago he talks about Odyssey batteries which are a good make and have a low impedance.

Could you please post the datasheet for the batteries you're using?

The gate voltage to the high side MOSFETs needs to be 10V higher than the positiver rail for them to turn fully on, if the positive rail is 51V, the gates need to be 61V.

There are ways to do this but it depends on what you're doing.

 

[ituaku]

hero999: **broken link removed**. see the pc1200. im not sure how he gets the 51V. all i know the battery will supply 48V.

audioguru: it's just a go kart thats why.

 

[audioguru]

hero999: **broken link removed**. see the pc1200. im not sure how he gets the 51V. all i know the battery will supply 48V.

audioguru: it's just a go kart thats why.

A lead-acid battery is 12.8V without a load, not 12V. Four make 51V, not 48V.
Four of those huge batteries weigh as much as me. That is a lot of weight for a little go cart to haul around.

Each battery has an internal resistance of 4.5milli-ohms so with a 400A load the total voltage drop in the batteries is 7.2V so the voltage to the wires (the wires will also reduce the voltage) will be only 43.8V.

With a current of 400A, each battery will be boiling or exploding with 720W of heat.

 

[Hero999]

400A is the peak current which shouldn't be a problem as the batteries are designed to supply a starter motor with 550A, even at low temperatures.

The maximum continuous current will be 215A which will drop the votage by 3.9V, still 208W of power is a lot for a battery to dissipate. Assuming 12.8V per battery, the battery voltage will drop to 47.3V

When travelling at a constant speed the current will be even less. If you're worried about transient voltage drop then add an expensive super capacitor across the motor controller.

 

[ituaku]

audioguru: now its clearer. well been thinking to cool down the battery. if i connect a heatsink across the battery lead would it dissipates the heat?

Hero999: im afraid ill be broke before the kart can moves

btw today i found a really cool hbridge driver from farnell. Ive read the datasheet n looks like it's compatible with my hbridge. The output voltage to the load is up to 80Vdc.
I just want to ask someone to check it for me again to make sure it really works with my hbridge. It's so gonna help me the hassle and saves me the time.

**broken link removed**

The one shown on farnell gives several prices but they all look the same. Browse for Products | Farnell United Kingdom. WHich is confusing me. is the packaging really affects the price? the one i wanted is the £4.10 since its the cheapest n the datasheet for all are the same.

 

[Hero999]

That looks like the kind of thing that will work.

The datasheets will be the same because the chip inside the all the packages is the same.

If you're comfortable with soldering surface mount parts then get the cheaper SOIC but if you're not then go for the more expensive DIP.

 

[duffy]

Let's not explode those batteries just yet. We're talking about like four thirty-pound batteries, right? Figure half the weight's water, so it's like trying to boil 60lbs of water. That looks to be about 9 megajoules of energy, so to boil a whopping 60lbs of water from a piddling 2,880 watts would take 3,125 seconds, or almost an hour. And that's an adiabatic system, the real one's going to be losing heat all the time, more as it gets hotter, so figure something well over an hour at a constant 400A before she explodes... probably not going to happen unless it's a fat guy trying to drive up the side of Pike's Peak.

 

[audioguru]

There is a lot of very heavy lead in a lead-acid battery. Much more than water.
I think the battery will boil quickly. It must carry a driver plus its own weight so its current drain will be high.

 

[duffy]

I subtracted the weight of the lead out in my estimate. A car battery weighs 40 - 50 lbs or more. Add in the weight of that lead and the thermal mass just increases. Figure more like 200 pounds, total, not 60 - that was just for the water.

Water's got a higher specific heat, but they are both in thermal conduction and the lead is going to have to wait until the water absorbs that same amount of thermal energy before it boils... and the lead is going to have to absorb some energy, too.

Looks to me like no matter how you cut it, the numbers are going to go up, not down.

 

[causalitist]

the 400A current will conduct for like a few seconds just to start the motor and after that the motor will just use 200A current...on the spec sheet, it shows that the module can go up to 500A current at Vgs 10V....thats y i thought of just using 1 module only...so does it mean i can just connect the module straight to my motor? I ask someone about whether the module have safety features n he replied this "A voltage headroom is required to allow for voltage overshoots which result from the circuit loop inductances. This is a factor of how good you layout and design is but 55V may not be enough headroom (check)". I dont understand what he means n i cant get in contact with him. hope someone can explain what this means.....another question, do u know how am i suppose to drive the hbridge? i tried looking for IC...so far im not sure which is suitable for this module...so any recommendations?

your using pwm for control, so the mosfets turn on and off many times per second, each time they turn off youll get a huge voltage spike in the wires from the batteries to the controller, due to the wire inductance. these spikes will fry your mosfets and they will go to "on" permanently, and you will be in for a very very wild ride!!!

this exact thing happened to me on a razor scooter with 48v of 60amp discharge lithium polymers and a big cim motor with it's timing advanced 20 degrees.. was fun, but a larger motor and i wouldn't have been able to brake until the circuit board was smoking.

THIS is the most important part of designing something like this, often overlooked by beginners.

there are many things you can do to reduce this... the most simple being choosing higher voltage mosfets so they can tolerate more.

solutions:
the right way is to put computer grade, high ripple amp, low ERS(resistence) electrolytic capacitors across battery leads AS CLOSE TO H-BRIDE AS POSSIBLE.. for 200amps you will need maybe
20 381LQ682M063J452 capacitors.
or 14 381LQ123M050K452 capcaitors

reduce length of wiring to batteries.. this includes series wiring between battieries as well... it adds up!

the "zener" mentioned would be a 1.5ke series transient voltage suppressor.

Vspike=I*sqrt(L/C) 4" 12Gauge=86uH.. 6feet=1.54mH
for me... my controller
(https://www.diyelectriccar.com/foru...c4420-13763.html?p=100728&posted=1#post100728)
will limit to 50amps (notice banck of capacitors one guy posted!)
my battery lead length is 6 feet... (4 feet is from 35 lithuim polymer cells in series)
say i just had a 1000uF capacitor 0" from the mosfets:

so Vspike=50*sqrrt(.00154/1000x10-6)=62v spike above operating voltage.
in your case, a wild ride!

im using a 10,000uf , 250v capacitor, but its huge so the closest i can put it is 4" from mosfets... so then i also have 2 680uf and 2 30uf metalized film capacitors directly on the fets.

you really should try to make a 50 amp controller for a smaller motor first.. because you WILL burn out your first controller. or, do ALOT more research first on managing transient voltage spikes.

your best bet would be to hook up a pwm chip for the h bridge that can limit the current... absolutely needed for this go kart.. you will pull insane amps for a millisecond on startup!

try out the circuit posted in the url i have above... the chip is like $0.50 and to start you only have to hook up like 5 pins... others are grounded or such.

 

[causalitist]

oh, and ditch the lead acid, this is what i got, they are so cheap! prolly cuz they are from hong kong or whatever... but 37v 4amp 40amp cont. discharge lithium polymers for $58 is good.. that costs triple that in america.

its <$200 to make a pack thats 55.5v 12amp/hour, 120amp continous, 180amp peak discharge .. smaller than a regular car battery, and like 20lbs less weight.

i just felt you guys would appreciate this post since i spent months looking for the best deal on an awesome battery for my project, and believe me, this is it:
**broken link removed**

 

[duffy]

Ok, THERE's the one that will explode, McDuck.

 

[Hero999]

I don't think the battery will boil. This isn't a standard wimpy SLA, it's a heavy duty battery designed for starting large engines.

At work we use two 26Ah Odyssey batteries to start a diesel generator.

215A is the continious rating od the motor, it's not going to be drawing 200A most of the time, especially if he restricts the speed to just 20mph, it'll probably draw something closer to 100A may be even less.

 

[ituaku]

causalitist: i'll have a try. maybe i'll have a good understanding of how it works. btw i dont wana get burnt nor burn my $$. the battery looks good n way cheaper than mine. will probly try out for my trial circuit.

the battery i chose is made for heavy duty vehicles (truck etc) so i guess it wont boil for just driving a kart with max speed 20mph. maybe cooling it with a fan will do