Battery Question

[hbcolf]

Hello, so I'm building an electric go kart for my senior project.
I'm planning on using a 36v 1000w motor and a 36v 1000w controller all on a racing kart frame. The only thing I haven't been able to figure out is the best fit battery wise for my set up.
I know the commonly used batteries are sealed lead acid batteries, AGM, or LiPO4. Each with their own advantages and disadvantages when it comes to weight and cost.
Also how many 'ah' should I aim for? I talked to a guy at Batteries Plus (I was hoping to avoid shipping costs for the batteries) and he said that 26ah would get me an hour of ride time, while the next step down in ah would drop me down to 30 mins of battery life.
I was just wondering if anyone had any preferences or favorites when it came to picking one type over another.

 

[itsid]

well easy... (mathematically easy.. real world mileage will vary)

So, 1000W motor.. that's hopefully a EV-motor, made for electric vehicles..
so the power label states the mechanical power at it's ideal efficiency.

Let's just say it does.. (like a My1020 for example)

So, we can assume a powerdraw (electrical power) of roughly 1250 W
at that load and speed (assuming an 80% efficiency of the motor)

-keep in mind, that cruising around will require LESS power than that, while accelerating can draw up to twice as much from your batteries-

1250 W / 36 Volts means the motor will draw 34.722 Amps on average.
(with roughly half of the time you put the pedal to the metal and the other half you're just cruising around)

So to run that kart for an hour straight you need ~35Ah of useable capacity.

Now it get's slightly tricky..
since lead acid batteries will die if they load drops below 10% really easily,
nearly any motor controller for an electric vehicle has a battery protection build in
that will shut down the motor as soon as the voltage drops below a certain treshhold.
(for a 36V controller, that's likely in the realms of 28V..)

The voltage of a lead acid battery is not constant.
it's between 13.2 and somewhere around 9.5 Volts depending on it's load,
let's just say the lower end is 9.5 at ~12%
that's where the controller will shut down, so 12% load will remain in the battery to keep it alive.
(thus 12% capacity will remain unused)

NOW back to square two, you know you want ~35Ah of capacity per hour of driving,
and now you know you need to add 12% to keep the batterie pack alive.
let's round up again and you get roughly 40 Ah per hour of driving time.

3 cells in series do not add capacity, just voltage, so you'd want 3x 40Ah batteries 12V each.
(again per hour..)
45mins is enough.. 30Ah will suit your needs..
you want 90mins? 60Ah is what you need.. etc...
just multiply by hours to dial in your timeframe and capacity

on a flat surface the needed power to cruise around at slow speeds is much much lower than 1000W.. maybe just 250 (depends on weight and road and wheelsize etc.pp.)

so yeah, half of that will get you an hour cruising, 26Ah will get you an hour mostly cruising around slowly... but in the end.
you want an hour of a bit more fun than just that I think

'sid

 

[hbcolf]

So what if I switched motors and got a 24v one instead, and then for batteries got two power chair batteries rated for 12v 35ah. Would they work?

The link:
http://www.ebay.com/itm/131212514902

 

[itsid]

So, in short you haven't read what I said above, did you

No..

first: there is absolutely NO need for a fancy SLA battery.. those are great, don't get me wrong.. but a simple motorbike bat will do the same for you and comes at a fraction of the price

second: if you get a different (lower voltage) motor you need more amps to have the same power... thus you need more batteries in parallel to add capacity.
or you won't reach the 30min driving time.

it's not Amps or Volts alone.. it's always BOTH!

3 x 12V @ 40 Amphours
2 x 12V @ 60 Amphours
1 x 12V @ 120 Amphours

always the same 1440 Watthours for a 30min ride.
(with enough juice left to keep even cheap lead acid batteries alive..)

keep in mind, that the controller WILL shut down, no matter what batteries you're using.

'sid

 

[nayr83333]

amp hours refers to how many amps a battery can provide for 1 hour. if you have 20Ah then you can provide 20 amps of power for 1 hour, or 10 amps for 2 hours, or 40 amps for 1/2 hour. the amount of amps you are using is your volts/watts. your 1000 motor divided by 36v means it will draw 27.7 amps at full load. if you get a 3x 30Ah batteries you should get an hour of time. itsid is correct in what he said. he was trying to get you real world numbers and may have complicated it a little. the numbers i gave you are strictly on math and your real world results will be different but it you want something to go off of this is a good start. with all that said, get the biggest baddest batteries you can afford because you are going to want to have fun on your cart, not wishing you were having fun watching it charge.

 

[Poboy kartman]

Thank goodness 'Sid is back......here's the extent of my electrical knowledge :

If you put a 9 volt battery on your tongue....it will shock you if it isn't dead!!!!

 

[itsid]

amp hours refers to how many amps a battery can provide for 1 hour. if you have 20Ah then you can provide 20 amps of power for 1 hour, or 10 amps for 2 hours, or 40 amps for 1/2 hour. the amount of amps you are using is your volts/watts. your 1000 motor divided by 36v means it will draw 27.7 amps at full load. if you get a 3x 30Ah batteries you should get an hour of time. itsid is correct in what he said. he was trying to get you real world numbers and may have complicated it a little. the numbers i gave you are strictly on math and your real world results will be different but it you want something to go off of this is a good start. with all that said, get the biggest baddest batteries you can afford because you are going to want to have fun on your cart, not wishing you were having fun watching it charge.

unfortunately.. that's not true in the real world.

first: a 20Ah battery is loaded with 20Ah, but can only supply maybe 18Ah without dying,

second a 1000W motor if meant as a EV motor is rated in it's "recommended powerband" and as "mechanical output power"
so it'll draw 1250 Watts of electrical power. when providng said 1000W mechanical power.

And, most EV motors in that range can draw up to 2700 Watts from a battery (providing ~1500W mechanical power)
but only for a short amount of time, then they'll get too hot and fail.

So no.. 27.7A is too low, the main reason why 36V controller for 1000W motors generally apply ~40Amps to the motor.
35Amps at the very least.

'sid

 

[hbcolf]

Thank goodness 'Sid is back......here's the extent of my electrical knowledge :

If you put a 9 volt battery on your tongue....it will shock you if it isn't dead!!!!

Thank you!! I'll keep that in mind while building

 

[hbcolf]

So, in short you haven't read what I said above, did you


'sid

Hahahaha!! no I did read it.

I just wasn't entirely sure about the battery situation yet.

And when you say motorbike battery what do you mean by that? Because when I just search google for a 12v 35 or 40ah battery it comes up with power wheelchair batteries for like $60-70 bucks a pop.

But when I just search for 12v motorcycle batteries it comes up with some really cheap batteries, but that makes me think that they are either: a) super low ah, b) going to wear out because they are cheap, or 3) are going to explode or something because they are just cheaply made

 

[itsid]

well..
motorcycle batteries are lead acid.. most likely.
SLA (sealed lead acid) for wheelchairs for example are the exact same technology...
with one major difference:
they're sealed, which means it's perfectly okay to tip them over, lay them on the side etc.
without the acid leaking out.

That's important safety precaution for a wheelchair, where a person is close to the tipped over wheelchair and in worst case cannot move away from the leaking acid.
If a biker cannot even crawl away from a tipped over bike, he's other things to care about than an acidburn

durabilty of gel batteries like the SLA you linked earlier might be better than a cheap lead acid cell.. but if you buy a quality product, you still pay way less and get the same durability.

you can also use small car batteries.. the reason I mentioned motor bike batteries is because they have much nicer terminals to work with in an EV environment
(and are typically smaller/lighter for the same Ah.. at the price of a little higher cost)
but you're right.. it appears that the 35ah is a bit large for an ordinary motorbike, so maybe that's why they're getting expensive, even unsealed.

'sid

 

[hbcolf]

well..

you can also use small car batteries.. the reason I mentioned motor bike batteries is because they have much nicer terminals to work with in an EV environment
(and are typically smaller/lighter for the same Ah.. at the price of a little higher cost)
but you're right.. it appears that the 35ah is a bit large for an ordinary motorbike, so maybe that's why they're getting expensive, even unsealed.

'sid

It can't just be any car battery can it? That would be too easy...

Should I look for a 35ah car battery? I did some google searches and it was only giving me 12v sla or gel batteries.

 

[itsid]

it can be just ANY car battery yes...

but keep in mind that the more Ah you get the heavier the battery will be, and you need at least three of them.
Take identical batteries, the LOWEST capacity will dictate your mileage.
but you can even buy any size you want, 35Ah is what you'd need for a 'guaranteed* hour' of driving, just divide the amp hours of your battery by 35 and you get a rough idea how many hours you'll be able to drive around with those cells...
like...
26Ah / 35A = 0.742 hours ~ 44mins
42Ah / 35A = 1.2 hours or 72mins etc.


'sid

* well... at least closest to a guarantee as you can get