First run results (48V 1800W on a large kart)

[Bmr4Karts]

I will admit that the shunt mod can be done better with wire and solder versus just solder like I did, but that does not deny the fact that the shunt mod does work and works very well. My controller is still only warm after driving around for half and hour. It is possible that the 48v controllers are overbuilt and probably share the same components as the higher wattage controller with a different low voltage cutoff. I have run my 48v controller at 70v for a short period and it took it just fine, however I doubt the motor was happy which is why I now own two 60v motors (and controllers). Still need to get around to playing with them though.

 

[scorn001]

A rule of thumb is that it's about 10% of the CCA (maybe 15)
and/or in the realms of their capacitance
means a quality automotive battery can be fully drained rather safely over the course of one hour
(a 45Ah battery delivering 45Amps, a 60Ah battery 60Amps, two paralleled 35Amp batteries 70Amps, a 550CCA battery 55Amps and so on)
it's not more than a rule of thumb though.. type matters
wetcells can deliver amps "more easily" than SLAs, more than AGMs, more than Gel cells
and marine batteries more than automotives without overheating.
(batteries too get warm if abused)

Now you got a 1800W motor (let's assume it's mechanical power rating)
it'll draw ~47Amps (about 2200 Watts electrical)
with just four batteries in serial for your 48Volts you want 45-50Ah batteries
with a 4s2p configuration of eight batteries 25Ah batteries should do and so on.

These are smaller SLAs and I'm not seeing a CCA like an automotive battery would have. The capacity is just listed in amp hours (22Ah) and a max discharge rate of "300A (5S)" - 5S meaning 5 second duration? At any rate, is seems like 60A should be well within the capabilities of this battery (not expecting it to last an hour at that rate of course), and I'm guessing that a 1.2-1.7V droop under that load is within spec maybe.

Battery Data Sheets:
Datasheet1
Datasheet2

So, just for clarification, what I'm seeing with both of my controllers is a draw of about 60 Amps at peak demand, so I'm thinking maybe that 1800W rating on each controller is an electrical rating. I don't remember what it pulled with just one motor attached but the controller does have a documented 32A limit.

So, it sounds like maybe, with my peak draw of 60A, ideally I would want 60Ah. 22Ah being too lean. 44Ah still being lean, but better - and droop would be less. 66Ah would be ideal with a little wiggle room.

For what it's worth, I've been running the kart through the paces, and pushing it pretty hard. Everything stays VERY cool, including the batteries. I only feel the slightest hint of warmth from the controller and everything just feels very cool.

 

[Bmr4Karts]

Are you measuring the voltage at the batteries? If so the voltage drop you are seeing is directly related to the batteries having voltage sag due to the load being placed on them. If the batteries remain cool you can choose to deal with the sag knowing it will only hurt your overall power (reduced voltage means reduced wattage) which would hurt acceleration and potentially top speed.

If the voltage is being measured at the controller than your power wires and connectors could be a part of the issue. Doubtful if you are running 8 gauge and the wiring is also cool.

 

[scorn001]

No, I have my kart power meter on the far side of a battery cutoff. Plan tonight is to put my handheld multimeter across the battery terminal and the far side of that cutoff switch to see what type of drop I have there across that segment at 60A.

I'll also take some measurements with my infrared thermometer as well.

 

[scorn001]

So I just got off of it. Made two passes up a gradual incline in my neighborhood maybe 1/4 mile long. Then I ran up a hill a couple of times in my yard.

Each time when accelerating I was measuring just over 64A, it would drop back to the 40s once I started getting up to speed. Voltage drop across the segment from the battery terminal through the cutoff switch was 131mV with that 64A draw. The voltage level reporting at the output to the cutoff (the end of the segment dropping 131mV) was hovering between 44V and 45V with at that 64A draw. Once the draw dropped into the 40s, the voltage level recovered to about 48V. Starting voltage with no load was 52.7V.

After the run which was just maybe around 5 minutes I checked temperatures.
Left motor: 92°F
Right motor: 85°F
Left Controller: 87°F
Right Controller: 84°F

Batteries ~ 67°F. Felt around on them in different areas and felt no warmth at all. Maybe a longer sustained draw would be a different story.

 

[itsid]

the good news is.. a properly geared setup on flat ground doesn't even need to draw the full amperage..
it should hit max rpm before it hits max amps..
that way not only the motors, controllers and batteries keep cool at top speed,
you also have enough grunt for the occasional hill climb without having to slow down
to say half speed (which feels awkward to most)

cruising speed is yet another animal (usually less than 200W are needed for gently cruising down a road)

The good news is the second datasheet..
it tells us that the manufacturer emptied the tested battery at a 60A rate
(8 minutes second diagram leftmost curve)
which to me suggests it's safe to do so.
(actually pretty impressing considering it's a small AGM battery)
Bad news is.. at that rate it can only deliver roughly 9Ah (a bit less.. but AGM.. so yeah)
it'll recover slightly I guess so you might be able to get another minute or two
of limping back home after you allowed them to rest for a good while.
NOT a good idea to do that repeatedly IMHO,
but the battery shouldn't overheat and burst into flames if I can trust that datasheet.

'sid

 

[scorn001]

the good news is.. a properly geared setup on flat ground doesn't even need to draw the full amperage..
it should hit max rpm before it hits max amps..
that way not only the motors, controllers and batteries keep cool at top speed,
you also have enough grunt for the occasional hill climb without having to slow down
to say half speed (which feels awkward to most)

cruising speed is yet another animal (usually less than 200W are needed for gently cruising down a road)

The good news is the second datasheet..
it tells us that the manufacturer emptied the tested battery at a 60A rate
(8 minutes second diagram leftmost curve)
which to me suggests it's safe to do so.
(actually pretty impressing considering it's a small AGM battery)
Bad news is.. at that rate it can only deliver roughly 9Ah (a bit less.. but AGM.. so yeah)
it'll recover slightly I guess so you might be able to get another minute or two
of limping back home after you allowed them to rest for a good while.
NOT a good idea to do that repeatedly IMHO,
but the battery shouldn't overheat and burst into flames if I can trust that datasheet.

'sid

Thank you Sid. I need to take a few spins around the yard, or better, have one of my kids who are about 1/2 my weight take it around the yard at a more reasonable speed and see how long it lasts. Originally I was thinking 1 motor with a peak of 32A under an extreme condition, more likely to be averaging around 20A or 25A, on 22Ah batteries - should be good. Now of course I'm seeing much more than that - but again, I was hammering it pretty hard.

The worse case I scenario I see is my kids favorite thing todo is on the rare occasion that we get significant snowfall, they love to be pulled around the yard on a sled by the gokart. So yeah, cold, wet, my 200lbs driving, and potentially pulling another 170lbs (2 kids) in a sled. Luckily it's a pretty long downhill with a short uphill part. Still, thick and deep snow, if we get it this year it'll be interesting.

I can add another 22Ah of battery for about $160, which will add another ~50lbs. I have plenty room, just not sure I want to keep adding 50 lbs at a time in weight. The Lithiums would be cool, but 60Ah of Lithium batteries looks like it's going to cost me about $700. Hard to justify that one.

Thanks again.

 

[Quinc]

Thank you Sid. I need to take a few spins around the yard, or better, have one of my kids who are about 1/2 my weight take it around the yard at a more reasonable speed and see how long it lasts. Originally I was thinking 1 motor with a peak of 32A under an extreme condition, more likely to be averaging around 20A or 25A, on 22Ah batteries - should be good. Now of course I'm seeing much more than that - but again, I was hammering it pretty hard.

The worse case I scenario I see is my kids favorite thing todo is on the rare occasion that we get significant snowfall, they love to be pulled around the yard on a sled by the gokart. So yeah, cold, wet, my 200lbs driving, and potentially pulling another 170lbs (2 kids) in a sled. Luckily it's a pretty long downhill with a short uphill part. Still, thick and deep snow, if we get it this year it'll be interesting.

I can add another 22Ah of battery for about $160, which will add another ~50lbs. I have plenty room, just not sure I want to keep adding 50 lbs at a time in weight. The Lithiums would be cool, but 60Ah of Lithium batteries looks like it's going to cost me about $700. Hard to justify that one.

Thanks again.

Check out the refurbished hover board batteries. Razor crowed is having great luck using them on the 1800w motors and they are cheap. (really cheap with a 15-20% off ebay coupon)

The lipo have 20-50% more usable aH over sla. So you would probably only need 30aH of lipo to = 60ah of SLA. And very little voltage sag if running 10C+

And longer life and possibly some great Cyber monday deals.

You cant afford not to get them.