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Old 02-04-2020, 03:54 PM
MotWat MotWat is offline
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Default Battery choice for electric go-kart

Hello everyone,

I'm working on an electric go-kart project and need to pick the batteries + their accessories. I have a budget of about €300-500 ($330-550). My motor is the 3 kW 72 V Golden Motor (https://www.goldenmotor.com/frame-bldcmotor.htm) and a 72 V, 80 A Kelly controller (https://www.kellycontroller.com/shop/kbs-e/). My main objective is to be able use the power the motor can provide, even if only for a short time. 5 minutes of ride time would be sufficient, but the more the better, of course.

Considering this, I have come up with the following plan:
Buy 4x 18.5 V 5S 5000 mAh XT90 LiPos (https://www.amazon.de/HRB-18-5V-5000...10&sr=8-5&th=1)

Put them in the configuration shown in the first attachment and then put them into the whole system as shown in the second attachment.

Buy accessories:
50 W charger (https://www.amazon.de/Netzteil-ORIGI...4&sr=8-11&th=1)
XT60 balance board (can't find XT90) (https://www.amazon.de/Crazepony-UK-L...0854053&sr=8-6)
XT60 and XT90 plugs + 10 Gauge wire to make adapters for the balance boards and connect the LiPos between each other (https://www.amazon.de/LHI-M%C3%A4nnl...98&sr=8-6&th=1 and https://www.amazon.de/LHI-Connector-...92&sr=8-5&th=1 and https://www.amazon.de/Silikon-Schwar...RJWJ7QGEN368K6)
2x Bat-safes for safe LiPo charging and storage (https://www.amazon.de/Li-Polar-BAT-S...9893319&sr=8-6)

What do you think? Is this setup of good value? Have I forgotten about anything important?

One thing that I am wondering about is that the LiPo I've picked advertises continuous 50 C, which would imply a continuous current of 250 A. As far as I know, this would require massive wires - around 0 AWG (https://www.powerstream.com/Wire_Size.htm), yet from the picture of the battery I can see that it has a 10 AWG wire. According to the chart in the previous link, that would allow for 55 A... So is it even safe to pull the 80-160 A that my controller would possibly allow? And what about the XT-90 connectors, that - according to the seller - are 120 A peak? I hope I'm misunderstanding something here, because otherwise the wires would be quite a bottleneck.
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065fc2c1773305bc16213b899aee4abfcd9a3134_2_499x499.jpg   KellyWiring.jpg  

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Old 02-22-2020, 09:22 AM
MotWat MotWat is offline
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Anyone?
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Old 02-22-2020, 10:49 AM
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I think my main problem with that is,
that you link to the seller not the manufacturer.
And what is of interest is NOT what some retailer thinks,
but what the manufacturer has put in his datasheet.

Finding the datasheet isn't trivial (especially not for amazon listings)

So, find the datasheet and link that if you do have a problem with reading that.

Without datasheets:
I doubt that that would be a good idea..
in fact I'm rather certain it's a BAAAAAD idea!

reason being: those look to be 18650 3P packs;
3P at the absolute best can provide 30Amps
(frankly.. likely to be closer to half that!)

the motor you linked draws peak 5750 Watts of power from the batteries @ 72V that's
80 Amps alright.. but pulling 80 Amps from those batteries will burn them down very violently.

So you need batteries that are made of high drain cells
(some Sanyo or Sony can provide a peak of 10A with a cont rating of 8A or such)
and you need a 10P pack
to get a continuous battery rating of 80Amps

And for 72V you need a 20S battery (or 4 5S batteries in series)
that makes a total of 200 18650 battery cells

each such high power 18650 costs at the cheapest possible source I can find 4.50 € (bulk bought)
makes 900 € just for the cells!
So any battery configuration cheaper than 900€ is NOT going to be sufficient if it's composed of 18650s
sleeving, wiring and BMS also costs money..

So that's twice your battery budget... and the absolute MINIMUM for 18650s
even if you take the bare minimum of 8P and hope it'll last for a few seconds without exploding,
you still need 160 cells (and spend 720 € minimum and that is already a bad idea in itself)

you need an alternative (lead acid I'm afraid)

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Old 02-22-2020, 11:09 AM
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Quote:
Originally Posted by MotWat View Post
Hello everyone,

I'm working on an electric go-kart project and need to pick the batteries + their accessories. I have a budget of about 300-500 ($330-550). My motor is the 3 kW 72 V Golden Motor (https://www.goldenmotor.com/frame-bldcmotor.htm) and a 72 V, 80 A Kelly controller (https://www.kellycontroller.com/shop/kbs-e/). My main objective is to be able use the power the motor can provide, even if only for a short time. 5 minutes of ride time would be sufficient, but the more the better, of course.

Considering this, I have come up with the following plan:
Buy 4x 18.5 V 5S 5000 mAh XT90 LiPos (https://www.amazon.de/HRB-18-5V-5000...10&sr=8-5&th=1)

Put them in the configuration shown in the first attachment and then put them into the whole system as shown in the second attachment.

Buy accessories:
50 W charger (https://www.amazon.de/Netzteil-ORIGI...4&sr=8-11&th=1)
XT60 balance board (can't find XT90) (https://www.amazon.de/Crazepony-UK-L...0854053&sr=8-6)
XT60 and XT90 plugs + 10 Gauge wire to make adapters for the balance boards and connect the LiPos between each other (https://www.amazon.de/LHI-M%C3%A4nnl...98&sr=8-6&th=1 and https://www.amazon.de/LHI-Connector-...92&sr=8-5&th=1 and https://www.amazon.de/Silikon-Schwar...RJWJ7QGEN368K6)
2x Bat-safes for safe LiPo charging and storage (https://www.amazon.de/Li-Polar-BAT-S...9893319&sr=8-6)

What do you think? Is this setup of good value? Have I forgotten about anything important?

One thing that I am wondering about is that the LiPo I've picked advertises continuous 50 C, which would imply a continuous current of 250 A. As far as I know, this would require massive wires - around 0 AWG (https://www.powerstream.com/Wire_Size.htm), yet from the picture of the battery I can see that it has a 10 AWG wire. According to the chart in the previous link, that would allow for 55 A... So is it even safe to pull the 80-160 A that my controller would possibly allow? And what about the XT-90 connectors, that - according to the seller - are 120 A peak? I hope I'm misunderstanding something here, because otherwise the wires would be quite a bottleneck.
You want those type batteries (wired in series you will only have a 72V 5AH bat pack) to (hopefully) power a motor that draws ~72V 80A (peak)
...or even maybe ~1/2 of that (~40A) just crusin'

We have kinda been discussing using small batteries like this, in this thread
http://www.diygokarts.com/vb/showthread.php?t=42483

Sit back & think about/compare the "different" situations

That size of a battery pack is designed to release ~5A for ~1 hour
...or ~10A for ~1/2 hour
(think, like the size motor in a RC car &/or power tools)

So, IMO to power a "bigger" motor (like yours) with a (let's say) ~40A draw
...it seems that it would probably be best to use a ~40AH battery pack
...or at least a ~20AH pack
(to NOT overstress the battery pack & get a decent amount of "run time")
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Old 02-23-2020, 08:24 AM
MotWat MotWat is offline
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Thanks for your response, guys!

Quote:
I think my main problem with that is,
that you link to the seller not the manufacturer. [...]
I have looked further into these packs and their manufacturer. It appears that the Amazon listing I have posted is from the manufacturer himself. Anyhow, the closest to the source of those packs I could get is HRB's official Aliexpress store (https://www.aliexpress.com/item/3296...3338352euQlEQa). These packs are also offered on ecplaza.net (https://yowoo.en.ecplaza.net/product...000mah_4297221). HRB is the brand of Shenzhen Yowoo Electronic Technology Co.,Ltd (https://www.tradesparq.com/users/290...hnology-Co-Ltd). I couldn't find a dedicated datasheet, but the characteristics of the packs on each website match the info on Amazon. As a side note and for what it's worth, the vast majority of opinions I could find on these packs are very positive.

How did you determine that these packs hold 18650 cells? They look like soft LiPo pouch cells to me. That's also how they are illustrated on the AliExpress page (see attachment).

If all of the above is true, would you consider this pack to be a good option after all?

Quote:
That size of a battery pack is designed to release ~5A for ~1 hour
...or ~10A for ~1/2 hour
(think, like the size motor in a RC car &/or power tools)
I am aware that I won't be running the kart for very long with 5 Ah of charge. But isn't the discharge rate (C) * charge (Ah) what I should use to determine the amount of amps I could draw from the pack without "over-stressing" it? The advertised 50 C continuous and 100 C burst are possibly overblown, but even with 30 C I could get 30 C * 5 Ah = 150 A, which would be more than enough, wouldn't it?
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Old 02-23-2020, 11:50 AM
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I was going by the image you posted not the link (I avoid following amazon links most of the time)
sorry, yes flatpacks..

I find it hard to believe that those should be able to provide 50C
working for up to 50C yes.. 50 times the cell capacitance as a constant discharge..
tough.

But sure, IF those values are not pulled out of thin air, but actually true.
No problem! 80Amps will not cause an issue.

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Old 02-24-2020, 06:11 AM
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Ah, I see. In that case, if nothing else comes up, I'll order the parts tonight.
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Old 02-24-2020, 10:12 AM
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Well you forgot everything you said above yourself I think..
wire gauge vs current, amp rating for the connectors etc..
that all actually rather impressively tells you to "do not trust their C rating" blindly.

I don't care about all that RC stuff anymore,
for the purpose of vehicle moving
(apart from maybe a skateboard, kids scooter or bike)
with carefully picked LOW power
I pass at all times.

Nearly everything in the RC world is a lie an exaggeration or or just the most useless
RC motor powerratings are useless, RC Controlelr ratings even more so
Kv ratings provide you with ZERO information that is of use,
C Ratings are mostly exaggerated if not a blunt lie
and so on and so forth
and the cheaper the product in question is the worse it gets

it's exactly like those 7gramm LiPo 18650 cells which state to have 10000mAh of capacity,
and inside is a tripple A NiMH...

So, yeah... without looking up exact wire gauges and their ratings myself
55Amps is the limit I'd put on that (11C so to speak)

and in my book that's not enough for a 80Amp peak draw...
not even for a few seconds
battery will be dead and empty in about 3minutes
and three minutes is way to long for a maximum to be exceeded
(touching the peak for ten seconds should be okay... 30 second rating is what many reliable manufactures give you)
3 minutes.. that's constant rating you need to obey.

If you're willing to take the risk, feel free..
but chances are the batteriy wires will melt eventually,
that might short the battery in question, and that is likely
to cause a thermal runaway state (bloating, blasting, burning....)

your health not mine, but I would suggest to get something way more reliable than RC stuff.

'sid
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Old 03-23-2020, 12:42 PM
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Sorry for the lack of response, I was real busy for the past month.

I have considered your advice, but in the end decided to go for RC lipos after all. However, instead of the lesser-known brand I have posted above, I bought four Turnigy 5000mAh 5S 30C Lipo Packs (https://hobbyking.com/en_us/turnigy-...html?wrh_pdp=7), which are very popular and have great reviews. The C rating seems to be more realistic too. Additionally, I've seen them used for go-karts, like in this popular video, for example:

Now I have all the parts and already started tinkering; but that's for another thread.
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Old 03-24-2020, 09:41 AM
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Quote:
Originally Posted by MotWat View Post
Sorry for the lack of response, I was real busy for the past month.

I have considered your advice, but in the end decided to go for RC lipos after all. However, instead of the lesser-known brand I have posted above, I bought four Turnigy 5000mAh 5S 30C Lipo Packs (https://hobbyking.com/en_us/turnigy-...html?wrh_pdp=7), which are very popular and have great reviews. The C rating seems to be more realistic too. Additionally, I've seen them used for go-karts, like in this popular video, for example:

Now I have all the parts and already started tinkering; but that's for another thread.

Don't do that. Those lipos will be over worked. The owner of the kart you linked did not keep that kart very long and his choice of cheap lipo batteries was a bad one. They will lose capacity quickly being overworked in a kart even with the multiple packs he had in parallel.

There are suitable lipos to use but they cost more. They don't come in 5s, so you would need some 4s and 6s for 20s. Also, you want at least 20s2p. 20s1p will run for like 4 minutes with your power.

If that's too much, you need to look into building a 18650 pack or using lithium batteries pulled from full size EVs and hybrids.

You did pick a good motor and controller though.

As for the charger, you need a way more powerful one unless you are fine with waiting a full day to charge your packs. That charger is 50w, my hobby charger is 1100w.
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Old 03-25-2020, 07:45 AM
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Later I might research more for a long-term solution, but unfortunately I have limited time for this project and I need to get going with the actual build. If the Lipos run for 4m and die off after a while, so be it. In the end I went for this charger, which has a bit more power: https://hobbyking.com/en_us/turnigy-...___store=en_us
At a 5 A charging rate I should be able to charge my 4 Lipos in two hours.

I'm curious why you think the two lipos you linked would be suitable and mine are not? I realize that they have a higher discharge rate (60 C) and the 4s has 8 AWG wire. But if you trust these specifications, then why not trust my battery's specs as well, which promise 30 C continuous? At 5 Ah that's 150 A continuous, which is 10 kW of power at 72 V. Isn't that more than enough?
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Old 03-25-2020, 08:00 AM
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That charger is better, you are correct about how long it will take to charge.

When choosing a hobby lipo, only trust that their C ratings are half of what they say they are. Those 30C lipos, are good for 75amps continuous. Your controller goes up to 80amps continuous, 160amps max. If you go with those weaker lipos program the controller to pull less power or you can do like you said and see if they die over time. Just know that when they die they will be puffy at best, catch on fire at worst. This wouldn't be as much of a problem if you were running packs in parallel so they could share the load.

Also, those Golden Motors are awesome and highly underrated. Your motor and controller combo will work great, but that motor could also benefit from an even more powerful controller which would then require even more amps from the batteries.
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Old 03-25-2020, 01:08 PM
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Ok, got it. I'll be very careful!
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