72v build

[Oldspot]

Hi all, I am a novice at this but learning fast!
I have just got a really cheap Yamaha G1, I like the chassis and suspension on these, it’s built like a tank and not a spot of rust (I’m from the uk).
I have a 4000w brushless motor and Votul controller with the ancillaries.
Taking out the broken gas engine and giving it a lift no problem
I plan to use it on my farm.
So my point- batteries, I want to go lithium, can I link 2 x 36v batteries together
What bms could,I use to balance the two batteries. The single 72v is very expensive and difficult to source in the uk.
What batteries would you,recommend.
Any suggestions and advice greatly appreciated 👍

[bronsonj]

Sounds like a fun project. Can you post more info about the motor and controller? With stock differential gearing that 4,000 watt motor may not be enough. My cart routinely uses well over 12,000 watts during acceleration. Due to inefficiencies of all the components I'd suspect the motor is doing 10k watts of work regularly. My motor peaks at about 300 amps on hard acceleration from a stop at 56v or 16,800 watts.

If you change the gear ratio then you'll have less speed but 4,000 watt motor can do all you want. The stock diff is probably a 12:1 ratio. What speed are you looking for? Some brushless motors are capable of incredible speeds. My stock (brushed) golf cart motor can handle 6k rpm. If you take your max RPM and expected final gear ratio (guessing 24:1) then you can put in your expected tire size and see your theoretical top speed. The calculator assumes the ":1" part so just enter 24 as the gear ratio if that's the ratio you expect to try first.
https://www.buggiesgonewild.com/speed-calculator.php

I used modules from a Chevy Volt for my cart. Lots of other people use Nissan Leaf cells. Some people have used lifepo4 cells but the max discharge of those is less so they have to build a larger pack.

If you buy pre-made battery packs they have a built in BMS. So the two 36V packs will each be protected, but it is far better to have one BMS over all the cells than two BMS each protecting half.

I just looked on aliexpress.com for a 72v BMS. A 24S, 72V bms for lifepo4 cells was 42USD for a 20 amp up to 46USD for a 60 amp.

Lots of people here either use the BMS for balancing and charging only, or people run the output current from the BMS to a solenoid and that solenoid will do the actual cutoff of the current. Either way, you won't need a BMS that can handle 200+ amps.

[Oldspot]

Great thanks, the G1 has a 8:1 ratio which at 5000rpm max 22 inch wheels is way too fast
The motor is chain drive so if I enlarge the sprocket by 50% making the ratio 12:1 gives me 27.5 mph. That’s fast enough but the motor may spin faster than my conservative 5k rpm
But if I don’t ask for that extra rpm my amp draw will not be so high.
I think a single battery is the way to go (for simplicity) ,I appreciate they are quite expensive but my atv drinks gas for fun and the pita is going to fetch more. I plan to sell my Honda atv to put towards battery.
Where can I get info on working out discharge rate so I know what spec battery to look at.
👍😀👍😀

[Oldspot]

The motor is a Qs 138 90 4000w motor
The controller is a Votul 200amp
People use them in motor bikes, they are programmable, have three speed settings. I rather hope with 72v it should have sufficient power.
I do have 2 motors and controllers if I want but I was hoping to learn with one before I go extreme!
Endless Sphere have quite a bit of info and help about the setup
But it’s quite a bit for me to get my head around especially the battery

[bronsonj]

Sounds like you have a plan of action and an interesting set of components.
The good news of all this is if you get the ratio wrong it's very easy and cheap to change sprockets.

From what I saw online the max RPM of that motor was 7,000. But that was no-load. So under load it should be substantively less. It will be interesting to see what you get for real-world results.

Good luck and post pictures as you build!

[bronsonj]

As for discharge rate of different chemistries, I went to Battery University initially but there are several sites with useful info online. Here is one page from Battery University that shows several different lithium chemistries:
https://batteryuniversity.com/articl...of-lithium-ion

As a general rule there are a couple things that can dramatically reduce the lifespan of the battery or even cause concern of fire (depending on chemistry). The "Big 3" are 1) discharging too low 2) charging too high and 3) high temps.

[Oldspot]

Thanks for the reply.
I am coming up with a bit of a plan. I can probably scrape together 6 deep cycle 12v batteries. I was going to set these up to bench test and program the controller.
If I succeed with that step then I can cobble them into the cart for a test run, I could then measure amps etc to see what draw I get.