looking for a 72v BMS that isn't crap, or maybe do something else?.

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CUDAcores89   1 W

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looking for a 72v BMS that isn't crap, or maybe do something else?.

Post by CUDAcores89 » May 06 2018 9:21pm

So in my journey now to take a "500w" ebay hub motor and run 3000w through it (motor did 1500w just fine with everything stock), I have decided I will need to upgrade my battery pack to 72v.

The battery pack will be a 20S 8P 18650 battery pack made from tested power tool cells.

On the hub motor, I am going to fill the thing with statorade and add my own makeshift "hubsink" to the motor since I don't want to spend $70 on the real thing. I think I have a previous thread about the mods I am making to the hub motor. It should also be mentioned my riding style is street riding where it's 95% flat roads as hills basically don't exist in the part of Michigan where I live.

Currently I use a 52v battery pack that is split into two 7S battery packs for charging and balancing the cells. For bulk charging, the battery is kept as a 14S battery.

Up until now none of my batteries have had no BMS, I have been the BMS. I have used RC cell loggers as a form of LVC that beep really loudly when a single cell voltage drops, and I have used an icharger 208B for balancing the battery when it get's out of balance.

Here's the problem, when you go to 20S batteries, 10S RC hobby chargers become really hard to find and 10S balance cables are not compatible with 8S RC cell loggers. Meaning that i'm going to have to try a different tactic for my battery this time around.

I had two ideas:

1. Go out and buy a 72v BMS.

Not sure which one to buy here, but I have been looking at this one on aliexpress. I plan to run a 72v at 40A system so hopefully the BMS won't burn up if it's rated double it's max current. The LEDs are a nice touch too:

https://www.aliexpress.com/item/20S-80A ... autifyAB=0

Generally I have stayed away from BMSes until now because they are expensive and even the good ones (aside from the REALLY expensive stuff from batrium) are known to explicitly fail for no good reason. I also have no way to view whether the BMS is doing it's job and the ones where you can do that (they usualy have an LCD to read each cell voltage) are $100+. Almost every single power tool battery pack I have ever taken apart failed because the BMS died, so I would rather have direct access to my battery pack so I can control exactly what it is doing.

Second idea:

Build a 72v (20S) battery without a BMS and only wire a single main charge cable Instead of separate dedicated charge cables, then have a 7S + 7S + 6S balance leads come off the pack for balancing. Basically instead of building one battery pack that is internally three little battery packs that can be split up to charge and discharge, I will just use a single charge lead and wire up a 7S + 7S + 6S balance lead to the battery that will allow me to monitor each cell individually

But now that we have our 72v battery pack, how will we balance the pack at the top-end?

To acomplish the balancing, I am looking at using these:
41VZVSGHsuL._SX425_.jpg
41VZVSGHsuL._SX425_.jpg (22.26 KiB) Viewed 492 times
Some of these have a "balance function" that simply bleeds each individual cell down to the same voltage even if the battery is sitting there doing nothing by holding down the furthest left button. Therefore when I do actually feel the need to balance each cell, I no longer need to split the pack apart and plug it into an RC hobby charger. I can just plug these things into the balance leads and they will collectively bleed the battery down till it is balanced. And unlike a BMS if one of them dies, swapping in a replacement is a 10 second job.

IT should be noted this is another type of cell monitor with a balance function, but the reason I don't want to use these is because I have bought two of them before, and they both burned themselves out:
AOK-RC-2s-6s-lipo-li-fe-Battery-Balance-Lcd-Voltage-Meter-Tester-and-Discharge-3.jpg_640x640.jpg
AOK-RC-2s-6s-lipo-li-fe-Battery-Balance-Lcd-Voltage-Meter-Tester-and-Discharge-3.jpg_640x640.jpg (89.93 KiB) Viewed 492 times

The LCD turned weird colors and the logger became ridiculously hot. These smaller ones don't balance as fast, but I have also never had one of the 7S ones burn itself out. They just get pretty warm. Even if my battery becomes super out of balance for some reason, I can just leave these balance loggers sitting for a few days and they will equalize the battery pack on their own.

If I had a 7S + 7S + 6S balance cable coming out of the pack, I would also still be able to use the 8S RC cell loggers as my form of LVC:
download.jpg
download.jpg (6.36 KiB) Viewed 492 times
But will option two work? Option one is obviously the cleanest, but then i'm going to have to pay $60 and wait for a BMS to come all the way from china. I already have all the parts on hand to do option two so the price to accomplish it would be $0, so I am leaning toward that.

Thoughts?

What do the guys that run 72v DIY batteries charge and discharge your packs saftely? I would love to know.

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eee291   10 kW

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Re: looking for a 72v BMS that isn't crap, or maybe do something else?.

Post by eee291 » May 07 2018 3:22pm

I bought two of those Aliexpress BMS and they were total garbage. Basically no or too high HV cutoff.

liamcaff   1 kW

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Re: looking for a 72v BMS that isn't crap, or maybe do something else?.

Post by liamcaff » Aug 21 2018 5:13pm

Which way did you go in the end?

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eee291   10 kW

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Re: looking for a 72v BMS that isn't crap, or maybe do something else?.

Post by eee291 » Aug 23 2018 5:40pm

Totally missed your reply, I went with a pair of Bestech BMS. You can only buy a minimum of two from them sadly, but their quality is pretty damn good.

Grider   100 mW

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Re: looking for a 72v BMS that isn't crap, or maybe do something else?.

Post by Grider » Feb 17 2019 6:22pm

bump for this post, as I am also looking for recommendations for a 72V BMS that isn't crap

Are these two from EBAY any good????:

https://www.ebay.com/itm/20S-72V-90A-BM ... :rk:1:pf:1

https://www.ebay.com/itm/72V-20S-100A-B ... :rk:3:pf:1

markz   100 GW

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Re: looking for a 72v BMS that isn't crap, or maybe do something else?.

Post by markz » Feb 17 2019 9:26pm

Grider wrote:
Feb 17 2019 6:22pm
Are these two from EBAY any good????:
Who knows, you willing to gamble?

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