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Old 05-03-2021, 11:37 PM   #1
1Fast7
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Default How to choose BMS

I just purchased my first golf cart and the batteries are old and crusty. I bought 3 batteries to do a lithium ion conversion. The cart is currently 36v and with the new batteries I want it to be 48v. I'll be upgrading the necessary stuff to do 48v conversion. Now I know I need a BMS 15s and the one I've picked out has a 30-102v operational range but that's all I know. How do I determine what amperage I need? What do you pair it up with to get the right amperage? The controller, the motor or the 150A continues 250A peak of the battery?
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Old 05-06-2021, 09:32 AM   #2
bronsonj
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Default Re: How to choose BMS

What batteries did you get? What kind of cart do you have?

For BMS amperage it depends on how you wire it. On some of the lithium installs the BMS is there for balancing and charge cutoff. If you do that, then you need a BMS that can easily handle your charger. So if you charge with 10 amps then get a 20 amp BMS.

You are not likely going to find a BMS that will handle the controller amperage output. If your controller can push 300 amps that'd fry a BMS. So what some people do is balance their pack and then set the LV cutoff in the controller so it stops long before it gets to a questionable voltage so if your cells drift then you still won't over discharge.

The other option is to wire the load through another solenoid and let the load leads on the BMS control the solenoid. Sergio did a write up a while back on that. If you do that I'd still have the controller LV set to a safe range and use the BMS as a backup. You want the controller to stop you more than you want the BMS to just halt voltage possibly damaging the controller (like on an RXV)
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Old 05-08-2021, 05:04 PM   #3
1Fast7
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Default Re: How to choose BMS

I have bought 3 Fiat 500e batteries:
Capacity 63Ah
Max 20.5v
Nominal 18.5v
Minimum 15V
Continuous current 150A
Peak current 250A
Each battery is 5 cells 3.75V nominal ea.


The BMS that I'm looking at is a smart ant bms.
The voltage is progamable from 30-102v.
Charge current 50A
Discharge current choice up to 450A


What I want is for it to stop charging when it's fully charged and for it to shut down when it gets to the minimum discharge voltage.
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Old 05-10-2021, 10:02 AM   #4
bronsonj
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Default Re: How to choose BMS

Nice! I have one of those Fiat 500E packs in my son's power wheels. Three wired in series and charged to 4.1vpc puts you at 61.5v. The Alltrax controllers are good up to 62V so you can get all three sets in there wired in series. If you discharge to 3.30vpc then your pack voltage swing is from 61.5v down to 49.5v. The controller you'd want would be the 300 amp Alltrax and you can limit that from their software to 250 amps to match the battery.

The Alltrax you can set to stop the cart at whatever voltage you want. Obviously that doesn't replace the BMS but you can set that to 50V for the LV cutoff then in the BMS you can set the stop voltage to 3.25 or 3.3vpc.

The Smart Ant looks like it'll suite your needs. I have no experience with that BMS but it sounds quite capable.
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