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Old 09-09-2021, 09:33 AM   #1
wamcneil
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Default Newbie 1995 Medalist 36v Lithium conversion

Hi all,
Just got my first golf cart. Free from a neighbor. I think there's a saying like 'there's nothing more expensive than a free boat'...
Anyway, it's got bad batteries and I'm trying to decide what to do.
Normally I'd just buy 6 GC batteries and be done with it, but I happen to be sitting on two 12v280AH lifepo4 batteries left over from an RV project. So in theory I would just need one more similar battery to make 36v. And I figure the net cost of this would be comparable to buying 6xGC flooded cell batteries.
These batteries are each built up from 4x3.2v prismatic cells with a BMS.
Is this folly? Should I just buy new lead batteries?
I've read through a few posts and it looks like I might have problems with the BMSs shutting down from overcurrent when starting the cart.
Anything else I might not be considering that would make this end in sadness?
Thanks!
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Old 09-09-2021, 11:44 AM   #2
bronsonj
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Default Re: Newbie 1995 Medalist 36v Lithium conversion

There are a couple issues. First, as you note, the BMS on that is likely only going to be able to pass 40 or 80 amps. That's very low, my cart requires about 75 amps of current to maintain speed on flat level pavement. Starting from a stop requires a lot more current.

The second issue is if you use 3 separate 12V lithium packs then they will become imbalanced with time. Some won't fully charge and others will be depleted early. If you can pull the cells out of them, get 4 more cells and a BMS that will handle all the cells at once then you'd have a good battery pack.

So your options are to spend about a grand on lead acid packs, spend some money on additional cells and a bms and create your own pack (probably spend less than a grand) or to buy a pre-built kit which will run you probably $1,600 to $2,000.

In addition to the other cells and a single BMS, you'll need a lithium charger if you create your own pack.

Another option is to sell the two 12v lithium batteries and use those funds to buy a pre-built lithium pack... That'd certainly take the sting out of a purchase like that.
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Old 09-09-2021, 02:42 PM   #3
Bama Brad
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Default Re: Newbie 1995 Medalist 36v Lithium conversion

Another option is to sell the two 12v lithium batteries and use those funds to buy a pre-built lithium pack... That'd certainly take the sting out of a purchase like that.[/QUOTE]

This is what I would do...I have a lithium cart and a FLA cart, Big difference with the lithium! I will not be buying anymore FLA's. Well worth the money for Lithium. I have not seen a single post on this forum where someone has regretted going lithium, most regret not doing it sooner.
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Old 09-09-2021, 03:24 PM   #4
wamcneil
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Default Re: Newbie 1995 Medalist 36v Lithium conversion

Thanks.
Yeah, I think it would be better to reconfigure them as a 12s battery pack and get one big 250a BMS.
There are some issues with these batteries that complicate selling them...
I think I'll take the cells out of the boxes and build a plywood enclosure on top of the original battery rack
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Old 09-13-2021, 10:07 AM   #5
wamcneil
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Default Re: Newbie 1995 Medalist 36v Lithium conversion

So I've been thinking about the BMS and maybe I don't need a super-high amperage BMS. I'll need to get ahold of a wiring diagram to plan this out, but since there's already a big solenoid in the motor controller circuit, maybe I can just leverage that with the BMS to disconnect via the ignition circuit?
So in this case I'd use a very small 12s BMS, like 50 amp. The BMS positive output would not go to the controller solenoid, but to the circuit that feeds the key. And then controller's solenoid would be fed directly from the battery's positive, bypassing the BMS.
If the BMS decides it needs to cut power, it would cut power to the ignition circuit, which in turn would de-energize the solenoid and completely disconnect the batteries, right?
On the solar forums I've read some references to a similar approach to controlling high current banks with solenoids instead of high-current BMS.
I'd need a BMS with a separate charging port to protect the cells while charging.
Any obvious holes in this plan???
Thanks
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Old 09-14-2021, 12:46 PM   #6
bronsonj
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Default Re: Newbie 1995 Medalist 36v Lithium conversion

There are a few people who wired a solenoid to the BMS like you're talking about. Get a $40 BMS and a $100 solenoid and you're good to go. Most I've seen have used a separate solenoid but I've wondered why not just interrupt the ignition circuit.

You absolutely need to run the charge current through the BMS to prevent overcharging and imbalance issues. But we're still talking about a little $40 BMS.

Sergio did a write up on the solenoid wiring. I think that was in the Extreme DC forum or the lithium subforum in there.

--- edit ---
I found the link to the wiring discussion.
https://www.buggiesgonewild.com/showthread.php?t=160302
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