Pulling super high amperage from Lifepo4?

[ericfx1984]

So I've been asked to build a lifepo4 battery system for a golf cart the golf cart is 36 volts... So I would guess that we would need about 12 cells

I keep running into is I have read that there can be anywhere between 800 and 1200 amps of surge when the golf cart initially starts moving this is very momentary.. but I greatly doubt that there are any battery Management systems that can handle this

I've been thinking about doing is using the battery management system only for the charging and balancing side of things... But then allowing the discharge side of things to bypass the battery management system

The only real disadvantage I see to doing something like this is not having a low voltage cut off

And I guess in the grand scheme of things even if the batteries were ran all the way down to nothing they would still by far Outlast sealed lead acid batteries which are also ran down to nothing

Looking for any ideas or insight that anyone has thanks

 

[time2roll]

800 seems way way high. Get a clamp-on DC meter to measure. Possibly look at the speed controller rating. I am thinking closer to 100 amps max.

 

[Zil]

There are heavy duty relays that will open at a low battery voltage. There are both mechanical and solid state.

 

[rickst29]

I don't see any pre-built super capacitor modules for 36V (the common voltages are 16V and 48V), but you could possibly add a 48V module to assist with motor start. LFP battery manufacturers hardly ever offer a "peak discharge current", but I will SWAG that EVE-280(K) cells would probably not be upset by a momentary load of more than 600A. The busbars and BMS, that's a different situation - and worthy of attention. Adding a super capacitor module would assist everything - the cells, the BMS, and the busbars.

What sort of wire size did they use in the OEM build, from lead-acid batteries into the motor?

 

[time2roll]

Ok I looked at a couple golf websites and they are using wrong terminology. They are talking in 12v amp-hours. Very confusing.
Not talking about amp draw through the actual wire but capacity to get through the day.
I looked at a few and it reads like this:


https://golfcartresource.com/what-to-know-about-voltage-and-amperage-in-golf-cart-batteries/

The Importance of Distinguishing your Battery Pack​

Now that we understand the differences between Amperage and Voltage, we can begin to break this down so we can locate the differences between various battery packs. For standard golf carts, we will typically see a Six Eight Volt Battery System. This can be visually determined by locating six batteries with 8 cells per battery. A cell is distinguished by a battery cap that is used for watering the batteries. With batteries used in Golf Carts, each cell will represent 2 Volts. Let’s take a look at the 5 most common battery packs we see in the market:

Battery System	Total Volts	Total Amps (Approx)	Travel Distance
6-6 Volts	36 Volts	1,350	Approx 22 Miles
4-12 Volts	48 Volts	600	Approx 12 Miles
6-8 Volts	48 Volts	1,020	Approx 19 Miles
8-6 Volts	48 Volts	1,800	Approx 35 Miles
6-12 Volts	72 Volts	900	Approx 17 Miles

*Battery Amperage calculated using Trojan Batteries at a 20 Amp-Hour Capacity Rate

 

[ericfx1984]

well its all wired up on 4 gauge, there is NO WAY it can pull that much... 36v x 1350a = 48600w? yeah that math is way wrong

 

[Zil]

LiFePo battery must have a low voltage disconnect. They don't just drain dead, they reverse polarity, a disaster.

 

[time2roll]

Fairly sure most controllers are PWM into the DC motor so there is no start surge like turning on an AC induction motor.
This is part of the reason golf car batteries have a RC rating with 75 amps in addition to the more common 25 amp.

 

[Substrate]

Yep - a few ways to do it. First, you need high-C rated cells that can actually handle surges. Ie, a box full of "Miady" cells won't cut it.

Options:
1) Balance on charge only, and YOU watch / determine how far you can go before destroying them. The hope here is that your motors won't run properly long before reaching 0 volts! Ie, when it gets sluggish, STOP. Possibly walk or trailer it back home.

2) Bottom-balance. Charge up to only a predetermined "pack level", usually not exceeding 3.4v at the first cell in the pack to reach it.

3) "Add-On". Some of the wheelchair guys do this with their existing lead-acid for extended range, using the LFP battery first. Some ideas here:

WheelchairDriver • View topic - PINNED - ADD ON PACK Step By Step 30ah Pack build / pictures

everything powerchair

www.wheelchairdriver.com

Should be something here to incorporate LFP in some way with your golf-cart application without a bms.

 

[ericfx1984]

What sort of wire size did they use in the OEM build, from lead-acid batteries into the motor?

Yeah its only 4 gauge, maybe less... the numbers dont add up

 

[Sailor K]

I think the numbers are way off. On a 36V older model, one guy measured peak amp draw at under 200amps. Most recommendations on motor controllers are for 300A, maybe even less. So for an older 36V unit, I'd say 300A draw is a reasonable absolute max. With #4 cable, can't draw that much. And the solenoids would blow as well. The max occurs primarily from a standing start pointing uphill, with a load. There are 300A BMS's available, but they get pricey the higher the amp max...

 

[Sailor K]

Seems to me that six 6V lead acid golf cart batteries are 200AH, but shouldn't really be discharged to under 50%. So I'm thinking 50AH Lifepo4 cells would be absolute minimum and 100AH would be pretty good. A 250A BMS would perhaps be OK? A 250A 36V Daly BMS from aliexpress is under $200 and 12 50AH cells also from ali would be about 350, 12 100AH cells from ali about 600. Total close to a new set of lead-acid batteries.....

 

[raurre]

I'm currently building a system you might want to look at Thread '230Ah Mobile Battery (aka) EZGO Golf Cart' https://diysolarforum.com/threads/230ah-mobile-battery-aka-ezgo-golf-cart.40927/

 

[crossy]

The carts we use to get around the big sites have 1000W motors (that's the only rating plate I can see as a rider), so at 36V thats around 30A or so. They go very satisfactorily with 5 meaty chaps plus the driver.

They make the "PWM buzz" so likely don't have massive starting currents.

Also. I got some used cart cells, 200Ah LiFePO4's not sure how many were used in the actual cart.

 

[Roll2]

So I've been asked to build a lifepo4 battery system for a golf cart the golf cart is 36 volts... So I would guess that we would need about 12 cells

I keep running into is I have read that there can be anywhere between 800 and 1200 amps of surge when the golf cart initially starts moving this is very momentary.. but I greatly doubt that there are any battery Management systems that can handle this

I've been thinking about doing is using the battery management system only for the charging and balancing side of things... But then allowing the discharge side of things to bypass the battery management system

The only real disadvantage I see to doing something like this is not having a low voltage cut off

And I guess in the grand scheme of things even if the batteries were ran all the way down to nothing they would still by far Outlast sealed lead acid batteries which are also ran down to nothing

Looking for any ideas or insight that anyone has thanks

2003 Yamaha G22 48v

 

[mikefitz]

Bottom balance the cells at 2.5 volts, low volts cutoff is less important as no cell can damage another. Add a BMS to protect from cell overvolts when charging, charge via the BMS.
Take the traction power direct of the cell array . Cell data sheet will indicate continuous and peak current.
Mike
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