building 2 more 72V carts

[Sergio]

Originally Posted by Volt_Ampere

Low voltage cutout can be dangerous on some carts so be careful with that. For me an alarm is sufficient but for the general public that just drives their cart, a cutout might be necessary. I have a bunch of golfing buddies that have lithium carts that they purchased that way - and they don't really have the knowledge we have here - and they don't really need it. One guy has been towed home at least once when he ran it down a bit too far.

I think the danger of the low voltage alarm by itself is if the alarm was triggered by a single low cell and not by the entire pack.

That could kill the pack and possibly cause a catastrophic failure if someone kept driving.

I am not sure everyone understands the circumstances when a low voltage alarm can be ignored for a short distance so for liability purposes OEMs should always disable the cart but isolating the pack is not the best approach.

[morvolts2]

Originally Posted by Volt_Ampere

Low voltage cutout can be dangerous on some carts so be careful with that. For me an alarm is sufficient but for the general public that just drives their cart, a cutout might be necessary. I have a bunch of golfing buddies that have lithium carts that they purchased that way - and they don't really have the knowledge we have here - and they don't really need it. One guy has been towed home at least once when he ran it down a bit too far.

just another reason to have a "fuel"(SOC) gauge

[Volt_Ampere]

I made my own SOC gauge that reads out in percent SOC. It's fairly accurate - uses the published discharge curve for Leaf cells.

[Pat911]

Pack should not be isolated but cart should be stopped from moving. Another valid reason for not passing the motor current through the BMS. An intelligent cutout would disable forward/reverse travel and other high drain devices such as lights and radio, whilst leaving regen breaking enabled.

An accurate SOC gauge is invaluable.

[Mike from Texas]

What is the consensus for a good quality SOC gauge?

I’ve been looking at this one.


Renogy 500A Battery Monitor, High and Low Voltage Programmable Alarm, Voltage Range 10V-120V and up to 500A, Compatible with 12V Lithium Sealed, Gel, Flooded Batteries https://www.amazon.com/dp/B07RP5B5P7...ing=UTF8&psc=1


If there is a better option I’m all ears.


Sent from my iPhone using Tapatalk Pro

[simicrintz]

Not sure it is better, but I like mine.

https://www.amazon.com/Programmable-.../dp/B0043BDFYA

[morvolts2]

I use a somewhat simpler one that can be set to full when the pack is fully charged.

it goes down to 2 bars of yellow,then red which flash when it reaches that level.

An easy way to trick the SOC meter to test, is to tap off lower levels of battery
pack and see what the SOC meter acts like. In other words a 12S battery fully charged maxes the meter,an 11S will put it at about 45v or 60% SOC,10S will put it close to what you would want for discharged. If you have an adjustable power supply for testing,even better. I dont like doing real world tests using completely discharged packs but if done once and done properly its each persons individual call.
The digital volt meter is about .4v high compared to all of my Fluke meters so at 49v its not fully charged,hence one bar down from top on the SOC meter.

[Sergio]

The only accurate SOC meters for Lithium batteries are the ones that continually measure AH.

Both of those suggested track AH to display an accurate percentage of remaining capacity.

Voltage based SOC meters are not accurate for Lithium batteries.

A lot of folks that use voltmeters to estimate remaining capacity get stranded as the voltage drops a lot faster as the battery gets near the 2/3 end of the discharge curve.

The AH remaining is an accurate measurement of the remaining capacity you have left.

[AussieInSeattle]

Originally Posted by Mike from Texas

What is the consensus for a good quality SOC gauge?

I’ve been looking at this one.


Renogy 500A Battery Monitor, High and Low Voltage Programmable Alarm, Voltage Range 10V-120V and up to 500A, Compatible with 12V Lithium Sealed, Gel, Flooded Batteries https://www.amazon.com/dp/B07RP5B5P7...ing=UTF8&psc=1


If there is a better option I’m all ears.


Sent from my iPhone using Tapatalk Pro

I'm using that Renogy one in my build here which uses Sergios approach with a 16S 120Ah LifePo4 setup:
https://www.buggiesgonewild.com/showthread.php?t=172654
Only just powered on the build last night to charge the pack so haven't configured the meter from a capacity standpoint yet but will document in above thread once I get there in the coming weeks.

[Volt_Ampere]

Measuring current in a cart is very tricky because it's got tons of noise on it. Current is chopped by the controller. That makes measuring AH by integrating current over time very difficult to do accurately. It requires properly designed filtering. My SOC does work only on voltage but it's nonlinear based on the published discharge curve for my Leaf battery cells. It's not super accurate but it's good enough and probably better than most. I could improve it by adding temperature compensation but I am satisfied with the way it predicts SOC now. My program uses a table lookup and interpolation to get a fairly accurate conversion from voltage to SOC.