Low Voltage Alarm

Brandon1107

I was wondering if anyone has a simple low voltage alarm that I can mount in my dash? My kids will be driving my cart and I'd like to have an alarm that goes off at 60% ROC so that they can make it back to the shop and plug it in. I found one for around $70 and I may get it but I wanted to see if someone had something cheaper that worked.

http://www.energyonthehook.com/Low-B...m-p/bfd001.htm

Thanks,
Brandon

BGW

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mutley

Simple $5 volt meter is what most use. Problem with a golf cart and this kind of device is that the voltage will drop way below the minimum while the motor is running even though the batteries might not actually be below the minimum.
Example.
48v cart, Max voltage is about 51v and lets say the minimum/alarm setting is 48v. A simple voltage alarm will work fine when there is no load or a small load. Put a golf cart motor to the pack and you will regularly see 46v across and battery bank when the motor is at full load, but stop or slow down and it'll jump right back to 51v.
There are ways to overcome this problem, but I can't think of any that would be really cheap. At $70, if that device doesn't have false triggers on a golf cart then it's not a bad deal.

I have a $5 voltage meter I just tell my son to come home if the voltage is 48v or below when he is stationary. ( I think 48.9v is the absolute minimum / 60%)

Brandon1107

Thanks mutley! Being new to electric carts I didn't think about the huge swings in voltage. I've got a digital meter to install from Scottyb so my kiddos will just have to learn to pay attention.

Thanks,
Brandon

scottyb

I too thought the low voltage alarm would be good but after trying several makes and voltages I gave up. They were not handling the swing in voltage well.
Something I never did try was locking the alarm out when the pedal was depressed. That may let the application work.
Otherwise, like you said they may need learn to return at a certain voltage reading like the adults do

cgtech

Anyone thought of this? A standard volt meter, a diode on the input. After the diode & before the meter a capacitor and a little resistor to slowly drain the capacitor. The voltage sags would take some time to actually reach the meter.

BobBoyce

That could work for a low voltage alarm as well. The diode in series would introduce a bit of voltage drop, 0.7V or less, depending on the diode type used.

Brandon1107

I wonder if running a normally closed relay off of the pedal that would open when you step on the throttle would work? Not sure if constantly cycling the alarm would shorten it's life.

Brandon

JohnnieB

I've kicked it around some a while back.
The tricky part is the battery pack voltage sag is almost instant while the recovery is very slow.
Attached is the recovery analysis I did after a 10 mile trip I took to do a range estimate. The SoC went from 68% when I first stopped cart to 82% an hour later.

If you picked a RC time constant of about 5 minutes, or so, and an alarm trigger voltage a bit lower than your actual desired value, it mighty work out okay for a low battery warning alarm.


As mentioned, the diode will drop the voltage on the meter a bit, but if you installed an analog meter as the fuel gauge, you could scratch a line on the bezel for the lowest allowable voltage. Of course, you ought to have a DVM readout that actually tracked the pack voltage also.

Attached Images

Voltage Increase after load.jpg (100.4 KB, 0 views)

cgtech

I think the opposite ought to be true (slow drop, near instant recovery). You would have to experiment with a resistor or a few tries, to find one that drains the capacitor of roughly 10v in say 5 seconds.

Brandon1107

Ok.this brings up an interesting question then. I will have a DVM installed, I also have 8 x 6v CR260s, 50% SOC should be 48.41 volts. I plan to stop if at all possible with 60% SOC or more (48.96). Am I correct in assuming the tank is "empty" the first time I see that number or lower when the cart is stopped?