Battery level indicator rebuild

Thread Starter

pluggedin

Joined Sep 30, 2020
2
Ok I have a 24v go kart that a lot of the circuits have been hacked out. I have rebuilt most of them, and the last remaining one is the battery level indicator.

All that remains is 3 LED's 1 red, 1 yellow, 1 green on a pcb wired in parallel, this I would like to keep as it fits into the dash. The go kart is 24V from 2 led acid batteries. The red LED needs to come on at about 21/22v the green at 24V and the yellow somewhere in the middle. I am looking for help with a basic simple circuit.
 

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Irving

Joined Jan 30, 2016
3,792
The link below is for 12V battery, but with some variation could be made to work for 24V. Would this do the job for you?
https://www.circuitstoday.com/3-led-battery-monitor-circuit
Good find, but sadly not going to work without additional mods for the TS as his 3 LED are all common anode....

Assuming a -ve ground system, here's a modded version with values for 24v though I'm not sure I'd do it this way, zener diodes at higher voltages can be soft, and if you want to increase the current to improve that then the wattages are getting higher... I'd prefer something based on comparators and voltage dividers... (diagram 2)... though its more parts...

1601473069367.png


1601473213648.png
 

Tonyr1084

Joined Sep 24, 2015
7,766
Good find, but sadly not going to work without additional mods for the TS as his 3 LED are all common anode....
Unsolder the three LED's and reverse them to common cathode.

If the TS doesn't mind three LED's being lit at the same time:
1601485418690.png
When fully charged, all three LED's will be lit.
When the voltage drops to 26.92 the green LED will go out (24ZV + 2.92Vf)
When the voltage drops to 24.01 the yellow LED will go out (22ZV + 2.01Vf)
When the voltage drops to 21.95 the red LED will go out (20ZV + 1.95Vf)

When the red LED goes out it's time to recharge. Would not recommend driving with no LED's lit. You won't know the state of the battery, and you can harm it if you pull the voltage too low.

[edit] The forward voltages of the above LED's are the values for the LED's in my possession. The numbers will have to be modified for whatever the Vf of your LED's are. They look old so they're not going to be the same as my super bright LED's.
 
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ci139

Joined Jul 11, 2016
1,898
the battery SOC depends on

  • it's age / condition
  • (at the close time proximity) -- the previously (at that cycle) applied discharging rate "history"
  • it's temperature

the "apparent SOC" also has to do with

  • the condition of the terminal/cable contacts
  • the cross section of the terminal cables
  • relative humidity
  • the cleanness of the battery / cabling surfaces

you likely would want to do the design on an MCU bases or buy one readily avail for your specific battery model (capable of testing and remembering the usage data)

otherwise it serves much no purpose
 

Tonyr1084

Joined Sep 24, 2015
7,766
@ci139 I think he just wants three lights. The TS hasn't said anything about charging; just rebuilding and reusing the three LED indicators that are OEM (Original Equipment Manufacturer). Beyond that I don't know exactly what the usage of the go-cart is going to be - whether for racing or for general fun.

My friend and I built a gas powered go-cart back when we were teens. 4 HP motor with belt drive. The clutch pedal was a deadman style clutch: had to hold it down to make it go. Gas and brake pedal were on the right and the clutch was on the left side. Revv it up and press on the clutch pedal and away you go.

With an electric cart it's going to be a lot different. We don't know if there's going to be dynamic braking. For that matter we don't know what motor the TS is going to use. At best - to answer the TS question about how to utilize the three LED's, I think the simplest approach is the best approach. Turn the cart on and the three LED's light up telling you the voltage in the battery. As the voltage drops the LED's extinguish as their respective thresholds are reached. The rider will know there's still power but will also know it's probably time to put it back on the charger.

Speaking of chargers, the TS may just have a smart charger for car batteries. Again, I'm assuming he's using AGM batteries. Marine batteries might be better, but they're far more expensive.
 

Thread Starter

pluggedin

Joined Sep 30, 2020
2
First of all thank you for your feedback, lot's of good ideas. There is a control module for the motor which shuts down at 20.5v to protect the motor, it also shuts off the motor during braking. There is no charge management system, the charger instructions are charge for maximum of 5 hours.
 

AlbertHall

Joined Jun 4, 2014
12,328
In the datasheet there is a high voltage version, cunningly using the LM3914 reference to regulate the supply voltage.
But I think the TL431 version makes more sense.
1601588062739.png
 
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