How to remove 5V from circuit so that my meter reads correctly after LiFePO4 upgrade on golf cart?

Question

A while ago, I upgraded my lead acid batteries to LiFePO4 for my golf cart. The state of charge ("SOC"/battery gauge/voltmeter) is for a 48V cart but the new batteries put out 53.x volts so it always shows as 100% full and more or less just functions as an on/off light.

I've been wondering if there is a way to trick the meter and step it down 5V so that the SOC meter will read correctly. I was looking at buck converters based on another question asked here but that seems like it would just always try to deliver a constant voltage.

This SOC is in a very small space so I really would like a very small solution so that I don't have to do a bunch of rewiring.

This is the SOC meter. I can't find any specs on what the actual meter voltage range is but I'm guessing pulling off 5V would put it right in range (if possible.)

EDIT: As requested, here's my best stab at a schematic:

I'm guessing for testing the quickest thing would be to pop a 4.7V Zener onto the positive side of the circuit.

Answer 1

You could try adding a series resistor so that the scale will be 0-56 V, but that might not properly drive the display, unless it has a separate power supply. The series zener, as suggested, might work better. But in either case, a capacitor across the meter might be needed.

A better inexpensive way to estimate SOC might be a running time counter, but it would depend on how consistent your usage is, and would need to stop timing when the motor isn't running.

The best DIY solution might be a current to frequency circuit and a counter, to measure A-h energy used. And there are inexpensive combination V / A / W / W-h meters.

Answer 2

A lead acid battery voltage drops proportionally as it discharges, while LiFePO4 batteries have a much steadier voltage until 85% of capacity is used, then a sudden drop off. Here's a discharge curve from batterypowertips.com

Your simplest bet is to just re-interpret what your SOC meter shows. If it shows anything less than 100%, you're on your last 15% of capacity from the LiFePO4.

You can buy small modules for a few dollars that read DC voltage directly, but I don't think it's worth the trouble of the mounting and rewiring. The voltage still won't give a reliable indication of remaining capacity until you're near the end. A coulomb counter measures cumulative current flow and would work, but you won't find a ready-made one to mount there.

Answer 3

You need a different measurement strategy for LiFePO4 fuel gauging

Lead acid batteries can be judged (reasonably accurately) from their voltage. This is what your current "fuel gauge" does. That approach won't work with a lithium chemistry battery (as you've discovered).

credit: Pilot Group UK

Lithium batteries have the very desirable attribute of maintaining their voltage over the majority of their discharge cycle, making the voltage-only approach mostly useless.

You need to measure energy instead

Energy is measured in Joule's and charge is measured in Coulomb's. The relationship is:

Joule per Coulomb = Volt

Commercially, energy based fuel gauges are known as "Coulomb Counters". Here's a relatively common and economical example from Renogy:

credit: Renogy, Inc.

It's a "Universal" battery gauge because you must tell it the size, voltage, and derating parameters of the battery you are actually measuring:

credit: Renogy, Inc.

...and then you connect it in the low side electrical return path so it can measure the voltage and current.

credit: Renogy, Inc.

Due the wide range of battery systems in use in carts, scooters, and other small electric vehicles, there are many OEM's for this type of panel meter and most units cost less than $100 all in.