Strange Battery Issue

Aaron67Camaro

So, I recently purchased a 2001 (early) EZGO PDS, 36V, with batteries that are about 18 months old. I brought it home, charged it up and tested it with a DVM the next day after charging it. it read 38.8 volts - which according to the charts posted here is a "100% Charge". I didnt run the cart for about a week other than just short trips taking my daughter around the cul-de-sac. I checked the charge each time with a DVM and each time the charge read 38.8V with 6.4V coming from each battery.

This leads me to today.

I went out to the garage and checked the charge - 41.8volts, with about 7.something volts coming from each battery. What are your thoughts on this? Could my DVM be going bad and causing an erroneous reading? I am hoping this is the case. How could each battery and the whole pack be reading higher than a few days ago when I checked it without having been on a charger?

By the way - I took the cart out and ran it for about 20 minutes or so and didnt notice any decrease in performance or anything like that. It is on the charger right now.

Any insight is appreciated!

Thanks

BGW

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gornoman

When my DVM gives me readings that defy logic, I replace the battery in it. Or, I try my "other" DVM.

Aaron67Camaro

Yeah, I plan on changing out the battery tomorrow and checking it again.

JohnnieB

A fresh battery in your DMM is a good start.

If you have a spare CR2032, or similar, Lithium (Lithium-Manganese Dioxide) battery around the house, they make a pretty good test reference for a DVM.
Open circuit voltage is 3.30V and the discharge curve is basically flat until it is almost depleted and then drops almost straight down.

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38.8V for a 18 month old 36V battery pack sounds a bit high, but it depends on what voltage the charger turns off at and how long the batteries have rested after the charger turns off.

The typical automatic charger shuts off in the 44V-46V range, but that voltage is a "Surface Charge" that depletes rapidly and erratically, so the batteries must be allowed to self discharge for several hours (6 hr minimum - 12 hr or longer preferred), or used for a short period of time, to eliminate the surface charge and get a meaningfully voltage reading to more accurately estimate the State of Charge.

The trick to using the SoC charts is consistency.
In other words, always let the batteries rest for about the same length of time or drive about the same distance before taking voltage measurements for 100% SoC purposes.

Batteries do have higher terminal voltages at higher temperatures, but I'm more inclined to suspect the DVM readings.

Hope this helps.