Digital Voltage Meter

[scottyb]

BS40, creating a benchmark to compare to is a good idea. Note that over time motor bushing wear and built up resistance in the amperage delivery system could adversely effect these numbers. The more you know about your batteries the longer they will last.
When they are new they are all equally rated for the job, the moment they are connected to the cart all bets are off.

[JohnnieB]

Maybe I can shed some more light on the subject, or confuse it more.

1. When to head back to the charger?
If you are going the travel the same distance over the same terrain at the same speed carrying the same load head back when the at-rest voltage is at 75% SoC.
If the inbound leg is different than the outbound leg, adjust SoC % up or down as required.
To get an accurate at-rest voltage while in the field, simply let cart sit about 10 minutes before taking voltage reading.
To get an accurate at-rest voltage after charging the batteries, either wait about 12 hours or take a 3-5 minute ride and wait about 10 minutes.

2. How much voltage drop under load?
In addition to all the variables already mentioned, there is also the Amp-Hour rating of the battery vs the Amp-Hour usage of the cart.
A 48V 400A controller will suck harder on a 4x12V battery pack than it does on a 6x8V pack or a 8x6V pack, so going up the same hill with the same cart with same souls on-board will take a 4x12V to a much lower voltage than a 6x8V and a lot lower voltage than a 8x6V pack.

The closest thing to a general rule of thumb that I've been able to find is don't routinely load the battery pack down to less than 1.75VPC (Volts Per Cell) and never load them down further than 1.50VPC.

For a 36V pack, that is 31.5V and 27.0V respectively, and 42.0V and 36.0V for a 48V pack, so that is a heck of a load and probably wouldn't come into play unless the batteries were in pretty bad shape or significantly discharged at the time.

I'm not 100% sure of this, but I believe 1.75VPC is the end voltage used for many of the non-destructive discharge testing protocols used to rate a batteries AH and run-time.
And 1.50VPC is when lead begins sloughing off the plates during heavy discharge.

I'm just guessing, but in my personal opinion: If your battery pack is relatively new and in fairly decent condition; And is dropping more than 5V (or 6V) under the most strenuous conditions you typically encounter, your battery pack is too small (Too few Amp-Hours) for your driving habits combined with the way your cart is set up.

[bigstik40]

Thanks, Johnnieb, for the additional clarification and also for putting some voltage numbers out there. Also, the 10 minute wait after running, before reading the resting SOC, is good to know. I never was 100% sure how to decipher exactly what my meter readings meant. RAY

[manwitaplan]

Thanks Johhnieb!

I really do value and appreciate the time you and scotty and the rest of the gang, devote to helping the common person.

Thanks for providing more information. This thread will be saved.

Manwitaplan

[JohnnieB]

Originally Posted by bigstik40

Thanks, Johnnieb, for the additional clarification and also for putting some voltage numbers out there. Also, the 10 minute wait after running, before reading the resting SOC, is good to know. I never was 100% sure how to decipher exactly what my meter readings meant. RAY

It's taken me a while to figure some of it out and I'm still learning.

Each battery pack and each cart is different, so the numbers I posted are guidelines rather than absolutes.
For example, the 10 minute rest after the cart has been driven may be longer than needed, or not long enough.
Just like the battery voltage is "artificially" high after being charged, it is "artificially" low after being discharged and it takes some time for everything to equalize and give true readings of the State of Charge.

If you watch your dash mounted DVM battery meter climbing a hill, the battery voltage will drop several volts while climbing, jump up a few the moment you reach the top and continue to slowly climb after you stop on top of the hill. If it was a modest hill, it may take 10 minutes before the voltage stops increasing, if it was a steep hill, it might take 15 minutes for it to cease increasing, and if it was a mere rise in elevation, maybe 5 minutes is all it takes.

The yardstick I use to get a ballpark guesstimate of my battery pack's SoC while in the field is waiting until the rate of increase is less than 0.1V per minute.
Being retired, I no longer wear a watch, so how long a minute is, is subjective.

[bigstik40]

I'm questioning this at this time as I had gotten used to watching my meter with a "very sick" pack, so was just wanting an idea what to expect out of a good pack. I realize that your numbers are just a guide and not absolute. My sick pack would hold at 35 - 33volts for a while, then quickly drop to 26-28 volts as the bad batteries started to drop out. I run in Florida on very level paved roads, so hills aren't an issue. I'm currently back up North for the holidays but will soon be back to breaking in and becoming familiar with my new battery pack. Thanks for all your help and wishing you a Merry Christmas and Happy New Year. - RAY

[kab69440]

If I ever see 36.5v resting, I'm gonna push it home.