Volt meter reading vs Alltrax monitor reading

[Sideways]

I have one of Scottyb's volt meters that is hooked up directly to the battery pack + & -. It is reading more than 2 volts different than the reading on the Alltrax SR monitoring page.
Is that normal or do I need to rewire something?
Thanks again

[JohnnieB]

It depends on when the respective voltage readings are observed and other things.

If it is prior to the solenoid clicking, they will be different and if the are issues with the high current cables, contacts and connections, they will be different.


The Alltrax monitor is looking at the voltage between the controller's B- and B+ terminals the dash mounted voltmeter is looking between the battery pack's B- and B+ terminal's which are not always the same points electrically. The B- terminals are only separated by a single cable, so they will be the same point electrically unless that cable or its connections are bad, but there's two cables/connection plus a set of contacts in the solenoid between the two B+ terminals.

Also, before the solenoid contacts close, the voltage between the controller's B- and B+ terminal will be several volts lower than battery pack voltage. Prior to contact closure, the monitor is looking at the pre-charge voltage on the filter capacitors.

[Sideways]

Thanks JohnnieB.
The cart was sitting at a standstill when I noticed the difference.
I guess my concern is if the pack goes below 50% according to Alltrax but not according to the dash mounted volt meter.

[JohnnieB]

Use the dash mounted voltmeter for SoC (State of Charge) estimates.

Also, the battery pack voltage is going to drop below 50% SoC during operation. Basically, the voltage of a 36V pack drops 2V per 100A (or more) of current draw, so whenever the motor is drawing more than 100A or so, the pack voltage will be roughly 36.2V (50% on SoC chart) , or lower, depending on how fully charged the batteries are at the time. However, that voltage is Under-Load and At-Rest voltage is use for SoC estimates.

At-Rest voltage is when the batteries have rested (neither charged or discharged) for about 12 hours after the charger shuts off automatically, or at an hour after the cart stops. (When out and about, 10 minutes rest is usually sufficient for making a SoC estimate. The SoC estimate will be a few % lower than it actually is, but that is on the safe side.)

While driving the cart, try to stay above 27V for a 36V battery pack. That is 1.5VPC (Volts per Cell) and physical damage to the plates can occur below that point.

[scottyb]

JB as always giving the best advice BGW readers can get

[Sideways]

Thanks Johnnie, would the same math be applied for a 48volt system while driving?

[JohnnieB]

Originally Posted by Sideways

Thanks Johnnie, would the same math be applied for a 48volt system while driving?

Yes and No.

The rest times to get an At-Rest voltage for an SoC estimate are the same and the numbers at the 2V cell level are the same, but there are six more 2V cells in a 48V pack and the amp draw for the same load is 33% less, so the voltage drop per 100A and max voltage drop allowable under load are different.

Max recommended voltage drop under load is 1.5VPC, so for a 48V pack it is 24 x 1.5, or 36V.

[Sideways]

Gotcha!
Thanks again

[Joelwdryden]

What should the 36v pack read at 100 percent charged after sitting 10 hours?

Sent from my Moto Z (2) using Tapatalk

[Jmzvet]

About 38.2 12-24hrs after charging cycle has ended. Credit goes to Carts Unlimited State of Charge Chart.