Question about high red zone on SOC meter

[scottyb]

I'm curious to know what is the pack voltage reading when the needle crosses over into the "danger zone"

[JohnnieB]

Technically, SOC gauges are (or should be) expanded scale voltmeters and the scale or light-bar should approximate the state of charge of a typical battery after a few months of normal usage. Unfortunately, the voltage drop vs percentage of charge is progressive rather than linear or exponential and there is disagreement as to when a battery is empty, or at least how far a battery can safely be discharged without permanently damaging it.

Basically, expanded scale analog voltmeters have been around for the better part of a century and are accurate, but the scale might have been laid out by someone that had never seen a golf cart or knew nothing about the discharge curve of a wet cell lead acid battery designed for deep discharge.

The lightbar type voltmeters may or may not be expanded scale and they only come in two flavors, linear and exponential (Unless you want to spring for an Application Specific IC [ASIC] or go discrete component) while the discharge curve progressively gets steeper as the battery becomes more discharged. (IE: Cell voltage drops 0.019v from 100% to 90%, 0.020v from 90% to 80%, 0.021v from 80% to 70% and so on, but the progression becomes erratic at 30% and under)

My suggestion for anybody with an analog SoC meter is to measure you battery pack voltage with a DVM and mark the critical points on the face of the meter with a scribe.

Hope this helps more than it confuses.

[JohnnieB]

Originally Posted by scottyb

I'm curious to know what is the pack voltage reading when the needle crosses over into the "danger zone"

Which "Danger Zone", top or bottom?

To find out, snap five 9V alkaline batteries together (they just snap into each other, no battery holder needed) and clip a 10k pot between the + and - at each end. Clip the - terminal of the meter to the - terminal of the 9V battery stack and the + terminal of the meter to the Wiper on the pot. Put a DVM across the + and - on the meter and adjust pot until needle lines up with the mark on the analog scale you are curious about.

Same technique can be used to locate percentage points on the face of the meter.

I've used this technique to measure various trip points, set points and scale accuracies over the years. It works well, but be careful about the batteries, 10 of them tied together have been know to relax certain sphincter muscles.

[ColbyKiller]

Originally Posted by JohnnieB

... be careful about the batteries, 10 of the tied together have been know to relax certain sphincter muscles.

Nice. Or really, not so much so.