Hi all,
estimating state of charge from a simple voltage measurement is close to impossible. There are too many variables: temperature, load current, and internal resistance to name a few. In the "good old days", tape decks had VU meters. Battery powered ones had a button to switch one of them to a battery meter - not with a voltage scale, but with a sector telling you full or empty - my Marantz CP430 did this. My Sony WM-D6C had four LEDs to show me battery voltage or level. You had to learn to understand what your recorder wanted to tell you - this has not changed in the last 30 or so years
![Cool 8)](/Smileys/throwback/cool.gif)
Another point to keep in mind is that current consumption of modern recording devices is not constant over changing supply voltage! Back then only a few parts of a tape deck were supplied with a stabilised voltage - the capstan motor, maybe the bias oscillator or sensitive preamps. The rest was simply powered off the batteries - mostly C or D cells (my marantz used three of the latter). So current drain dropped with dropping voltage.
Not so nowadays: our portable computers we call "decks" or "recorders" often use an elaborate array of high-efficiency switchmode converters to power the different subcircuits. The interesting part is that a DC/DC converter has the astonishing property to present the battery with a partially NEGATIVE input resistance! In simple terms: since the converter tries to keep the input power constant, current rises when voltage drops. I will give you an example:
Assume a recorder with four phantom powered mics, drawing about 3W of power overall. A good, modern, power supply of the switching type has an efficiency of 90%, so the battery has to deliver 3W/0.9 = 3.333W. Battery type is a Lithium polymer cell with 3.7V nominal, voltage goes from 4.2V (full) to 3.2V (empty). Now we can calculate current drawn from the cell:
4.2V: 0.79A
4.0V: 0.83A
3.8V: 0.88A
3.6V: 0.93A
3.4V: 0.98A
3.2V: 1.04A
Those numbers are also valid for the typical USB power bank.
Problem: the internal resistance of most electrochemical systems rises when they get discharged. There is a tipping point when the battery simply cannot sustain the ever rising current demand of the converter - the problem tonedeaf described.
My advice: keep backup batteries handy, use external power (power banks with as much capacity as possible), check your rechargeables often (there are chargers that can check capacity - since they discharge to a certain switchoff voltage, high internal resistance also shows as low capacity). Keep battery contacts clean, wipe with denatured alcohol: this keeps contact resistance and power loss down. If you record outdoors in cold climate: keep your rechargeables warm - put them in your pocket.
Greetings,
Rainer