Circuit Analysis – Powering and Using 4 Small Voltmeters for 3 12V Batteries in Series

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I bought some small voltmeters hoping I could use them with my kid's Razor dirt bike. The bike uses 3 12v lead-acid batteries in series to provide 36 volts to the motor. Ideally, I'd like to use the voltmeters to measure total voltage of the batteries in series, as well as the individual voltages of each of the batteries. I'd also like connect the voltmeters to the power switch, so they only measure voltage when the bike is powered on.

The working voltage of the voltmeters is 4 to 30v, and they can measure 0 to 100v.

The voltmeters have 3 wires. I ran the positive wire to the power switch, I ran the ground to the ground of each battery, and ran the measurement wire to the positive terminal of each battery. This seems to work fine for the first 2 batteries, because they are powered by 12v and 24v. The voltmeters for the last battery and the overall system end up getting 36v, which is too much for my tiny voltmeters.

The power switch is a single throw dual pole switch. Is it possible for me to power the voltmeters with 12v and use them to measure the system as described above?

UPDATE based on great feedback. I didn't know about reducing voltage with a resistor ladder. This is basically what I have now, just without the resistors. Will this work? I was afraid it may cause a short, because it seems like the batteries are both in series and parallel this way.
CircuitLab Schematic x29j4mhb4zs9

UPDATE – using 2 470ohm 5W resistors worked to divide the voltage, but the resistors generated too much heat. I measured 190 and rising after a few minutes. That seems dangerous. I'm going to drop back to 1 voltmeter powered by one battery, measuring all 3, and tied to one of the poles in the DPST switch.

Best Answer

Old-timer here. You're making this harder than it is.

The reason you're making it hard is that you insist on using powered meters. They make simple, unpowered meters that simply power themselves on the voltage they are measuring. They draw little enough that your far bigger problem will be battery self-discharge. But even then, turning them off is easy.

But it's easy with powered meters too; we'll just need a DPST relay and in one case we'll need a voltage divider out of 2 resistors.

First, for the "whole pack" meter we do exactly the same thing - go from bottom battery rail to switched side of the switch.

But for the powered meter, won't that be overvoltage? If you're averse to getting a better meter, then simply add a resistor ladder - a size X resistor from top rail, to meter power supply and a size X resistor to bottom rail. Start with a high value resistor like 10K, and if the meter refuses to power up, add lower value resistors in parallel (both top and bottom at once) until you find a happy number. When you're testing, it's easier to lower resistance than raise it, that's why I say start with a high value.

Why not 1 resistor for the resistor ladder? Because we can't count on the voltmeter behaving like a resistor (having the same impedance in any mode).

For the 3 per-battery meters, we make a "meter ladder" as it were. And we tie the meters ladder to the battery ladder. Since you are worried about the current draw of the meters, we need a DPST relay interrupting the ladder (the top of the ladder is already interrupted by the power switch).

If powered, these voltmeters simply take from the voltage they are measuring.


simulate this circuit – Schematic created using CircuitLab

I have no idea what's up with the editor's numbering of meters and relays.
The resistor ladder is only there for a powered meter; values unknown TBD. Consider analog meter or meter with higher supply in.
The relay can be eliminated if meter draw is too small to care about (compared with natural battery self-discharge).
The switch could be replaced by 3PDT switch to eliminate relay.

By the way, I gotta put this in here... OP took my resistor ladder concept and found a way to eliminate the relays. This is really good IMO. Although note that depending on the meter, there may be 24x7 leakage through the meter's measurement section, as Sphero discusses in a comment to another answer.

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