@▷ Battery Monitoring By LM324 | Diagram for Schematic

Battery Monitoring By LM324

It has been stated in other pages on this website that it is a very good idea to have two separate batteries, one for the drive (weapons and wheels), and one for the radio control receiver and other electronics (see Speed controllers and EMC sections). It is very useful to know the state of charge of both of these batteries so that you do not go into a bout with uncharged batteries, but do not have to continually charge them when they are already charged (which can degrade the batteries).

This page describes two methods of determining the state of charge of the batteries.

We can see from the discharge curve of the lead acid battery, that if we compare the terminal voltage with a fixed value of about 1.75V per cell (10.5v for a 12v battery, 21v for a 24v battery), we will get an idea of when the battery is in its terminal decline point. Note however that the discharge curves depend on how much current is being drawn from the battery at the time. This is because while drawing current, there will be a small voltage drop across the internal resistance of the battery. Hence at large current drains, the battery monitor may indicate a flat battery when it is actually OK. Therefore the monitoring should only be done (or only taken notice of) when no current is being drawn from the battery.

The voltage of the lead-acid reaction when fresh is typically around 2.15v per cell. A further comparison threshold can be put at around 2.0v per cell (12v, 24v) to indicate that the battery is down to about 25% charge left. These limits are a bit arbitrary, so we should allow them some trimmability.

A circuit diagram of the battery monitor is shown below:
The comparators are powered from the battery itself, so to get a battery voltage input which isn’t at the same value as the comparator supply (comparators would not work well like that), the battery voltage is divided by the trimmer resistors. This is compared with a value generated from the precision voltage reference. Opamps are used for the comparators since no pullup resistor is then required (comparators generally have open-collector/open-drain outputs which require a pullup resistor to make wired-OR functionality easier. They are also normally much faster then opamps at comparing, but we do not require any speed, or the wired-OR function).

The oamp after the trimmer and the opamp after the precision voltage reference are connected as buffers, and are there to ensure that the trimmer and reference do not have to supply any reasonable current which would affect their output voltages.

The 2.0V comparator LED is connected to the output of the 1.75v comparator. This ensures that when the battery voltage falls to below 1.75v and that comparator’s output goes low, the 2.0v LED will extinguish, and only the 1.75v LED will be lit. The series resistors on the LED should have the value

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