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Reading your Mobility Scooter Battery Gauge Correctly

Posted by Warren Chew on

One of the common service calls we receive from new mobility scooter users is that their battery depletes very quickly after using for a short while. But vast majority of the time, it is because they are not reading the battery gauge correctly.

Problem 1: Reading and Driving

The first problem is that they read the battery gauge while the scooter is moving. This is wrong, because the battery needle or light drops when the scooter is using power. The more power it uses e.g. to climb uphill, the more the battery needle or lights drop towards the red zone.

This is because battery gauges are essentially voltmeters. They read the voltage of the batteries, in order to estimate the amount of charge left in the battery. But when you drive the scooter, the voltage drops in order to supply the current needed to power the motor. And when you stop, the voltage recovers to it's natural level, but slowly.

So in order to get an accurate reading on your scooter, please do the following:

1) Stop driving the scooter, but leave the ignition key in the ON position. Do not turn OFF the machine.

2) Wait for about 30 - 60 seconds, let the battery gauge go back to its natural level.

3) Read the battery gauge. At this point, the reading should be more accurate.

Battery Gauge

Problem 2: Residual Charge

The 2nd common problem is that many battery gauges appear to show that the batteries are not full after driving for a very short while. For many models of mobility scooter, such as the Spitfire EX, there will be a 2 - 3 mm gap between the needle and the end of the of the green zone (see picture above). This leads to the misconception that the battery drains very quickly.

In reality, for these scooters, that is where the needle points when the battery is full. For those with LED lights, you may see 1 or 2 green lights going off (depending on how many lights the gauge has) after driving for a short while. But it doesn't mean the batteries are running out.

This again goes back to battery indicators being essentially voltmeters. Right after charging, there is a "residual charge" left on the terminals of the battery. This is just a small amount of charge left on the surface of the terminals, and is not at all reflective of the amount of energy stored in the battery.

12.8 V for a lead acid is considered full, but the residual charge can lead to readings of above 13.2 V. So if you read the battery gauge soon after charging, you may see the needle go all the way to the edge of the right zone, or even further beyond, so the battery appears very full.

But once you start driving for a very short while (we're talking in minutes here), or if you leave the batteries alone for a few hours, the residual charge dissipates from the terminals, and the needle drops somewhat, sometimes leaving the 2 - 3 mm gap shown in the picture.

So do not worry just yet. If you really want to test if your batteries are good, just drive it around the block at full speed for 1 hour. For most scoters, this means between 6 - 8 km. If after 1 hour of continuous driving, and your battery still isn't flat (i.e. needle at the yellow zone), your batteries are probably good.


Problem 3: Looking at the Battery Gauge while Charging

Many elderly users look at the battery gauge when charging to decide when to unplug the charger. This leads to under-charging of the batteries, which will lead to the batteries deteriorating.

In order to charge the batteries, the charger supplies a current at a voltage higher than that of the batteries. So the battery indicator, being a volt meter, always shows "full" while the batteries are being charged, even though the batteries may actually be flat.

So if they go out at this stage, they will have the misconception that the battery is full (see problem #2, residual charge). But in reality, their batteries could be very low, so the battery gauge needle move from the green zone to the yellow zone or even the red zone very quickly.

All battery chargers come with an LED light, which should show orange or red while charging, and turn green when fully charged. That LED light is what should be used to tell whether the batteries have been fully charged, and not the battery gauge.


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