Not all 9V Batteries are created equal. CAT+ Dopplers need the best

9V Batteries - Not all 9V Batteries are created equal. CAT+ Dopplers need the best
We always recommend that you buy your CAT+ batteries from us. That's because we've tested hundreds of brands. We know which work - and which don't! (Thames Medical have no affiliation with and do not sell Duracell products.)

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Over the 26 years of the CAT+ Doppler we have tested hundreds of 9V batteries and have (so far) only found two that are reliable: The Duracell Ultra and our own Vapex rechargeable, which can be found here.

Here’s why.

The 9V Label Is Misleading

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The label 9v on a battery doesn’t actually apply to the voltage it gives! Yes, this sounds ridiculous however it’s more related to the size and shape of the battery (also known as PP3 or MN1604). The 9v is a nominal value and only passingly relates to the voltage delivered. We have tested brand new 9v batteries that only deliver 6.5 volts! While this is fine for a smoke detector or calculator, it causes problems for a CAT+ Doppler.

The CAT+ Doppler needs at least 7.6V to work, any less and the orange/yellow low battery warning light will illuminate, and the sensitivity, volume and sound quality will go. The closer the battery gets to 7.6V, the worse the CAT’s performance will be and the brighter the check battery light will shine.

Issues With Rechargeable Batteries

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Our Battery Recharger ensures the longest life possible for Vapex batteries.

Rechargeable batteries come with two labels: Voltage (V) and Capacity (mAh).

Again, 9v relates to the physical size – not the actual delivered voltage. mAh relates to the length of time a battery can deliver its power, and generally, the higher the number, the longer it will deliver its power. But if it can’t deliver more than 8.0 volts it may last a long time but it still won’t drive the CAT+ Doppler.

Understanding Voltage and Capacity

An easy way to understand this is to think of a bucket of water with a hole in it. The water coming from the hole falls onto and drives a water wheel. The volume of water in the bucket is the capacity (mAh), and the size of the hole is the voltage (V).

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In this diagram, the smaller hole in the bucket represents a smaller voltage. The capacity is the same for both diagrams, so the bucket on the right will drain faster.

If the hole is small, then the water will last a long time but will only trickle from the bucket, and the resultant stream may not have enough force to turn the wheel. If the hole is bigger, more water flows. This may allow the wheel to turn but will drain the bucket faster.

A battery’s capacity does not tell you the amount of energy it stores or the actual energy it can deliver. Even with good capacity, it’s not possible to know how much energy the battery stores without knowing the voltage. This is because a higher voltage will deliver more energy for a given capacity.

The maths is simple: Energy (Watt-hours) = Capacity (amp-hours) x Voltage (volts)

Power Delivery Profiles

Depending on the chemistry, different rechargeable batteries have different power delivery profiles and can drop a couple of volts almost instantly but then plateau before fading. So even if they start at 9V after charging, they may drop to 7.6V within minutes and then stay at 7.6V for hours. This might appear good, but is no use in the CAT+ Doppler.

Here is just one example of a battery profile this one is from Duracell for a 9V Duracell disposable battery:

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Why We Recommend Vapex Rechargeable Batteries

Rechargeable batteries also only keep their profile and ability for a number of charges, sometimes as few as 100, sometimes as many as 1000.

The Vapex batteries we use will last in normal practice for between 2 and 3 years, however, we recommend that they stay on charge all the time they are not in use. This way they don’t register a charging cycle and stay topped up and ready to go.

We have measured Over 10V from our batteries in our workshop and we know that we can run them at a voltage above the critical 7.6v for hours. This is why we use the batteries we do, and why we recommend you use them too.

If you would like to read more about batteries, take a look at some of the articles from some other reliable sources below:

Electrical Engineering – Why isn’t Voltage considered in mAh ratings?

HB Battery Specialists – The Quick and Easy Guide to 9V Batteries

Kodak Batteries – What is the Difference Between Voltage, Current, Capacity and Power?

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