Discussion: Getting Rid of 9V Batteries for my Pinpointer

LiPo with 9V voltage booster for Minelab Pro-Find 35

I really like batteries, with the exception of the 9V battery. It’s expensive and has poor performance with respect to its internal resistance and capacity. In my opinion, nothing designed after 2010 should be using a 9V battery. Maybe some of you out there can think of a reason as to why a particular device, such as a multimeter or smoke detector should still use a 9V battery, but I doubt that’s possible. And if it is possible, it will be an exception.

I also enjoy metal detecting. However, my metal detector (Fisher Research Labs F2) and pinpointer (Minelab Pro-Find 35) both use 9V batteries. My next metal detector will use AA (Garrett AT series or Fisher F75) or a built in lithium (a la Minelab Equinox), but until then, I’m stuck with 9V batteries…or so I thought.

What’s a Pinpointer?

For you readers out there that aren’t familiar with metal detecting, a pinpointer is a handheld device that helps pinpoint a metal object in a hole or plug you just dug. You don’t have to have it while metal detecting, but it saves a lot of time.

The majority of the pinpointers on the market today use 9V batteries. The three major exceptions are Fisher Research Labs’ F-Pulse, White’s Bullseye TRX and XP’s MI-4 and MI-6.

I set out to create an add-on that would allow my Pro-Find 35 to use something other than a 9V battery. This blog post sets out how I did it.

The Set up

A picture is worth a 1,000 words, so I’ll start with pictures (they're upside down for some reason).

The connectors I’m using are Deans and the blue thing on the right is the voltage booster itself. Here are some of its more notable specs and pics:

Minimum operating voltage: 2.5v

Weight: 0.4g

Maximum quiescent current: 2ma

Minimum input current: 1.4a

Switching frequency: 1.3 khz (PWM control)

Here’s a link to where the item itself, in case you want to order one or learn more about it:

https://www.pololu.com/product/2116

Below are some pictures of it installed in my pinpointer:

Performance

I have not tested this in real world conditions, just indoors using the “hour glass and coin” method. Below you can see how it performs with the stock 9V battery:

I didn’t mark the cardboard, but when using the nickel and the voltage booster, the sensitivity improved by about 1-3 mm, i.e. the hourglass enlarged by about 1-3 mm in all directions except the middle bottleneck.

As for run time, that depends on the battery used. I’m currently using what’s in the pictures and it has 750mah in capacity at 3.7 nominal volts. Fully charged it’s around 4v. Using some rough math, this voltage booster is the rough equivalent of a 9V battery with 330mah of capacity. The typical 9V alkaline battery has about 550mah. Therefore, you can expect about 60% of the run time of whatever you’d get with an alkaline 9V battery. But there are at least two caveats to my numbers.

First, the efficiency of the voltage booster. I don’t know what it is, but I wouldn’t be surprised if it’s around 85%-90%. If I’m wrong, feel free to comment.

Second, the relatively low resistance of my set up. When using the stock 9V battery (it’s heavy duty, not alkaline), the battery has trouble delivering the current necessary when the pinpointer goes off. I know this because the LED slightly blinks rapidly. But this voltage booster set up does not result in the LED light slightly blinking at all.

I think my use of a much higher performing battery than the 9V battery (heck, almost any battery will do better than a 9V) at least partially compensates for the efficiency loss of the voltage booster. I think this compensation effect will still exist when using an alkaline battery instead of a heavy duty, although the effect will be less.

What Are the Advantages and Disadvantages of the 9V Booster?

The sole advantage is that you don’t need to use 9V batteries any more. This can make organizing your battery supply that much easier and save money on an expensive battery type. For some of you metal detectorists out there, this might allow you to completely remove 9V batteries from your metal detecting set up.

Numerous disadvantages include:

- You need a special charger. A typical R/C hobby style charge that can handle LiPo cells should suffice.

- It’s a bit janky, at least compared to a self-contained 9V battery. I intend to clean things up a bit as I tweak this design. I should be able to get most of everything to fit inside a gutted 9V battery.

- At least slightly lower capacity/run time. I don’t know this for sure, but I will assume this is the case for now. Under real world conditions, I wouldn’t be surprised if the run time is about the same with my 1S 750mah LiPo battery.

- If you want to avoid using a special charger and use drop-in AAA cells, you need to permanently modify your pinpointer. In my case, I’d need to cut a large hole in the end cap and effectively lengthen it to accommodate the AAA battery holder like this one:

It won’t be hard to do, but I don’t want to do anything to permanently modify my pinpointer while it’s under warranty.

Bottom Line

For most of you, it won’t be worth using this voltage booster. If you already have a hobby R/C charger and soldering tools and supplies, then you might enjoy this little project and can save some money in the long run by getting rid of 9V batteries

But if you hate 9V batteries as much as I do, you’ll definitely want to consider this.