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Measure battery voltage without removing cover!

James Copple

James Copple

Guest
Oct 5, 2014
40
8
57
#1
Hi,

Before every gig, I have been changing my on-board batteries for fear they will go flat at the wrong moment. I was getting bored of this hassle and expense, and so I made a modification to my guitar which I wanted to share to others in the same situation.

I soldered one end of a piece of wire onto the positive battery wire leading to the PCB and the other end onto a screw that holds one end of one of the pickups. Now I can measure the voltage between the head of this screw and the bridge (which is grounded). It takes seconds and I have no need to change the batteries or remove the cover so often. It's great and it was easy.

Just make sure the screw you choose isn't grounded or you'll short your batteries.
 
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#4
If you are that worried maybe you should play passive basses. It seems to me if you have to change your batteries that offten you are leaving you bass plugged in over night. They simply can't stay alive if you do that. I'm a big fan of active/passive switches. I think you would like that too.
 
#5
The OP is apparently that worried about dead batteries, but that doesn't necessarily mean he should play passive basses, although the active/passive mode is always a good idea...

I've gone this route with all 4 of my basses...

Bass Battery Boxes

Makes changing batteries a snap. And this...

John East BLI-01 LED

This is a small LED battery indicator. It only flashes when the battery(ies) is/are getting low and even then, you have several hours before it dies. I've gone for months wondering if the LED is working, then one day, there is goes. And with the battery boxes, changing the batteries is very easy.

Not cheap, but elegant. And I like elegant...:D
 
#6
Geri O said:
The OP is apparently that worried about dead batteries, but that doesn't necessarily mean he should play passive basses, although the active/passive mode is always a good idea...

I've gone this route with all 4 of my basses...

Bass Battery Boxes

Makes changing batteries a snap. And this...

John East BLI-01 LED

This is a small LED battery indicator. It only flashes when the battery(ies) is/are getting low and even then, you have several hours before it dies. I've gone for months wondering if the LED is working, then one day, there is goes. And with the battery boxes, changing the batteries is very easy.

Not cheap, but elegant. And I like elegant...:D
Click to expand...
I only metioned it because some people don't know they like passive basses because they have never played a good one. If you do like them why hassle with batteries?

I have to +1 the battery box idea. Makes changing batteries fun... almost.
 
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#8
James Copple said:
Hmmm... I tried that and could not measure a voltage as you describe.
Click to expand...
There might be some other issue then; I just checked a bass with this method a moment ago to be sure I wasn't crazy, worked just fine.

What instrument is it? A few use a weird method whereby the hot of the 9V is switched, and those won't show voltage with this trick.
 
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#9
walterw said:
There might be some other issue then; I just checked a bass with this method a moment ago to be sure I wasn't crazy, worked just fine.

What instrument is it? A few use a weird method whereby the hot of the 9V is switched, and those won't show voltage with this trick.
Click to expand...

There's a battery meter for sale that's attached to a plug that can measure the battery strength, I cannot for the life of me remember who makes it now. I'm sure Walter knows...

And it's mentioned that it doesn't work with all basses because of a weird way those basses are wired. You might be running into that.
 
#12
Geri O said:
I must be uber-paranoid.

I changed mine every 4 months or so until I got the weak battery LED indicator.
Click to expand...
I left a battery in for almost two years one time. Yes it was my first active and I had no idea the battery was in there until it started to die. I know I left it pluged in several times too. I don't understand how it lasted so long.
 
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#13
OOD said:
I left a battery in for almost two years one time. Yes it was my first active and I had no idea the battery was in there until it started to die. I know I left it pluged in several times too. I don't understand how it lasted so long.
Click to expand...

It's because of the very low current draw of the preamps. Actually, I worry more about a battery's self-discharge, which leads me to a question I've wanted to ask for a while.

Does it seem like batteries made for the last few years or so suffer from self-discharge more than in years past? Especially with AAs. Might be just me.
 
#14
If you are really worried about your battery life and especially if your bass does not have an active/passive switch that will let you play on with a dead battery, then having some means of checking your battery voltage is a very good thing. The OP's method is fine and in theory it will work with any bass. The method of checking the battery through the output jack does work on my active Fenders and on the Ibanez SR505 I once owned. There is indeed a meter built especially for this application, that link goes to Amazon but of course it is available at many places and I am not shilling for Amazon, use the one you like best. Oh and of course eventually that link will break so for the benefit of future generations of bassists the meter is called the Batt-O-Meter and is made by Keith McMillen Instruments. As noted above you can also test batteries in most basses with a simple voltmeter connected between the "ring" and "sleeve" of a stereo (aka TRS) cable plugged into your bass. Whether you use the Batt-O-Meter or an ordinary voltmeter there will be some voltage loss in the preamp when you measure the battery voltage in this way. On the basses I have checked it is between half a volt and one volt of loss, so you need to add that value to the voltage that you measure to get the true battery voltage. You can compare inside (directly across the battery) and outside (through the output jack) measurements to get the value for your particular bass. This value could vary a fair amount from preamp design to preamp design and obviously the Batt-O-Meter either ignores this or uses some average value to estimate the voltage.

Personally, I have been using a home made circuit to monitor the battery voltage in my basses and light an LED in some pattern to notify me that the batteries are approaching their end of life. Essentially the same thing as the John East device although this is the first time I have seen mention of it. Works pretty well in my 9 V bass. Not so well in my 18 V bass. I think I need to increase the trip voltage in my 18 V bass. Being cheap I set it pretty low, maybe 6 or 7 volts. Trouble with that is that the two batteries in series in an 18 V bass do not have identical capacities and so for the second time now I have seen my bass go dead (it has an A/P switch so I am good) without triggering the circuit. What appears to be happening is that one battery is still above my trip point while the other is not only dead, zero volts, it gets reverse charged by the current the other battery is sourcing. When I first plug in the bass the good battery is powering it normally and my LED gives me the good to go sign. I've been playing a lot in passive mode lately and I ignore the battery monitor while playing. While I am playing the dead battery's reverse voltage rises and eventually must trigger my circuit but I don't notice this while playing in passive mode. Typically by the time I stop playing the dead battery's voltage is so high the circuit no longer functions so the circuit is still not giving me any warning. By the next time I play the bass the dead battery has recovered enough to allow the cycle to repeat. So a word the wise DIY battery monitor builder: set the trip point ABOVE the 9.5 V a single fresh battery gives you. I think this will guarantee that your circuit will function as you expect and by the time the series voltage gets down to 10 or 12 volts, you have gotten all the good out of those batteries. If you try to push the trip point lower to get more out of the batteries you might as well not bother to build the circuit!

If you believe the manufacturers the shelf life of current alkaline batteries is better than it ever has been, for the name brand batteries anyway. If you believe them....
 
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#19
I encourage you all to get a volt meter of some sort. I often see people say, change your battery every 6 months, or once a year or whatever. The simple fact is that onboard electronics vary enormously in current draw and gain, pickups vary enormously in output, players vary enourmously in technique and practice habits. And batteries themselves vary in energy density. All of these things affect battery life.
 
#20
iiipopes said:
Got a link?
Click to expand...

ddnidd1 said:
This is the Batt-O-Meter. Where is it $10?
Click to expand...

The link Khutch gave goes to Amazon.ca and it shows as $10.93 + free shipping (over $25.) I don't know where it goes for you guys but that is what I get.

EDIT: Looking at the "view more" part, they seem to retail around $40. I wanted free shipping... so I just grabbed 3.
 
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