Voltage to change batteries

[Togomi]

Hi,

 

Which is the minimum voltage to change the batteries in an active bass if it works at 18V?

It's Fender Precision American Elite.

 

I forgot to check them and yerterday I had to change to passive.

I've checked batteries today and they have 10,42V. Too low.

 

One IA says that it works well till 12-14V.

I would like to check in order to avoid what happened yesterday.

 

I've had to open back tap. Is there any way to check it without opening the back tap?

 

Thanks in advance

 

[HeadlessBassist]

Usually you can tell when they're going, as your signal starts to aurally disappear through several frequencies. Some instruments emit a high pitched whine when the batteries are getting low, but not usually Fenders. How long have they been in the bass / how long on average is it plugged in? I usually habitually change the batteries in my active basses annually, and I never leave the cable plugged in when I'm not using the bass, as I don't want to get caught short on a professional engagement.

 

Nine volt batteries usually stay at 9 for a very long time. I'd say that yes, 12-14V would be the minimum for an 18V preamp to run at maximum efficiency.

[Hellzero]

It all depends on the opamp used, some will still work with very low voltage...

[Woodinblack]

9V batteries stay higher than 9v most of their life. if I was measuring a battery to see if it was good, and it was a normal PP3, If I measured it and it was below 9v I would throw it away.

[Hellzero]

9.6 to 9.8 Volts is the standard reading of a brand new quality (9 Volts) battery.

[Stub Mandrel]

The problem is that at the end of their useful life 9V PP3 batteries tend to 'fall off a cliff' in higher current use. Hence the familiar rapid onset of fartiness halfway through a set.

 

They usually show well above 9V off load even when fairly well discharged.

 

What really matters is the on load voltage. A better quality battery e.g. a good alkaline one, will go to a lower on load voltage before it suddenly drops.

 

The load varies between preamp designs.

 

A preamp taking a smaller current will cause less voltage drop and therefore work with a weaker battery.

 

They can also recover a bit when unused.

 

So the voltage level a gig or two before the battery fails depends on the quality of the battery, the preamp current requirement and the frequency of use.  This graph is at an unrealistic 100mA discharge rate. Much lower rates would be found in mist of our applications (except old school digital effects).

 

 

 

The theoretical ideal would be to measure the voltage with the preamp on and keep records for each battery brand until you know when it's likely to fail.

 

Thinking about it, a sensible compromise would be to use older batteries for practice/rehearsal and keep fresher ones for gigs. You could keep track of which batteries are freshest by measuring their voltage.

 

I'll probably start doing this!

 

 

 

P.S. I have an active Hohner with a red LED for power on. The LED probably uses more power than the preamp... 

[Woodinblack]

43 minutes ago, Stub Mandrel said:

They can also recover a bit when unused.

 

I have never known that, they seem to go down when unused for me

 

43 minutes ago, Stub Mandrel said:

Thinking about it, a sensible compromise would be to use older batteries for practice/rehearsal and keep fresher ones for gigs. You could keep track of which batteries are freshest by measuring their voltage.

 

Its not an economy I understand - I get a box of 10 batteries off ebay and if I ever measure anything less than 9V I throw the battery out (recycle obvs) and put a new one in, maybe environmentally not as good, but I would never put a known dying battery in a bass for a practice or any other reason, I do have a list of when I last changed them though, there are a few batteries that are several years old.

 

43 minutes ago, Stub Mandrel said:

P.S. I have an active Hohner with a red LED for power on. The LED probably uses more power than the preamp... 

 

I have a hohner with a red LED for power on.. however, it has no power or circuit board!

[Togomi]

Thanks everybody.

 

I've seen a video on youtube and one guy used a stero jack/TRS to check the batteries without opening the cover tap of the batteries.

 

Does it work always or only with some basses?

 

https://www.youtube.com/watch?v=kyUWE0JLpVU&t=307s

Edited yesterday at 12:00 by Togomi

[Hellzero]

For those interested, there's this perfect device, called Keith McMillen Batt-o-Meter, which can test batteries inside an instrument or pedal, outside them and even self test itself, which means it was really well thought.

 

Not very expensive and, to me, worth the price.

 

You can get it for £24,4 GBP from Thomann (and others): https://www.thomann.co.uk/keith_mcmillen_batt_o_meter.htm

 

Here is an old review:

 

 

[Stub Mandrel]

7 hours ago, Woodinblack said:

I have never known that, they seem to go down when unused for me

 

Typically, the voltage of an alkkaline cell rebounds when taken off load quite fast, then it slows down over a longer period, so if you measure when they come out of the bass after a gig it may have gone up a bit by the next one. It doesn't really affect the capacity but can mean a battery you thought was flat can appear to work OK for a while. Obviously, over months/years they self-discharge which is a different effect.

[dudewheresmybass]

precisely this

4 hours ago, Hellzero said:

For those interested, there's this perfect device, called Keith McMillen Batt-o-Meter, which can test batteries inside an instrument or pedal, outside them and even self test itself, which means it was really well thought.

 

Not very expensive and, to me, worth the price.

 

You can get it for £24,4 GBP from Thomann (and others): https://www.thomann.co.uk/keith_mcmillen_batt_o_meter.htm

 

Here is an old review:

 

 

 

[Togomi]

I've got a multimeter, so I am not gonna buy anything.

I would like to know the recommended voltage to change the batteries and if it is ok to measure the voltage with a trs Jack.

 

Greets

[tauzero]

You can get a reading from a multimeter between ring and sleeve which will be proportionate to the battery voltage but not equal to it.

 

I replace batteries when they're somewhat under 9V, if I spot they're below 8V I'll replace them otherwise I'll replace them when they die.

 

KMI Batt-O-Meter incoming, I'll report on it when it's here.

[MOSCOWBASS]

The Batt o meter only reads 9 volt batteries in your bass. If you have 18 volt series batteries you have to take them out the bass to test them.

[Hellzero]

23 minutes ago, MOSCOWBASS said:

The Batt o meter only reads 9 volt batteries in your bass. If you have 18 volt series batteries you have to take them out the bass to test them.

To have a precise reading, yes, otherwise, it will show it's maximum reading voltage, which is 10.23 Volts.

 

"The highest voltage the Batt-O-Meter can test is 10.23 volts. If you have a device/ instrument that runs on two 9 volt batteries, the Batt-O-Meter will display the highest voltage it can recognize (10.23 volts), although the voltage may be higher."

[Togomi]

But are you talking about Batt-O-Meter by Keith McMillen or any multimeter?

 

I tested them connected in the bass, in my American Deluxe Jazz Bass, and voltage was 14,60V, without disconnecting the batteries.

I disconnected both, and the voltage was 8.17V in both (16.34V).

 

I tested in American Elite Precision and connected 10,61V, disconnected 11.65V (5.80+5.85). I had to change this bass to pasive last Saturday.

 

So, it's not a good idea to test batteries connected to active previous?

 

I wanted to test them regularly without openning the tap. 

Is it not possible like in this video with a TRS jack?

 

https://www.youtube.com/watch?v=kyUWE0JLpVU&t=371s

 

Thanks a lot

Edited 8 hours ago by Togomi

[tauzero]

4 hours ago, MOSCOWBASS said:

The Batt o meter only reads 9 volt batteries in your bass. If you have 18 volt series batteries you have to take them out the bass to test them.

 

That's OK, I saw that. My most recent acquisition is the only one of my basses which is 18V, so it'll be fine for 97% of them (and the 9V powered Variaxes as well).

[tauzero]

1 hour ago, Togomi said:

But are you talking about Batt-O-Meter by Keith McMillen or any multimeter?

 

If you use any multimeter, it will read the voltage between ring and sleeve. That will depend on the impedance of the multimeter and of the preamp (the preamp impedance is actually the inverse of the current draw). A digital multimeter will have a very high impedance. If a preamp has a current draw of 1mA then it will have an effective impedance of 9kΩ and with a digital multimeter with an impedance of perhaps 1GΩ the potential divider between the two will result in the multimeter reading pretty much the same as the battery voltage - however, this is under no-load conditions. To read it under loaded conditions, you'd need to plug a mono jack plug into the output socket to switch the bass on, and then measure the battery voltage with the battery attached to the bass.

[nekomatic]

1 hour ago, Togomi said:

I tested in American Elite Precision and connected 10,61V, disconnected 11.65V (5.80+5.85). I had to change this bass to pasive last Saturday.

 

So, it's not a good idea to test batteries connected to active previous?

 

I wanted to test them regularly without openning the tap. 

Is it not possible like in this video with a TRS jack?

As the video says, there may be things like reverse polarity protection diodes that cause a difference between the voltage you measure from a stereo plug and the actual open-circuit voltage of the battery. The lower the current that your preamp needs to operate, the greater the voltage error might be. Also by the time your battery gets below 7 volts or so it is essentially dead so even a very small current may cause its voltage to drop significantly. 
 

At the end of the day though it doesn’t really matter whether what you measure is exactly the actual battery voltage, only that you figure out what voltage measured at the jack corresponds to ‘dead battery’ versus ‘not dead yet battery’, which you have to do by trial and error because it will depend on your preamp circuit anyway, then compare your measurement with that threshold. 

[Jo.gwillim]

On 23/06/2025 at 12:59, Togomi said:

Thanks everybody.

 

I've seen a video on youtube and one guy used a stero jack/TRS to check the batteries without opening the cover tap of the batteries.

 

Does it work always or only with some basses?

 

https://www.youtube.com/watch?v=kyUWE0JLpVU&t=307s

It's usually worked for me except on my stingray special, I think there's some extra jiggery pokery in there. 

[Jo.gwillim]

There's 2 great things you get on an acoustic guitar with a pickup that you never see on bass guitars:

A battery level indication and tuner. 

Why are these guys things on acoustic guitars but not on basses?

[Hellzero]

1 hour ago, Jo.gwillim said:

It's usually worked for me except on my stingray special, I think there's some extra jiggery pokery in there. 

It's because of the way the output jack is working: it's functioning with an on/off switch inside, so you won't be able to read the voltage with a TRS jack as it does nothing here except turning the preamp on with its tip. 😉

[Stub Mandrel]

5 hours ago, Togomi said:

I tested them connected in the bass, in my American Deluxe Jazz Bass, and voltage was 14,60V, without disconnecting the batteries.

I disconnected both, and the voltage was 8.17V in both (16.34V).

 

I tested in American Elite Precision and connected 10,61V, disconnected 11.65V (5.80+5.85). I had to change this bass to pasive last Saturday.

 

Both  a good example of how far the batteries can discharge in a low-current application and still function (the Precision ones were probably below 8V before they failed completely). It also hints at the rapid collapse in voltage when the batteries fail.

[Stub Mandrel]

Hmm ... an obvious experiment is to substitute a variable supply for the batteries and see how low it can go before the preamp fails.