Battery inside guage vs meter at batteries?

[gnijman]

I very recently bought two new deep cycle batteries, both filled correctly and all charged up (14.7 DCV across the 2 in series) and installed them in my FC 25.

I have the AS plugged in to my 30A service at its pad with the Store/Use switch on Use. After I disconnect shore power to check the battery level and immediatly check the meter over the sink it reads 12.7. When I then go outside right away and put my own meter on the batteries directly I get 14.7 DCV, which is what they read fresh from the battery dealer.

Two questions:

1. When the meter over the sink reads 12.7, is that 12.7 DC volts?

2. Why is the AS inside over the sink meter 12.7 when the batteries read 14.7 on the meter at the batteries?

Thx for any assistance.

Gary

[uncle_bob]

Hi

Ok, let's back up a bit:

Batteries when being charged have a higher voltage on them than when not being charged. That's just how charging works.

Current flowing through the wiring in your trailer induces voltage gain's / drops relative to the battery. Placing voltage meters at different points will give different numbers on the exact same battery at the exact same time.

Battery voltages are temperature dependent. A "full" battery at 70F has a different voltage than one at 100F or at 40F. Lead acid's come in a variety of plate alloys. This impacts the way they change with temperature. There is on single table that works with all lead acid's.

Battery voltage "settles" after charging. This is true with all batteries, not just lead acids. A typical recommendation is to wait at least a half hour to let things settle.

Battery voltage meters come in a variety of accuracy levels. Some are off by a tenth others may be off by a half volt or more. To some degree this is just random chance. Your wall mount meter may work better than mine .... or not ...

The best way to check battery voltage is with a multimeter. You measure directly at the battery posts. In terms of battery health, this is the *only* measure that matters. Even a dirt cheap (< $30) multimeter is likely much more accurate than the normal wall mount meter.

Basic lead acid voltages:

Anything over 13V means the battery is being charged. You are reading the output of the charger. Charging means current through wires. Anything below 12V means stop using. ( this assumes a 70 degree temperature, change the voltages by maybe a half volt for 100 or 40 degrees ).

So what's this all mean?

Voltage is a terrible way to judge how "full" a battery is. It will give you a vague idea, but that's about it. There will always be weird cases that cause the magic meter to lie to you.

If you actually want to know what's going on with your battery: Get a coulomb counting battery monitor. The Victrom BMV 712 is one, there are others.

Fun !!!

Bob

[gator.bigfoot]

Originally Posted by gnijman

I very recently bought two new deep cycle batteries, both filled correctly and all charged up (14.7 DCV across the 2 in series) and installed them in my FC 25.



I have the AS plugged in to my 30A service at its pad with the Store/Use switch on Use. After I disconnect shore power to check the battery level and immediatly check the meter over the sink it reads 12.7. When I then go outside right away and put my own meter on the batteries directly I get 14.7 DCV, which is what they read fresh from the battery dealer.



Two questions:



1. When the meter over the sink reads 12.7, is that 12.7 DC volts?



2. Why is the AS inside over the sink meter 12.7 when the batteries read 14.7 on the meter at the batteries?



Thx for any assistance.



Gary

Are you sure have the 2 in series? If you bought 12v batteries you would get double that in series. I would say they are in parallel. If you measure 14.7 then you should get 14.7 inside as well otherwise it would indicate a very large voltage drop somewhere. If your wiring is good this shouldn't happen. Most likely there is a discrepancy between your meter and the one built in. Also load versus no load could be what you're seeing. So when you measure the voltage at the batteries is that wired in place and the store use switch on? Try to measure at the same point that the built in meter is taking its measurement from. This would take some digging, but if you have an older model it is most likely near the converter. You might see some wire that looks like ethernet or phone cable that is headed to your built in monitor. That would be where they have connected it. Try to find some drawings of the electric circuit or post some pictures as there a ton of people here that will be familiar with your model and be able to help you.

[gnijman]

Originally Posted by gator.bigfoot

Are you sure have the 2 in series? If you bought 12v batteries you would get double that in series. I would say they are in parallel. .

Sorry, but I failed to mention that they are two 6v batteries wired in series ...

[gnijman]

Originally Posted by uncle_bob

Hi

Ok, let's back up a bit:

Batteries when being charged have a higher voltage on them than when not being charged. That's just how charging works.

Current flowing through the wiring in your trailer induces voltage gain's / drops relative to the battery. Placing voltage meters at different points will give different numbers on the exact same battery at the exact same time.

Battery voltages are temperature dependent. A "full" battery at 70F has a different voltage than one at 100F or at 40F. Lead acid's come in a variety of plate alloys. This impacts the way they change with temperature. There is on single table that works with all lead acid's.

Battery voltage "settles" after charging. This is true with all batteries, not just lead acids. A typical recommendation is to wait at least a half hour to let things settle.

Battery voltage meters come in a variety of accuracy levels. Some are off by a tenth others may be off by a half volt or more. To some degree this is just random chance. Your wall mount meter may work better than mine .... or not ...

The best way to check battery voltage is with a multimeter. You measure directly at the battery posts. In terms of battery health, this is the *only* measure that matters. Even a dirt cheap (< $30) multimeter is likely much more accurate than the normal wall mount meter.

Basic lead acid voltages:

Anything over 13V means the battery is being charged. You are reading the output of the charger. Charging means current through wires. Anything below 12V means stop using. ( this assumes a 70 degree temperature, change the voltages by maybe a half volt for 100 or 40 degrees ).

So what's this all mean?

Voltage is a terrible way to judge how "full" a battery is. It will give you a vague idea, but that's about it. There will always be weird cases that cause the magic meter to lie to you.

If you actually want to know what's going on with your battery: Get a coulomb counting battery monitor. The Victrom BMV 712 is one, there are others.

Fun !!!

Bob

Great information Bob. Tomorrow I will ensure I take my readings after the batteries have rested post-charge and will post the results. I have a good multi-meter courtesy of my electrician brother.

Thanks,

Gary

[GammaDog]

Here in the forums, questions sometimes generate more questions. Actual answers come later.

Neither a multimeter at the battery terminals nor the wall mounted monitor measures the battery voltage. Both measure the 12 volt system voltage. If the only source on the system is the batteries, then you are measuring their contribution. If there are other sources (e.g.: your trailer’s converter or the charge from your tow vehicle through the 7 way plug) you are measuring those sources.

Your lead/acid AGM batteries shouldn’t be capable of delivering 14.7 volts all by themselves. Are you sure you were reading the battery voltage and not the voltage applied to the batteries by your converter? What brand converter are you using? Is it perhaps set for charging Lithium batteries (which are typically charged at 14.4-14.6 volts) as opposed to lead/acid which typically charge at lower voltages and typically with a “profile” of changing voltage?

12.7 volts is a respectable reading for a fully charged lead/acid battery bank that is not under load and has been resting at a normal temperature (see Uncle Bob’s post above).

[gnijman]

Originally Posted by GammaDog

Here in the forums, questions sometimes generate more questions. Actual answers come later.

Neither a multimeter at the battery terminals nor the wall mounted monitor measures the battery voltage. Both measure the 12 volt system voltage. If the only source on the system is the batteries, then you are measuring their contribution. If there are other sources (e.g.: your trailer’s converter or the charge from your tow vehicle through the 7 way plug) you are measuring those sources.

Your lead/acid AGM batteries shouldn’t be capable of delivering 14.7 volts all by themselves. Are you sure you were reading the battery voltage and not the voltage applied to the batteries by your converter? What brand converter are you using? Is it perhaps set for charging Lithium batteries (which are typically charged at 14.4-14.6 volts) as opposed to lead/acid which typically charge at lower voltages and typically with a “profile” of changing voltage?

12.7 volts is a respectable reading for a fully charged lead/acid battery bank that is not under load and has been resting at a normal temperature (see Uncle Bob’s post above).

I have the stock Parallax 8300 converter that shipped with our 2016 FC 25 bought new.

I left the Use/Store button in Store for the last 24 hours. With the shore power disconnected I just now switched to Use and after hearing the converter “whirr up” I checked the meter inside and it read 12.7. I went to the batteries (two 6 volt in series) and with a multimeter read the DC voltage and got 15.8 DCV. Yep, no typo, 15.8.

With shore power disconnected I then switched to “Store” and as expected the inside meter registers nothing. At the batteries it read 15.2.

It’s pretty warm and humid today (26C or roughly 82F).

Gary

[gator.bigfoot]

Originally Posted by gnijman

I have the stock Parallax 8300 converter that shipped with our 2016 FC 25 bought new.



I left the Use/Store button in Store for the last 24 hours. With the shore power disconnected I just now switched to Use and after hearing the converter “whirr up” I checked the meter inside and it read 12.7. I went to the batteries (two 6 volt in series) and with a multimeter read the DC voltage and got 15.8 DCV. Yep, no typo, 15.8.



With shore power disconnected I then switched to “Store” and as expected the inside meter registers nothing. At the batteries it read 15.2.



It’s pretty warm and humid today (26C or roughly 82F).



Gary

Ok perhaps we should back up a bit. Lets measure the whole thing again and leave the shore power disconnected. Do you have solar? If so can you measure in the dark to ensure that the solar controller isn't also contributing to the readings.

If you have two 6 volt batteries can you measure across each battery as well. That way we know what each battery is contributing to the total amount. The sum of each battery should equal the total voltage across the 2.

[gnijman]

Originally Posted by gator.bigfoot

Ok perhaps we should back up a bit. Lets measure the whole thing again and leave the shore power disconnected. Do you have solar? If so can you measure in the dark to ensure that the solar controller isn't also contributing to the readings.

If you have two 6 volt batteries can you measure across each battery as well. That way we know what each battery is contributing to the total amount. The sum of each battery should equal the total voltage across the 2.

With shore power disconnected we get the same results. My last post did Mention that shore power was not connected.

No solar panels on this AS. I have portable ones that I can set up at a campsite if required but none hooked up through all this testing and the panels are in the garage, stored.

I did measure the voltage across each battery separately and each read one-half of the voltage across two in series.

Gary

[GammaDog]

It sounds like something is “off” here. Maybe your multimeter, maybe your batteries, maybe your converter. Maybe all three!

The very first sentence in this thread reads “ I very recently bought two new deep cycle batteries, both filled correctly and all charged up (14.7 DCV across the 2 in series) and installed them in my FC 25.”

Two, suitable, six volt, lead/acid batteries in series should not produce 14.7 volts when fully charged. Can you elaborate on the batteries? E.g.: manufacturer, model, specs?

Did you ever run tests like this on your original batteries? What voltage readings did you get?

Did you (or whoever installed the new batteries) make any other wiring changes, or was this a simple heart transplant? Since the original Interstates were 12 volt in parallel, at least some wiring change was needed. Ca you post pics of the new battery tops and wiring?

Have you tried a different multimeter?

When you ran the test with shore power plugged in and the use/store switch in “store”, then you turned the switch to “use”, the gauge should have shown something higher than 12.7 volts. Depending on the model and vintage of the Parallax converter you have, 13.4 or higher would be expected with shore power on and the converter running.

[uncle_bob]

Hi

If you are measuring a resting lead acid at 15.8V, your multimeter is broken.

If you are measuring the output of your converter with an accurate multimeter and it reads 15.8V, your converter is broken.

Bob

[gnijman]

Originally Posted by GammaDog

It sounds like something is “off” here. Maybe your multimeter, maybe your batteries, maybe your converter. Maybe all three!

The very first sentence in this thread reads “ I very recently bought two new deep cycle batteries, both filled correctly and all charged up (14.7 DCV across the 2 in series) and installed them in my FC 25.”

Two, suitable, six volt, lead/acid batteries in series should not produce 14.7 volts when fully charged. Can you elaborate on the batteries? E.g.: manufacturer, model, specs?

Did you ever run tests like this on your original batteries? What voltage readings did you get?

Did you (or whoever installed the new batteries) make any other wiring changes, or was this a simple heart transplant? Since the original Interstates were 12 volt in parallel, at least some wiring change was needed. Ca you post pics of the new battery tops and wiring?

Have you tried a different multimeter?

When you ran the test with shore power plugged in and the use/store switch in “store”, then you turned the switch to “use”, the gauge should have shown something higher than 12.7 volts. Depending on the model and vintage of the Parallax converter you have, 13.4 or higher would be expected with shore power on and the converter running.

I just used my multimeter on the AS, our SUV and my truck batteries and got this:

AS 16.0
SUV 15.5
Truck 15.5

I think my multimeter may be shot and will borrow a neighbour's tonight. He's a electronic's whiz so his should be good.

But to answer your questions:

- I had 2x 6v batteries in series from the dealer. They put them in. I put the new 6v batteries in myself after taking a picture of the original wiring and replicating it exactly using the existing cabling after cleaning the cable ends and adding a dialectric wipe. If the original wiring by the dealer was correct I am confident I have replicated it correctly.
- my multimeter read these batteries new from the store at 14.7, sitting on my garage floor (one terminal joined + to - with the original short cable and the other +/- bridged by my meter.
- When I ran the test with shore power plugged in and the use/store switch in “store”, and turned the switch to “use”, the gauge showed 13.5 or higher, as expected with shore power on and the converter running

I hope the issue lies with the multimeter. Will advise results of test with neighbour's tester.

Thanks again for this great help and your time.

Gary

[uncle_bob]

Hi

That meter is toast. Head over and replace it. If you want one that likely will last a long time, get a Fluke. If you want to save money, that's fine as well.

Bob

[gnijman]

Originally Posted by uncle_bob

Hi

That meter is toast. Head over and replace it. If you want one that likely will last a long time, get a Fluke. If you want to save money, that's fine as well.

Bob

Bob, you are correct. My friend had two meters and both read 12.7 at the batteries when mine read 15.5. The meter inside (no shore power connection) read 12.7 also. Mystery solved.

Thank you all who replied as I’ve learned a lot. Airforum members have a great wealth of knowledge and are giving of their time.

Now to buy a new meter (available in Canada).

Gary

[uncle_bob]

Originally Posted by gnijman

Bob, you are correct. My friend had two meters and both read 12.7 at the batteries when mine read 15.5. The meter inside (no shore power connection) read 12.7 also. Mystery solved.

Thank you all who replied as I’ve learned a lot. Airforum members have a great wealth of knowledge and are giving of their time.

Now to buy a new meter (available in Canada).

Gary

Hi

AFIK Fluke sells worldwide. Once they open the boarder, their main factory is just a bit south of you

Bob