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Voltmeter & Waterlevelmeter

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TheSwede73

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Hi!

This is probably somewhere, but i will ask anyway!
2 things.

Number one, i need a voltmeter for a campingtrailer, a simple way to check the status of 2 batteries.
i will parallell connect two 12v batteries for better stamina, is that okay just to do or should i consider things when doing it?
The meter will have a push button for checking so that it doesnt drain power.

Number two, building a waterlevel meter, simple one with transistors and led's, pushbutton activated too.
i need to know what kind of transistors that works for this.

THANKS!!!
 

tcmtech

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Check out EBay for digital panel meters. They have a store called asiaengineer that sells nice LED and LCD panel type voltmeters for around $10 including shipping. They are very easy to install and work with.
I buy them fairly often now they are easy to modify for other digital readout applications too! The circuit on them is based on a standard 3.5 digit AD converter IC so the circuit schematics are easy to get a hold of.
 

Sceadwian

Banned
It's generally okay to parallel batteries but they both have to be brand new and of the same brand and model. You'll need to fuse each battery separately. What kind of water level are you measuring? You can get simple floats that turn on and off any 120V device, but they need a bit of room so they don't get stuck.
 

MikeMl

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I just put a LCD 3 1/2 digit panel meter in my trailer. It was intrinsically +-200mV full scale, so I just used two 1% resistors to scale the input voltage by 1/100. I even strapped the decimal point so that it reads 12.65 etc.

It is the kind that operates on 5Vdc, so I used an LM78L05 to get that off the same battery that it is metering.

btw: I also wired in a -40A +40A zero-center analog ammeter. It shows the rate of charge when plugged in, and the rate of discharge when in the boonies...
 

TheSwede73

New Member
Thanks!

I just bought a 3 1/2 digit voltmeter from Ebay, its the kind that dosen't require a power source itself, should i go with that or make some additions?

What i basically need is a voltmeter that shows 4 digits..so i dont get the last one... i have two 12 volt batteries, brand new, never used, same brand.. to parallell, for the caravan..
The charger will be the best in the world, the Ctek, swedish made charger, plugged in whenever possible to the batteries.

I just need one voltmeter, right? to show the voltage for the moment.. the best thing would be for it to switch over to the charger or alarm when going to low.

You shouldn't discharge them more than to 10 volts or something, right?


Thanks for all the help! i'm surprised that people just help eachother this much, but i guess that's what it's like to have an interest that you burn for..
 

tcmtech

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Thanks for all the help! i'm surprised that people just help eachother this much, but i guess that's what it's like to have an interest that you burn for..
Careful where you show off your modern day tech stuff. There are some cultures that WILL BURN YOU for it! :eek::D
(Voo Doo devil machinery) :p

The one you got should work perfect.

The 3.5 digits read out as 1999 so you can read up to 19.99 with a 20 volt rated unit. Self powered, Nice! Ya spent the extra few dollars for the good stuff. :p
 

TheSwede73

New Member
Ah, great!, i though the "1/2" digit was a indicator of some kind "v" or something..

Well, i wont post my finished stuff then :)

but not many swedes are members here, so i think im safe :)

Thanks!!
 

MikeMl

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Thanks!
...
I just need one voltmeter, right? to show the voltage for the moment.. the best thing would be for it to switch over to the charger or alarm when going to low.

You shouldn't discharge them more than to 10 volts or something, right?
.
What type of batteries? If flooded-cell lead-acid, then you should not discharge below 1.75 V per cell (10.5V for 6 cells), and only if it is rated for deep-discharge. If it is a (car) starting battery, then you shouldn't be using those for this application.
 
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bryan1

Well-Known Member
For a 12 volt lead acid battery dont take it much below 12 volts for a good long life. Depending on the brand etc lead acids are all in the ballpark 13.8 volts fully charged and 12.8 volt at rest ( unplugged from the charger). The term 'outdoor battery' will most likely mean deep cycle which is designed for RE use NOT car starting. A SG meter is the best option for checking state of charge and if abit unsure of the care make sure you do plenty of research as it would be a shame to kill the batteries after a year.

For getting the battery info if you dont have it try the makers website or ask where you bought them from.

Most of all have heaps of fun and it doesnt take much to get that bug of living off the grid.

Cheers Bryan
 

TheSwede73

New Member
Hi!

Thanks for the advice!

Its a battery for campingtrailers, boats..etc, its called something like "holiday battery" or "freetime" battery here in Sweden, if translated :)

Im buying the super charger from Ctek, it has a reconditioning mode and everything you need to ensure the batterys life, it even works as a battery if something happens with the normal battery.

THe Batteries aren't so expensive, but i would like them to last for some time yes.. it's 2 x 75Ah, going to be parallell connected.


What's a SG meter?
 

MikeMl

Well-Known Member
Most Helpful Member
...
What's a SG meter?
Battery Hydrometer to measure Specific Gravity (SG) of the acid. A battery's State of Charge (SOC) is a direct function of the SG, corrected for ambient temperature. This is the most reliable way of determining the SOC of a flooded-cell (removable caps) lead-acid battery.

Google for a table of SG readings vs SOC, one here
 

tcmtech

Banned
Most Helpful Member
For unknown batteries I have typically put the running load,whatever it will normally power, on them and just let them go.
The voltage will drop very slowly for a while and the reach a point that it drops off much faster. The start of that faster drop rate is typically the point the load should be disconnected.
 
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