monitoring battery

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=122288
Printed Date: May 16, 2025 at 1:26 PM

Topic: monitoring battery

Posted By: hideo namigata
Subject: monitoring battery
Date Posted: June 13, 2010 at 10:23 PM

Hi,

Recently, I have replaced my alternater and wated to monitor that the alternator is working well and charging my bettery at its best condition.

To check, I am thnking of installing aftermarket Voltage meter directly connecting to Battery + with the use of the relay and want ot double check if the wiring shown below is correct or not:

85 Acc wire(from radio)

86 Ground & Votage Meter (-)

87 Voltage Meter (+)

30 Batter wire(fused 10A)

Thank you in advance for your kind advise.

Replies:

Posted By: oldspark
Date Posted: June 13, 2010 at 11:47 PM

Normally #85 would be GND but that only matters if internal spike-suppression diodes are fitted, else for conventional reasons.

The #86 to accessories else to IGN if you want to monitor voltage whilst cranking (a good way of monitoring battery and starter condition).

#87 & #30 are fine,
The fuse can probably be much smaller (as long as it handles the meter's current - probably far less than 1A if electronic; micro-Amps if a meter). The fuse is to protect the wiring rather the the meter (since the meters usually can't be fuse protected).

If it's an analog meter (electro-mechanical - not lit nor electronic), you can probably leave it connected straight to the battery - they usually take less current than alternator "S" connections and battery self leakage, and far less than alarms and door remotes. (In that case solder a (say) 1k resistor instead of a fuse - that's more reliable etc.)

Posted By: hideo namigata
Date Posted: June 14, 2010 at 1:25 AM

Oldspark, thank you for the comments.

Posted By: howie ll
Date Posted: June 14, 2010 at 11:46 AM

Why use a relay? Just connect constant to ignition 1 (Doesn't dump whilst cranking) and cold to a good ground.

Posted By: t&t tech
Date Posted: June 14, 2010 at 5:18 PM

You know Howard i was thinking the same thing! KISS! LOL!

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COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF. PSALMS 37:5

Posted By: hideo namigata
Date Posted: June 16, 2010 at 7:05 PM

Thanks guys for all the comments.

I maybe wrong but when monitoring battery, connecting DIRECTLT to battery + ? and if monitoring alternator then connet directly to the Alternater D+ ?. The difference between direct and indirect maybe very small but still you will have less resistance connceting directly to the battery.

So, it is all depneds on what do you wnat to monitor?,

Posted By: oldspark
Date Posted: June 16, 2010 at 7:26 PM

Yes - directly to battery. (The voltmeter is a high-impedance device and takes very little current - often well under 1uA (micro-Amp), though digital meters required extra current to power themselves.)
Hence if a plain meter - don't use the relay; leave it permanently connected. Or if it consumes insignificant power (some LCD voltmeters consume less than a LED).

Monitoring should be done across the battery as the charging system is designed to provide maximum battery life - hence (usually) never more than 14.4V longterm and usually above 13.8V etc.

But monitoring can also be across the alternator to check its performance - ie, does it output voltages that may damage loads (which may not be detected at the battery due to alternator to battery voltage drop - especially during surges.)

Many vehicles have voltmeters across their ignition supply (gauges etc) which may be quite different to the battery (higher or lower)- especially when bad connections exist.

Posted By: hideo namigata
Date Posted: June 17, 2010 at 9:38 PM

Hi, I have last thing to check before doing over the weekend.

The voltmeter that I have ordered is digital one without any wires. and My cars' battery is located in the trunk.

Should I use AWG 24 or AWG 18 between Battery(+) and Voltmeter ?

Thanks in advance

Posted By: oldspark
Date Posted: June 17, 2010 at 9:43 PM

Unless it is a powered device (ie, digital), the gauge won't matter. A hair will do fine (if it's one of those older silver ones - especially if gelled for insulation).