How to test if starter battery is being discharged

[dmksails]

Hi All -

My boat has a house bank and a starter battery. Theses are controlled with a battery selector switch with the typical 1 - 2 - Both - Off selections. I also have a charger with separate leads to each bank.

I would like to verify that when the switch is in the House bank or Off positions that there is no drain on the starter battery. Any suggestions for how to do that?

[S/V Illusion]

You can check continuity with a simple voltmeter. Or voltage. Or just look at the back of the switch.

[donradcliffe]

There may be loads on the starter battery that bypass the selector switch, like a bilge pump or a CO detector. The most reliable method is to use a clamp on ammeter on the battery cable.

[cofc]

If you don’t have an amp clamp you can charge the starting battery until the charger enters float, let the voltage settle for a couple of hours, and take a voltage reading. Then come back the next day and take a second voltage reading. If they’re the same, nothing is discharging your battery. If they’re not, something is drawing current other than the starter.

[Compass790]

Originally Posted by dmksails

Hi All -

My boat has a house bank and a starter battery. Theses are controlled with a battery selector switch with the typical 1 - 2 - Both - Off selections. I also have a charger with separate leads to each bank.

I would like to verify that when the switch is in the House bank or Off positions that there is no drain on the starter battery. Any suggestions for how to do that?

If you have a clampmeter put it around the starter battery cable or pull a starter battery cable off & check for current flowing between the battery terminal & the lead. Set your multimeter to amps.
If its leaking a small amount of current the clampmeter is not very accurate, need to use multimeter leads.
Thats how I do it but I'm no electrical expert.

[Scorpius]

Originally Posted by Compass790

If you have a clampmeter put it around the starter battery cable or pull a starter battery cable off & check for current flowing between the battery terminal & the lead. Set your multimeter to amps.
If its leaking a small amount of current the clampmeter is not very accurate, need to use multimeter leads.
Thats how I do it but I'm no electrical expert.

Be careful using a multimeter to read current directly off a battery. If it's significant it could easily burn out the multimeter. Most can't handle more than ten amps.

Before using a multimeter I'd put an old-fashioned 12v incandescent light bulb in series with the battery lead (I have, in my electrical tool box, one with two pieces of wire soldered to it for just such occasions). If it lights up, you are drawing significant current: dim: an amp or so. Bright: two amps or more (assuming an automotive tail light bulb). If it's bright, don't use the multi-meter on amps. If it's dim or out, it's safe to use the multimeter.

Also make sure to switch your multimeter OFF amps before using it for anything else or putting it away. If, for example you go to measure voltage somewhere (like across the same battery) and the meter is on amps it will self-destruct for sure and you might get burned fingers in the process.

[Cadence]

Originally Posted by cofc

If you don’t have an amp clamp you can charge the starting battery until the charger enters float, let the voltage settle for a couple of hours, and take a voltage reading. Then come back the next day and take a second voltage reading. If they’re the same, nothing is discharging your battery. If they’re not, something is drawing current other than the starter.

Or the battery has seen better days,

[NorthCoastJoe]

Using a volt meter set on amps should be OK to check for vampire draws the current should be low enough for the meter to handle it. Disconnect a battery terminal and put the meter inline. With everything off you should get zero. An alternator with blow diodes can create a vampire draw or a bad starter solenoid, I have seen both happen.

[Boatyarddog]

Originally Posted by Scorpius

Be careful using a multimeter to read current directly off a battery. If it's significant it could easily burn out the multimeter. Most can't handle more than ten amps.

Before using a multimeter I'd put an old-fashioned 12v incandescent light bulb in series with the battery lead (I have, in my electrical tool box, one with two pieces of wire soldered to it for just such occasions). If it lights up, you are drawing significant current: dim: an amp or so. Bright: two amps or more (assuming an automotive tail light bulb). If it's bright, don't use the multi-meter on amps. If it's dim or out, it's safe to use the multimeter.

Also make sure to switch your multimeter OFF amps before using it for anything else or putting it away. If, for example you go to measure voltage somewhere (like across the same battery) and the meter is on amps it will self-destruct for sure and you might get burned fingers in the process.

Most multimeters employ a fuse built in, and will blow that before the meter.
You can use a test lite as mentioned, pull the start battery + lead and connect your ground(black) lead to the + battery terminal at the battery and your + (red) lead to the cable you removed.
Set your scale to auto volts or 20 volts dc.
Should be less than .5 volts, closer to .3 or 0vts.
If your using the test lite, again, hook it up in series, should not light or be very dim.
Any more and you've got a draw.
SV Cloud Duster

[Thumbs Up]

The simplest test would be to look for a spark when connecting or disconnecting either battery terminal.