Battery level on my 308

Sounds a bit low to me. Do you get this reading after disconnecting the ground?

 

It is a low reading. A fully charged battery is 12.6 volts. At 12.4 it is 75% wich is an acceptable usable battery. 12.0 volt is 25%. If your charger is full time on, your battery may have slowly evaporates the water and the electrolyte is below the plates and she is sulfated.

 

Too low for sure. The rule of the thumb as my old physics prof used to say.

These are general voltage ranges for six-cell lead-acid batteries:
Open-circuit (quiescent) at full charge: 12.6 V (2.1V per cell)
Open-circuit at full discharge: 11.7 V ***YOUR TEST VOLTAGE IS HERE ***
Loaded at full discharge: 10.5 V.
Continuous-preservation (float) charging: 13.4 V for gelled electrolyte; 13.5 V for AGM (absorbed glass mat) and 13.9 V for flooded cells
1.All voltages are at 20 °C (68 °F), and must be adjusted −0.0235V/°C for temperature changes.
2.Float voltage recommendations vary, according to the manufacturer's recommendation.
3.Precise float voltage (±0.05 V) is critical to longevity; insufficient voltage (causes sulfation) is almost as detrimental as excessive voltage (causing corrosion and electrolyte loss)
Typical (daily) charging: 14.2 V to 14.4 V (depending on temperature and manufacturer's recommendation)
Equalization charging (for flooded lead acids): 15 V for no more than 2.205 hours. Battery temperature must be absolutely monitored.
Gassing threshold: 14.4 V

 

How did you manage to start the car with a discharged battery? The 14 volts is alternator voltage not battery voltage. If you just disconnected the cable and got 12.4 volts versus 11.7 with the cable connected then your car is drawing 15 amps or so while turned off or your battery is toast.

 

Are you taking the measurement directly on the battery or at a convenient location somewhere else on the car?

 

I think your battery has nothing to do with the 'slight miss,' You're overlooking the obvious!

Have you checked ignition wires and spark plug extenders? Also, might find bad connections in the Digiplexs'.

 

11.7 volts = fully discharged or a relatively heavy load on a partially charged battery while you are checking the voltage. 12.4 volts with no load =1/2 charged (approximately). If the car is easy to start with those voltages I would suspect your voltmeter. No 12volt lead/acid automobile battery will start a car with 11.7 volts across the terminals prior to starting. You can always take the battery to an auto parts store and have them test it under load.

 

He said his battery has 12.4 volt with nothing connected to it. It's 75% charged and usable. Then whem connected it drops, telling something is using power. To make sure nothing is using power from your battery while at rest you should measure the current draw. Make sure everything is OFF. unplug your negative post. Plug a ampmeter in serie between the battery post and the cable. The reading shouldn't be more than 35 miliamps on most older cars. Try it on the higher scale first in case you have a high current flow then move to a lower scale. As per what you described I suspect you'll have a fairly high reading. To find the circuit that is using the power you then unplug one fuse at a time to find the faulty circuit. It can also be a blown diode in the alternator.

 

Good advice, Claude

I'd be interested to know the current draw on our cars (without alarms and any car radio draw). It should be just the clock if I'm not mistaken.

 

The 35 miliamps usually include the clock, radio, ecu memory, etc. A very old car would have zero current draw. Wait until the reading is steady, giving time for the capacitors to get charged.

 

Does a 308 from ~1980 really have something as advanced as ECU persistent memory?!

 

The problem is, most cheap multimeters only have current ranges which go up to 250ma or so. So this means unless you have one with a high current range, if there is a problem causing a large current drain it will immediately burn out the meter.
If you suspect there is a large current draw it better to isolate the problem by measuring the voltage and pull fuses until the battery voltage goes up to the disconnected voltage.

In fact the clock alone, which should be the only load, draws around 300ma which will overload many meters.

 

One quick and dirty way to see if you have parasitic draw on the battery is to wire up a small bulb in series with the battery. If it glows then you have draw. If it glows bright you may have a short somewhere! It is not a quantitative test but a way to protect against blowing a meter. Also, using this with the fuse pulling method mentioned above works as well. In my terrible drawing you can see I use the connector to splice in the bulb so you do not need to disconnect the battery terminals. Key out of switch. If you turn the key you will blow the bulb like a fuse.
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It's usually the clock, that runs it down if car is not in use.

 

The meter I use has a 20A range. But you can also use a clamp meter. Just make sure it is for DC as most are for AC.

Crowndog, the light is a good trick. I use this tool I made to plug in serie in the fuse port to find intermitent short to ground. I wiggle the wires and look at the lamp. very easy to see.

 

I have the trickle charger on mine plus leave the red disconnect Connected, so that way I feel I have a bit of current coming in and a bit going out, all things equal. So far it works well for me.