Switching large currents

I don't see how you reach that conclusion. If a battery is worn out and can only take a limited charge, it might still be enough to start the engine if the battery has that level of charge. Take out 8 amp hours and it might not be.

So the problem can either be inadequate charging, or limited ability to hold charge.

Sylvia.

You either have a bad battery or alternator.

Check the voltage at the battery with the engine running, it should be near 15 volts. If it is under 14 it is definitely not charging enough. I like t o see about 15.2 volts. In the old days that much of a charge would create the need to keep on adding water to the battery, but these sealed cells can take it, and have to because of the computers and all that holding memory. Also, some cars actually pull more current running than starting reverse o f how it was in the old days.

Now if you got more than 14.5 when running, shut it off and see what you go t. You should have over 13 volts. It might drop to 12 overnight but should have the current behind it to start a modern car. The trick here is to meas ure the battery voltage in the morning BEFORE you try to start the car. You might have a bad battery.

If you are really reading 50 mA constant drain, that is not that much. Figu re it out, do the math if you want to be scientific about this. How many am p/hours is the battery ? From that you can figure out how long it will take 50 mA to drain it. Granted, when the thing sits for a year it will have a dead battery, but overnight ?

Bottom line is you most likely simply have a bad battery. You should make s ure the car is charging properly though because if not it can cause a bad b attery. And if the voltage goes over about 15.6 it is overcharging and will wreck batteries in time. You might enjoy being able to crank the radio for hours while you mow the lawn or grill the steaks or whatever, but you'll o nly get a couple of years out of it. Normally a battery in a car lasts eigh t years, sometimes more. In fact take that into consideration as well, how old is that battery ?

One trick, but only do it ONCE is to put epsom salts in it, but the modern ones you would have to drill holes. Then you have to seal them. A big PITA really. I know batteries cost money but you might just have to bit the bull et and buy one. And if you are in the northern hemisphere is will be alot w orse in the winter. Cold kills. When the battery charge is low the electrol yte is less acid and more water which can freeze and wreck it forever. You' ll have to put it on a charger to go to the local convenience store for smo ke ad beer or it won't start.

Bottom line, 50 mA should not kill that battery for at least a week, more a ctually. There is also the possibility you have a bad starter motor. For th e most part newer cars do not pull as much to start as the older ones. this is due to a more radical camshaft. That allows them to have a higher theo retical compression ratio which they can make run smooth by the use of bett er spark and fuel injection. When the RPMs go up though it has power and th e volumetric efficiency is great. So really it doesn't take that much to st art them like a 455 in 1970. I've seen cars and looked at the wires coming off the battery and thought "There is no way" because they were so thin. Bu t they work.

Self discharge is probably more than that in a lead acid.

An *excellent* point! But it doesn't apply in my case as the vehicle is a station wagon.

Good point. Well, it's nothing to do with the battery because it's happened with previous batteries too. And I do *not* buy cheapo batteries. I'll double check the measurement and report back here if my recollection of 50mA turns out to be incorrect.

The alternator output is fine, since the voltage across the battery at tickover is ~13.5V. But as someone else said, it might be the alternator that's causing the issue when the motor's not running. I'll have to look into that.

No, it's charging fine when the motor's running.

Thanks, Paul, I never thought of that! Brilliant!

I'll double-check the measurement with a different meter.

My brother had a similar problem in his Mercedes. The culprit was a bad thermostat on a rear view mirror heater.

I had a small wall wart 15 volt unit that I connected to the battery of my truck when my alternator was not doing the job. The battery didn't let the voltage rise to an unacceptable voltage and the 15 volt unit gave it the full 100 mA or whatever it is rated for. I ran this for some weeks until I finally got a good alternator to put in. I think this thing was less than $4. It's still in my glove box just in case.

Doesn't an SCR have a minimum hold current? One large enough to handle

I assume "tickover" means when it is running. I think 13.6 V is a bit low. Most references say it should be a bit higher. Does it stay at

Either way, you really want to check the voltage with the engine not running. Fully charged it would be around 12.6 volts. If it is about that after you run it enough for the battery to be fully charged, then you have a drain. If the voltage is lower you may have a charging problem or the battery could be bad. Since you have seen this with other batteries I guess that would mean the alternator is not putting out enough current/voltage.

When I lost a diode in my alternator all was well until I ran the thing at night with headlights on. Not enough current to run everything. I put the scope on the battery and it was easy to see one of the three phases was out.

Idling speed/lowest possible engine speed. That is like when you're sitting at a stop light for example; a few hundred RPM only.

Dunno. I'll check.

I'm sure it's more of a drain issue than a charging one. I will double- check the drain rate when it's daylight again.

If that is running, that is not enough. It takes 14.4 minimum to charge the battery properly.

It should be 14.2, although some alternators put out up to 14.5, which is actually a bit high and tends to boil the batteries dry. Anything less than

Jon

If you battery is dead after a couple of weeks at 50 mA, I'd say it is in poor condition.

Let's say your battery is 60 Ah. It would take 25 days to drain half of it, in which case you should still be able to start just fine.

Have you checked the charge voltage? The best battery in the world is no good if it doesn't get charged.

I was given a choice of two different capacities: standard and HD. I went for the HD and if memory serves me right it's 105Ah. I'll check that's the case tomorrow when I'm re-testing it, though.