The Stovebolt Site Forums The Shop Area The Electrical Bay battery or alternator?

Put in a new 12 volt battery last month. Last night sitting with just the parking lights on and engine NOT running, the truck would not start. Jumped the battery and it started right away. Drove it a mile with headlights on and parked it. Dead again.
Jumped it this morning and its seems fine. Started it about 6 times no problem. When I put the volt meter on the battery it reads 11.45 and when running it showed about 11.25 volts.
I will recheck grounds and belt tension but I believe they are okay.
Is there a way to check the output of the alternator on the truck?

a 12V battery should read over 12V, about 13.8 static and as much as 14.4 when running .... I'd first check all cable connections for cleanliness and tightness if that was the last thing touched - next take it to an auto elec shop to check alt output under different conditions [could be the regulation circuit] and get the battery load tested as well - if it holds a charge for a short time but not overnight it could be a bad cell or other battery internal problem - defective batteries do happen

Bill

It has sat idle since my last post and now the battery still shows 11.40 static and running it is 13.35. I put the headlights on (static) for about 30 seconds and the battery would not turn over. I waited a few minutes and it started.
I will pull the battery and take it back, same with the alternator for testing but its a long drive across town. All the connections are tight and clean and the belt is tight.

You need to put a charger on the battery let it charge overnight. Then check the voltages. It should have about 12.5 static & go up to about 14 when running.

Not sure if this is relevent - the shop that rebuilds my alternators and starters told me you should never use the alternator to recharge a low or "bad" battery because it could fry a diode or two. That's what happened to me before I had an alternator rebuilt.

The shop owner said to fully charge the battery before running the engine with the rebuild - and make sure the battery is good (holds a charge overnight, or a couple of days).

Hope this helps.

I have heard that before as well. Anyway the auto parts checked the battery and said it was taking a charge okay but as it was only a month old gave me a new one. I had the alternator looked at and it was the problem so I changed it.
The irony of it all was I was in a deserted area when it failed and had to take a bus home and wake up the wife. When I was looking at the battery warranty today it said that it included ROADSIDE SERVICE!! I won't tell her though.

Bovine Scatology! An alternator is self-regulating and cannot overload diodes by recharging a low battery! The internal resistance of the stator windings determines how much maximum current the alternator can produce, and the diodes will have at least some overload capacity- - - -sometimes a lot of it! What an alternator won't do is recharge a sulfated battery, one that needs more voltage than the alternator can produce before the regulator kicks in to get the charge started. For instance, a badly sulfated battery might need 16 volts or more just to get a trickle charge going and get the charging process under way. A battery that has been sitting for weeks or months with no cycling has probably become sulfated at least a little- - - -OK for one or two starts, but little or no reserve capacity. The fix is a full discharge, recharge, drain again, recharge, repeat as required.
Jerry

On the contrary Hotrod, I repair alternators for a living and I will say that 90% of alternator failures are due to Bad or Low batterys. An alternators electronic regulator senses output voltage only, so a regulator will feed the fld circut full power to try and bring the system up to the voltage set point, regardless of the Amp output. So in many cases and alternator will work itself to death trying to charge a battery. Diodes will come un-soldered and stators will get so hot the varnish will burn off.
Always use a battery charger to charge your battery. A generator is more likely to slow charge a battery that is if you want to drive for the same length of time that you would slow charge your battery for with a charger.... anyone care to drive for 8-12hrs?? LOL I'll put the coffee on when you get up here to Canada.

Regardless of field magnetism, a stator's internal resistance limits its maximum output. Maybe some of the recent design alternators are cutting the safety margin too close, but the ones I worked on, particularly the big diesel rig alternators, had plenty of over-engineering. As the weight went down, along with the amp capacity going up, I'm sure today's units are working a lot closer to the breaking point than they used to! An old 75-100 amp 10-SI alternator was pretty close to bulletproof unless a battery cell got shorted and put it into full output 100% of the time! Voltage spikes caused by disconnecting a battery cable to "test" the alternator used to fry regulators pretty regularly, though!
Jerry