Testing alternator
 

Okay had a battery to go south so after reeplacing and charging both batteries took truck down to Auto Zone to get them to run an Alternator check, guy asked me which battery was the primary battery well of course I had no idea and told him so he proceeded to connect his tester and told me one battery tested 35 amps and the other 70 amps,
Have the following questions:
1. Is there a primary battery?
2. Does the alternator charge one battery at a time depending on the
batteries condition?
3. Do these results seem okay?

VOM meter shows 13.7 volts on both batteries no load.

they need to test with the truck running and both hooked up when they do the alt test // if load testing they are tied together so unhook the pass side ( easiest and test seperatly )
but I don't suggest running the truck ,, when in doubt pull them out and bring them in to the bench .
do you have the batt light going on?

Buster, you have been 'ad

The batteries are hooked up in parallel.

No "primary".

They should be testing identical if your cabling is not defective and both batteries are brand new, fully charged.

Disconnect both, check shape of cables, esp. resistance.

Then fully charge them, load test them.

IF they pass.

Alternator output --- use a clamp on ammeter to read output with everything running and get voltage at same time.

If the batteries are being drained down one more than the other (and hence a bigger charge going in), chances are, the cable to the other battery is not carrying the weight when heavy drain (starting) is occurring. It can be dirty . bad terminals connections etc.

That explains why it is discharging a lot less, and have less amps going in when alternator is charging --- because it is less discharged, it takes less amps.


See link below.

Both batteries were tested out of truck, replaced one battery and the other tested Ok. All cables are clean and in good condition. Assume you mean "had" not "ad"
I know they are in parallel and was just wanting confirmation that there wasn't some charging circuit I did not know about maybe a solid state switch that switched from one battery to another depending on state of battery charge.
I am aware that the best thing is to replace batteries in pairs but at $75.00 each that just is not an option at this time.

Hook up a volt meter and monitor voltage while you are driving etc.

With full load, every electrical gadget running, at 70mph, you should be somewhere north of 12.8 to 13v

If you are below 12, you got a problem.

Likewise if you are above 14.2

Test the latter (max voltage) with every electrical gadget turned off.

batteries at A$75 each ? nice ... to test your alt ,, make sure your truck has been running for 2 minutes ,, the glow plug drain will confuse the Auto zone tester

What issue are you having?

ps your correct no switching circuit ,isolator etc .. they are hardwired together

Okay that all makes sense thanks guys will check tomorrow..

What you risk:

a) bad battery frying good one

b) alternator frying

c) FICM frying

2 batteries at the same time is soooo cheap compared to the alternatives.

Advice so far has been great, just to add my 2 cents worth. Battery cables can wick battery acid down into the cable, under the insulation. Every thing looks great, will read voltage, but wont carry any amps.
Owners of some diesel have rotten cable clamps to the battery, guess its scary to cut and replace ends, normally there is enough to pull and use new HD truck battery ends.
A cheap, under powered battery can and will effect the second battery. think of old D cell flash light, 1 dead, 1 good cell battery, put them together you get light, but only for a short time before both are bad. Buy the correct or better CCA batteries, replace them in pairs.
Example: Customer wanted one battery, the other did check okay, after the 1 replaced and the truck started the weak one started to swell, smoke. I turned off key but it was a direct internal short. I had to protect myself with a plastic creeper, looking through the handle hole, so I would not have it blow up in my face. (which has happened in the shop) Finally got it loose and tried to carry it outside but the plastic was getting soft and molten, dropped this jewel in a snow bank, and for a hour it boiled, melted, churned. Moral of story, a 5 year old battery is not a good combination with a new one.
HILLBILLY TESTING FOLLOWS, Okay fast check to see if one or both cables. Use a heavy set of jumper cables, connect to alternator 12v output post (12-14 volt) put ground on alternator mounting, connect to battery. If charging, the bad harness or cable ends. One can isolate each battery to do the test as well.

Low voltage is DEATH to diesel starter and FICM

okay, more like a dollars worth of advice instead of 2 cents. hope it helps add to the other great responses.

Where do you get a battery for $75 each?? I do know you want at least 850CCA each for these things...anything smaller is a waste of time.

Crank up your time machine and set it for 2010?