Again, this is my first diesel and looking under the hood lastnight I noticed there is two batteries. Why?

 

2 bats because:

1. Glow plug system (8 GP's) = 8 electric heaters = lots of current
2. Diesels use compression to ignite the fuel = hard cranking = more current

Hope that clears it up. BTW - If you replace batteries replace them as a set of two as a weak one will drag the other one down with it.

DG

 

Also - lots of Amps required to move a 3000 # motor up to cranking speed.

 

So that splains it. Good question gman. Thanks dagrin99

 

In addition, when the batteries start to go dead, it will still turn the motor over but not have enough juice to fire the 115v injectors.

 

+1. Happened 2 weeks ago. New batteries ran $170

 

The injectors are NOT 115V, only the glow plugs are... and they only stay on for two minutes after you turn the key on. The two batteries are for the glow plug system and to turn the monster over. Diesel burns from heat and pressure, not spark, and it takes a huge amount of compression to compress the mixture to fire. Hence the two batteries to start... especially when it's cold!

 

these truck have solenoids on the injectors. that run off battery power and signal from the idm. look at my pics to see one if you never have.

so in a nutshell you need them for
heating
cranking
and firing the injectors
good day mates

 

They glow plugs are 12v, they just draw lots of amps. The injectors ARE 115 volts...thats what the injector driver modules function is....to produce the firing voltage for the injectors.

 

Actually, the glow plugs are 12 volts. The injector solenoids are around the 115 volt mark, but that voltage is not supplied directly by the batteries, but is stepped up by the IDM.

The engine could be started by one battery if it were a huge battery with a LOT more CCA than our meager, car-sized batteries have. By paralleling two of them, a lot less voltage droop is experienced during the starting process. Our batteries are a lot more available commercially, so it's what Ford chose to use. Makes it a lot easier for us to find replacement units.

Pop

 

Cookie, you posted while I was composin'.

Cookie's absolutely right!

Pop

 

Like the others have said....2 batteries are for the injectors they need quite a bit of AC voltage (I thought is was 130v...but 115v could be correct). It takes about 1000 amps at 12v DC to get enough juice to fire the injectors. One really strong battery could fire the engine, but that would be pushing things. When the batteries run down....the engine will crank over very fast and still not start. The glow plugs do not draw too much power.

 

OOPS my bad... injectors are 110V! But the glow plugs are 110V too. Bet me on this one! All that crap under the valve cover is.

 

Injectors on the 7.3L draw very little power, it takes almost nothing to signal the injector and to return the signal to the control module, oil pressure does the work. But the glow plugs are 8 little 115v heaters and take a serious draw on batts to come up to temp. Clarificaton... the amps from the injectors are very low, the amps used by the glow plugs very high. It's the amp draw that drains the battery not the voltage (in and of itself)

The glow plugs and the starter (because of the high compression) is what puts the draw over the capacity of most normal batts. Vehicle batts with high cold cranking amps but low reserve capacity (normal in autos and light/medium duty trucks) should never be drawn down below 20% of capacity (therefore you need two to compensate), they are made to take a lot of small drain and recharge cycles.

Deep cycle on the other hand can take up to 80% drain without damage, but they usually have lower CCA available, therefore not necessarily powerful enough for the draw of starting a big diesel. They also don't like numerous small drain and recharge cycles, they prefer deeper draw and slower charge than normal auto batts.

If the glow plugs were taken out of the mix, one battery would be able to handle it.

 

The glow plugs are 12 volt heaters, not 115.

Pop