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Old 10-30-2020, 11:20 AM   #1
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Use Battery Boost every time to start

I have never been impressed with the way the CAT C9 400hp engine starts when cold. Cold for me is 50~60 degrees. It does start but is very slow compared to anything that I have experienced.

So yesterday just for the fun of it I hung a clamp tester on my lithium house battery and set it to remember max current.

Held down the boost switch for 10 second then started the engine.

The engine turned over like it was already warmed up!!

The amp meter read 201 amps, a significant contribution to the starter which is rated at 5KW or about 416 amps, not to mention the initial contact current. Note the boost relay is rated for 200 amps and that circuit has a 200 amp resettable fuse/beaker. The Battery is a big 8D rated at 1450 CC


Now you may say I have a low battery but the battery is good just a year old and not ever run down. Cables are all tight and not corroded and this has been unchanged for more that a year. There is a small chance my starter is tired, but know it has worked this way for 1.5 years that I have owned the RV.

So why not use the battery boost every time. I was told one company recommends it in the manual.

The boost solenoid does connect both set together anyway after the oil pressure comes up to charge both battery sets.

I only see upside to this process, higher voltage to the starter motor so less lugging and lower current. Faster starting engine can only be better for it. Less load on the starter battery. And if you are in real cold environment then the lead acid battery may only provide 50% of the ratting.

Many larger trucks will run with 2 -8D batteries

Give it a try and say what happens for you.
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Old 10-30-2020, 11:28 AM   #2
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I would check the voltage at the battery when cranking then the voltage at the starter when cranking you may have a bad connection or a bad cable
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Old 10-30-2020, 11:41 AM   #3
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I would check the voltage at the battery when cranking then the voltage at the starter when cranking you may have a bad connection or a bad cable
If the cable was in fact bad then there would not be any change with the boost because the boost cable goes to the battery terminal, not the starter.
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Old 10-30-2020, 12:25 PM   #4
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Yes it will start faster because of the increased amps, but mostly because of the increased voltage of the lithium batteries. Not sure if your alternator is charging your lithium batteries fully when solenoid connects both together though.
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Old 10-30-2020, 12:28 PM   #5
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The starter motor is not an induction motor like your A/C motor. Lower voltage causes increased current in induction motors. Higher voltage at starter motor causes increased current.

In the good (bad) old days, starters and batteries were designed to work together. The 12 volt battery voltage might be expected to drop to 9 volts and the starter motor may have been designed to work at 9 volts. This arrangement was a way to maximize power output of the system. This strange concept defeated me in physics class. It took a second time through for me to understand it.

Higher voltage and thus higher current increases heat in the motor. As long as starter run time is short, the heat will not be a problem.

I can't say about modern starting systems. Lots of things have been changed since I was last involved in such things.

A lithium battery has a shallow discharge voltage curve. It will deliver high currents with only minor voltage drop. I would expect it to provide a high proportion of the current needed when used in parallel with a lead acid battery.

Hopefully you are not overloading the lithium battery. Wire and contact resistances between lithium battery and lead acid will cause some voltage drop and limit the lithium battery contribution.

I wish you good luck and happy trails ahead!
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Old 10-30-2020, 01:00 PM   #6
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The starter motor is not an induction motor like your A/C motor. Lower voltage causes increased current in induction motors. Higher voltage at starter motor causes increased current.

In the good (bad) old days, starters and batteries were designed to work together. The 12 volt battery voltage might be expected to drop to 9 volts and the starter motor may have been designed to work at 9 volts. This arrangement was a way to maximize power output of the system. This strange concept defeated me in physics class. It took a second time through for me to understand it.

Higher voltage and thus higher current increases heat in the motor. As long as starter run time is short, the heat will not be a problem.

I can't say about modern starting systems. Lots of things have been changed since I was last involved in such things.

A lithium battery has a shallow discharge voltage curve. It will deliver high currents with only minor voltage drop. I would expect it to provide a high proportion of the current needed when used in parallel with a lead acid battery.

Hopefully you are not overloading the lithium battery. Wire and contact resistances between lithium battery and lead acid will cause some voltage drop and limit the lithium battery contribution.

I wish you good luck and happy trails ahead!
Yes higher voltage and current will get hotter. The work it does (Watts) is what will move the crank. Higher voltage trades off for lower current but since the performance of the starter increases current will still be higher.
It's all good for the starter and the engine.

RE Lithium, yes it operates at a higher voltage but will still drop at high current loads. My set will allow for 300 amp through the BMI and the wires will carry that load for a short burst.

RE Alternator and lithium - To maximize the charge rate 14.6 volts is preferred but it does just fine at 13.8 to 14 volts and after some time running I see the batteries are not taking any more current.
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Old 10-30-2020, 01:34 PM   #7
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There could still be a problem with the cable connected to the primary start battery or a problem with this battery. If you crank it using only the secondary battery how fast does it crank
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Old 10-30-2020, 01:46 PM   #8
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Yes, start by verifying all connections at the chassis battery as well as positive and ground at starter are clean and tight.


Then call Caterpillar with your engine serial number and ask for their specs on minimum CCA for your C9. My suspect is that a single 8D may be marginal. Throw in a poor connection or two and perhaps slightly under-sized wiring and ..............
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Old 10-30-2020, 02:13 PM   #9
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Seems that most coaches that I see are set up with two GP31 batts at 1000ish CCA ea even though most only have 12mo warranties people have reported getting well over 10yrs on them.
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Old 10-30-2020, 03:03 PM   #10
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There could still be a problem with the cable connected to the primary start battery or a problem with this battery. If you crank it using only the secondary battery how fast does it crank
I am unable to disconnect the 8D easily. The House battery is lithium (560AH) and by itself would not be able to handle the initial load
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Old 10-30-2020, 03:06 PM   #11
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Seems that most coaches that I see are set up with two GP31 batts at 1000ish CCA ea even though most only have 12mo warranties people have reported getting well over 10yrs on them.
The one 8D is 1450 CC. The Lithium set is 560AH, no clue what the CCA would be but the BMI would limit to 300 amp.

More battery can only be good.
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Old 10-30-2020, 03:49 PM   #12
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When I picked up my coach from Country Coach they told me after the wait to start light goes out to hold the Battery Boost for 5 sec then start. My 8D is going on 9 years old plus I use Amsoil 5w40 Syn oil for quicker starts and have no problem starting in cold weather.
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Old 11-01-2020, 06:58 AM   #13
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Check the negative cable and connections on the starting battery's. Once had a similar issue and the cable was bad. You can use a set of jumpers to go from the negative on the starter battery to a chassis ground and see if it works better.
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Old 11-01-2020, 08:35 AM   #14
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My c7 will start with the chassi batterys every time but i always use the boost switch when starting spins and starts quicker less time on starter,
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