Deep cycle cranking

[Thos]

I would like to use the two SOK batteries in my motorhome to start the engine. I can run a 4ga cable from the positive post on my batteries to the place on the engine where the original engine cranking battery's positive cable was connected. Ofcourse the original engine cranking battery was grounded right to the engine.
Well, my two deep cycle SOK batteries have never needed to be grounded to anything, so I guess I don't need to be grounding them to anything now. I'll just run the positive cable to them and not hook-up a negative cable to anything because it could end up draining them unnecessarily. Is that OK ?

 

[Hedges]

??

Please sketch out the circuit you plan to use to crank the starter.

Look up "Kirchhoff's Current Law" if you have questions.

 

[Rocketman]

One issue may be your bms…

How many amps can pass through your bms?
How many amps do you need to start cranking the starter?

 

[McCarthy]

Don't do it. SOK's biggest battery has a max discharge current of 100 A. Assuming your system is on 12 volts, that's only 200 amps for 2 batteries in parallel. Most starters pull 400 to 600 cold amps. If your motohome comes with a big inline 6 cylinder / V8 motor, its even more.

Chances are the BMS in the batteries will interrupt the high amp draw and eventually go up in smoke. The battery cells will also lose a lot of cycle life.

Lead acid batteries suck, but they still have a place as starter batteries.

Also, read SOK's warranty terms:

What does this limited warranty not cover?

Product that was used for applications other than which it was designed and intended for, including repeated engine starting

 

[sunshine]

I would like to use the two SOK batteries in my motorhome to start the engine.. Is that OK ?

Interesting! Maybe a case for -
supercaps

 

[Thos]

Don't do it. SOK's biggest battery has a max discharge current of 100 A. Assuming your system is on 12 volts, that's only 200 amps for 2 batteries in parallel. Most starters pull 400 to 600 cold amps. If your motohome comes with a big inline 6 cylinder / V8 motor, its even more.

Chances are the BMS in the batteries will interrupt the high amp draw and eventually go up in smoke. The battery cells will also lose a lot of cycle life.

Lead acid batteries suck, but they still have a place as starter batteries.

Also, read SOK's warranty terms:

What does this limited warranty not cover?

Product that was used for applications other than which it was designed and intended for, including repeated engine starting

I've got two of SOK's 206 amp/hr 12v batteries in parallel. And my motorhome is an old Dodge with a huge 440 cubic inch V8. So it sounds like I will be a big loser if I try this.

 

[Hedges]

There are jump starters with tiny batteries.
At Harbor Freight, looks like only the smaller 650A one (which I have) uses lithium iron phosphate, while the others are lead-acid

Jump Starters - Harbor Freight Tools

Harbor Freight jump starters deliver maximum cranking amps ideal for weak or drained batteries in all weather conditions. Compact design. Ultra-long life.

www.harborfreight.com

Other brands have high amperage lithium jump starters.
Maybe these can't last for thousands of start cycles?
They are meant for just brief cranking, then time to cool off. Not as long as larger starter batteries.

I would guess that the cells in a LiFePO4 battery can handle high currents for a short time, but drawing through BMS circuit is the limitation.
DIY batteries one could bypass BMS for cranking.

Lead-acid is good as a starter battery. I'm mostly using AGM, but I'm not sure automotive alternator/regulator is set to charge them at high enough voltage. Nor does the usual driving cycle provide proper absorption time.

 

[HRTKD]

I've got two of SOK's 206 amp/hr 12v batteries in parallel. And my motorhome is an old Dodge with a huge 440 cubic inch V8. So it sounds like I will be a big loser if I try this.

I don't even use my LiFePO4 battery bank to start my generator. Forum members have measured just over 200 (surge) amps to start similar generators. Yes, the BMS in my battery bank is rated for that. But it's at the margin. I chose to retain a small lead acid battery to start the generator.

 

[Rocketman]

This is one of the reasons I choose the Batrium bms when I designed my MotorHome battery. I wanted to retain the boost ability of using the house battery to help start the chassis (engine). I have a diesel motor. In the owners manual it said to get a battery with 1000CCA (cold cranking amps). For my house battery I have eight 272ah cells in a 2p4s- so at 1C discharge I should have 540a. The manufacturer spec sheet says I can do a 2C discharge for 10 seconds - so I should (hopefully) be able to start the engine with the lithium house battery.

I have a contactor that can handle the amps (as long as the bms doesn’t tell it to break the circuit). I also have to replace the class T fuse.

There is a lot of design and planning that goes into maybe using a lithium battery to help start an engine. (I hope I never find out if it is really needed). My guess is it will be able to help a lot because it will just need to help - not do it all.

Good Luck!