Starting a big diesel with lifepo4

[wholybee]

Originally Posted by SOLAR SUPPORT

The initial inertia of the moving parts of the engine, such as the flywheel, crank, pistons, connecting rods, and additionally the compression pressure that the piston must overcome, forces the starter motor very hard for a few seconds at the start of the first movement. If you cannot provide the high power required to start the starter motor in the first few seconds, the starter motor can not start.

If the required current can be measured during the first start, the CCA value of the battery required for this job becomes clear. These small-capacity Lifepo4 batteries, which are used as starter batteries, must have circuit elements that can accumulate energy and discharge instantly. A power-increasing circuit that allows the 50 Ah Lifepo4 battery to produce 1450 A, albeit for a short time.

As I said though, it is highly unlikely that anywhere near 1450A is needed. What is needed is a higher voltage. A battery with 1450CCA will deliver a higher voltage at 500A than a 750CCA battery. So, a 1450CCA battery might be specified and used, even when the needed current is much lower.

LFP batteries produce huge currents, a small LFP can produce 1000-3000A. A large one 10kA-30kA. No "power-increasing" circuits are needed, only the removal of the BMS, which is limited in its power handling.

Here is a 60Ah 1000CCA(limited by the internal BMS) LFP that is designed for cranking. It is likely plenty to start the OPs engine:

https://dakotalithium.com/product/da...ttery-1000cca/

[skipperpete]

By “big diesel” do you mean Cat C18 big. 16V92 Detroit diesels can use a lot of initial battery power the get em rotating but they start very quickly after that first hit, how big is your engine?

[Wotname]

A couple of points -

Many appear to be confusing CCA with the maximum amps a battery can deliver.

1. CCA by definition is the number of amps a battery can deliver for 30 seconds at 0F (-18C) without the terminal voltage dropping below 7.2V.

So a 720 CCA can deliver a continuous current of 720A for 30 seconds at 0F and the voltage will remain above 7.2V.

It might be able to deliver say 1,200A for 3 seconds or it might not. A new 720 CCA battery will deliver a much higher max current than an old 720 CCA.


2. The initial current draw on a large diesel can be enormous, well in the order of 1000+A but only for say 100 milliseconds or less. Once the engine has turned over through compression on the first cylinder, the current requirement drop away as the starting rpm is reached. A 200 start rpm has completed 3 revolutions after 1 second and the mass inertia obtained means the current required by the starter motor has steadied out.

If the OP's engine requires two 720 CCA batteries in parallel to get a fast start, I guess the initial draw could exceed 2,000A but only for tenth of a second.

So an LiFePO4 battery will need to be able to pack a real punch for a second or so.

Which pretty much what Solar Support has posted upthread!

[NPCampbell]

There are a number of diesel big rigs using ultracapacitor starting modules in lieu of a bank of multiple 12v FLAs. They are a little pricey but they last forever, aren't affected by cold and they dump current considerably faster than deep cycle lithiums or FLAs.

[tanglewood]

Originally Posted by Wotname

A couple of points -

Many appear to be confusing CCA with the maximum amps a battery can deliver.

1. CCA by definition is the number of amps a battery can deliver for 30 seconds at 0F (-18C) without the terminal voltage dropping below 7.2V.

So a 720 CCA can deliver a continuous current of 720A for 30 seconds at 0F and the voltage will remain above 7.2V.

It might be able to deliver say 1,200A for 3 seconds or it might not. A new 720 CCA battery will deliver a much higher max current than an old 720 CCA.


2. The initial current draw on a large diesel can be enormous, well in the order of 1000+A but only for say 100 milliseconds or less. Once the engine has turned over through compression on the first cylinder, the current requirement drop away as the starting rpm is reached. A 200 start rpm has completed 3 revolutions after 1 second and the mass inertia obtained means the current required by the starter motor has steadied out.

If the OP's engine requires two 720 CCA batteries in parallel to get a fast start, I guess the initial draw could exceed 2,000A but only for tenth of a second.

So an LiFePO4 battery will need to be able to pack a real punch for a second or so.

Which pretty much what Solar Support has posted upthread!

Agreed Re initial surge current. But if the engine mfg asks for 700a for 30 seconds at 0F, shouldn’t the LFP start battery be able to deliver the same? That in addition to the initial surge?

[wholybee]

An LFP can deliver a much higher current, for longer time, and at a higher voltage, than a lead. Even a very small one. That is the point. A 25Ah LFP will deliver thousands of amps, at nearly 12V, until it is depleted. If I am doing my math right, converting Amp hours to minutes, (25Ah x 60) would mean nearly 1500 amps for 60 seconds, and will maintain nearly full voltage. Let's be conservative and say 10V for 30 seconds, as I haven't tested it myself to see what the actual numbers are in real life.

A 200Ah lead battery(about the size of a 1450CCA) will deliver ~1500A for 30 seconds at 7.2 volts. The lead battery isn't nearly depleted having used only 10-15Ah, but the voltage will sag such that it isn't useful after 30 seconds of cranking and will need to recover. An LFP 1/8 the size beats it hands down, by providing a much higher voltage for the 30 second period. The engine will crank faster, and start easier, with a 25Ah LFP than it will with a 1450CCA lead.

The limiting factor of LFP is the BMS. The BMS on most drop-ins limits current to ~100A-200A. A few starting batteries now exist with high cranking amps, and that is probably simply from adding heatsinks to the BMS so it will last longer before it is destroyed by heat. If you get rid of the BMS, LFP is an awesomely powerful starting battery. The 100Ah LFP staring batteries being sold are undoubtably MUCH better at starting a large engine than a pair of 750CCA lead batteries, regardless of the CCA ratings because of the higher voltage.

[Nicholson58]

Originally Posted by Fuss

To start my motor, I need paralleled 2x 12v 750 cca batteries.
Has anyone changed to lifepo4 to start a big diesel?

We just installed Blue Heron lithium iron phosphate replacements. Large house bank and starter bank for the 115 Hp Westerbeke 6. Works just fine.

[Trimaran Ken]

Originally Posted by Fuss

Maybe a battery could bypass the BMS when cranking and use the BMS when charging, and also when installed, have the ability to switch off the charging...my FLA start batteries at the moment are not charged with an alternator so this change would not be difficult.

You would be crazy to bypass the BMS. It is there to protect the battery and the internal components.


In the best case, the internal wires connecting the individual cells would blow like a fuse with that much current. If a worst case that wires would overheat and create a fire inside the battery.

[Scubaseas]

Originally Posted by Fuss

A few more thoughts,
-Lifepo4 as a starter is very different to deep cycle. As a starter, it needs to deliver the CCA but can be kept for example at partial charge for long life. So my charging regime can be switched on and off as I think fit. (assuming that these start batteries only have the starting job)



-Also, its amount of use in a typical day/week is not that much so I could imagine that some huge current abuse for starting might reduce its life and cause some damage but possibly not enough to make its life alot less... it might reduce its life from 15 years to 10 years for example.


-So a question would be...what if I had a 12v 90ah Lifepo4 start battery with no BMS like the Winston WB-LP12V90Ah and I tried to start my motor, would it deliver enough cranking amps? I imagine that it would and these cranking amps would be over the spec of 900cca for max 10 seconds in every minute. Maybe with Lifepo4, the actual cranking amps required are alot less than FLA. My FLAs get drawn down to 11 volts turning the starter where as Lifepo4 will not, so the CCA with Lifepo4 will be less than with FLA for the same result.


- It might be that 1400 CCA in FLA batteries is equal to 900 in Lifepo4. Does anyone know this??

Having bare cells without any BMS sounds like a really bad idea.

[Scubaseas]

Originally Posted by Fuss

To start my motor, I need paralleled 2x 12v 750 cca batteries.
Has anyone changed to lifepo4 to start a big diesel?

What engine do you have that needs that much amps?

I'd check the draw on the starter and voltage drop at the starter when cranking and compare it to the engine manufacturers specs. You really shouldn't need 1500CCA to start a diesel. Even big and cold ones.

[mqbenedict]

I have a Universal 4 cylinder 36 hp. I had no trouble starting it by attaching one of these to the battery posts. Not a huge engine.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

I keep it on board just for that purpose and was happy to have it as a last resort recently.

Good luck,
Mark

[PaulCrawhorn]

An easily replaceable BMS is key.

Wire a small amp one to control external relays rather than flowing Starter current through the cheap Chinese mosFETs

Best of both worlds.

For longevity let it cycle between 3.24 to 3.28 no need to go outside that range unless deep cycling

maybe once a quarter do a full 3.0 to 3.5V cycle for maintenance

[Ballsnall]

Sounds like a dodgy negative return cable, unless you're diesel is in a large tug boat?

[SVFaith]

I believe Dakota has a 12 x 135ah, 1000cca that they market as house and starter battery.

[Phil_Fr17]

Originally Posted by Fuss

To start my motor, I need paralleled 2x 12v 750 cca batteries.
Has anyone changed to lifepo4 to start a big diesel?

Use a Winston 90A, no BMS.