Lithium for starting

dlochner · Jan 9, 2026

The problem is many LFP batteries cannot safely produce the inrush current necessary to run the starter. The BMS and the internal wiring cannot handle the current and the BMS will shut down. Watch some of the LFP battery tear downs by Panbo, MarineHowTo, and Will Prouse and you will see the construction differences.

Some newer LFP batteries can handle the inrush current, these are usually more expensive. The battery must be able to handle 250a-300a of current.

jssailem · Jan 9, 2026

I am still learning about LiFePO4 batteries. What I have explored suggests the issue would be the BMS.
The BMS has limits for OverCurrent conditions. It shuts down. Your starter may require more current than the BMS is designed to permit. In order not to harm the battery, it shuts down.

Any BMS shutdown is a cry for help. Not to reset and try again. It is a signal that something is wrong and you need to look at the system, resolve the anomaly, then hit the reset button.

Suggested reading:

Why Did My BMS Trip? Understanding Common Protection Faults

BMS tripped again? Uncover the 4 hidden reasons your energy storage system shuts down. Fix over-voltage, current, and temp faults to restore power.

www.anernstore.com

The Role of Battery Management Systems (BMS) in Preventing Battery Failures

To maximize performance and safety, a Battery Management System (BMS) is a critical battery system component. The BMS monitors and manages various aspects of battery operation, ensuring efficient and reliable performance. Understanding its role can help users prevent battery failures and extend...

www.relionbattery.com

Scott T-Bird · Jan 9, 2026

I don't quite understand how you will be charging the house bank if you disconnect your shore-power charger. I suppose your main charging source could be your solar panels. But if you are relying on the new Mini to charge the house bank via a new DC/DC charger, I think you will not be adequately charging the house bank. You won't be running the engine long enough when you are sailing, and I don't see how the charging current when the alternator is running will be enough. But I'm pretty confused about this matter myself.

Project_Mayhem · Jan 9, 2026

If a BMS prevents lithium from being used in a starter battery, how do we have lithium jumper packs?

dlochner · Jan 9, 2026

Project_Mayhem said:
If a BMS prevents lithium from being used in a starter battery, how do we have lithium jumper packs?

All BMS's are not alike, some BMS's have high discharge rates. Nor are the innards of a LFP battery the same. Take a look at the video below on the Wattcycle 314 Mini by Rod Collins. He opens up the battery and shows why it is a better battery than a cheaper comparison battery.

Early BMS's seemed to have a continuous discharge limitation of 100a, even high end Victron NG batteries have this limitation. In some cases batteries placed in parallel can be used to start the engine if the total or the maximum discharges is higher than the inrush current for the starter. We have 3 100a LFP batteries with 100a max continuous discharge and they can start the engine and run the windlass because they are in parallel and the total allowed discharge is 300a.

There is also an instantaneous or burst rating for somewhere between 3 and 10 sec which is much higher than the continuous rating, but I would not rely on that rating for starting an engine, because if it didn't start the engine the BMS would shut down.

Stu Jackson · Jan 9, 2026

In his excellent article, written years ago, Maine Sail wrote:
Current Handling

The current rating of the internal switch that protects the battery is quite often too small for the task on many cruising boats. Drop-in LFP batteries routinely use multiple tiny little MOSFET switches as the batteries BMS protection ON/OFF switching. Unfortunately these FET’s often can’t handle the typical loads imparted by many cruising boats. On board devices such as bow thrusters (400A +), windlass’ (100A to 300A+, large inverters 150A to 300A +, electric winches 75A to 300A +, electric cook tops, massive alternators, chargers or large inverter-chargers are very very common on-board cruising boats these days. These are exactly the devices many boat owners are hoping to see a gain in performance from when switching to LiFePO4.

Drop-In LiFePo4- Be an Educated Consumer

Lead is Dead! (Except for Starting Batteries) Warning: This article is long and detailed—it may require two reads to fully grasp the nuances and important details. You might be wondering why I’m declaring that lead is dead. The answer is simple:

marinehowto.com

********************************************

Folks, Maine Sail's article has been linked many, many times before. I remain amazed that those of you who continue to express lack of understanding of LFP don't just re-read this article. It explains the limits based on the BMS, which differs in capacity between different manufacturers. You used to be able to buy a lead acid battery. Now you buy a LFP battery AND its BMS. The BMS is the "limiter." Contrary to some who just refuse to "get it," this is not rocket science.

Richard19068 · Jan 9, 2026

Watt Cycle does have a 100AH "dual purpose" battery.

12V 100Ah 1200CCA Dual Purpose Marine Cranking Battery with Bluetooth

High Capacity & Long Lifespan: Powered by advanced LiFePO4 technology, this 12V 100Ah battery ensures superior performance with a 10-year lifespan and over 6,000 deep cycles @80% DOD, making it perfect for both marine and automotive applications.

www.wattcycle.com

Johnb · Jan 9, 2026

The real issue is not that the BMS has a set current limit that will shut down. What really limits the battery output is that the current flows through the components that the BMS uses to switch off. Unless designed to do so, they cannot carry starter motor and similar loads.
Read post #8 above and the linked article.

dlochner · Jan 9, 2026

Richard19068 said:
Watt Cycle does have a 100AH "dual purpose" battery.

12V 100Ah 1200CCA Dual Purpose Marine Cranking Battery with Bluetooth

High Capacity & Long Lifespan: Powered by advanced LiFePO4 technology, this 12V 100Ah battery ensures superior performance with a 10-year lifespan and over 6,000 deep cycles @80% DOD, making it perfect for both marine and automotive applications.

www.wattcycle.com

If you look at the specs for this battery, it has a 100a continuous discharge rate, which means it can deliver 100a continuously until it runs out of power.

It also notes that Discharge rate, which is the short term instantaneous rate. The rates are all high enough and long enough to work as a start battery. For example, it can deliver up to 1200a for 20 seconds before shutting down.

In contrast, the 314ah Mini only lists a Max Continuous Discharge of 200a and no instantaneous discharge rate. Although a larger capacity battery, it shouldn't be used for starting a diesel.

[Bluetooth-Free Version] Wattcycle 12V 314Ah Mini LiFePO4 Deep Cycle B

Reliable deep-cycle lithium battery with long runtime and 10-year lifespan. Perfect for RVs, solar systems, and off-grid power.

www.wattcycle.com

Ya gotta read the fine print on these batteries.

dlochner · Jan 9, 2026

danm1 said:
Battery makers don't make it easy.

Wattcycle 12V 314Ah Ultra Bluetooth LiFePO4 Battery with 300A BMS

Experience unmatched power, endurance, and protection with Wattcycle’s flagship 12V 314Ah Ultra Mini Battery.

www.wattcycle.com

Footnote in features (not specs) says instant 600a 20s output capacity "perfect for starter + motor applications" but nowhere in the rest of the description does it indicate dual purpose. I guess only its hairdresser knows for sure.

Could be the marketing folks didn't get the memo or they don't want to hang their hat on it as a starting battery.

marcham · Jan 11, 2026

Your 3GM starter, if OEM is 1200 Watts..... 1200 W / 12V = 100 Amps.... that's roughly the current draw while cranking. When you first push the button/turn the key, the current will be much more than that. Do you use the decompression levers? If so, it will draw less current to start (turn the engine over a few seconds with the levers raised to warm the cylinders).

I know that my 3JH3E cranks at about 60-80 Amps, the initial inrush current I don't know... It's also a 1200W starter (Hitachi). I do know that it will start with only ONE of my SOK 314 Ah batteries.

I normally start with my AGM, but can also run the starter from the house bank.

Note that the higher voltage and lower voltage drop (less internal resistance) of the lifepo4 will start the engine faster but also cause more heat in the starter.

I'd have no issues with using a properly spec'ed Lifepo4 start battery,especially if weight or space is an issue. If you are not concerned about space or weight, then save some $$ and get an AGM from Costco.

Stu Jackson · Jan 11, 2026

marcham said:
Do you use the decompression levers? If so, it will draw less current to start (turn the engine over a few seconds with the levers raised to warm the cylinders).

For almost three decades I have heard this "suggestion" related to decompression levers.
Those levers are ON the engine. Most START buttons or keys are in the cockpit. It would take TWO people to do this.

marcham · Jan 11, 2026

Stu Jackson said:
For almost three decades I have heard this "suggestion" related to decompression levers.
Those levers are ON the engine. Most START buttons or keys are in the cockpit. It would take TWO people to do this.

In colder weather (no glow plug on any 3GM30F I've used)... open the decomp levers, pull the fuel shut-off, spin the engine for 10 seconds. Close the levers, return shutoff to run position, start the engine. Below 10 deg C, this makes a huge difference, especially when 4000 + hrs.... Maple Bay can't be that much warmer then up here in the North

BrianQ · Jan 12, 2026

Stu Jackson said:
For almost three decades I have heard this "suggestion" related to decompression levers.
Those levers are ON the engine. Most START buttons or keys are in the cockpit. It would take TWO people to do this.

A handheld remote starter switch makes it a one-person task.

jssailem · Jan 12, 2026

Or a really spry, quick-footed crew member able to be in nearly two places at once.

Scott T-Bird · Jan 12, 2026

I watched a video on YouTube last night that suggested that DIY LFP battery installation will make your boat uninsurable. It even suggested that once you have initially installed LFP batteries and try to replace them again with LA batts that the evidence of the installation will make your boat uninsurable. It was a video that listed common problems with various boat models that are the most problematic with insurance companies. It concluded with the LFP commentary, saying that it is the number 1 issue to make your boat impossible to sell or insure.

dlochner · Jan 12, 2026

Scott T-Bird said:
I watched a video on YouTube last night that suggested that DIY LFP battery installation will make your boat uninsurable. It even suggested that once you have initially installed LFP batteries and try to replace them again with LA batts that the evidence of the installation will make your boat uninsurable. It was a video that listed common problems with various boat models that are the most problematic with insurance companies. It concluded with the LFP commentary, saying that it is the number 1 issue to make your boat impossible to sell or insure.

I think I saw that same video. It's not entirely accurate. Insurance companies vary, my company BoatUS/Geico said they didn't care when I went to LFP. A friend has a different company and they insisted on an ABYC compliant installation by an ABYC certified Marine Electrical Technician. The best advice is to call the insurance company ask and be sure the LFP installation is as compliant as is reasonably possible.

Insurance companies are always looking at risk and potential loss. As the number of LFP installations increase and the perceived risk decreases I suspect the restrictions will also decrease.

I would not be surprised to see restrictions on other forms of Li-ion batteries. E-Bike and scooter batteries seem to be an issue lately, there have been a couple of fires from e-bikes in local landfills. Airlines now prohibit or restrict Li-ion power packs for cell phones and other devices. Some of the batteries used for electric dinghy motors may also come under scrutiny.

Lithium for starting

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SBO Weather and Forecasting Forum Jim & John

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SBO Weather and Forecasting Forum Jim & John

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