CONTINUOUSWAVE

I'm planning to install a LiFePO4 battery (like a NOCO NLP20, Ideal, or similar powersports/starting LiFePO4 battery in my boat with a Suzuki DF40 outboard to serve as the sole battery for both starting and running accessories like lamps and electronics.

There is conflicting information out there [on the internet] regarding charging LiFePO4 batteries directly from an outboard engine alternator. Some say it works fine, while others warn about potential damage to the alternator or the battery BMS kicking in [and disconnecting the battery].

Q1: based on your first-hand, direct experience, will there be any damage to the battery or the alternator if using a LiFePO4 starting battery with a Suzuki DF40 engine?

Q2: can a LiFePO4 battery be connected directly to the battery charging output of a Suzuki DF40?

Q3: should a DC-to-DC charger/convertor be used between the engine alternator and battery to ensure safe and proper charging without stressing the alternator?

Any insights from those with real-world experience would be greatly appreciated.

My advice is to contact Suzuki tech support.

Although there is a good chance they will just advise that their outboards are not suitable with LiFePO4 batteries.

Note that in the past Mercury advised against using LiFePO4’s, then they acquired Renogy, a maker of LiFePO4 batteries (and approved using them)

The charging profiles for LiFEPO4’s is very specific and will have different values than lead-acid batteries.

When I replace my current AGM lead-acid batteries with LiFePO4 batteries, I will be using a DC-to-DC charger to get the correct profile.

I don't have any first-hand direct experience with either your engine or the battery you propose. But I am familiar with the electrical circuits involved.

From the information on the NOCO website at

https://no.co/nlp20

they seem to be quite confident in endorsing their battery for engine starting, particularly on motorcycles. Also, I would be surprised if on a motorcycle there was room to accommodate a DC-to-DC voltage convertor for battery charging.

The general concern with LiFePO4 or any battery with a battery management system (BMS) is for the battery to disconnect itself from the load when too much current is demanded or too much voltage is supplied during charging.

Typically during engine cranking the starter motor is initially in a stalled state (no rotation), and the current flow will be very high until the motor armature begins to rotate, as you essentially have a dead DC short as the load on the battery; the current will only be limited by the resistance of the wiring and the internal resistance of the battery. The peak current might be several hundred Amperes, but only for a short time, that is, assuming that the starter motor has enough power and is getting enough voltage and current to begin rotation.

Exactly how a particular LiFePO4 battery and its battery management system (BMS) will react to such a high but very short initial load is difficult to predict.

To know exactly how much current your particular engine (Suzuki DF40) will draw at initial start is also difficult to predict. If you have access to a high-accuracy clamp-on DC Ammeter that records peak current, you could attempt to measure the actual current that occurs now with your lead-acid starting battery. However, LiFePO4 batteries are often mentioned as being able to deliver very high peak currents, possibly even more than the battery you have now, so there is, again, not a perfect way to predict how much initial surge current might occur with the LiFePO4 battery.

I believe the real concern about charging a LiFePO4 battery from the battery point of view is that an alternator whose output voltage is set for a lead-acid battery will not provide a sufficiently high enough voltage to completely re-charge a LiFePO4. The result is the battery never gets back to a full charge state. On the other hand, unless you will be spending a lot of time using auxiliary loads without the engine running, you probably do not have to obsess about the battery not getting back to 100-percent state of charge. You could alway attach a 120-VAC powered charger to the battery when the boat is out of the water on the trailer or at a dock, thus getting the battery back to 100-percent.

The DC-to-DC convertor device is usually intended to regulate and raise the output voltage of the engine alternator to be slightly higher so it is in the range recommended for the LiFePO4. Note that these devices are not magic. They cannot create electrical energy. If they raise the voltage then there MUST be a drop in current available. And the voltage conversion is not 100-percent efficient, so some power is also lost in the process.

ASIDE
All really "good" batteries will come with specifications for the charging voltage and the float voltage they are intended to work with. I looked at the NOCO website to see if they listed any information about their NLP20 battery and its preferred voltages, but I did not find any data.

I recommend you check http://www.panbo.com where a lot of information on LiFePO batteries is found.

Jefecinco wrote:I recommend you check http://www.panbo.com where a lot of information on LiFePO batteries is found.

The PANBO website is a good source of information about boat electrical and electronic topics, but it tends to skew toward much bigger and much more sophisticated boats than typically are the topic of SMALL BOAT ELECTRICAL.

Back to the problem with LiFePO4 batteries on small boats:

The principal concern with a lithium-chemistry battery in the possibility of LOAD DUMP when the battery management system (BMS) decides it should disconnect the battery from the circuit.

A BMS will disconnect a battery whenever it thinks the battery might be harmed. The circumstances could be any of the following:

• the internal temperature is too cold;
• the internal temperature is too hot;
• the current being drawn from the battery is too high;
• the charging voltage is too high.

When the BMS disconnects the battery, the battery was part of the load on the engine alternator that was supplying current to the battery. When the battery is removed we have a sudden change in the load, known as a load dump.

If the alternator was supplying, say, 25-Amperes of current to charge the battery, and the battery suddenly disappears from the circuit, the alternator (and all other loads connected to it) must be able to tolerate what is likely to be a rather big spike in voltage resulting from all that electrical power looking for some place to go and finding almost nothing connected.

Unless the alternator was designed for this type of load dump and its components hardened and selected precisely to be able to tolerate this big voltage spike, there can be and may very probably be some harm done to the alternator.

One brute-force solution is to always include one lead-acid battery in parallel with the lithium battery as a safeguard, so that lead-acid battery is always in the circuit, ready to absorb electrical current when the BMS decides it needs to disconnect the lithium battery.

If the engine alternator is on an outboard engine, then you can be at risk for damage to the alternator if a load dump occurs, unless the alternator was specifically designed to tolerate this type of event.

It appears that Brunswick has designed some of their newest and most powerful (and most expensive) Mercury-brand outboard engines to have alternators that can tolerate a load dump, as they are now no longer warning that a Lithium battery should not be used. And, curiously, Brunswick has bought a lithium battery manufacturer (RELiON) and nows sells Lithium batteries to use with those high-power, newest model, very expensive outboard engines.

Another means to protect an alternator from damage in a load dump situation is to install a large avalanche diode protective device. The avalanche diode is intended to be placed across the alternator output terminals. The diode will sit there, doing nothing, until the voltage across the diode exceeds its threshold voltage, and then the diode becomes conductive, shunting the excess voltage through the diode until the excess voltage is gone. The diode is designed to be able to tolerate very high currents without being damaged itself. Thes devices cost about $100. Exactly how your outboard engine alternator will react to the the avalanche diode jumping into the circuit remains to be determined. In theory, the alternator will be more likely to survive that action than it would to survive trying to absorb the voltage spike itself. Also, these devices are designed to be placed right across the alternator output connections, which may be a bit difficult on outboard engines that are not using belt-drive automotive-style alternators.

That, in a nutshell, is the problem with using a battery with a BMS with a legacy outboard engine.

If this gets explained somewhere on PANBO in a better, clearer, and more easily understood way, then great for PANBO. Go there and read about it, too.

In the context of this particular thread, the need to use a Lithium battery is due to a change in the seating in the boat having created insufficient room for the existing lead-acid battery. A simpler solution might be to move the existing lead-acid battery to a new located, perhaps closer to the stern.

This illustration (from the thread about the new seating arrangement) shows the problem with fitment under the helm seat once the helm seat is lowered.


Fig. 1. A boat battery that will not be able to remain under the helm seat once that seat is lowered.

ALTERNATE SOLUTION
Get an AGM battery with a nice case color. It won't need the battery box because it won't spill acid. You might be able to find an AGM battery with a sufficiently low profile to fit under the lowered helm seat height. Also, most AGM batteries will be sealed so you do not need to mount them in the usual vertical orientation. The battery can be oriented in such a way that it fits under the seat. This solves all this worry about the BMS and Lithium chemistry batteries.

I don’t see any special mandate to adopt lithium chemistry batteries in most small boat applications, particularly in the case of a boat with only one battery. Any weight saved will not be particularly significant, and the concerns about harm to the outboard engine alternator are very real and could create significant repair costs.

ASIDE
I have been using premium-quality ultra-high-purity lead-acid AGM batteries on my boat for about the last 15-years. I get ten years of useful life for the starting battery.