You could try putting a 100 W automotive headlight bulb in series.

 

Lithium starting battery for an outboard motor. 100 AH.

It can be charged using most any constant voltage power source. With the voltages varying.

Now we are talking. For a Lithium battery, with 14.5v nominal voltage, dropping to 12v means goodbye battery. Its cells (likely 4S) are severely damaged and the damage is irreversible.
But the most important caveat is that the charging cannot start from 14.5 volts. the voltage must be increased gradually as the battery fills up. Adding resistors can prevent overheating and chemical deterioration but not a good solution for an expensive battery. The proper method is trickle charging.
How much does this battery cost anyway?

 

The lithium battery has 12 volts nominal voltage. Resting fully charged voltage is 13.2.

Starting battery for outboard motors.

Constant voltage alternator output. Nominally 14.5 volts. No trickle charging. In a number of applications the current flow is limited by the alternator.

Typical cost is several hundred dollars.

 

The lithium battery has 12 volts nominal voltage. Resting fully charged voltage is 13.2.

Starting battery for outboard motors.

Constant voltage alternator output. Nominally 14.5 volts. No trickle charging. In a number of applications the current flow is limited by the alternator.

Typical cost is several hundred dollars.

Not sure how this battery ended up on your outboard but I suspect you had an acid battery before this. Am I correct? cuz the generators near these motors may not have a built-in protection and Lithium batteries are significantly different from those old acid ones in terms of internal resistance. Acid batteries can stay connected to charger with little issue (as long as the voltage does not exceed the battery's max voltage) and they do not have a low series impedance like Lithium batteries.

 

Not sure how this battery ended up on your outboard but I suspect you had an acid battery before this. Am I correct? cuz the generators near these motors may not have a built-in protection and Lithium batteries are significantly different from those old acid ones in terms of internal resistance. Acid batteries can stay connected to charger with little issue (as long as the voltage does not exceed the battery's max voltage) and they do not have a low series impedance like Lithium batteries.

There are marine rated lithium starting batteries that are considered as drop in plug and play replacements for lead acid batteries. Mercury Marine, the largest outboard motor maker in the world, has approved the use of lithium batteries meeting certain specifications with some Mercury Marine models. Those models having conventional belt driven alternators and those having permanent magnet alternators. All of which are constant voltage output devices. The kind that have been around for years.

Marine rated lithium starting batteries are designed such they will not disconnect the alternator from the battery no matter the state of charge of the battery. How they do this is unknown to me.

As you note, the characteristics of lithium batteries allow for considerable current flow. Much more than lead acid batteries. Notwithstanding Mercury Marine's approval of lithium starting batteries, a few nervous nellies are concerned about high current flow damaging their alternator. I am trying to help them with a solution.

 

".... , has approved the use of lithium batteries meeting certain specifications with some Mercury Marine models" gets you off the hook if you install (I figure you are a mechanic?) ONLY if you install on those "certain" models. This should conclude the case. Hanging a light bulb you can do if it is your boat, not customers'.

 

Fortunately there is no regulatory agency that controls what one can do and not do on most boats. Unlike the FAA where darn near everything has to be approved by the government.

 

For a Lithium battery, with 14.5v nominal voltage, dropping to 12v means goodbye battery. Its cells (likely 4S) are severely damaged and the damage is irreversible.

12V in a 4S means 3.0V per cell. That is absolutely a standard discharge cutoff for lithium-ion battery packs. No damage whatsoever. Especially considering cutoff happens under load, so cells return to >3.0V immediately after shutting down.

 

You could try putting a 100 W automotive headlight bulb in series.

Doesn't this method, and large resistors, not help at all with what OP is trying to accomplish?

My experiment is to reduce the current so as to not cause harm to the alternator.

 

So far I have used an automotive bulb with 3 ohms of resistance.

Keep in mind that a regular incandescent bulb will have a low starting resistance (when cold) but will quickly go up in resistance as the filament gets white hot.

14.5 volts sound like a backup power for a PC or something?

Car batteries and charging systems have traditionally been such that a wet cell Lead Acid battery will draw a lot more current after start-up. Depending on the condition of the battery that charge voltage can go up as high as 15 volts. 14.5 is common among old worn LA batteries.

What type of battery is it?

Lithium starting battery for an outboard motor. 100 AH.

Lithium has a specific charge profile. Failure to follow that profile runs the risk of ruining the battery and even worse, fire.

and those having permanent magnet alternators. All of which are constant voltage output devices.

Permanent magnet alternators have to be regulated by some means other than just being a simple permanent magnet type alternator. If you take a hobby motor (Perm-Mag) and spin it slowly it will produce a smaller voltage than when you spin that motor at a much higher RPM.

I am trying to help them with a solution.

So we're not helping you we're helping your client(s). Hope you understand the liability issues.

 

Lithium has a specific charge profile. Failure to follow that profile runs the risk of ruining the battery and even worse, fire.

He's obviously using a commercial 100AH enclosed battery with a BMS which manages the charging and handles faults.

 

That's the piece folks are missing here. Marine lithium batteries have integral BMSs that limit current (eg. to 100A for a 100AH battery) and also provide over and under temp cutoffs. For example:
https://www.amazon.com/12V-100Ah-Group-24-Rechargeable/dp/B0FKTD38WN

So the question is, will the BMS allow a current draw that's risky for the alternator? You can only answer the question by having and comparing the specs for both the BMS and the alternator. I don't know but I suspect the BMS will vary from battery to battery, so there's not just one simple answer.

Although I recommended the headlight hack, I don't really think it's a good idea as a 'permanent' installation on a boat unless it can be accomplished in a safe way. I wouldn't want a breakable lightbulb on my (little) boat but maybe on a bigger boat the bulb can be in a protected spot?

 

BMSs that limit current (eg. to 100A for a 100AH battery)

BMS amp rating in this case almost certainly means amps output. Charging amps would be significantly lower. There are lifepo4 cells inside. I highly doubt any manufacturer with a warranty would charge them at more than 0.5C. It's probably limited to 0.3 to 0.2C.

Although I recommended the headlight hack

Doesn't this keep current out of the alternator the same?

 

He's obviously using a commercial 100AH enclosed battery with a BMS which manages the charging and handles faults.

So then the TS is pursuing a worthless endeavor. Since there's a BMS there should be no need to control current before the BMS controls current.

Again, this is not a project for himself it's for his client(s). Watch out!

 

My clients are just fellow boaters. Looking for a solution to what I believe is not a problem.

Alternators are both types. Permanent magnet and electromagnet.

 

Doesn't this keep current out of the alternator the same?

No, it drops voltage supplied to the BMS thereby strangling it if it tries to call for more current.

 

... there should be no need to control current before the BMS controls current.

The need is to potentially protect the alternator. Like the TS, I suspect it's not a real problem.

Watching the voltage across a shunt resistor, maybe 0.1Ω so that current can be calculated, could alleviate the fear of excessive current being drawn on the alternator. It wouldn't be that hard to watch the voltage in normal usage and then you'd know that the thing isn't really calling for 100A. I bet it's rarely more than half that.

 

There is no exact calculation for this because there are some unknowns. But let us do some ballpark calculations.
Assume that the charging voltage Vs = 14.5 V.
Assume that the battery voltage VB = 12.5 V.
There is an overhead of 2 V.
If the charging current is 20 A, then the total resistance in the circuit is

R = (Vs - VB) / I = 2 V / 20 A = 0.1 Ω
There is power loss somewhere in the circuit of 2 V x 20 A = 40 W.

You can increase the series resistance with a resistor or a diode. Either would have to be rated at about 50 W.
The resistor would have to be 0.1 Ω rated at 50 W and mounted on a very large heat sink or fan or water cooled.

Or you can use a 20 A diode mounted on a heat sink and cooled as per the above.

Well my results were not as expected.

Used a 1 Ohm 50 watt resistor. It dropped the current from ~ 20 amps to less than 2 amps. More reduction than wanted.

Got some 0.5 Ohm resistors on order. Might parallel them to simulate a 0.25 Ohm resistor. Back to the laboratory.

 

20 amps to 2 amps: under what conditions? Did you start with a discharged battery? Did you measure current while battery was charging? Did you leave the 20 amp charge on the battery for a period of time sufficient to charge the battery before you added the resistor? Dropping from 20 to 2 amps doesn't readily make sense.

 

20 amps to 2 amps: under what conditions? Did you start with a discharged battery? Did you measure current while battery was charging? Did you leave the 20 amp charge on the battery for a period of time sufficient to charge the battery before you added the resistor? Dropping from 20 to 2 amps doesn't readily make sense.

Battery was a 50% state of charge.

Current measured while charging. With and without the resistor in the loop. 20 amps with no resistor and 2 amps with the resistor.

Why do you say that it makes no sense?