How to not blow up your alternator when charging lithium batteries

[DeepFrz]

I came across this video posted by Victron Energy and thought it might be of interest to our members.

https://www.youtube.com/watch?v=jgoIocPgOug

[mlydon]

Not really an accurate test - a typical alternator off the crank pulls runs double the rpms due to the pulley ratio, so a fast idle (1500 rpm) will turn the alt at 3000, and a 2500rpm cruising throttle runs it at 5000......a lot more fan cooling than shown. That said, all small case alts still need to be de-rated in order to keep temps down.

Matt

[RaymondR]

They don't have to be lithium.

Connecting an auto alternator which is rated for a single say 100 amp hour auto battery to 600 amp hours of house batteries tend to produce black deposits on the inside of the engine box.

Hint: In an emergency you can limit the current output by wiring a push bike spoke in series on the alternator output. Slide the wires apart until the smell of smoke disappears. Hang the push bike spoke up clear of anything flammable and whatever you do don't touch it.

[YANDINA]

A much cheaper method of limiting current is simple series resistance.

This increases the series resistance of the Lithium battery causing it to behave like a lead acid battery. If it tries to draw too much current the added voltage drop across the resistor causes the alternator voltage to rise which in turn causes the regulator to cut back and reduce the current to a safe level.

As the battery reached full charge the charging current drops off to nearly zero so in turn the drop across the resistor also drops off to zero and the battery still gets the full charge voltage.

The downside is that charging will take a few minutes longer but that is a small price to pay for protecting your alternator.

(See next post for resistor value.)

[YANDINA]

Originally Posted by RaymondR

They don't have to be lithium.

Connecting an auto alternator which is rated for a single say 100 amp hour auto battery to 600 amp hours of house batteries tend to produce black deposits on the inside of the engine box.

Hint: In an emergency you can limit the current output by wiring a push bike spoke in series on the alternator output. Slide the wires apart until the smell of smoke disappears. Hang the push bike spoke up clear of anything flammable and whatever you do don't touch it.

And be careful where the hot cables will fall if you melt through it.

A safer method is plain copper wire. 10 gauge wire is about 0.001 ohms per foot so 5 feet would produce 0.005 ohms. At 100 amps that is 1/2 volt drop. With a discharged Lithium battery charging at about 13.75 volts that will raise the alternator voltage to 14.25 causing the alternator to cut back output thinking it is a fully charged lead-acid battery. The heat in the cable is 50 watts which spread out over 5 feet will be well below maximum insulation rating of 105C. The 5 feet should be well distributed, not bundled together.

[mlydon]

both of those options sound like really bad ideas....you're passing current through an under-rated cable in order to drop voltage.....

much better to derate using an external multi-stage regulator like the balmar. keep your cables cool, the regulator knows your battery's true voltage, etc.

Matt

[RaymondR]

The metalurgical specs on SS are not far from those of the resistance wire they use to wind wire wound resistors. and if the cabling is sufficient to carry the full rated amp capacity of the alternator it is not going to heat up by one placing a series resistance in the circuit.

I have also used long, small diameter SS machine thread screws as series resisters or shunts on wind generators with crimp connectors on the wiring so that the resistance can be adjusted.

Once upon a time high wattage rheostats were easy to source but I have not seen one of those for years.

If you want a manually adjustable current control you can wire an adjustable DC speed controller in parallel but if you go this route you need to monitor the battery voltage and manually adjust to keep from damaging the battery.

Of the various means the bike spoke has proven the most practicable as I keep a stock of spares on hand for my bike anyway.

[Coquina]

FYI - trying to make a big resistor out of stainless wire or something is stupendously bad advice.
Please buy a regulator with the appropriate settings for the batteries you are using and temperature sensors. My Balmar regulator will back off the charge rate if either the alternator or battery gets too hot.

[RaymondR]

Originally Posted by Coquina

FYI - trying to make a big resistor out of stainless wire or something is stupendously bad advice.
Please buy a regulator with the appropriate settings for the batteries you are using and temperature sensors. My Balmar regulator will back off the charge rate if either the alternator or battery gets too hot.

Some of us are quiet capable of devising solutions from first principles and safely implementing them. And then there's others that really shouldn't get out of bed in the morning because by doing so are both a danger to themselves and the rest of humanity.

Now that I'm a helpless old dude with a truncated energy budget I appreciate my new boat having a Balmar alternator and smart regulator but in my former youthful economy cruising mode on my steely I tended to use the cheapest solutions and tended to get a good deal of gratification from bike spoke type solutions.

The voltages one deals with with a 12V alternator are quiet safe so the only hazard created by the bike spoke resistor is excessive heat depending upon how much resistance one includes and I don't recall having to take any special precautions against fire hazards.

[CaptainRivet]

sorry sailors, if you have the money for a big lithium bank get a proper high current alternator with an external regulator.

Yes they cost some additional money but thats the only right way to operate an alternator save and use all the energy that your alternator supplies with a lithium battery. Thats why you get them, more AH and more C capability to charge.


And don't forget to have a dump load ready (the hot water boiler via the inverter is a very good choice for this) if the lithium are disconnected or are full, otherwise you destroy your electrics...

[markcouz]

Why not a simple current sensing circuit at the alternator output, and when it drops below a predetermined value you cut power to the field terminal?

[markcouz]

Originally Posted by markcouz

Why not a simple current sensing circuit at the alternator output, and when it drops below a predetermined value you cut power to the field terminal?

Ignore above post, should have read the thread 1st. I was referring to the issue of a bms shutting off charge current which will cause alternator voltage to rise and blow diodes.

[zboss]

Originally Posted by mlydon

Not really an accurate test - a typical alternator off the crank pulls runs double the rpms due to the pulley ratio, so a fast idle (1500 rpm) will turn the alt at 3000, and a 2500rpm cruising throttle runs it at 5000......a lot more fan cooling than shown. That said, all small case alts still need to be de-rated in order to keep temps down.



Matt

And even then, active cooling is recommended.

There are way way more modern methods of dealing with alt temperature and charging components than using resistors or other tricks, without artificially constricting your output. For example, using an alternator with a remote rectifier and using a blower on the alt and rectifier to circulate cool air over it. As far as the BMS shutting off the battery you can just go to a starter battery first or using a device from victron or another company to manage that by switching off the alternator field.

These strategies are well established.

[RaymondR]

Originally Posted by markcouz

Why not a simple current sensing circuit at the alternator output, and when it drops below a predetermined value you cut power to the field terminal?

Needs a time delay circuit to delay the charge control relay until an excitation dump relay can dump excitation current to avoid a spike.

[YANDINA]

Originally Posted by markcouz

Why not a simple current sensing circuit at the alternator output, and when it drops below a predetermined value you cut power to the field terminal?

That is essentially what your regulator does and can do so instantly.

The problem is although you no longer have field current, the inductance of the circuit means the magnetic field takes time to dissipate and excessively high voltages can occur momentarily if there is no load on the alternator.