Is a DC to DC charger necessary?

[larrie]

Is a DC to DC charger necessary? That was the question I got asked today.

We were discussing how to install a lithium house battery system and my buddy asked why he couldn’t just hook the battery, through an isolator and fuse, to the alternator and not go through a DC to DC charger.

Knowing enough about lithium systems to be dangerous, I did not have an answer for his question. Can one of you explain it to me so I can tell him. Thanks.

[Scalf77]

When hooked directly to the standard vehicle alternator you have a couple of issues.

1. You are at the mercy of the output voltage of the alternator. Does that voltage match the specified charge voltage of the Lithium battery.
   
2. Alternators are not great chargers, but have basically been designed for lead acid batteries. They generally rely on not overcharging the battery because the voltage regulator will decrease the current based on level of charge or SOC of the battery. For lead acid batteries this is around 80% to 85% SOC. A three stage charger would kick in at this time and go into the "absorption" charge phase. Here, we keep the voltage steady, and apply a decreasing but steady current to fill the battery to 100% SOC.
   
3. When we apply the alternator to to the lithium battery, because of the difference the voltage will charge the lithium battery to 100% SOC. AT this time the charger would need to be turned off or go into a float voltage specified by the Lithium battery. This results in a possibility of overcharging the lithium battery and producing catastrophic results or at minimum reduce life span.
   
4. It is possible, especially if the alternator is directly connected to a lithium battery, that you will overheat the alternator
   
5. The DC to DC charger is one way to get around this issue, as it just uses the power from your alt/battery and provides a charge profile that is for the lithium battery.
   
6. There are other methods such as a LiFePO4 Battery Isolation Manager (BIM). This will both limit the chances of burning up the alternator and limiting the charge to the lithium battery so as to not overcharge.

-greg

I probably should have finished my coffee, before answering this, but I think I hit the top points.

[boywonder]

Since Greg hasn't finished his coffee yet....

Lithium batteries really need a smart charging profile, and alternators are dumb. A DC-DC converter takes the alternator output and makes it a smart charging profile for the lithium.

Undercharging you won't get full capacity and overcharging could be dangerous and at a minimum will likely continuously trip the battery's BMS (battery management system) which does a few things.....one of which is to protect against overcharging.

Lithium chemistry has a max voltage of 3.65V per cell, or 3.65x4=14.6V for a 12V battery. The battery BMS will disconnect the battery at 14.6V; this is typically a "last resort" protection. You can set up a typical DC-DC converter to do the final absorb charge a little under 14.6V, typically around 3.6V per cell or 14.4V.

Lead acid chemistries are quite robust and can handle the dumb charging from a typical alternator/voltage regulator, although they are also happier with a smart charging profile.

[larrie]

Thanks. I think that I understand it. Will have to digest it some more before relating it to my friend.

[Lilnuts2]

TY Scalf77 & boywonder for putting that in laymen's terms.

[campIV]

And that, among a couple other reasons, is why I hit the easy button and replaced my lead-acid battery with an AGM. ��

[chubba]

I can't explain why, but having killed my alternator in Prince William on a holiday weekend before ours was installed, I can say yes, you need one

[boywonder]

Originally Posted by chubba

I can't explain why, but having killed my alternator in Prince William on a holiday weekend before ours was installed, I can say yes, you need one

....because Lithium batteries can have very low internal resistance and pull lots of amps from your alternator when charging...and when the engine is idling the alternator overheats because it can't supply the amps when it's spinning at idle speed.

Then the overheated windings in the alternator melt some of the insulating varnish between them and the windings short out.

The typical DC-DC converter limits the amount of current flowing to the battery(s) in addition to providing a multi-stage charging profile to keep the battery(s) happy. The Largest Victron Orion model limits current to 30 amps.

[chubba]

Ha, thanks for the explanation for others, but you misunderstood - it's all over my head 🤣 I have many areas of expertise, but electricity isn't one.

Originally Posted by boywonder

....because Lithium batteries can have very low internal resistance and pull lots of amps from your alternator when charging...and when the engine is idling the alternator overheats because it can't supply the amps when it's spinning at idle speed.

Then the overheated windings in the alternator melt some of the insulating varnish between them and the windings short out.

The typical DC-DC converter limits the amount of current flowing to the battery(s) in addition to providing a multi-stage charging profile to keep the battery(s) happy. The Largest Victron Orion model limits current to 30 amps.

[Vanimal]

Originally Posted by boywonder

Since Greg hasn't finished his coffee yet....

Lithium batteries really need a smart charging profile, and alternators are dumb. A DC-DC converter takes the alternator output and makes it a smart charging profile for the lithium.

Undercharging you won't get full capacity and overcharging could be dangerous and at a minimum will likely continuously trip the battery's BMS (battery management system) which does a few things.....one of which is to protect against overcharging.

Lithium chemistry has a max voltage of 3.65V per cell, or 3.65x4=14.6V for a 12V battery. The battery BMS will disconnect the battery at 14.6V; this is typically a "last resort" protection. You can set up a typical DC-DC converter to do the final absorb charge a little under 14.6V, typically around 3.6V per cell or 14.4V.

Lead acid chemistries are quite robust and can handle the dumb charging from a typical alternator/voltage regulator, although they are also happier with a smart charging profile.

With that said then is a secondary alternator worth it or is one big enough amp alternator sufficient fof DC-DC lithium charging. I ask because I have the Ambo package...2 alternators.