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Experiences charging LFP from alternator WITHOUT a DC-DC charger

No, a relay or 'combiner' of any kind, including a solid state relay, will not limit current. Follow the advice and use a b2b charger. Limit the current to a value that is consistent with the design limits of the Prius system, I suggest a 30 amp b2b charger. Note this is the output current, depending on conditions the input current could be nearer 40 amps.
The whole concept is strange , I cannot undstand why a 280 Ah lithium battery needs to be used/charged in a Prius.

Mike
 
@mikefitz I never suggested 280ah. I’d stick with 100ah max. It would be only for emergency use. But I don’t think that would change your advice.
 
Do you mean that you would only charge it in an emergency (like if solar wasn't working or an AC charger failed) or that you would leave the battery connected to the Prius and only use the battery in an emergency?

The reason for the DC-DC charger is not related to the size of the battery. If lfp batteries are not charged properly, you can damage both the battery or the charging equipment. The DC-DC charger protects both your $1000 battery, and several thousand dollars damage to your Prius.

You can certainly try other solutions, and they work for some, but also some suffer damage. A Prius just seems an expensive thing to risk.
 
@wholybee Thanks. Initially I I certainly didn’t realize that transferring the continuous charge applied to one 12v Lead Acid battery to a discharged 12v Lifepo4 would be so complicated. 12v to 12v sounds easy. But I’ve come to accept it’s not.

It appears to me that the DC -> AC inverter -> DC charging is the more obvious route for temporary emergency charging despite the losses. At least the AC inverter could then also be used inside the home.
 
It’s interesting to see the alligator clips on this device coming out in a bit. I bet the charge speeds are tiny however.

 
Does anyone have medium or long term experience charging a LFP house bank from the alternator without a DC-DC (B2B) charger.

Another anecdote for onlookers: my campervan (Promaster, base 180A alternator) originally had a 220Ah FLA GC2 house bank. Due to frequent boondocking and infrequent driving 95% of the charging is by solar, but I did install a VSR (w/toggle switch for manual disabling). At 50% DoD the bank pulled ~42A through the isolator, which is pretty close to the C/5 charging max associated with FLA.

The FLA bank gave years of good service and was replaced with a single 100Ah LFP. Posts on the Promaster forum from people who have actually tried charging lithium from an isolator indicated it could work reasonably well. I decided to test the existing VSR setup before buying a ~20A DC-DC charger.

Hand on the toggle switch and ready for The Worst I enabled the VSR; the 100Ah LFP pulls ~30A at my normal morning SoC (35-55%), falling to ~20A as battery voltage rises. Both the current and voltage are within the battery manufacturer's charging guidelines.

The VSR is disabled by default. When driving in darkness or crappy weather I'll give the LFP some alternator love then shut the VSR off again with the toggle from the driver's seat. At this point I see little need to buy a DC-DC charger.
 
This spring, I will use my LFPs to run a small microwave in my van.

My plan

Vehicle Battery 60A connection > 12V inverter (160w) > 100w 12v 120v Charger > LFP battery.

Why? I already have the above and I won't have to by anything new.

DC > AC > DC conversion loses will be there but for my ramen budget and very little use (warm precooked meals and coffee) I am good :)
 
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