DC/DC to charge LiFePo4's (Lithium) batteries in van?

[Milton477]

My D4 is not charging the Lithium batteries in my caravan at a rate that I would expect given the size of the D4 alternator.

The setup I have looks like this: D4 Cranking battery --->Traxide DBS --->twin Optima 55Ah under bonnet, --->Anderson Plug on tow bar --->360Ah Lithium in van via 25 mm2 cable, van chassis as earth.
400W of solar (24V panels) is also connected directly to the van at all times. I have tried disconnecting the solar while charging from the D4 to see if there is a difference.

I understand that ordinarily we don't need a DC/DC charger to charge van batteries with the D4's smart alternator system as long as the batteries are lead acid/AGM type chemistry it seems.
An almost flat AGM battery is around 12V or even a bit less depending on your point of view whereas a Lithium with 25% charge left is still up over 13V.

The bigger the voltage differential between the alternator & the batteries the more charge, yes?
I have seen 50A going into the Lithiums from the alternator when they were down near 12V.

Assuming I start driving with the vehicle batteries all charged & the Lithiums in the van at 50% & 13.3V, cloudy day so little solar.
The initial alternator charge according to Gap IID Tool is over 120A at 14.6V & drops within a minute or two to around 90A at 14.4V then some time later down to 80A at 14.2V where it sits. I'm guessing that around 10A is going to the van at this stage as the electronics of the D4 requires 70A to run.
2 hours later when I stop, the Lithiums have increased by only 3 or 4 percent. The fridge has been running in the van too.

I am thinking that a DC/DC device may be necessary because of the unavoidable volt drop between the DBS & the van & the relatively high discharged Lithium voltage.
In other words, the voltage differential between the alternator & the van batteries is not sufficient for the alternator to keep the charge rate up.

Anyone have any thoughts?
Multiple DC/DC chargers in parallel?
Big cable direct from crank battery to van?

[drivesafe]

Hi Milton and you should not be trying to charge the lithium batteries direct from the alternator.

It can be done but the problem is that when ever you turn the motor off, while your caravan is connected to your D4, your lithium batteries, which have a rested voltage of 13.2v, will be discharging back into your lead acid batteries, which have a fully charged rested voltage of 12.7v.

Even if your cranking and auxiliary batteries are fully charged, they will still continually drawing a considerable amount of energy from your lithium batteries.

And I suspect this is part of your problem.

Also, to fully charge lithium batteries, the final stage of the charge must be around 14.4v.

if the lithium batteries are low, they will cause a fair bit of voltage drop, even with your thick cable. But once the lithium’s get close to their fully charged state, the voltage drop should be minimal, and your lithium batteries will eventually fully charge.

With lithium batteries, being charged with a lower voltage, they will only charge to a specific capacity.

With your alternator voltage and what your solar can put in, I am surprised you can’t achieve a fully charged state, if you drive long enough, and the actual drive time needed will depend on the discharged level of your lithium’s when you start the drive.

Lots more to cover, but this should give you a starting point.

[RobA]

LifePO4 batteries,like all, require a specific charging and maintenance cycle. You need a battery management system capable of undertaking the full mainrenance cycle to keep them in the correct state of charge. Faiking to do that will kill them quickly. Not a cheap outcome. Any alternator is not capable of doing a full cycle to suit the battery chemistry.

We run 300AH of LifePO4 in our AOR using a RedArc BMS30 which does the job perfectly.

Rob

[PeterJ]

Hi Milton, I am reading between the lines of your post here, so if I have got it wrong then my bad, sorry, but what I hear is a LFP user that has not thrown out all the baggage from the bad old lead acid days. 340Ah of LiFeP04 at 50% charge, no problem. 50 to 60% charge is a good place for LFP to be, unlike lead acid. You still get 13V+, can pull 200A out of it without it breaking into a sweat if you need to (say for an inverter) and can run the battery down to 15% before any real change. How do I know this? because it's exactly what I do. The leap to LFP is fantastic, but you must change your paradigm, it's not Flooded Lead Acid (FLA) don't use it or manage it like you would FLA. You don't mention your power needs in the van but my suggestion, make your life simple, forget about trying to charge the van battery from the car. Set the van up as a stand alone entity (700w solar total would be a good minimum starting place and probably much cheaper upgrade than adding a DC/DC converter) that way you can manage the LFP properly, trying to hybrid LFP/FLA is going to end in tears, (and probably a bruised wallet) they are poles apart in management. The car uses FLA the van on LFP so deal with each as different systems, which they are, and happy days.
Happy to discuss ideas via PM if you wish, I don't want to hijack your thread and I hasten to add it took me awhile to get the brain lead free as well, carried a generator around with me for 12mths before I finally realised I did not need it. I did go through all the various "what if's" when I made the leap and looked at charge rates, with my battery charger (90A) and the Yam 2kVA genny 3 hours to fully charge, but as I said, never needed it. This is an example of what I mean by changing the paradigm. If you get a decent charger, say 100A or bigger, plug into 240 supply or use a genny and 2 to 3 hours you are fully charged. FLA can not do that. Nowadays, get up in the morning, 50% charge, that works, on goes the inverter, on goes the George coffee machine, hit the microwave to heat the milk for brekkie. Easy. Stop for lunch on a hot day, turn on the van A/C, no problem. Have fun, push the envelope on the LFP but throw away the FLA learned behaviour.

Peter