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Pros / Cons of using a DC to DC CC/CV to charge a LiFePO4 from the alternator

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corp9592

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Hello,
I am planning on building a DIY LiFePO4 battery (4S 100Ah) to put on a van. After some research I came across different charging solutions such as a Renogy DC-DC Battery Charger or Victron Orion-Tr ones. I was wondering if I can achieve a not-so-good-but-cheaper solution by using these CC/CV modules that can handle up to 20A with proper cooling:
1622133041856.png
The cons are obviously (but not limited to):
- This is just a buck converter, so if the main battery / alternator is giving me 14,4V I'll get at most 14V out of it, if not less.
- If the alternator is a "smart" one I may even go lower to 13,8V after a while with the van started, therefore the charging voltage would be lower.

So my questions are:
  1. If I am OK at charging the LiFePO4 battery at a lower voltage (let's say 13,5 or 14V) would this device work?
  2. Efficiency asides, what if I put a boost converter before to convert the alternator voltage to, let's say 16V and then regulate it back to 14,6V?
  3. Could I put a pair of boost-buck converters set in parallel in order to get higher current without damaging these little devices.
    1. (my main concern is regarding the voltage outputs of the 2 buck converters not matching 100% and causing some problem)
The following picture may help:
1622134038907.png
The green circle is my concern.
Thank you for your time!
 
Did this work for you ? I suspect that efficiency would be an issue.
I have bought 4ea 20 amp 12 volt to 15 volt buck converters, I had not planned out the heavy gauge wires I needed for this.
I did test just one and it seemed fine except it drew more than 25 amps. It got fairly hot, I am going to get 1/2" thick piece of aluminum to mount them on.
 
I’ve been wondering this too a bit too. Scan guage on my van (2019 promaster) tells me alternator often sits 13.9 for long periods but is at 14.5 other times.

I’m planning todo some sort of charge controller from alternator to house battery back for variety of reasons including isolation of my solar panel mppt flowing / mixing with my starting battery/ alternator. Have not seen if many of the dc to dc chargers will boost and buck or only buck. Seems we would need one that does both?
 
corp9592 said:
I am planning on building a DIY LiFePO4 battery (4S 100Ah) to put on a van.
Click to expand...

Generally speaking, the amount of charge gained from vehicle alternator charging is not worth the expense to set it up, and the risk to your vehicle's alternator.

Vehicle charging was designed to accommodate the small SLA batteries for emergency operation of electric trailer brakes, and a few 12v interior lights in a cargo trailer or camper van.

Unusual situations exist which might change that calculus, like a delivery vehicle or long-distance transport van which runs continuously for 10 -15 hours per day, but for most people, the cost and introduction of additional failure points is not worth the tiny amount of charging gained.

Perform an accounting audit model. How much charging per day you will typically gain? Weigh that against the expense of getting it and the potential expense of adding points of risk/failure.
 
Hi all,
Thank you for your answers.
The original plan was to do this to my friend's van, but I preferred to buy an off the shelf DC-DC charger (AliExpress) to save me the trouble of bad soldering, bad connections, etc.
On the other hand, I myself have a van with a smaller battery (30Ah) and I tried that idea. Originally I had a DIY charger but without the step-up converter, and this caused me some problems due to the alternator not always being at 14.4V.
I have to say that the setup in my original diagram (without the second charging channel) works amazing, but bear in mind that this only pulls 5A from the alternator, since my energy needs are really low (small inverter, LED lightning and small appliances).

Please do not hesitate to ask for more info if I missed some details.

Regards.
 
I have seen some of the car audio folks (dyi lithium face book group) who put a manual potentiometer and diodes on the alternator sense line. This allows you to manually tune the voltage level coming out of the alternator.
Agreed on the comment above about being cautious on how much you plan to draw from alternator but fact is a lot of vans / vehicles have 200amp capacity alternators. Now running that much current at idle they surely will overheat .. but lets be conservative and shoot for 40-60amps.

I just looked at the victron programable buck/boost .. OUCH $650 .. but could take 12v @50amps to 14 or 24 or whatever you program it to.

That said there clearly are cheaper ways to do this.
 
junkycosmos said:
I have seen some of the car audio folks (dyi lithium face book group) who put a manual potentiometer and diodes on the alternator sense line. This allows you to manually tune the voltage level coming out of the alternator.
Agreed on the comment above about being cautious on how much you plan to draw from alternator but fact is a lot of vans / vehicles have 200amp capacity alternators. Now running that much current at idle they surely will overheat .. but lets be conservative and shoot for 40-60amps.

I just looked at the victron programable buck/boost .. OUCH $650 .. but could take 12v @50amps to 14 or 24 or whatever you program it to.

That said there clearly are cheaper ways to do this.
Click to expand...
Oh yes I have done that in my audio days but also did few 16v separate setups when I was sponsored by MTX. They had there amps modded to take more voltage.

I see this could be a backup solution to charge your batteries say rainy days not much solar for some time. But it is nice to drive somewhere and during the drive top off the batteries before you make it to camp or wherever.
 
if not using dc-dc, the LFP would draw too much current from the sourse (alternator or lead acid). I had my BMS tuned to disconnect when current exceeds 60Amps. I did tests and on half discharged LFP it was over this figure (60A) and the trigger disconnected the sourse.
So I decided to order dcdc for travel van from Aliexpress with 25Amps max current. It really works, gives 14.5V and limits current to 25A. The only issue so far that after some time it gets hot. Manufacturer notes that it has overheat protection at 60 or 80 deg C. But who knows...
Since I dont want to put a huge heat sink Ive ordered 80mm fan and temperature sensor with relay. Hope this could solve the issue. If not, maby would have to buy 10Amps version-uts the same dimensions as 25
But this type of charging for me is an extra, emergency, not for full time.



-2126669241568617560.jpg
 
Last edited:
When I started using this dcdc charger I had issues with heating after maybe 10minutes. With 25A current it gets hot very quickly.
The list of parts
80mm fan with dgbb
Grill for fan I purchased at local store.
Sensor and relay witch programming box
The most tricky thing is to drill since when using 80mm fan and 25A housing some holes are directed at ribs of radiator.
I fixed the sensor (1 m wire)with poxipol little bit away from air flow of the fan.

Now I see AE offering more advanced chargers equipped with fans etc
 
Car's alternator provides something between 13.8 and 14.5 volts. So you don't need a step up converter. Just connect your LiFePo4 battery through solar PWM charge controller directly to a car battery.
 
You are right regarding charging voltage - its all ok. My task is just to limit charge current and not overload alternator and wires since without any box low soc LIFEPo4 would draw too much current. In my case with 310ah battery it took more than 70A. I guess it could take more but i had bms cutoff current at 70A.
Regarding solar charge pwm as I understand they have also max input ratings for solar batteries.
 
brewmatic said:
Yes PWM controllers have max input rate for voltage. They also have max amp rate they can provide for battery charging.
Click to expand...
Thanks vor advice! I have couple of those exactly same as on the Amazon but 10A, and never used them. Will try to make tests if they heat or and check the current.
 
FWIW..the Kisae /30/50 amp DC to DC is pretty cost effective and works well with lithium.. we use them in our Winnebago Ekkos even though they are equipped with a Balmar 170 amp second alternator hooked to the house batt.. it gives us redundancy and a boost. it is consistent and works fine.. the out put can be adjusted, and the internals will pull back the load if temp becomes an issue.. I also have the renogy DC to DC but Renogy's charge profile sucks for lithium.. so I had to build a user profile so it wont hold the charge at 14.4 all the time, which it does on the stock stetting... I put a 13.6v float in it. but the Kisae has all the stuff built in.. the Victron Orion is also a great product.. they just work.. I know we like to do a DIY thing and save bucks, but anything that holds the voltage of a lithium at max voltage indefinitely is ultimately damaging to the battery.., and finding the charge conversion is hard with the currents involved heat becomes the issue..
 
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