DC to DC charging. Can I use a constant-current voltage regulator from AliExpress?

Hello Dear DIY Solar Community,

my Van Build currently has two 100W Panels with a Tracer MPPT and a flooded lead acid battery. While Driving a simple relay is what connects my solar-battery to the starter battery of my vehicle using a 200A relay. This solution is simple and works.

Currently I am considering upgrading to a LiFePo4. While the solar system should work just fine with the lithium battery I need a better solution for charging the battery while driving. I have been searching for different DC to DC chargers for a while now.

I am supprised of the lack of cheap bang-for-the-buck DC to DC chargers, especially considering the relatively low cost of MPPTs for comparison. Also I did not find any Chinese devices that are made for that purpose. For that matter it seems that my only option is buying an expensive Victron or Renogy device.

While researching Aliexpress and co. I found this device (https://www.aliexpress.com/item/4000156542513.html?spm=a2g0o.cart.0.0.28703c00Iv2uq0&mp=1) that takes an input of 8-36V and gives an output of max. 13.8V at a constant current. From my understanding all the DC to DC chargers are basically constant current sources with a few more fancy control options. I do not need fancy stuff like Low Temperature Cutoff on the DC-to-DC since I would be using a BMS with such capabilities anyways.

On a sidenote I think 13.8V might be a little high but I think a diode would do the job of dropping the voltage to an appropriate level just fine.

Do you think that I could safely use this device instead of a name-brand DC to DC charger, without risking any damage to my battery?
Or do you know any other options for DC to DC charging, espeacially good bang-for-the-buck options.

Thank You all for your help

As I have been looking into the same sort of I do not have enough details to provide. But interested to see what post come about here.
I have been looking at the KISAE DC to DC with built in MPPT but I would rather have without the MPPT.

They have one for Lithium batteries with 12.6V output as well (same page). If it limits the output voltage to 12.6V it should act as a CC-CV charger. Looking forward to seeing other opinions though.

If you only need ~20A, you may want to check out the Victron Orion 12/12-18. Quite a bit less $ than the 30A smart version if you don't need bluetooth. Adjustable output voltage set by potentiometer.

As far as I understand I need a 13.6V CC-CV charger for my LiFePo4 battery chemistry, is that true?

Yes the 18A version of the Victron energy would be my other choice. I think ~20A should be fine in combination with my solar system.

t-muehlberger said:

As far as I understand I need a 13.6V CC-CV charger for my LiFePo4 battery chemistry, is that true?

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Yes I think you are right, 12.6V is too low. Not sure why the Lithium version is only 12.6V.

I think they might be referring to a different lithium chemistry

I guess the 13.8V version might work but it might be too high

Got the same setup and thus question here.
Most batteries I was looking at shouldn't have an issue with 13.8V

I was looking at the 12V
Aluminum Case BLS Cells

They are rated with 3.65V a cell for charging - that is 14.6V for the pack.

My alternator never puts out more then 14.4V so I was even thinking about directly wiring those... but they are drawing a huge amount of current.
Please advise from someone with more experience.

You do not want to directly connect the alternator to the LiFePO4 battery, even if the voltage was compatible. The low internal resistance of the battery will draw too much current and can burn out the alternator. Need some way of regulating the current like the DC DC converter.

sduser said:

You do not want to directly connect the alternator to the LiFePO4 battery, even if the voltage was compatible. The low internal resistance of the battery will draw too much current and can burn out the alternator. Need some way of regulating the current like the DC DC converter.

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It would be parallel to the existing engine battery? This battery would also supply current to the LiFePO4 as well?

When the vehicle is running, the alternator will charge both batteries (at some level) if they are directly connected in parallel. Current will flow from the alternator into both batteries proportional to their internal resistance and cable resistance. Need to regulate the current to the LiFePO4 battery to avoid damage to the alternator.

tl'dr: you can directly connect the car to charge the LiFePo4 battery, or via a DC-DC power supply, if and only if you have some way of disconnecting it when the LiFePo4 is fully charged. i.e. a BMS. You may or may not get a full charge depending on the behaviour your car's electrical system, and you may or may not damage your car's alternator depending no whether the current is sufficiently limited (as noted by sduser) - ymmv.

I haven't been so bold myself, but this person has, on a Mercedes Sprinter : https://windinmyface.com/idx-bike.html#Sprinter_Electrical (they don't do a great job of describing their actual setup, you may have to wade through a number of pages before you realise they've just patched their BMS-managed LiFePo4 battery directly to the car's alternator, and whack up to 80A or more into the battery, the battery's BMS cuts the charge off when it's good and ready; https://windinmyface.com/Sprinter-Alternator-Primary-Behavior.html). They quote a Mercedes mechanic that the Sprinter's alternator is "bullet proof".

Probably the most important aspect of LiFePo4 charging is terminating the charge: a constant current power supply won't do that. A constant voltage power supply can in so far as you could set the "constant" voltage to the desired terminal voltage and let it rip (a CV supply can't generally supply infinite current, so will in practice be current limited. Whether your CV power supply burns up in the process is dependant on the quality of its design ).

For a 12V vehicle and house battery, I'd recommend the Renogy DCDC chargers. They're not crazy expensive for what they are.

bdl said:

tl'dr: you can directly connect the car to charge the LiFePo4 battery, or via a DC-DC power supply, if and only if you have some way of disconnecting it when the LiFePo4 is fully charged. i.e. a BMS. You may or may not get a full charge depending on the behaviour your car's electrical system, and you may or may not damage your car's alternator depending no whether the current is sufficiently limited (as noted by sduser) - ymmv.

I haven't been so bold myself, but this person has, on a Mercedes Sprinter : https://windinmyface.com/idx-bike.html#Sprinter_Electrical (they don't do a great job of describing their actual setup, you may have to wade through a number of pages before you realise they've just patched their BMS-managed LiFePo4 battery directly to the car's alternator, and whack up to 80A or more into the battery, the battery's BMS cuts the charge off when it's good and ready; https://windinmyface.com/Sprinter-Alternator-Primary-Behavior.html). They quote a Mercedes mechanic that the Sprinter's alternator is "bullet proof".

Probably the most important aspect of LiFePo4 charging is terminating the charge: a constant current power supply won't do that. A constant voltage power supply can in so far as you could set the "constant" voltage to the desired terminal voltage and let it rip (a CV supply can't generally supply infinite current, so will in practice be current limited. Whether your CV power supply burns up in the process is dependant on the quality of its design ).

For a 12V vehicle and house battery, I'd recommend the Renogy DCDC chargers. They're not crazy expensive for what they are.

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Thank you @bdl that confirms what I was thinking. I would be using a BMS anyways and the purpose of the DC to DC is mostly to limit the current.

What voltage should I go for on the DC to DC power supply? Do I want it to be slightly higher or slightly lower than the cut-off voltage of the BMS?

I am assuming for the battery it would be best to be cut-off as soon as it is full and have the rest of the system just running of the DC to DC. But does this mean that I have a short loss of power, the second I turn off my vehicle and the battery is also still turned off? How quick are BMSes usually turning on again after the external voltage drops?

t-muehlberger said:

... I would be using a BMS anyways and the purpose of the DC to DC is mostly to limit the current. What voltage should I go for on the DC to DC power supply? Do I want it to be slightly higher or slightly lower than the cut-off voltage of the BMS?

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I would expect that RCNUN DC-DC constant current "charger" you posted earlier to do the trick - essentially as you've noted, its purpose would be to limit the current drawn from the car. The implication of separate "lithium" and "lead acid" versions seems to imply a voltage limit as well (i.e. it's really a current limited CV supply, which is good); with a BMS might want the higher voltage 13.8V version and rely on the BMS for cutoff; conversely the 12.6V lithium version would be a safe bet.

t-muehlberger said:

I am assuming for the battery it would be best to be cut-off as soon as it is full and have the rest of the system just running of the DC to DC. But does this mean that I have a short loss of power, the second I turn off my vehicle and the battery is also still turned off? How quick are BMSes usually turning on again after the external voltage drops?

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Depends on where you connected things. For example, if you put a single BMS on the battery side of both the charge and load connections, yes you'd lose power to the load when the BMS disconnected due to overvoltage from charging. If however you use two appropriately configured separate BMS' for each of the charge and load, your load could operate independently of the charger (the load BMS would want to have a higher, or even disabled, upper limit; conversely the charger BMS would have a lower or disabled lower limit cutout). I haven't tested this arrangement, validation is required ;-).

I got this for alternator charging (haven't tried it yet because the SMBS0 is not currently available, and I am doubtful if I should get a Chargery BMS8T instead):

Telun 30A LifePo Car Charger



It is advertised specifically for the purpose of charging a LiFePo4 battery, and there are separate models for 12V/4S and 24/8S applications. The documentation is not great but the device seems to be able to limit the maximum power drawn from the alternator/starter battery. Since there is no enable/disable switch (the built-in switch only affects the display), I intend to attach a solid state relay to control it from the BMS.

anschluss said:

I got this for alternator charging (haven't tried it yet because the SMBS0 is not currently available, and I am doubtful if I should get a Chargery BMS8T instead):

Telun 30A LifePo Car Charger

View attachment 18623

It is advertised specifically for the purpose of charging a LiFePo4 battery, and there are separate models for 12V/4S and 24/8S applications. The documentation is not great but the device seems to be able to limit the maximum power drawn from the alternator/starter battery. Since there is no enable/disable switch (the built-in switch only affects the display), I intend to attach a solid state relay to control it from the BMS.

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great find!

I reads like as if they have bunch of Constant Current Diodes (current limiting Diode) in there, probably they are just adjusting how many of those Diodes they use to define the charging current.

Didn't know that they make limiting Diodes in high Amp capacity these days. The ones I know had been in mAmp

bdl said:

I would expect that RCNUN DC-DC constant current "charger" you posted earlier to do the trick ...

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I was just looking at the other RCNUN DC-DC converters and realised one practical problem: switching it on and off. You probably only want the DC-DC converter to run when the engine's running; devices like the Victron and Renogy DCDC chargers have a "D+" (alternator running) enable input that you connect to the ignition / engine running signal from the car (the Victron Orion-T devices also have an algorithmic approach that attempts to deduce the engine-running condition based on battery voltage, I guess if you couldn't access a suitable D+ point). The RCNUN, and from the looks of it the TELUN, don't have this feature so you'd need to finagle some method of turning it on only when the engine is operating.

You can buy the Renogy DCDC1212-20 in Australia for about $110 AUD incl. postage when on sale, with warranty; the RCNUN is $65 delivered from China (practically, no warranty). By the time you implement an on/off mechanism for the RCNUN, you're going to be close to the Renogy in cost.

You probably only want the DC-DC converter to run when the engine's running

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To this end, I will put this device in series with the BMS HVD output in front of the Solid State Relay:

CD60L Under Voltage Protection Board