Using Buck converter 12V to 19V thru PWM solar charge controller to charge batteries

[ultrakiev]

Hello everyone.
I'm making portable solar generator power bank for camping in my SUV.
100Ah lifepo4 with Daly 60Amp smart BMS, Renogy 20 PWM controller. And 100w "Allpowers" portable solar panel case.
I'm thinking how I could charge my battery bank while I'm driving without buying actual DC-DC converter. Just looking for something simplier alternatives.
I'm thinking to get 12V in to 19V out 20A buck converter (12V input range 10-19V) with overcurret protection. I want to run separate wires 8-10 GA from car battery directly without ignition control because I will be plugging in buck converter manually while I'm driving, want to make it simple.
Buck converter will supply power to my solar input connector ( solar panels disconnected while driving) to PWM controller to mimic solar panels input.
My question is: do I missing something? Can I run into problems with such set up. Whole idea is to get 15-20Amps charging current into lifepo4 bank while I'm driving utilizing my PWM controller.

 

[HRTKD]

Welcome to the forum!

Do you have a 7-pin outlet for towing? You could run that 12v supply to the PWM controller. Do that first, to see if it will provide enough volts/amps to the PWM to get a charge to the battery. If so, that may be a simple solution that requires no extra wiring or boost converter.

 

[ultrakiev]

Thank you.

My PWM controller rated at minimum 15V input voltage. But it's just specs, in real word it still should work for some bulk charge ( not tested yet) , but I won't be able to reach proper voltage 14.4-14.6 to my lifepo4 bank. There is many discussion about this kind of setup and many , many guys still recommend to use dc-dc charger.

Actually I wired my 7pin by myself with Aux positive double 10G wire. It should handle 60A plenty of extra capacity. But I'm thinking to cut this wire at the plug for now and run it in the trunk so I can connect my portable power bank. I'm not towing RV anymore .

I'll try both ways with buck converter and without.

Direct charging via PWM just of 12v plug working perfect for my current setup, but it's different chemistry - SLA not lifepo4. And small 18 Ah, so Its not pulling too much amps thru this socket even when it's fully discharged.
I use this SLA bank to inflate my boat and power up 12V sonar.

 

[HRTKD]

A dedicated DC-DC charger is the ideal solution. It's smarter than the PWM controller.

 

[ultrakiev]

I know DC-DC it's ideal solution, but this setup is more self educational purpose.
I'm just worry about how well buck converter could do current protection/limiting and if it can handle 20A continuous load, as well as my 20A PWM controller, of course it's better to be 30A controller so it's not working on it's 100% duty cycle. But reason why I went with 20A because it's waterproof. For my setup I need it waterproof.

I'm still waiting for all parts to arrive including buck converter. I will do some tests on this setup. I'll post them here when I'm done.

 

[HRTKD]

There has been a lot of recent experimenting with different inputs into solar charge controllers. Most of it has been around AC-DC power supplies going into a solar charge controller. Like laptop power supplies.

There was a post this week about a forum member that caught his buck converter on fire. It was an extreme case though. The buck converter was used to power an inverter, which greatly overloaded the buck converter.

 

[mikefitz]

Can I run into problems with such set up. Whole idea is to get 15-20Amps charging current into lifepo4 bank while I'm driving utilizing my PWM controller.

Perhaps.
I expect the converter and/or PWM controller to fail.

The buck converters typically dont have current limiting in the sense they will hold the output volts constant at a certain current, they fold back, so may be incorrectly protected, overheat and fail. The PWM has no current limit feature, its just a semiconductor switch. Even in the converter survives, the excess current through the PWM will melt the semiconductor switches. The PWM is designed to operate with a current source with limited current, the solar panel. Feeding with a voltage source with crude current limiting may/will result in failure.

Use a DC to DC battery charger with correct settings for lithium. I know they are expensive but your vehicle and lithium battery need to be protected and correctly treated.

Mike

 

[ultrakiev]

Thanks for reply Mike.
I didn't have any experience with buck converters. I thought they are current limiting.
Specs showing actually it's overcurrent protected not showing it current limiting ( you are right).
Since I ordered it already from Aliexpress. I'll try it anyway I will hope Buck converter current protection will be able to "cut out" power.
But I have a feeling I will ends up getting DC-DC converter.
Do you know maybe anything about Daly smart BMS may I limit current without triggering overcurrent protection in BMS, limit it to 15A to prevent PWM controller and Buck convert overheating.

 

[HRTKD]

None of the BMS are current limiting. It's either on or off.

 

[ultrakiev]

Got respond from the seller. There is no protection on this unit for high current ):. Already ordered DC to DC charger. But I will be using this converter to charge my laptop efficiently without consuming power on DC AC inverter.

 

[MT_Mike]

I use a HiTEC RDX1PRO 100 watt programmable AC/DC charger to charge up my home brew 96AH and several 16AH Lifepo4 battery packs. I use the 96AH battery for trolling motor and ham radio gear and the smaller 16AH batteries for low power ham radio or C-Pap use when camping off grid. There is a 11-18 VDC input which I plugged into my 7-pin trailer tow accessory circuit and use it to charge up the lifepo4 band deep cycle lead acid batteries while traveling. When camping I use solar to charge the two 12VDC deep cycle lead acid batteries and the HiTek charger to charge the 96AH & 16AH lifepo4 batteries. This charger does and excellent job and allows for up to 10 programable battery charge profiles. I also use it to discharge lifepo4 batteries to 80% for storage.

 

[rickst29]

Got respond from the seller. There is no protection on this unit for high current ):. Already ordered DC to DC charger. But I will be using this converter to charge my laptop efficiently without consuming power on DC AC inverter.

This is 100% true. For Boost Converters of this type, you MUST limit power consumption at the "demand" side. But that doesn't make this kind of configuration "impossible".

In my more complex MMPT-based scheme, I limit the maximum output amps (into the battery), in order to assure that the under-the-hood Boost Controller will never be overrun (or even run above 90% of rated capacity). Output Amps * 14.5 Volt LiFePo4 "CV" charging voltage, plus power lost via "Voltage Drop" through the Bargman cable/TV wiring/Trailer wiring, plus about 5% maximum power loss within the MPPT.

Real-world math of my configuration, which has been running for many years: The 36 Volt Converter is rated at 15A maximum output current (540 Watts). My 90% limit allows for up to 486 watts into the Bargman Cable. After calculating losses on the wiring path, I can receive a maximum of about 460 Watts at the MPPT controller, which is right next to my batteries. (@ about 13.3A, ending a bit less than 35 Volts into the MPPT. The low amperage and higher voltage creates much lower "voltage drop" and power loss.)

With ~5% consumed by the MPPT conversion, my scheme can support maximum of 437 watts (30.1A) into the batteries. IIRC, I have actually programmed my MPPT for a maximum of 28A, providing an even bigger "margin of safety". My configuration is here.