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MPPT + DC-DC = (cheap) B2B

Solar DC-DC B2B Hack Battery Charger

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#1 rando

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Posted 11 September 2019 - 03:02 AM

Here is another 'hack' I have been working on, somewhat reminiscent of the DIY Lithium Battery.    This time the goal is to get fully configurable bluetooth enabled battery to battery charging for $30.

 

Cliff Notes:  Add a 12V-24V boost DC-DC converter between the truck battery and the solar panel input of an MPPT charge controller - charge controller will convert the 24V down to the correct voltage to charge your battery.

 

A couple of important caveats:

  • This is a 'hack' and not supported by Victron, myself or anyone else, try it at your own risk and only if you know something about electronics - you could break something.
  • This will only work with an MPPT charge controller with a common ground, do NOT try this with PWM charge controller, it will end poorly.
  • I have tried this and verified it works with one particular charge controller (Victron 100/20) and a generic DC-DC converter, it will likely work with others, but proceed carefully.

I have a 160w solar panel and a 150ah LiFePO4 battery, so far I have done fine relying primarily on solar to keep the battery charged and meet my needs.   My camper is connected to the truck with a Blue Sea Battery Link ACR (stock FWC) through 6AWG wire in the truck to the stock 10AWG(?) in the camper.   Between that and the Toyota 'smart' alternator I only get ~5A charge current while driving.   While I don't need more charging, I have thought about increasing the charge current while driving just to have some redundancy.   So far I haven't jumped on commercial B2B chargers for a couple of reasons - they are expensive for what they are, the one programmable one is even more expensive, and they are unidirectional - you can only charge the house battery from the truck battery.  My truck/camper is an adventure mobile only and sometimes sits for long periods, so having the camper solar charge the truck battery is important. 

 

The Victron MPPT charge controller is essentially a buck only computer controlled DC-DC converter.  It really doesn't care where the input voltage comes from, just that it is at least a few volts higher than the output voltage and can source the load current.   The idea here is to step the 13.xV from the alternator up to 24V using an inexpensive DC-DC converter, then feed that into the MPPT charge controller to step down to the ideal voltage for charging my batteries.   There are several pros to this:

  • The MPPT 100/20 is bluetooth enabled, completely programmable and already programmed to coddle my fancy battery, including a low temperature cut off.
  • I already have the MPPT controller, and the DC-DC converter is only $28 plus $2 for a switch, 
  • I can wire it up so there is a switch for unidirectional battery to battery charging while driving/camping, then switch it over to parked mode where it goes back to being bi-directional and charging my truck battery via solar. 

Besides the caveats listed previously, the biggest con to this is that you need to have a specific type of ACR and charge controller for this to work.   In addition to the charge controller being MPPT,  to use a cheap non-isolated DC-DC converter is must also have a common ground.  This means that the solar panel -ve terminal must be tied directly to the battery -ve terminal.  This is the case with the Victron 100/20 and is likely the case with other Victron units.  I don't know if this is the case with other charge controllers, but you can check with an ohm meter.   The other requirement is that if you want this to be automatic, your ACR must not have an under voltage lock out.   When the ACR is connected to the DC-DC converter it won't see any voltage on the house battery, so it will kick in the low voltage lock out and not engage.   

 

The diagram below shows how to wire this up and the required parts (in the dashed box) are:

12V - 24V 20A 480W DC_DC Converter $28

20A or 30A SPDT Toggle Switch $2

 

In my testing, I have seen a solid ~19 amps going into the battery, drawing ~11A @ 24V from the DC-DC converter and about 20A from the alternator.   With the toggle switch in the 'up' or 'in use' position it works like any other B2B charger, with the toggle switch in the 'down' or 'storage' position it works like the stock ACR set up. 

 

Untitled Diagram.png


Edited by rando, 11 September 2019 - 03:55 AM.

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#2 Vic Harder

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Posted 11 September 2019 - 03:40 AM

sweet!


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#3 Stokeme

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Posted 11 September 2019 - 06:04 PM

Interesting, even as a non electric guru, so here are a few simpleton thoughts. Alternator is a scary word to all the Battery boys. Will this protect against any possible issues inherent to an alternator connection?
When I turn the power off in my Camper, I assume the Batteries are not being drained, especially, since I disconnected my truck. With the system resting, & connected, wouldn’t this still be a daily drain in V? I certainly see the pro for a solar charge to the Truck if the house can be bypassed completely.
I carry a specific compact lithium battery charger for my truck. I have 120Wx2 on my roof & flex 100W portable with 50ft of 10awg shielded cable. I should have a 25ft option. Since I have a Grandby I can install another 120W roof panel. I also have a 20amp LFP4 Charger (clearly an on grid option). I am 12V only. I plan to install a external/internal AC through plug, though I am not sure about the safety aspects involved with that.
All this being said I need Sun. I would always being looking at Battery replenishment options. There seem to be many if’s with the above hack. Always looking to know, but with ⚡️, I don’t know what I don’t know.
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#4 rando

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Posted 11 September 2019 - 07:17 PM

I am not sure I totally follow all your questions, but I will take a stab at it:

 

Will this protect against any possible issues inherent to an alternator connection?

 

I am not sure which specific issues you are referring to, but this completely isolates the camper battery from the alternator (when in B2B mode).    As long as the alternator output is between 10 - 20V and can source the current (~20A) then the battery should see what ever the MPPT is programmed to deliver.

 

With the system resting, & connected, wouldn’t this still be a daily drain in V? 

 

No.   This is only connected to the battery when your ACR/isolator is on - which is when the truck is running.   As soon the truck engine stops, the ACR opens and the DC-DC converter shuts down.   The solar charging works as normal. 

 

 I plan to install a external/internal AC through plug, though I am not sure about the safety aspects involved with that.

 

You definitely want to be careful wiring up AC in the camper, but this would not affect that in anyway.

 

There seem to be many if’s with the above hack

 

This is true - you need a specific set of equipment for this to work.   But if you have that equipment it appears to work pretty well. 


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#5 Zirdu

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Posted 12 September 2019 - 12:42 AM

I have a Victron 100/30 mppt solar controller, and have my "12 volt nominal" panels hooked up in series.  So my solar panels put out approximately 34 to 37 volts to the Victron controller.  Is there a problem with adding a 24 volt input from the DC-DC converter to the Victron unit, which is already getting a 35 volt input from the solar panels?   They always tell that when you add solar panels, to match the solar panels carefully, so they feed the solar controller with equal voltages, not two very different voltages.  Wouldn't the Victron controller just ignore the lower voltage?  Not sure how all this would work.  If you need to match voltages, is there such a thing as a 12 volt to 36 volt DC-DC converter?  If so, wouldn't that work better with my setup?


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#6 rando

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Posted 12 September 2019 - 01:43 AM

I am not actually sure what would happen if you were to try this with series panels.   As solar panels are current sources, not voltage sources, they are hard to conceptualize.   My guess is that under low illumination, the solar panel output would sag to pretty close to 24V and the DC-DC would provide the current.   Under full sun you would probably pull all its power from the solar panels, but this is all just a guess. 

 

I don't see inexpensive 12v to 36v converters which can source enough current, but I do see 12v to 48V converters that would work:

https://www.amazon.c...r/dp/B01LYVSL53

 

If you are thinking about this, the first thing to do would be to verify that the MPPT 100/30 also has a common ground.   With the charge controller off, carefully measure the resistance between the battery -ve and solar panel -ve.  If it is zero ohms (you may need to measure the resistance of your multimeter leads first and subtract it off) then it should work. 


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#7 Vic Harder

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Posted 12 September 2019 - 02:31 AM

Wait, in Zirdu's case, would the alternator not be in parallel to the solar panels?


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#8 rando

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Posted 12 September 2019 - 02:42 AM

In all cases the output from the DC-DC converter (not the alternator) will be in parallel with the solar panels.   As long as the voltage from the DC-DC converter is a bit above the panels, the charge controller will pull the necessary power from the DC-DC converter, not the panels. 

 

I am not sure what would happen if the output from the solar panels is higher than the DC-DC converter output, which would happen in Zirdu's case if he used a 12->24V converter, but not if he used a 12->48V converter.


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#9 wiredinoc

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Posted 04 October 2019 - 04:49 AM

Whoa! I can't believe I found someone as nutty as I to try this setup! Just found this post on google and signed up.

 

I have a teardrop camper running a 170ah lithium, victron MPPT, inverter, charger, low voltage cut off, and a bvm 712. I have done exactly what you have laid last year. The reason I wanted to do this is to have a proper lithium charger between my alternator, and the stupid expensive battery in the camper. No way was I going to let that thing roll connected directly to the car battery. I also do multi leg trips, and keeping the battery topped off is important.

 

I currently have the 10a 48v buck/boost device you just posted from amazon. The most I've ever seen the victron solar charger put into the battery is 2-5 amps, hooked to the car, engine on, with a half dumped battery.

 

You mentioned you have the boosted negative lead grounded to the 12v frame? I have mine going into + and - on the controller, along with my 100 watt suitcase panel. I'm wondering if this is the issue, or should I invest in a 24v 20 amp buck/boost?

 

Any help is greatly appreciated. Thank you!!


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#10 wiredinoc

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Posted 04 October 2019 - 05:03 AM

* I also don't have an ACR. 12v+ coming off the jeep is going directly into the input on the buck converter. I am cognizant to unplug the trailer if I'm planning on parking for more than a few hours.

 

This is my setup:

 

Jeep + to trailer - into the buck/boost input. Negative buck/boost input to frame.

Buck/boost output @ 48v + and - to MPPT PV +/-, along with my suitcase panel

 

Nothing connected to the load +/- on the MPPT. Victron unfortunately has hardware problems with this in these controllers.

 

Confused. Do I need to ground the buck/boost - output to the frame?

 

I really want this to work. It's a killer solution. 

 

THANK YOU!

 

 

2019 03 29 13.25.03

 


Edited by wiredinoc, 04 October 2019 - 05:39 AM.

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