Do I need a DC-DC Charger for my vintage motorhome Lifepo4 solar install?

[kbeefy]

Hi.

I've built 3 simple (successful) solar systems on my camping rigs so far, 100-300 watts, PWM controller and FLA batteries.
FWIW previous builds have been a VW Vanagon, Race car trailer, and cabover camper.

I just bought a small-ish motorhome and am looking to take the next step in 'off-grid' and solar.
I'm planning on 4-600w of glass panels, MPPT controller and 200-400Ah Lifepo4 battery (DIY EVE or chinesium/chins).

I just ordered a 50A Renogy DC-DC charger/MPPT controller.

The MH already has an old Heart Interface 2000w inverter/3 way charger. I know that probably won't be adequate for fully charging the lithium battery but it should bulk charge it for now until I can afford something nicer, and suffice for my minimal AC needs.

The MH is 1978, I don't know the output of the alternator but I'm sure it's pretty low (63-80A?). Even if I upgrade to a higher output alternator the belt would become the limiting factor.

The MH also has a (gas powered) 6kw Onan genset, I am unsure of the DC output available from the Genset.

I am assuming, at this point, that the maximum current to the house batteries with the engine running would be 15-20A. I don't think the GenSet would do any better, unless possibly via AC and a inverter/charger. Not sure if there would be any benefit at that point.


Should I ditch the DC-DC charger? If not, what would the benefit be?
I forsee 110v 3 stage/smart charging and solar being the most beneficial.

 

[kbeefy]

 

[MisterSandals]

I'm sure it's pretty low (63-80A?).

I know Victron makes a nice small DC-DC that is only 18A. I suspect trying to pull 50A would not be a good thing. Even with solar charging, the DC-DC will try to pull 25A which may also stress that small alternator.

I am assuming, at this point, that the maximum current to the house batteries with the engine running would be 15-20A.

Why do you assume this?

 

[kbeefy]

I am assuming, at this point, that the maximum current to the house batteries with the engine running would be 15-20A

Why do you assume this?

Mainly speculation and pipe dreams, but also due to the relatively low output of the alternator and line losses due to the run between the house bank and the alternator. I plan on upsizing the cable size to the house batteries.

Do you think I'm wrong about this? I'm a newbie and basing this on smoke and wishes at this point....

 

[camelCase]

I assume this is what you purchased: https://www.renogy.com/dcc50s-12v-50a-dc-dc-on-board-battery-charger-with-mppt/

If yes, than it will charge at up to 50 amps from a combination of solar and alternator power. If you're not getting any solar, the DC-DC portion of the charger could pull over 60 amps from the alternator/starter battery (due to conversion inefficiency). If you want to run this, I would highly suggest a better alternator and a "big three upgrade" with heavier cabling.

Another thing to know is that unit has a maximum allowed voltage of only 25V on the solar side. That means you can only use 100w panels and they must all be wired in parallel.

 

[kbeefy]

I assume this is what you purchased: https://www.renogy.com/dcc50s-12v-50a-dc-dc-on-board-battery-charger-with-mppt/

Correct!

Since it is limited to 25v input, would it be better to use a more capable Solar Charge controller and a separate DC-DC Charger?
Or, ditch the DC-DC all together? I am unsure when (if ever) a DC-DC charger is a waste.

 

[MisterSandals]

I'm planning on 4-600w of glass panels, MPPT controller and 200-400Ah Lifepo4 battery

The Renogy DC-DC will not charge more than 25A from solar:
600W / 14V charging = 43A

would it be better to use a more capable Solar Charge controller and a separate DC-DC Charger?

With the amount of solar you intend to have and the limited alternator capacity, there are far better options than the Renogy.

I am unsure when (if ever) a DC-DC charger is a waste.

It largely depends on your usage, how much solar you harvest and the amount of driving you do. Not enough info for anybody to make that call yet.
I have friends who drive a lot and charge mostly with alternator. I have other friends who boondock for days at a time and charge mostly with solar. You need to figure out what YOUR expected use case will be and get the tools for that application.

If you go with separate solar and alternator chargers, you can install the one you expect to use most and see if you need the other.

 

[kbeefy]

The Renogy DC-DC will not charge more than 25A from solar:
600W / 14V charging = 43A

With the amount of solar you intend to have and the limited alternator capacity, there are far better options than the Renogy.


It largely depends on your usage, how much solar you harvest and the amount of driving you do. Not enough info for anybody to make that call yet.
I have friends who drive a lot and charge mostly with alternator. I have other friends who boondock for days at a time and charge mostly with solar. You need to figure out what YOUR expected use case will be and get the tools for that application.

If you go with separate solar and alternator chargers, you can install the one you expect to use most and see if you need the other.

600w of panels doesn't mean 600w of power, unless I make it to Mexico. I anticipate 300w at best.

With limited alternator capacity, is a DC-DC charger ever useful?

 

[MisterSandals]

With limited alternator capacity, is a DC-DC charger ever useful?

Depends on how much you alternator has to spare. Most DC-DC chargers will pull what they need from the alternator until it burns out.
Also depends on how much driving you do (and how much you need that solar does not supply).

600w of panels doesn't mean 600w of power, unless I make it to Mexico. I anticipate 300w at best.

True, your location and how you mount/orient the panels makes a big difference.

 

[camelCase]

You mentioned a 63 amp alternator which hints at a GM Delco 10SI or 12SI type alternator? You can convert to a later CS130 type without much difficulty and double your amperage.

https://www.summitracing.com/parts/PWM-478021

 

[ianganderton]

600w of panels doesn't mean 600w of power, unless I make it to Mexico. I anticipate 300w at best.

Where are you planning to use the RV in the winter?

Have you done a power audit to estimate what’s could be your average watts consumption per day?

This is the basic conundrum of any off grid system:

1 - How much power will I use?
2 - how will I generate it?

 

[kbeefy]

OK, I canceled the Renogy order.
I happen to have a 40A dc-dc charger still new in the box from a previous project.
I'll use it if I upgrade my Alternator and wiring.

As far as power audit and general useage, I'm all over the place. My previous builds have all at one time or another left me underpowered in certain situations. Usually in the winter. As this motorhome has a 3 way fridge that will cut my power needs down quite a bit, the heater will likely be the largest draw.

Now I'll go shopping for a MPPT controller.....

 

[kbeefy]

Where are you planning to use the RV in the winter?

Nobody knows at this point.

I spent 2 months in NZ backpacking and hitchhiking around both islands in the early '90's. Beautiful country!

 

[camelCase]

If that's a Renogy 40A DC-DC charger, there is an option to run it at half power by connecting a jumper. I would highly suggest doing that if you don't upgrade the alternator.

 

[ianganderton]

Nobody knows at this point.

Then no one can answer your question ?

So I could understand what my solar needs might be I’ve looked at how it would work if I spent a winter in Queenstown snowboarding

I created this spreadsheet so I could work out different scenarios around power use, solar irradiance, solar pv, DC2DC charging and batteries

Van CALCS

Power Audit Van Conversion -- Electrical loads Estimate Calculations based on 24 hr period. 1 - Electrical Loads: Inverter Item,Usage,Time (min),Wattage,Energy (W),Peak Amps,% of use,Ah,Notes Laptop,240V,1,275,45,206,4,15%,17 Toaster,240V,8,1000,0,83,0%,0,12 minutes is about 6 slices of toast so...

docs.google.com

I got my irradiance data from this link

Solar Irradiance Calculator

www.solarelectricityhandbook.com

I really like the simplicity. Just pick a city ?
Then multiply the solar hours by the planned watts of your array to estimate average daily pv generation

Here’s what Dunedin in the lower South Island looks like

 

[kbeefy]

If that's a Renogy 40A DC-DC charger, there is an option to run it at half power by connecting a jumper. I would highly suggest doing that if you don't upgrade the alternator.

It is, thank you. I'll look into it.

 

[Supervstech]

Love those old EM-50 campers!

A good conversion for this camper is a 6.5 van turbodiesel... with oil pan and pickup change...
Anyway, many alternator and vbelt to serpentine conversions out there.

 

[dirt diggler]

The Renogy DC-DC will not charge more than 25A from solar:
600W / 14V charging = 43A

With the amount of solar you intend to have and the limited alternator capacity, there are far better options than the Renogy.


It largely depends on your usage, how much solar you harvest and the amount of driving you do. Not enough info for anybody to make that call yet.
I have friends who drive a lot and charge mostly with alternator. I have other friends who boondock for days at a time and charge mostly with solar. You need to figure out what YOUR expected use case will be and get the tools for that application.

If you go with separate solar and alternator chargers, you can install the one you expect to use most and see if you need the other.

from the renogy spec sheet
Max. Solar Input Power: 660W @ 50A

From personal experiance : 47A from solar, peak in summer in the UK.

 

[kbeefy]

from the renogy spec sheet
Max. Solar Input Power: 660W @ 50A

From personal experiance : 47A from solar, peak in summer in the UK.

I assumed he misread the 25v limit. Ultimately that is the reason I decided to ditch that unit and go with a stand alone MPPT controller.

Well, that and the fact that I already own a 40A dc-dc and the stand alone MPPT was $100 cheaper.

 

[kbeefy]

Love those old EM-50 campers!

A good conversion for this camper is a 6.5 van turbodiesel... with oil pan and pickup change...
Anyway, many alternator and vbelt to serpentine conversions out there.

Good for acceleration and spending money, not good for stinky diesel and noise/vibration.