Some RV system advice for 48V systems

[Brad Taylor]

Hi All - This is my first post.
I'd like to ask a question about a solar system I'm designing.  I want to upgrade my 1995 VW Eurovan camper to support solar and the KID controller looks ideal. My problem is that I'd like to charge the battery from 3 possible sources

o Solar
o AC landline
o 12V Alternator

This all works fine if I stick with a 12V battery. However If I go to a 48V battery I will need to replace my AC charger and I wont be able to connect it to the alternator.  The problem is compounded if I go with a Lithium battery as it seems like multiple charging sources is asking for trouble.

So I'm considering dropping the AC to ~100V DC using a AC/DC converter and connecting it directly to the KID controller DC input with a manual or automated switch.  Would this work with the Kid? and has anyone else done this? Or is there a better was to add AC charging?

The other question is charging via the Alternator. If I connect the Alternator DC voltage (14V) directly to the Kid DC input will it work?

Thanks in advance
Brad

[ClassicCrazy]

It seems like first you need to decide on which battery you want to use before you can answer some of the other questions.

How many watts of panels can fit on top of that van  ?  Or are you going to just pull the van up next to a ground mounted solar system and plug it in ?

Larry

[vtmaps]

So I'm considering dropping the AC to ~100V DC using a AC/DC converter and connecting it directly to the KID controller DC input with a manual or automated switch.  Would this work with the Kid? and has anyone else done this? Or is there a better was to add AC charging?

Welcome to the forum.  This is a FAQ.  Doing what you suggest can work, but it doesn't always have a happy ending. 

MPPT controllers need to sweep... that means they vary the load they present to the DC source and the MPPT expects the voltage of the source to drop a bit as it is loaded.  If the source is too "stiff" there may be damage to the controller.  Some folks have solved the problem by putting a resister in series with the output of the source.

--vtMaps

[Brad Taylor]

How many watts of panels can fit on top of that van  ?  Or are you going to just pull the van up next to a ground mounted solar system and plug it in ?

I'm considering 4x 100 W panels (Renogy 32 cell flexible panels 21x41in) with a 17V operating point. These would be attach to the top of the van, but could be set up to be portable as they are very light weight. http://www.renogy-store.com/Renogy-100W-12V-Bendable-Mono-Solar-Panel-p/rng-100db.htm

[Brad Taylor]

So I'm considering dropping the AC to ~100V DC using a AC/DC converter and connecting it directly to the KID controller DC input with a manual or automated switch.  Would this work with the Kid? and has anyone else done this? Or is there a better was to add AC charging?

MPPT controllers need to sweep... that means they vary the load they present to the DC source and the MPPT expects the voltage of the source to drop a bit as it is loaded.  If the source is too "stiff" there may be damage to the controller.  Some folks have solved the problem by putting a resister in series with the output of the source.

--vtMaps

Since these DC  sources are either shore power or from the alternator, I don't see a problem adding some series resistance as efficiency from these sources is not an issue. Just the wiring will probably be enough resistance. Any value I should be shooting for? 100mOhms would give a volt of drop at 10A. Also do you know if the Kid can work with an input voltage as low as 12-14V from the alternator?

[ClassicCrazy]

If you use lead acid batteries then it would be pretty straight forward , especially if you stuck with 12v because that way you could run a wire right to the batteries from alternator. But you won't get much charging into the batteries if they are full because the car alternator is set to cut off at 14.2 v .

If you use lithium batteries you need to use a BMS or some other electronics to make sure they stay within the proper voltage ranges, don't get over discharged, and that each cell stays in balance .  It really depends upon which type of lithium cells you get.  The alternator isn't going to help with that.

The easiest would be to use lead acid batteries and an inverter which also will charge. That way you could plug in AC anywhere and the inverter would do the regulating. Also you could have the Kid charging the batteries from solar  any DC input from your panels up to the limit,  and drop it to the voltage of your batteries.

Lithium batteries are better but you have to figure out how to charge and take care of them properly.

[Jones]

Hi Brad,

I went through the same set of questions about multiple charging sources last year.


The first thing I did was to get a really accurate estimation of the daily power required.

With the figures worked out I saw that by building some extra capacity in to the battery bank that things got less complicated and less expensive if I could charge using just solar.


If it helps I use four almost identical 100w panels, (great choice) on what I think is a really simple and effective RV system.

No a/c charger, no alternator link. Straightforward and super easy.


We run completely off-grid with a Kid, 2 Rolls batteries (S12-240AGM)  plus a 1200w inverter.

After looking at lithium cells and associated electronics too, I settled on the Rolls AGM as they seem to be more of a known quantity with good cycle life and tested 80 percent DOD if necessary.

Unlike the lithium units I checked out they didn't need an initial (and maintenance) charges from a dedicated lithium AC charger. Just connect and forget.



In almost 300 days we've used power from the alternator twice.   (when parked in the shade of a house for a week whilst staying in the truck.)  No complex wiring or smart switching, just by using jump-leads for an hour or so.

I didn't want to risk cooking the alternator at all with a perminant link- although one option was to swap to a unit actually intended to charge a house bank.

A high output marine alternator looked good, but seemed overkill.



For the battery bank voltage we kept it the same as the truck, in our case 24v, so we can take a charge if the sun has not showed for a few days, and also in an emergency jump-start the wagon, or top up the starting batteries if needed.





Really happy that I chose to just let the Kid handle everything.  It really is a solid and trusted piece of kit. We have it linked to a Whizbang and BTS.

The system has been in full time use scince October. In April we added a 1100w electric kettle to help burn the excess power now that the UK has a bit more sunshine..

All the best and good luck with the design.


Stuff I use:
http://www.wattuneed.com/en/supples/249-mx-flex-solar-panel-sunpower-100wp.html

http://www.wattuneed.com/en/victron-energy/815-inverter-victron-phoenix-1224481200va.html

http://www.rolls-battery.com/content/specifications-renewable-agm

The Midnite kit came from Arizona Wind and Sun who shipped  to England in under 48hours.  Amazing service.

[JeanTunell]

Please can you tell me about the Kid device and also about your solar system? What kind of charger are you looking for? What problem you are facing for 48V? May be your current charger is not sufficient to give result for 48V.

[Westbranch]

Please can you tell me about the Kid device

http://www.midnitesolar.com/pages/kid/index.php

hth