Solar charging of multiple banks - how?

[jdege]

I'm trying to design a solar charging system for my boat.

It has two 48V electric motors that draw a max of 80A, and two 48V 100Ah batteries.

It will have four 12V 100Ah batteries in parallel for the house bank.

It has, as of now, no solar installed. That I'll be dealing with this spring.

What I want is to be able to switch the solar from charging one bank to another.

I currently have three banks, two 48V and one 12V. I've considered wiring the two 48V in parallel so I'd have only one 48V bank, but no matter what I do I'll have at least two banks.

What I want is to be able to redirect the solar charging into whichever of the banks needs charging. It may be, for example, that I've not been running the movie for a few days, and I need to charge the house bank, of perhaps I've been motoring a lot.

I've seen 12V charge controllers, and 48V charge controllers, and I've seen one that can be configured for either.

How feasible would it be to install separate charge controllers for each bank, then switch the solar input between them?

Or would it make more sense to wire all the solar input to the charge controllers then switch the connections between the charge controllers and the batteries?

 

[42OhmsPA]

I would look into using DC-DC converters wired to the banks.
Dedicated 12V and 48V SCC for each bank. Parallel the 48 banks.
Use the converters to transfer power from 48V to 12V or 12V to 48V. Put a quality shunt on each pack for accurate SOC tracking.

I have no experience in the marine world but that's how I would go about it. Swapping wires/cables and reprogramming setting will get old quick.

 

[GXMnow]

I would recommend paralleling the two 48 volt banks. Setup the solar to keep that charged. Then use a DC to DC 48 volt to 12 volt charger to charge the 12 volt system from the 48 volt system.

The only real tricky part is making sure the DC to DC 12 volt charging system does not drag down the 48 volt system if there is not enough sun. If the DC to DC charger does not have this protection built in, you can add something like a low battery protect which will cut off the power to that charger when the 48 volt battery falls below a set level.

 

[yabert]

I've seen 12V charge controllers, and 48V charge controllers, and I've seen one that can be configured for either.

Many SCC can charge 12V and 48V.
So, of course I suggest as other a DC-DC from 48V to 12V, but on a boat it can be useful to be able to swap / disconnect few cables to charge the 12V.
Example in case the 48V battery or the DC-DC have problem you can still rely on the 12V for emergency.

 

[jdege]

I've seen people suggest using an MPPT controller as a DC->DC charger.

Any idea how well that works? Because it would give me the option of rewiring to charge the 12V bank at need.

(Once you get a cruising boat into the back of beyond, being able to rejigger things can be important.)

 

[yabert]

I've seen people suggest using an MPPT controller as a DC->DC charger.

Maybe, but with Victron Orion DC-DC starting at 70-110$, why don't get one more device?
That add one more choice when something go wrong.
I'm in the same boat than you with two 48V batteries + 12V and I look at the Orion 20A.
20A can seem low power, but over 24h it can transfer over 6 kWh from 48V to 12V

 

[GXMnow]

(Once you get a cruising boat into the back of beyond, being able to rejigger things can be important.)

If you are concerned about being able to manage an equipment failure at sea, then you should have a few redundant devices. I suggest splitting the solar onto a pair of charge controllers, but have each one large enough to take the whole array. If one fails, move all the panels to the working one. If the DC to DC fails, you can move one array to the 12 volts battery to put some charge there. The Victron 48 volt capable charge controller can all be changed to 12 volt with the app. And maybe even have a pair of the 48 to 12 volt DC chargers running in parallel, so if one fails, you are still working. You could also bring along a 12 to 48 volt CC CV boost converter that can be set up to push 10 amps from the 12 volt system to the 48 volt batteries. That way, if you have no sun, but the engine is turning the 12 volt alternator, you can still charge the 48 volt batteries.

 

[MattMan119]

I'm trying to design a solar charging system for my boat.

It has two 48V electric motors that draw a max of 80A, and two 48V 100Ah batteries.

It will have four 12V 100Ah batteries in parallel for the house bank.

It has, as of now, no solar installed. That I'll be dealing with this spring.

What I want is to be able to switch the solar from charging one bank to another.

I would parallel them for sure, giving you one larger bank and a way to charge them at the same time.. If you must break them up you can put switches to each bank and allow you to charge #1/#2 or Both ..

I currently have three banks, two 48V and one 12V. I've considered wiring the two 48V in parallel so I'd have only one 48V bank, but no matter what I do I'll have at least two banks.

What I want is to be able to redirect the solar charging into whichever of the banks needs charging. It may be, for example, that I've not been running the movie for a few days, and I need to charge the house bank, of perhaps I've been motoring a lot.

I've seen 12V charge controllers, and 48V charge controllers, and I've seen one that can be configured for either.

Yes Victron has many that can go from 48v to 12v, but remember the charge limit is based on amp rating to the battery being charged, not to speak of the moving of crap around from 48v to 12v, and also keep in mind what happens when you forget to switch the charge voltage..

How feasible would it be to install separate charge controllers for each bank, then switch the solar input between them?

Or would it make more sense to wire all the solar input to the charge controllers then switch the connections between the charge controllers and the batteries?

The best bet is to use the 48v to maintain the 12v or eliminate it all together.. So you can use 48v to 12v stepdown for the 12v unless you have high demands, then this is more tricky.. If you are just using lights, and other small load 12v items this will work well.. Many options.

If you need to high amp pull of starting motors or other big spike pulls, you will want to have a second charge controller and panel system to charge the 12v ..

Sharing becomes a problem when you need to change from 48v to 12v with a few items :
1) You have to login to Bluetooth and manual adjust the charge voltage (and don't forget ever)
2) You will may need to purchase a way oversized charge controller cause 20a in 48v will give you 960w charging, but with 12v it will only give you 240w
3) you will need to match the voltage input requirements for both 48v and 12v on this controller.. might be able to get it to work but very limiting.

2 charge controllers could allow both to charge at same time, and eliminate any chance of releasing magic smoke cause of a rushed switch to wrong voltage/bank ..

 

[jorby]

I have one of these controllers. It will auto detect the operating voltage when the battery is connected, so it could connect to each battery by using a bank1/off/bank2 switch. There are a couple of things to pay attention to:

1) The manual says that the battery should be connected before pv (to allow auto detection and parameter setting) and disconnected ONLY after pv is disconnected (to prevent possible damage).
2) The "LOAD" output function should not be used as the voltage will change along with the selected battery and you might blow up your 12v load.
3) The battery banks would have to be the same technology to use one of the "standard" charge profiles. I have not tested it to determine whether it can store different settings in the user config profile for different voltages. If it can, then the batteries could be different technologies.
4) When switching the battery bank, you would have to pause in the off position until the display went blank to ensure the controller would reboot and detect the selected battery voltage. A label at the switch indicating the sequence of operation (pv off, battery off, wait # of seconds, battery on, pv on) recommended!
5) I have actually charged 48v @100a (5kw) into lifpo4 bank with it. It will only do 1250w with 12v @ 100a.

 

[GXMnow]

I have one of these controllers.

What is the input PV voltage range? MPPT and absolute?

 

[jorby]

What is the input PV voltage range? MPPT and absolute?

The range is not specified as a "range". It is specified as 3v above battery to maximum 250v for this particular model. The input overvoltage protection point is specified as 250v. I don't know what it does when "protection" occurs as they do not say, but I assume it stops or will not start charging. Whether it will blow up if input exceeds 250v is unknown. If it has a disconnect relay, it is possible that there is headroom before damage would occur because a simple voltage sensing circuit ahead of the relay would prevent high voltage from reaching critical components. I will have to test it. It is not in use any more because I have a new inverter with 3 controllers built in. I will have to hook it up and play with it. I think I will try the dual battery switch idea with 12v vs 24v while using a power supply as the "pv" source. Maybe tomorrow.

 

[Rocketman]

The way I would deal with wanting one solar array to be able to switch its power into one of two banks is this way:

Each bank has a mppt SCC attached to it that is sized for the array. Then I would have a on-off-on switch that controls contactors. When the “left switch” is on the power goes to the left mppt - when right if goes to the right mppt. Since center is off - there will be a time delay when switching. If possible find contactors that will switch both the positive and negative wire - or use two per side.

These are several different ways to do it…

 

[Rocketman]

Just to add to my thoughts… put a solar on-off disconnect switch so you can shut the solar down - then switch over to the different battery - then turn solar back on.

 

[jorby]

Just to add to my thoughts… put a solar on-off disconnect switch so you can shut the solar down - then switch over to the different battery - then turn solar back on.

Yes, i mentioned that before. I think any controller would need that to be safe. They always need somewhere to put the energy. Meanwhile, I still intend to test mine as previously stated.... just to see how it will work.

 

[MattMan119]

Yes, i mentioned that before. I think any controller would need that to be safe. They always need somewhere to put the energy. Meanwhile, I still intend to test mine as previously stated.... just to see how it will work.

I noticed with my Victron it auto defects when you connect it out of the box, then when I switch it requires manual confirmation to change it .. I would make sure that it really is going to auto change the charge voltage.. But, i did not play with it that much maybe it would have or maybe i was missing something!

 

[jorby]

OK. I tested the controller with 2 battery voltages. I forgot that you can't change much from the front panel display. You have to buy the optional BT-02 module and use the phone app, which is easy to figure out. Qr code for app is in the manual. If you set it to lithium mode, the auto-voltage-detection function is disabled. If you select one of the other pre-programed lead acid charge profiles, it will do auto-voltage-detection, but won't let you change the charge voltages. If you select "other lead acid" it will do auto-detect and let you specify all the settings. After setting up appropriately for 12v lifpo4, I disconnected from the 12v battery, waited for the display to go out completely, then reconnected to 24v. It came up in 24v mode. Then I checked the app for the settings and found that the previous voltage settings had been doubled exactly. I then went back to 12v and the settings returned to those I had originally set.

So it will work as I previously described, but with three caveats:

1) Both batteries must be the same type.

2) It will not auto-detect 36 volt batteries.

3) There is a big capacitor on the battery side of the controller which creates a spark when connected. The spark increases with the square of the voltage so a precharge function (inrush limiter) would be nice idea. It might not matter to a high power switch, but I don't like to charge a capacitor that hard. I would use a 10 ohm resistor and a pushbutton* from each battery positive. You would press and hold the button for the battery that you are switching to, operate the switch, then let go of the button. That will eliminate the big spark and protect the capacitor while allowing the capacitor voltage to rise quickly enough for the auto selection to function properly.

* You would need one button for each battery. Perhaps a better approach would be a SPDT/center off/ spring-return-to-center switch. That sounds crazy but they are available.


1)