Adding second separate 12v battery

[user 423]

I want to add a second 12v 105ah battery to my solar setup. The present batt is 1 year old so I won't run them in parallel but use them independently.  One would be used while the other is charging.

I've spotted a Blue Sea 4-pole switch that will let you choose one or the other or both batteries to feed to the bus.

I'll also need to switch the feed from the controller to each battery so will I need another switch?

Or is there a way to use the 4-pole switch to do both?

 

[wayne49]

Simple, but possibly not the best, answer is to connect the solar positive to the same lug as the bus positive on the Blue Seas switch. Both the solar and the load would then switch together.

Caveat!!!! Only rotate the switch after the sun has gone down, or before dawn, when there is zero power coming from the panels. Or cover your panels whenever you want to switch.

 

[Tony's Dream]

I am interested in this answer as well.

 

[Trebor English]

If you have an MPPT comtroller it could cause a problem if you turn off the battery connection at just the wrong microsecond.  The current (energy) in the inductor might cause a "load dump" voltage spike.  If you have a cheap PWM charge controller, no inductor to step up current while reducing voltage, there is no source of energy to fry the controller or attached electronics. 

The problem to solve is to avoid paralleled different batteries discharging by passing current between them.  If they are charging or under load there won't be much, if any, current passing between batteries.  Paralleled with no charge current or load current is what you want to avoid.  Two single pole single throw on / off switches between the batteries and the bus will allow turning off one battery or the other to split the pair when there's no load or charge.

 

[user 423]

Simple, but possibly not the best, answer is to connect the solar positive to the same lug as the bus positive on the Blue Seas switch. Both the solar and the load would then switch together.

Wouldn't the output from the solar then be going only to the battery in use?

How would the depleted battery be charged?

 

[user 423]

Paralleled with no charge current or load current is what you want to avoid.  Two single pole single throw on / off switches between the batteries and the bus will allow turning off one battery or the other to split the pair when there's no load or charge.

Isn't it recommended not to parallel SLA batts of different ages?  Two SPST would take care of the output from the batts but it seems I'd also need another set of switches to connect the output from the controller to the batts (one at a time).

It's been awhile but I seem to remember 'stacked switches'. Rotating the switch would energize one set of contacts at the same time another set were disconnected. I guess that would be like combined 3-pole switches operated simultaneously?

BTW: I do have a PWM controller, so less chance of frying. Making the switch in low light would be the way to go like W49 says.

 

[B and C]

AFAIK, all controllers need to be connected to the battery first so the controller knows the system voltage (I only have a MPPT controller). Trying to switch in this way may have unintended consequences (ie., I wouldn't do it). The controller won't have any battery connected during switching. If you must go down this road, disconnect the solar panels, switch batteries and reconnect the panels.

Not best practice but just parallel the two and accept a possible shorter lifespan from the bank. You kinda made this decision when you started with one battery and needed two.

 

[user 423]

Trying to switch in this way may have unintended consequences (ie., I wouldn't do it).  The controller won't have any battery connected during switching. 

That's true. There would be a momentary no-connection.  The Blue Sea switches seem to have a 'make before break' feature built in that ensures that battery #1 is connected before disconnecting #2 so there's no interruption.

This should work for the controller-to-battery circuit also.  So I guess I'll need two switches.

 

[user 423]

Not best practice but just parallel the two and accept a possible shorter lifespan from the bank.  You kinda made this decision when you started with one battery and needed two.

Yeah, dang budget...LOL.  I may just go ahead and hook them up parallel to see the end result.

That would bring up another question though. I just have one 100 watt panel and don't plan on adding another, so would that be enough to charge 2 batteries? Probably have to run the generator more?

 

[Trebor English]

If you use more electricity a second solar panel might be what you need.  A second battery doesn't generate any more.  Maybe get the second solar panel first.  Then if you still need more overnight add the other battery.

 

[highdesertranger]

not all Blue Sea switches are make before break. some are, some are not. make sure you read before buying.

when you are charging the age of the batteries doesn't matter. it's leaving them connected together overnight when you will run into problems.

a simple on/off switch on each battery will do it too. but they have to be big enough to handle the load.

highdesertranger

 

[XERTYX]

I think I know what HDR is saying. Connect the charge controller to one of the batteries and then connect the other battery via a switch (rated high enough) to the first battery.

I'm kinda curious about this as well. I've been toying around with building a small solar system to use around the grill at night for lighting. And I bought a clearance 8Ah AGM battery used for deer feeders at tractor supply. Ive been watching them for a second markdown to snag the rest of their stock.

Then 2 days ago I was at Wally world and they had 3 7Ah AGMs marked down to $5.60 so I got all 3. But I'm worried about mis matching them.

I already have some small used panels from years ago and a cheap pwm controller that came with my 25 watt usb output flexible panel kit I bought.

So would this be the best way to connect it for the OP as well as myself?

Panel- Controller- Battery A- switch- Battery B

Initially i was thinking for my purposes with 3 7Ah and 1 8Ah battery this setup.

Panel- Controller- 7Ah parallel Bank- Diode- 8Ah Battery

I was thinking of using the 8Ah battery just for powering USB devices and the 7Ah bank only for lighting so I figured a blocking diode would be the best way to accomplish this and avoid mismatching the batteries. But a diode wouldnt be a good fit for the OPs problem I'm sure cuz it would need to be pretty damn beefy.

 

[B and C]

If one solar panel can't keep your one battery charged and you are wanting to switch back and forth, you will probably need another panel too. If you add another panel and a cheap controller instead of a switch, you would have two separate systems to run different loads off of. Just trying to give you ideas.

 

[user 423]

If you use more electricity a second solar panel might be what you need.  A second battery doesn't generate any more.  Maybe get the second solar panel first.  Then if you still need more overnight add the other battery.

The panel I have now is mounted on a smaller luggage rack that was already on my van's mid-high top. It is the perfect size to mount a single panel.

Adding a second panel would be a no-go for me at this time as I have knee and rotator cuff problems.

So now I'm checking for sales on portable panels.

BTW: Another reason I want to use and charge the batteries separately is that I want the 2nd. batt to be an AGM instead of SLA.

 

[B and C]

Different charge profiles so a second charge controller and panel unless you want to reprogram the existing charge controller each move.

 

[user 423]

^
Having a basic PWM from Renogy is an advantage in this case.

It simply has 3 indicator lights for each batt type: Sealed,Gel and Flooded.

You just toggle a switch to set it to the type you have.