Joining 2 solar systems on 2 rigs

[noteven]

Hi we haven’t had questions from me, The Solar Dummy, for a while...

I have a truck camper and an enclosed cargo trailer, and a Big Rig. I am thinking about this scenario:

Install some mounted solar and 6v batteries in the camper.

Install some mounted solar and 6v batteries in the enclosed trailer which is used for bike transport and expanded living space, bike gear, easy chair, rainy day bench racing, and stuff carrying.

Have a portable solar setup to use with The Big Rig, and use with the truck camper/enclosed trailer to get out from under the trees or to have more capacity. The batteries from the enclosed trailer would go in The Big Rig as I don’t use both at the same time. 

So my questions: 

Will my rigs go up in flames if I connect a set of 95% state of charge 6V’s to a 87% state of charge set after driving back to camp?

Is it ok to connect and disconnect the trailer solar/batteries to the camper set as side trips like runnin to town or going fishin without the enclosed trailer occur?

And then later set up the portable and connect it too, but not always, and so on? 

If all controllers and panels were matched equipment is this better? 

[Kirk W]

2 minutes ago, noteven said:

Will my rigs go up in flames if I connect a set of 95% state of charge 6V’s to a 87% state of charge set after driving back to camp?

1

There will be a power surge between the batteries to get them to a balanced charge state but I'm not sure how bad it would be or the impact on your batteries. Perhaps one of the resident electrical engineers has some background in this sort of thing and can help you out. Parallel batteries will always seek to share the same state of charge. 

[Lou Schneider]

Shouldn't be a problem.  You don't get enough of a surge to worry about when you turn on a converter into a dead battery pack, connecting two packs with different SOC shouldn't do any harm either.

In normal use, you're talking about a 2-3 volt difference between a full charge and a full discharge, so even a few tenths of an ohm of wire and connector resistance between them will limit the current to safe levels.  Any surge will only last a short time, until the lower pack gains a surface charge to match the higher pack's voltage.

But I would put a disconnect switch and fuse or a circuit breaker between the two packs so you can break the connection before you plug in to protect the plugs and sockets against wiping burns, similar to turning off the circuit breaker before you plug or unplug from shore power.  The fuse or breaker should be sized to the wire capacity, to protect it against overcurrent in case you inadvertantly plug a fully charged pack in one vehicle into a completely drained or shorted battery pack in the other.

[oldjohnt]

H again noteven, I enjoyed your earlier questions hope you got help here??

Okay, there's obviously no problem having two independent systems, truck and trailer etc. and its the heart of your question as to what happens when you parallel them together ??? The size and solar capacity and charging system of each bank need not be matched, its that below that makes a difference. 

As in nature, systems seek balance, the same will occur when you suddenly join two battery banks, they will tend to equalize and the current flow depends on 1) The state of charge and difference in the two banks 2) The size, type and capacity of each bank  3) The size and distance of the interconnecting cables 4) The total resistance  of all the cables and switches and connectors  etc involved in the parallel connection.

The lower SOC bank will draw down the higher,,,,,,,,The greater the difference in battery bank capacity and greater the difference in their respective SOC's THE GREATER THE INITIAL CURRENT SURGE/DRAW,,,,,,,, The bigger the cable and shorter the distance and the use of sufficient current rated connectors and switches is an advantage (less voltage drop and less resistance).

That all being said I don't envision major problems, its not too much different then  connecting a battery charger to a battery OTHER THEN depending on the amp hour and CCA rating of your battery bank(s) a huge battery bank has the capacity to deliver a lot more initial surge amps into a load then a modest sized battery charger, so sure, its a bit more risky then connecting a charger MORE STORED ENERGY CAPACITY IN A HUGE BATTERY BANK . 

I would use overcurrent protection and disconnects to avoid any long term catastrophic current draw DO THAT AT BOTH BANKS.

I say go for it using big enough wire and quality connectors/relays/switches with adequate overcurrent protection. If the wiring is too small and one bank is way lower then the other, there can be significant current and overheating SUBJECT TO current and wire resistance. More resistance from small wiring may limit current,but it drops voltage and could overheat if there's enough current for long enough time !!!

NOTE when designing a system I prefer ALL batteries be the same type and brand and design and rating and age etc etc etc and Id still like that so each system is the same, however, yours is not quite the same situation so you can get by.

John T Electrical Engineer but so longgggggggggggg retired n rusty NO WARRANTY !!!!!!!!!!!!!

[noteven]

Thanks everyone - ya I was thinking neither bank should be at too different a soc from the other. the camper charges from the truck while driving plus it’s own solar, the trailer back at camp is charging by solar and neither has a huge load. I was thinking of using fuses at the bank + terminals, disconnect switches  and Anderson style connectors with fairly hefty cable. I was thinking of figuring out the wire R and matching the cable R of the two banks...

Like I said a bit of a dummy but I do know DC electricity is happy to return to earth at a high rate of current until something catches fire or the batteries are flat...

And batteries like solar cause it can keep them topped up properly at high voltage low current for a long and happy life..

From what I can see in on line research is solar controllers seem to get along...

 

 

[ALLOY]

 How will SOC/voltage of each bank be determined?  The voltage will vary depending on load and while under different states of (bulk, absord, float) charged.

A battery balancer or a 12v to 12v charger would take the worry out of connecting the banks.

The batteries will be in boxes and vented to the outside?