Charge controller through an isolator

[Wobbly Wheels]

Howdy all,

I have a pair of batteries that will feed the DC bus and inverter through a battery switch as two separate banks. On the charge side, I have an isolator that my 110V charger will be connected to. I will also have 60W of solar, primarily to maintain the charge while in storage.

I would like to connect the solar charge controller to the same terminal on the isolator, but do I need a diode between the solar charge controller and the isolator to prevent the battery charger's current from backfeeding the controller ? I assume that the controller is already so equipped....but you know what they say about assumptions.
I'd rather not let the smoke out of the box

Thx

[MtnDon]

I'm not following why you have an AC powered battery charger connected to an isolator in the first place.

In my mind an isolator is used on the vehicle end of a power connection between the vehicle alternator/battery and the trailer battery. It is there to prevent the trailer battery and trailer power demands from depleting the vehicle battery.

Any charge controller, even the cheapy little bricks have diodes in them to prevent battery discharge.

So what's the purpose, the intent, of this isolator you are connecting the battery charger to? I'm probably misinterpretting something.

[Wobbly Wheels]

Thanks for your thoughts on this and yeah, that's definitely where one most often sees isolators used.

In my installation, the isolator will allow me to split the charge current between both banks without having a depleted battery drawing down a more charged battery.
My two batteries for the trailer have two different capacities - the isolator will prevent them from trying to equalize to each other, since any other method of charging both will see them tied together. The isolator acts as a check valve allowing current to flow into the batteries but not from one to the other - that's what the diodes do. Isolators are an integral part of multi-bank chargers but, since I am using a single-bank automotive charger at the moment, I want to be able to charge both without having to backfeed the charger through the battery switch to charge them separately.

Its input terminal will also provide a common point to connect multiple charge methods: an AC charger, jumper cables, solar, genny, etc. Because the charge controller and the AC charger are both connected to the isolator's input terminal, I need to figure out if the controller is up to the job of blocking that current from backfeeding either itself or the panels. I assume it is, but...


Not sure how clearly I explained that...

[MtnDon]

Well, even the $20 Chinese brick I use on a battery maintainer solar charger has diodes to prevent discharging the battery. I believe you would be hard pressed to find one that did not have diode reverse protection.

I have a personal dislike about the isolators that use diodes. They all have a voltage drop from the input side to the output side. 6/10 of a volt difference maybe, but enough to make it difficult to achieve a true 100% charge in the isolated battery. Have you thought about that? Or is your isolator one that does not have that issue?


It may be just me, but I think it would be simpler to have two identical batteries in parallel.

[Wobbly Wheels]

My isolator is a Vanner 51-140, which uses a Schottky diode array specifically to reduce the drop across silicon diodes. They are the standard for use in ambulances which have strict certification limits on electrical loads. It's total overkill for what I need, but since I have it...

On the surface of it, two parallel batteries would indeed be a simpler solution. Unfortunately, my experience in boats (marine tech) has shown me that there is no such thing as two identical batteries - even brand new off the shelf - though at some point we will venture into splitting hairs on that topic. My batteries are cores that are service replacements from police boats that get replaced every two years regardless of condition. I reimburse the shop for the $15 core charge and take the 'old' batteries after charging and load testing them. As a result, even though the batteries might test like new, I will never be able to match the capacities: I get whatever I get. Using an isolator means I can put whatever I want into each bank, including brand new batteries later on, and swap them out as needed when they do pack it in.

Not knowing much about solar applications specifically, my query is about the diode blocking in the controller. I assume it is sufficient, but I'd prefer to know for sure since charging voltages coming from the charger are higher than the controller would ever see when tied only to a battery. At the same time, it would be reasonable to expect (and design for) a controller to be tied to multiple charging sources without transfer switching.
Is that voltage difference greater than the typical breakdown voltage of the diodes ?
Should I install a heavier diode inline ?
Or am I over-thinking it ? (I've been known to do that, lol)

[Dale M.]

How about a "relay" as an isolater for charging circuits ... You could isolate solar from the AC charger....... You can make a self locking circuit for relay (ac powered) so any time you disconnect AC , it drops relay off and used solar to charge battery... Simple single pole double throw relay (switch) and gives you choice of either charge method... And you choice of what you charge method used would be determined by if you do or do not have shore power (and manual override)...

As for your question, I would use blocking diode on all charging sources to prevent feed back into other chargers if is not internally protected for feedback...

Dale

[MtnDon]

it would be reasonable to expect (and design for) a controller to be tied to multiple charging sources without transfer switching.
Is that voltage difference greater than the typical breakdown voltage of the diodes ?
Should I install a heavier diode inline ?
Or am I over-thinking it ? (I've been known to do that, lol)

I have no idea if the built in diodes would be up to higher voltages. I would guess they have some cushion but how much? No idea.


Please allow me to play devils advocate for a moment. Are the "free for the core charge" batteries worth all the extra hardware and trouble when a proven solution is two 6 volt golf cart batteries in series? That gives approx the same amp-hour capacity (a little more I believe) and they are true deep cycle batteries with thicker lead plates.

I've used that configuration in RV's since the late '70's. GC-2's, the way we use them, last 4 years for certain with 5 years being common. Sometimes solar charged as well as house powered or generator powered battery chargers.

[Dale M.]

Are the "free for the core charge" batteries worth all the extra hardware and trouble when a proven solution is two 6 volt golf cart batteries in series? That gives approx the same amp-hour capacity (a little more I believe) and they are true deep cycle batteries with thicker lead plates.

I have only seen you say this.... How do you prove this...... Yes I agree it may be good solution for high amperage (capacity) usage, but then again most of my life I have been associated with batteries in parallel for "capacity" and have never seen a negative comment about this other than "no two batteries are equal".... Geez.... The telecommunications companies installations have thousands of batteries in parallel, if it was a problem, they would have moved away from concept years ago....

Dale

[Wobbly Wheels]

Are the "free for the core charge" batteries worth all the extra hardware and trouble...

I appreciate the critical eye
When I already have the hardware, a resounding yes
And it's no 'trouble' since these are exactly the sorts of problems I enjoy solving. If I'm working on a client's boat (for example), I don't get the option to try something just to see if it will work.
Kinda bent perhaps...but to me that's recreation

Dale, in thinking about the relay...it occurs to me that a standard 12V 30A cube relay with the coil connected across the charger's DC output, and the solar charger run through the NC contacts would serve as an automatic transfer switch, no ? When the charger is putting out DC current, the lead to the charge controller would be opened. I would need to use the NO contacts to provide a dump for the controller's output when they close, but an LED ought to do that (that's what was supplied with the kit for that purpose) and would also serve as an indicator.
Hmmm, interestinger and interestinger....thanks for your input, guys - love kicking this stuff around....

[MtnDon]

I have only seen you say this.... How do you prove this...... Yes I agree it may be good solution for high amperage (capacity) usage, but then again most of my life I have been associated with batteries in parallel for "capacity" and have never seen a negative comment about this other than "no two batteries are equal".... Geez.... The telecommunications companies installations have thousands of batteries in parallel, if it was a problem, they would have moved away from concept years ago....

Dale

Here's a link that may help explain things...
http://blog.thesolarbiz.com/keeping-you-and-your-batteries-alive/

Telecommunication batteries spend most of their life sitting at float charge, waiting for the power mains to go dark. Any lead acid battery is happiest when maintained at a full charge. That's how the telecom industry avoids problems. Telecom use is not the same as being discharged and recharged daily as in typical RE use, with some sunless days meaning no complete recharge at all. A travel trailer battery set lies somewhere in between telecommunications and RE use; periods of no use (and maybe on constant float with a good multi stage charger giving periodic equalization charges) interspersed with periods of medium to deep discharge.

[MtnDon]

Are the "free for the core charge" batteries worth all the extra hardware and trouble...

I appreciate the critical eye
When I already have the hardware, a resounding yes
And it's no 'trouble' since these are exactly the sorts of problems I enjoy solving. ....

I appreciate the satisfaction that comes from a problem solved successfully. As I have aged I tend to try and make systems more simple so others without the required knowledge can obtain satisfactory results with the least input. That's all.