Wiring for MPPT solar controller with battery charger

foothjim · October 2019

Hi. New here so expecting to be severely roasted on this question but here goes... I'm looking at rewiring and upgrading and old home made off grid setup in my hunting cabin to include a couple PV panels but want to revamp the way it's wired. Currently using a newer 45A Progressive Dynamics RV power converter/charger to charge two 12V battery banks (2 golf cart batteries each in series). An inverter runs off the batteries and feeds a couple outlets, and there are a couple of DC circuits that run directly from the batteries. Power for the charger comes from a gas inverter generator. It's set up pretty simply now with a marine battery switch that ties the two banks together for charging and lets me use one or the other so one bank can be kept fresh for night time to run the fridge while the generator is off. Charger is currently wired downstream of the selector switch so that if the generator (& charger) is running and the switch is off, the batteries are disconnected completely and the charger is able to run the circuits directly.

Sometimes if one bank is discharged quite a bit (fridge on all night for example) and the other is still good, I'll start the generator and keep the selector switch on the bank that's low to give it maximum amps from the charger, then a few hours later switch it to "All" to top off the other bank and use that one for the night. I try to alternately use each bank and let the one just charged rest before using it, not sure how much that even matters though.

What I'd like to do is a couple things at once. I'd like to simplify the charging process by using a battery isolator, and also add a pair of PV panels with an MPPT controller to the system. My main question is how can I connect the DC battery charger/power supply to the batteries along with the MPPT controller without frying it if both are running at once? Do they have to be physically disconnected from one another (switched etc so only one or the other is charging the batteries) or can they both be connected to the batteries (at the isolator input) and charge simultaneously? Does there need to be a diode in the circuit for the MPPT controller ? Just wondering if both were connected together will the MPPT misread the state of charge in the batteries and back way off on the amps it outputs.

My first crack at a wiring diagram looks like (ignoring fuses/breakers & grounds for the moment, and setting aside my question on the MPPT and charger connection):
MPPT controller and battery charger + leads connected [somehow, pending previous question] at the input terminal of the isolator.
Outputs from the isolator going to battery bank 1 and bank 2 + respectively.
Cable from bank 1 to selector switch input 1
Cable from bank 2 to selector switch input 2
Selector switch output to inverter + and 12VDC fuse panel + input.

In this case, "ALL" on the selector wouldn't be helpful since it would bypass the isolator so can't think of a reason to use it, but I could still use each bank separately like I've been doing which works well for powering the fridge at night as I mentioned.

So please let me know what the collective here thinks about my first couple questions and my proposed diagram. I can make up something graphical if my description sucks. I'm prepared to be majorly trolled and called a moron etc which seems to happen any time I ask a question in places like this but hopefully some nice folks can correct me and help me out without going there, although I know it's oh so tempting and satisfying..).

Estragon · October 2019

With two batteries often at different states of charge, I'd likely use a separate controller for each. Mppt may not be needed if using 12v nominal panels and wire runs will be fairly short (eg < ~50-75'). Pwm tend to be much cheaper than mppt.

Having multiple charging sources on a battery isn't generally a problem. Physically making lots of good connections directly to the posts can be a problem though, so there's often a single + and - wire (per battery in your case) to buss bars used to connect all the other DC stuff.

MichaelK · October 2019

I think you should just gut out your existing system and start over. You need more capacity, and sticking with 12V is really holding you back. I'd switch to 24V and get larger batteries, like L-16s. Six 30V grid-tie panels producing about 1500W would keep them happily charged. A single string of larger batteries at 24V instead of 12 would solve all of your problems, and need less attention on your part.

Estragon · October 2019

If OP was starting from scratch, I might agree. That's not the case though. There's a "newer" 12v charger, unspecified 12v inverter that apparently runs the fridge and any other AC loads, and some native 12v loads.

IMHO, the benefit of going to 24v is not such that it justifies ripping out what's already there unless there are other problems or unmet needs not mentioned.

That said, I'n not a big fan of keeping separate 12v batteries/banks. On a boat or RV, it makes sense to separate a starting battery, but for a stationary application it complicates things with limited benefit.

icarus · October 2019

MichaelK said:

I think you should just gut out your existing system and start over. You need more capacity, and sticking with 12V is really holding you back. I'd switch to 24V and get larger batteries, like L-16s. Six 30V grid-tie panels producing about 1500W would keep them happily charged. A single string of larger batteries at 24V instead of 12 would solve all of your problems, and need less attention on your part.

In principal I agree, in practice not so much. For the kind of use, L16 are way too much for such a small system. The classic error people make is over battery, under PV/charge.

all considerations begin with the loads. Accurately calc the loads in AHs or WHs then work backwards. Figure your seasonal use pattern, and decide how much genny use you are willing to live with. Size the battery bank for those loads, then size the PV and generator charge regimen to fit. 4 Golf cart batteries have a total of ~450 ah of capacity in a 12 volt series/parallel config. Of that, figuring a discharge to 50% (I prefer a discharge only 20% (80%) will give 250 ah of power (12 volts) or about 3kwh of power. That, for a small cabin, with properly designed loads, led lights etc is plenty. To get 250 ah back from solar, assuming moderately good conditions, you might need ~1kw of PV if you are going to try to draw that on a daily basis and use no genny to top up. 500 watts, with 3hours of genny time at 50 amps would accomplish roughly the same thing.

Bottom line, all three components need to be balanced. Using a battery bank that is too big is wasteful and quite expensive on a per kwh basis. Consider golf cart batteries cost ~$150-200@ for 225ah (~$.80ah) L16 cost ~$330 for ~420ah, ($.78ah). Both batteries are likely to ‘age out” before they will cycle out. I did a calc when I built my current system. 4 t105s or 2 L16s. If the T105s lasted four years, and the l16s 8 I would be even as per cost. I am now on year 12 on the T 105s.

Bottom line, buy a good set of batteries (One set!) build the charge regimen around them to suit, assuring proper charge currents are available from either solar of generator. Buy a good controller to keep the batteries well managed when you are not there, and buy a good 4 stage charger, like an Iota, and get a good battery monitor system. You can build it piece by piece assuming you have the genny and a good charger, then you can add a good controller, and the PV that you need or want. It sounds like the heart of your system now is the genny and the batteries. PV is the icing.

tony

PS there is nothing wrong with a 12 vdc native system. Of course large inverter loads are harder to manage, but it doesn’t sound like you have large 120vac loads. Native 12 vdc allows you to use all kinds of 12 vdc native stuff like car radios wired into the system, LP fridges with 12vdc igniters, We have lived off grid forever, gradually evolving from Kerosene lamps to small 12 vdc reading lamps, to 24/7 solar battery power. Much of the house still runs on 12vdc without inverter losses. Bottom line...KISS

T

foothjim · October 2019

icarus said:

all considerations begin with the loads. Accurately calc the loads in AHs or WHs then work backwards. Figure your seasonal use pattern, and decide how much genny use you are willing to live with. Size the battery bank for those loads, then size the PV and generator charge regimen to fit. 4 Golf cart batteries have a total of ~450 ah of capacity in a 12 volt series/parallel config. Of that, figuring a discharge to 50% (I prefer a discharge only 20% (80%) will give 250 ah of power (12 volts) or about 3kwh of power. That, for a small cabin, with properly designed loads, led lights etc is plenty. To get 250 ah back from solar, assuming moderately good conditions, you might need ~1kw of PV if you are going to try to draw that on a daily basis and use no genny to top up. 500 watts, with 3hours of genny time at 50 amps would accomplish roughly the same thing.

The feedback is much appreciated from all who responded so far. A little more background. This setup has evolved and improved over the last 25 years so there are some old/legacy components and wiring I'm always gradually replacing and improving. Budget is the name of the game here; the cabin is used relatively few days out of the year so we're trying to keep it fairly inexpensive component wise. The batteries are 215Ah Duracell GC2 golf cart batteries, a deal at $90 or so from Sam's. The previous set we had before we replaced them lasted several years so they did age out vs cycle out. The inverter is a 1500 pure sine wave, can't remember make but it's new. Largest load is the refrigerator, a 10ft^3 electric which is also new and very efficient, so much so we abandoned our search for a good gas fridge which are insanely expensive. The inverter runs 2 overhead light fixtures in 2 rooms all evening, all with 13w 110VAC LED bulbs (all lights in the cabin are now LED) and is also connected to a couple of dedicated outlets so no large loads. There are 2 outlets wired directly to generator power via a small load center for larger loads like the microwave or vacuum cleaner since there was no reason to run all that through a much larger, more expensive inverter just for the occasional usage of those items. There are some "native" 12v LED area lights, bunk bed lights, and porch lighting as well as in the outhouse which are only used as needed. Water pump is 12v/RV so 5A or so but a transient load. Only other thing is a 30" LCD tv which doesn't use much power and is not on often. I had thoughts of wiring all the lighting direct 12v by using 12v LED bulbs in the fixtures so they'd all work if the inverter went poof, but it doesn't matter much since it has to be on all night anyway to run the fridge.

Generator is either a Honda EU2000i or an equivalent Predator inverter generator. We usually start it up after we all get back from the woods in the evening during hunting season and it will run until bedtime which is typically 5hrs max. There are times we are not at the cabin in the evening until later on (trip to the tavern or restaurant) so generator time is reduced since we don't like to have it running when we're away. Out of an overabundance of caution, I disconnect everything at the switch so nothing is powered and no wires in the cabin are live. I agree a single bank of batteries would be ideal, but the system we have for keeping one bank "fresh" for night time has worked well for us in the past. I'd be concerned with a larger bank (all four) not fully charging in that case and always being behind the 8 ball on generator time. Maybe a larger charger would be the solution to that - Four (vs two) GC2s charging plus running all the lights and refrigerator as well as other transient loads I'm guessing would use up all the charger's capacity currently. When the bank being charged is pretty low the charger gets fairly warm and the fan runs. Any ideas on a conservative/worst case number on charging amps for one of these batteries? I could check but don't have that large an ammeter at the moment.

The idea of the PVs would be to allow them to catch up the batteries to an extent so they have some amp hours put back in during the day to somewhat minimize generator time, not replace it completely, so it would be more of a bonus add-on to the system. I'm thinking of a pair of 300W panels to start out might put out a good chunk of what the battery charger can do under good conditions.

Any thoughts on my question about the wiring of the isolator feeding both banks from the 2 sources (PV and battery charger)? I've never used one before and wondering if I'm understanding it's function and how to connect it correctly.

Estragon · October 2019

As a general proposition, my understanding is the battery isolator keeps banks separate while discharging, and combines (parallels) when charging. My further understanding is not all isolators do this in quite the same way, so it may help if you could post a link to the particular isolator you have (or better, are considering).

A pair of 300w panels (likely needing mppt), properly tilted in full sun, might put out 30-35a for a few hours around noon.

If the inverter has a search mode, it doesn't need to be "on" all night. In search, it uses less power to check every few seconds to see if there's a load. When the fridge wants to run, the inverter wakes up to run it. It may not really save enough to be a consideration in whether to do DC for the AC LEDs though. I have both.

Wiring for MPPT solar controller with battery charger

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