Using an Automatic Charging Relay with solar

[dhandzy]

I've been using a Renogy MPPT controller with my solar panels for a few year where the controller is connected to one of my two batteries. I use the 1-2-BOTH switch and set to BOTH to ensure the charge gets to both batteries. But that means the battery that's not directly connected to the MPPT controller gets less current because it's going through lots of resistive wires before getting to the batter.

I'm looking into adding an automatic charging relay (ACR) so both batteries get equally charged. My understanding that an ACR just connected to both positive terminals of the two batteries and to their common ground and it does not connect to anything else (like the MPPT controller). Do I need to be concerned about impacting the alternator's function or is this just as simple as I make it out: add the ACR to the battery setup and I'm good to go?

 

[dlochner]

I've been using a Renogy MPPT controller with my solar panels for a few year where the controller is connected to one of my two batteries. I use the 1-2-BOTH switch and set to BOTH to ensure the charge gets to both batteries. But that means the battery that's not directly connected to the MPPT controller gets less current because it's going through lots of resistive wires before getting to the batter.

I'm looking into adding an automatic charging relay (ACR) so both batteries get equally charged. My understanding that an ACR just connected to both positive terminals of the two batteries and to their common ground and it does not connect to anything else (like the MPPT controller). Do I need to be concerned about impacting the alternator's function or is this just as simple as I make it out: add the ACR to the battery setup and I'm good to go?

An ACR, like the one from Blue Seas, is connected from one positive battery post to the other. There is one small DC- wire that needs to be fused and grounded to the DC- bus. The Blue Seas ACR is dual sensing so it doesn't matter if the switch is in the 1 or 2 position, but should not be in the Both position.

There is no effect on the alternator or the Solar controller. There is also no effect on the Solar controller, if it providing a charge current to one battery the ACR will allow the other battery to absorb some of that current.

The one consideration is how many wires are attached to the battery, no more than 4 per post. It may be necessary to add a DC+ bus to accommodate all the wires and fuses.

 

[thinwater]

I've been using a Renogy MPPT controller with my solar panels for a few year where the controller is connected to one of my two batteries. I use the 1-2-BOTH switch and set to BOTH to ensure the charge gets to both batteries. But that means the battery that's not directly connected to the MPPT controller gets less current because it's going through lots of resistive wires before getting to the batter.

I'm looking into adding an automatic charging relay (ACR) so both batteries get equally charged. My understanding that an ACR just connected to both positive terminals of the two batteries and to their common ground and it does not connect to anything else (like the MPPT controller). Do I need to be concerned about impacting the alternator's function or is this just as simple as I make it out: add the ACR to the battery setup and I'm good to go?

Measuring the voltage drop across the wires while charging and at float. I'm betting it's not 0.02 V when charging at what are very low amps for the cable size. They'll even out when it's on float and not changing. This feels like a complication for no measurable gain.

Measure it. Battery cables are big.

 

[Davidasailor26]

I agree with thinwater - the charging differences are probably negligible as-is. This isn’t like a parallel lithium bank with large loads where the cables could cause a more notable voltage drop, and even a small voltage difference could equate to a big state of charge difference.

The ACR would save you the inconvenience of switching the 1-2-B, and prevent you from forgetting to switch back to one battery and running both down overnight, but from a charging health standpoint I don’t think there’s much to gain.

 

[jssailem]

MPPT controller with my solar panels for a few year where the controller is connected to one of my two batteries. I use the 1-2-BOTH switch and set to BOTH to ensure the charge gets to both batteries.

Lets look at the idea in simple terms.

Battery A = House, it is on Switch 1. It gets "First Priority" when you connect the battery to capture solar power. At some point in time, you fill the battery.

Battery B = Start battery. It is on Switch 2. When Bat A is full, you turn the switch to #2, and the solar power goes into Bat B.

You are not feeling energetic, so instead of deciding #1 or #2, you choose BOTH. Now you have the two batteries in PARALLEL. Solar power is going to both batteries at the same voltage and about the same level of current. If one battery reaches "FULL", then all of the juice goes to the other battery.

With the switch in BOTH, both batteries are full, the MPPT recognizes this state and shuts down from absorption to float. Any solar power is dumped if the MPPT has an overcharge protection circuit. If not, then you could see the batteries continue in an overcharge state and begin to boil out Hydrogen from the batteries.

If your cables are of proper size and the distance from the Positive bus to either battery is short, then there is minimal "CURRENT" wasted in this setup. There would be a negligible voltage drop if the circuits are short in length and large-sized wires are used.

NOW for the ACR (Automatic Charging Relay). The ACR is going to be positioned between the two batteries. It will replace the function of manual battery charging selection. It will do this automatically. You sit back and sip a cold drink. The ACR takes care of the task. It does this by monitoring the voltages of the circuits.

The ACR checks and sees that the MPPT is producing solar power. It is monitoring the voltage of Battery A. When the voltage is above the ACR threshold (typically 13-13.6 volts ACR dependent), it closes the connection between Bat A & B. This means current that can not be used by Bat A is shuttled to Bat B "Automatically". Both batteries are getting charged (in parallel) from the single source. The benefit is that it maximizes solar energy use without needing a separate charger for Battery B.

That is the basic answer. More elements are involved in optimizing the setup, choosing proper overcurrent protection, and controlling the voltages for bulk/absorption and float charge conditions. Setting these would be dependent on the details of your systems and hardware.

 

[Stu Jackson]

Do I need to be concerned about impacting the alternator's function or is this just as simple as I make it out: add the ACR to the battery setup and I'm good to go?

Where does the alternator output go?
If it goes to the C post on the 1-2-B switch, you are using the switch as a charge directing switch. If the switch is off, your engine should be off, too.
All the ACR will do for you is to avoid having to leave the 1-2-B switch in the B position when you leave the boat.

Here's some reading for you:

OEM 1-2-B Switch Wiring History Alternator/Batteries & "The Basic" 1-2-B Switch BEST Wiring Diagrams

1-2-B Considerations (New 2020 - Rod finally got around to diagramming what I had done in the above link in 2009 )
1/2/BOTH Switch Considerations

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: Basic Battery Wiring Diagrams

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
http://forums.catalina.sailboatowners.com/showthread.php?t=137615

This is a newer primer for boat system wiring design with a thorough diagram: Building a Good Foundation (October 2016)
http://forums.sailboatowners.com/in...dc-electrical-foundation.181929/#post-1332240

What are ACRs, Combiners & Echo Chargers? (by Maine Sail) [scroll to the top]
http://forums.catalina.sailboatowners.com/showthread.php?p=742417 and Battery isolator / voltage regulator / batteries

Making Sense of Automatic Charging Relays (2019)
Making Sense of Automatic Charging Relays - Marine How To

 

[marcham]

I've been using a Renogy MPPT controller with my solar panels for a few year where the controller is connected to one of my two batteries. I use the 1-2-BOTH switch and set to BOTH to ensure the charge gets to both batteries. But that means the battery that's not directly connected to the MPPT controller gets less current because it's going through lots of resistive wires before getting to the batter.

I'm looking into adding an automatic charging relay (ACR) so both batteries get equally charged. My understanding that an ACR just connected to both positive terminals of the two batteries and to their common ground and it does not connect to anything else (like the MPPT controller). Do I need to be concerned about impacting the alternator's function or is this just as simple as I make it out: add the ACR to the battery setup and I'm good to go?

Difficult to understand what you've got going on here..... Can you provide a wiring schematic of your boat, along with fuses and wire sizes? What's connected to what? What do you mean by lots of resistive wires? The small current of solar panel would experience negligible voltage drop through the wires between the batt switch and the batteries. Are the batteries the same chemistry?

Are these 2 house batteries or a house and start battery? where is the alternator and shore power charger hooked up? if the 2nd battery is a start batt, it really doesn't need a solar charger so long as the boat is started every 2-3 months.

Normally, only your starter / alternator / dc panel would be switched with the 1-2-off-all batt switch. Other charging sources should be directly connected to a battery (via an appropriate fuse).

Do you want the ACR active when the alternator is running? when shore power is charging?

We need a clear understanding of how your boat is wired and what your goals are, then we can make some informed suggestions on options.

 

[Stu Jackson]

Do you want the ACR active when the alternator is running? when shore power is charging?

ACRs are usually active when ANY charging source is present.

 

[marcham]

ACRs are usually active when ANY charging source is present.

Not necessarily. You can wire them so they are only active with ignition on, you can wire them on the battery side or the load side of the switch, all of which will produce different operating conditions.

 

[Captain Larry-DH]

Any solar power is dumped if the MPPT has an overcharge protection circuit. If not, then you could see the batteries continue in an overcharge state and begin to boil out Hydrogen from the batteries.

One of the primary functions of the MPPT controller is to cut off the excess charging. They all do this. If it’s off it’s off. Excess solar power doesn’t need to be dumped elsewhere, although it can be.

 

[thinwater]

I've been using a Renogy MPPT controller with my solar panels for a few year where the controller is connected to one of my two batteries. I use the 1-2-BOTH switch and set to BOTH to ensure the charge gets to both batteries. But that means the battery that's not directly connected to the MPPT controller gets less current because it's going through lots of resistive wires before getting to the batter.

I'm looking into adding an automatic charging relay (ACR) so both batteries get equally charged. My understanding that an ACR just connected to both positive terminals of the two batteries and to their common ground and it does not connect to anything else (like the MPPT controller). Do I need to be concerned about impacting the alternator's function or is this just as simple as I make it out: add the ACR to the battery setup and I'm good to go?

The first thing I would do is measure the amperage on both sides. Takes seconds.