Bat. Switch vs. Auto Charging Relay

[karlt5503]

I have a question regarding the automatic charging relay(ACR). I just installed a second battery. in my boat to run only my amplifier. I will be running 2 batterys, one for engine, the other as a house, for the amp only.I have read all the threads about the bat. switch and what they do. My problem is that I dont want to run the wires for the bat. switch all the way to the helm where I can access the switch, but also dont want to open the engine compartment to get to the switch every time either. It looks like the ACR does about the same as the switch, but automatically. So, my question is, would I not be better off just installing an ACR over the bat. switch? Wouldn't this charge both bats and not allow one bat to discharge the other while anchored and listening to some tunes? Not sure how many amps the alt is, but its on a Volvo 5.7, so I think I would be ok charging both bats.

Thanks for any help or advice.

 

[itsonlymoney]

Re: Bat. Switch vs. Auto Charging Relay

installing an ACR, VSR or other type of isolator will take human error out of the equation when it comes to recharging an accessory or seperate system battery. You'll still need to run the motor enough to provide the charge or use a onboard/seperate charger to complete a charge after a day on the water or eventually your battery will become discharged.

Alot of what you seem to ask does depend on you discharge rate and size of your alternator. your batteries reserve capacity will play a role also.

 

[Silvertip]

Re: Bat. Switch vs. Auto Charging Relay

You are missing the flexibility of the battery switch but in your situation and with your wants, the ACR is a simpler install. The battery switch allows you to run everything on your boat from either or both batteries and it also allows you to start the engine from either or both batteries. Yes -- it does require a more comlex install with what you want to do. An ACR will do you just fine but the ACR gives the starting battery priority so the house battery may have issues if you do not keep it topped off and a short 10 minute run back to the dock will not do that with a deeply discharged battery.

 

[karlt5503]

Re: Bat. Switch vs. Auto Charging Relay

Thanks for the advice guys, I appreciate it.
Silvertip, I had a question about your comment. Are you saying the house battery will not get charged until the starter battery is fully charged? The alt. will not charge both batterys at the same time? Just wanted to make sure I understood. I do plan on charging the house battery fully when I get home, but would like to have the alt. keep enough juice in it to play the stereo and amp while on the water.

We should be running the engine(alt) 3/4 of the time while skiing, wakeboarding. But would probably anchor 1/4 of the time, listen to some tunes.

Thanks guys

 

[Silvertip]

Re: Bat. Switch vs. Auto Charging Relay

The start battery will generally get charged first and then the ACR switches to the house battery. Below is a description of a Blue Sea ACR

Combines when the higher battery has remained at the required voltage for at least 30 seconds. Disconnects when the voltage drops below the charging voltage to prevent accidental discharge of a battery bank.

An isolator on the other hand will allow charging both batteries simultaneously but there is a voltage drop across the device as well. With a battery switch, you charge or run off of whatever battery (#1, #2, or both) you have selected and there is no voltage drop. The point I'm trying to make is that if you do happen to run down the house battery (and you will with a high power amp), don't expect that battery to be topped off in a short run back to the dock. Use whatever system suits you but just make sure you understand the pro's and con's of each system.

 

[bruceb58]

Re: Bat. Switch vs. Auto Charging Relay

The ACR will switch to combined once the battery voltage of the start battery reaches a certain voltage. In real life, the start battery voltage will reach this voltage almost immediately unless it is really discharged which is seldom. I use an ACR and it connects almost immediately after the engine is started. Once the ACR is connected, the alternator will be charging both batteries with the more discharged battery getting more of the charge.

If the house battery is extremely discharged, the result of the ACR connecting the two batterries may drop the voltage of the start battery to a point where the ACR will disconnect. What will then happen is the ACR connecting and disconnecting over time but slowly bringing up the house battery until the combined voltage is above the "stay connected" voltage threshold. The reason this occurs is to prevent a failed house battery from discharging your start battery.

 

[karlt5503]

Re: Bat. Switch vs. Auto Charging Relay

Got it. Thanks for the advice guys. I think I might go with the isolator. Outside of the small voltage drop, looks like it will serve the same purpose but cost less. I don't think I will anchor too long as to run down the house battery real low. Hopefully I can keep them both charged by running the engine periodically. As long as I dont run down the starter battery I should be good.

 

[chrclo]

Re: Bat. Switch vs. Auto Charging Relay

I have an isolator and love it. When you wire it up upgrade your alternator charge wire to something like a 2awg (which is what I have). With that size wire it will take care of the voltage drop. I think there are some wire/voltage drop calculators on the 12volt.com you can use to increase the voltage to the batteries and regain that loss. I currently have between 13.9 and 14.5 volts to both of my batteries (this is the same voltage I had before I installed the dual batteries and isolator). Ohh and upgrade the battery ground wire too.

 

[Silvertip]

Re: Bat. Switch vs. Auto Charging Relay

You do not regain lost voltage. Voltage drop over a diode for example is lost as heat. Adding bigger wire on the output does not bring that voltage back. It merely prevents further loss.

 

[bruceb58]

Re: Bat. Switch vs. Auto Charging Relay

I would not recommend using an isolator. Your battery will never get fully charged.

 

[bruceb58]

Re: Bat. Switch vs. Auto Charging Relay

Ohh and upgrade the battery ground wire too.

Why would you upgrade the battery ground?

Bigger wire is not going to make your battery fully charge with the isolator. If you think about it for a second, the last amount of current that is needed to top off your battery is actually very low.

10 AWG wire is plenty for a standard alternator output.

 

[karlt5503]

Re: Bat. Switch vs. Auto Charging Relay

Man, now Im really not sure what to do. Bruce, why would my bat. not ever fully charge? I understand the voltage drop, usually .7 volts for a diode. But your saying no matter how long I run the engine (alt) the bat will never fully charge?

 

[bruceb58]

Re: Bat. Switch vs. Auto Charging Relay

It will never fully charge. The alternator will sense locally that the battery is up to a certain voltage and the battery will actually 0.7V lower. Only way to prevent this from happening with the isolator is to remote sense one battery but then what is going to happen to the other battery!

 

[klicknative]

Re: Bat. Switch vs. Auto Charging Relay

IMHO, the best setup for him would be an isolator solinoid. His boat has a 350 Chevy stern drive so it should have at least a 60 amp alternator. I used to run an isolator on my pickup but switched to a continuous duty relay to keep the camper battery charged. The best way to hook all this up is to run the pos switch terminal to a wire that's hot with the ignition on and the ground terminal to a switch on the engine that turns on when the engine builds oil pressure. If the boat has an hourmeter it may have an oil pressure switch allready. The wire from the "batt" to the relay can be hooked up to the starter or the alternator, wherever there is a circit heavy enough to charge the 2nd battery that is a "constant hot". I like to use the oil pressure switch so the only way the starting battery is connected to the house or trolling motor batterey(s) is when the engine is allready running.

A Perko battery switch does have the advantage of being able to combine the batteries but if everything is working like it should the starting battery will be fully charged when you are ready to go. Also, you'll have to run #2 battery cables to the switch, batteries and starter but with the relay you ony need #8 or #10 from the engine to the relay and then to the 2nd battery.

 

[bruceb58]

Re: Bat. Switch vs. Auto Charging Relay

The isolator solenoid that you just described is exactly the same as an ACR except that the ACR connects the battery once the voltage is up above a certain level. This prevents a dead alternator from allowing both batteries to discharge or a battery with a faulty cell from bringing down a good battery.

 

[chrclo]

Re: Bat. Switch vs. Auto Charging Relay

Thanks Silvertip, you are correct. I will have to clarify my previous post. You are not gaining voltage with a larger wire just lowering resistance. My alternator charge wire was 15 feet of 10awg wire then 6 feet of 4awg wire (for the stock system) which gave 13.9-14.7 volts of charge at the battery (according to the volt meter). I then installed the isolator and upgraded my alternator charge wire to a 2 AWG at 5ft in length thereby lowering the resistance. I now recieve 13.5 to 14.XX (I can't remember the exact measurment right now). But my point is if you have a system like mine you can lower the resistance over the wiring by upgrading the wire. This is done in the car audio world all the time, there it is called upgrading the big three. Hope this helps to clarify.

 

[bruceb58]

Re: Bat. Switch vs. Auto Charging Relay

chrclo,

It is not necessary having a 2 AWG charge cable. The large voltage drop is only going to happen initially when the current is very high. Yes it will lengthen the time it takes to charge the battery but that time will be negligible. Once the current demand drops, so will the small voltage drop. Getting rid of the isolater would be WAY more beneficial.

10 AWG is plenty....8 AWG is even overkill.

The reason that the audio world "thinks" they need large wire is that there are huge current demands for very short periods and the voltage drop during those times is detrimental. I stress the word "thinks". A alot of what the audio world thinks they need to use goes against a lot of engineering common sense...like buying Monster cables!

 

[chrclo]

Re: Bat. Switch vs. Auto Charging Relay

Thanks for the repy Bruceb58,
But in taking voltage readings with the 10 awg + the 4awg + the isolator the voltage drop was about 1.5v (or so). By spending a little on the 2awg with the short run I was able to cut that in half. For me it was worth it to see the multimeter readings creep into the acceptable range. I should have wrote all this down when I did it. lol

 

[bruceb58]

Re: Bat. Switch vs. Auto Charging Relay

How long after you started the engine did you take those measurements? Also...what was the drop through the isolater compared to the cable.

 

[chrclo]

Re: Bat. Switch vs. Auto Charging Relay

About 10 minutes or so. I made sure I had the alt in charge mode (i.e. rev the engine a bit). The temp in the engine compartment was not that high and I did get the .7 volt drop from the isolator. I belive I was in the low 14 volt range at the alt charge wire and the high 13s at the battery.