Dual battery ACR and Current to drained battery

[lonesouth]

On a dual battery setup. If you drain the house battery while anchored, when you crank up on the starting battery and head home, what is the current to the house battery through the ACR? Since they are in parallel, the house battery is now a load to both the alternator and the starting battery until the batteries are balanced and then both are a load to the alternator. The question is, since the starting battery is capable of delivering several hundred amps, what is the current flow to the house battery in a discharged state?

 

[Silvertip]

Re: Dual battery ACR and Current to drained battery

I think you misunderstand how an ACR works. Most ACR's "default" to the starting battery at any time, making sure it is always charged. During that period current does not flow to the house battery, nor can current flow from the starting battery to the house battery because the ACR prevents current flow to the house battery. Only after the starting battery is charged does the ACR switch alternator output to the house battery. That maximum current flow will be whatever the maximum current output of the alternator is. If you have a 100 amp alternator, the charge rate will be whatever the battery accepts and the voltage regulator in the alternator allows. Only if the ACR has a "combine" feature can you charge both batteries at once. The regulator in the electrical system cannot determine which battery it is monitoring. If they are in parallel and one battery (typcially the house battery) is deeply discharged, setting the ACR to "Combine" will draw the voltage down to the point where the regulator output goes high until the "system" voltage is such that the regulator will now begin to taper down the charge rate.

This is why I prefer a dual battery switch. You can manually select where you want the alternator output to go whether that be BAT 1, BAT 2 or BOTH. Likewise, you can manually select whether you want BAT 1, BAT 2 or BOTH to power everything on the boat. If switches cause confusion or are difficult to incorporate into the boats electrical system because of location, spacing of batteries, or any other reason, then an ACR is the next best option. But that's just my preference.

 

[bruceb58]

Re: Dual battery ACR and Current to drained battery

That's not how the Blue Sea ACRs work and most others are similar. If either side senses a charge voltage, it will connect the two batteries. It does have a feature that if one of the battery voltages is too low, it will not allow them to connect. There is nothing that determines if one battery is fully charged before connecting.

http://bluesea.com/files/resources/instructions/990170140.pdf

 

[Silvertip]

Re: Dual battery ACR and Current to drained battery

There is a detailed discussion on this topic on the Continuous Wave forum (not that they are any more in the know than anyone here) but there are a couple of ways this device has been designed and I will admit I have not seen them all. Blue Sea ACRs work as you say and have for several years. But the sense circuit in some of the other ACRs (perhaps even older Blue Sea units did not cause the ACR to switch until that circuit sensed something over whatever the threshold was (something over 12.6 volts) for that ACR. Hence the initial default to the Starting battery. I guess since the start battery doesn't get discharged very deeply, it reaches the threshold quickly so the combine occurs relatively quickly regardless.

 

[lonesouth]

Re: Dual battery ACR and Current to drained battery

Silvertip, that is mostly correct, however, a battery acts like a long term storage capacitor. The way the ACR is wired in, there is no switching of where the alternator power goes. Rather it defaults to the starting battery and charges it to top off, then closes to parallel the two batteries. However, at this point, if the house battery is lower than the starting battery, the HB will act as a load to the SB and the alternator. The alt will run to charge the combined parallel battery.

The question is, how much current is going through the ACR. Since the SB is essentially a storage capacitor, and capable of up to 700 amp discharge, it is important to know that those 700 amps will not dump into the HB through the ACR.

If the internal resistance of the HB is such that it will limit the charging rate by limiting current, then this may not be an issue. However, I do not know what the resistance is and what the current potential is.

Bruce, that is a good thing to know, about the voltage. That may be there to prevent the situation where the charge current(dump from the SB) is greater than the ACR can handle. I would guess the ACR has an inrush limit of ~200.

 

[bruceb58]

Re: Dual battery ACR and Current to drained battery

Between the internal resistance of the battery when it is discharged, other resistances between the cabling and the ACR itself and the voltage limit, the inrush current will be fine and won't last that long. Really no different with what happens when you try to jump start a dead battery.

The threshold is 13.6V after 30 seconds or 13.0V after 2 minutes. Typically, if the start battery had enough capacity left to start the engine, these threshold will be reached fairly fast. Another example would be if you start an engine with a completely dead battery by jumping it, remove the jumper cables and measure the voltage, you would be way over the threshold and the battery would be still taking a huge charge.

 

[lonesouth]

Re: Dual battery ACR and Current to drained battery

Care to speculate what the charging current to the house battery is, assuming the HB has a voltage around 11 volts.

 

[bruceb58]

Re: Dual battery ACR and Current to drained battery

I can't...but I will tell you that if you let your house battery get down to 11V a few times, you will be replacing that battery. 11.1V is 100% discharged. I would never let it get below 12V which is 50% to 60% discharged if you want to make it last awhile.

 

[lonesouth]

Re: Dual battery ACR and Current to drained battery

Here is the explanation from classicbroncos.com

The current that charges a battery is dependant on two things, the voltage pushing that current (alternator or second battery) and the resistance of the battery (and cabling) itself. When a battery discharges it's internal resistance decreases. As it charges back up it's internal resistance increases. That's why a discharged battery will take a large charge current initially and then slowly decrease the charge current over time.

How much current will it take? Well batteries are very good at providing massive amounts of current when they are fully charged (higher voltage) if that current is going into a very low resistance. However when discharged, they are not (should not) exhibit a zero ohm resistance. If they did there would be no way to recharge them. Since a discharged battery has some degree of resistance it will not consume every bit of current that is capable from the alternator or second battery. Think of the battery as a current self-regulator, as long as it is not damaged. Damaged batteries can approach zero ohms and will destroy alternators also. I'd say it's safe to assume the battery charge current can be anywhere from zero to possibly 200 amps for a short time if everything is in good condition (other than discharge).

If you use that ACR relay just wire it up like it shows in the instructions here. Run your alternator output directly to the battery you want for starting and the ACR will take care of charging the second battery when it needs it.

Disclaimer: In no way should you ever attempt to use an ohmmeter on a battery. Your ohmmeter will melt and kittens will die.

 

[jhebert]

Re: Dual battery ACR and Current to drained battery

If you install a Blue Sea Systems automatic combiner relay (ACR) following their instructions, there will be two fuses in the circuit that connects the two batteries through the ACR. The current through the ACR cannot exceed the fuse ratings, which typically are just below the current rating of the ACR, which is usually limited to 50 to 100-amperes.

The observation that when the discharged house battery is combined in parallel with a fully-charged starting battery that the house battery represents a load is exactly right. The current flowing into the house battery will come from the starting battery and any charging current from the alternator in the boat engine.

In designing an ACR system one element that must be considered is the size of the batteries and the current output available from the charging system. If the batteries are of much larger capacity than the charging system, when a discharged battery is placed in parallel with a charged battery, the load may be too great for the charging system to maintain. The result will be that the battery voltage of the paralleled batteries will drop below the threshold of the ACR setting. This causes the ACR to drop the connection. Once the connection drops, the battery voltage of the charged battery goes back above threshold, closing the ACR. The sequence repeats, causing the ACR to chatter.

To prevent this the ACR uses two voltage thresholds. A higher threshold voltage allows the ACR to close the circuit (called the ON threshold), which then remains closed until a lower threshold voltage is reached (called the OFF threshold), which causes the circuit to open. Having a two different thresholds reduces the chatter. This is known as adding hysteresis to the control circuit.

However, if the charger cannot push enough current to hold up the voltage on the paralleled batteries to stay above the OFF threshold, the circuit will chatter on and off. In designing an ACR set up you must take this into account.

It is also possible to add time delays to suppress the chatter. I do not recall precisely what Blue Sea Systems do with their device.

I evaluated the option of using an ACR versus the option of having an isolated secondary charging output from the engine. I decided the isolated secondary charging output approach was much simpler, and actually cost less than the ACR approach, particularly when you factor in all the add-on connectors, fuses, lugs, and so on, that are needed to properly install the ACR to the manufacturer's specifications. The add-on components cost more than the ACR itself.

I think the article to which an allusion is made previously in this discussion is the one I wrote at

Automatic Charging Relay (ACR)
http://continuouswave.com/ubb/Forum6/HTML/001232.html

I also described the installation of the isolated auxiliary charging output on my engine in

Auxiliary Battery Charging Kit for E-TEC
http://continuouswave.com/ubb/Forum6/HTML/002758.html


--Jim Hebert
CONTINUOUSWAVE.COM

 

[lonesouth]

Re: Dual battery ACR and Current to drained battery

do you know if they make a kit for the johnson v4? a brief seach turned up nothing.