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Author Topic:   Automatic Charging Relay (ACR)
jimh posted 01-31-2007 11:10 PM ET (US)   Profile for jimh   Send Email to jimh  
Several vendors offer automatic charging relay (ACR) devices, also called voltage sensitive relay (VSR) devices, for use in marine electrical systems where there are two batteries and only one source of charging current. An ACR or VSR operates on a simple principle:

--a high-current capacity single-pole, single-throw (SPST) switch contact is controlled by a voltage sensing circuit;

--a threshold voltage is established which is above the normal terminal voltage of a fully charged battery, or around 12.9-volts;

--if the sense voltage is above threshold, the SPST switch is closed; otherwise the switch is open.

The ACR is typically wired so that an auxiliary battery is placed in parallel with the main battery (by the switch closure) when the sense voltage is above threshold, that is, whenever there is charging current available and the main battery voltage has reached full charge.

This simple circuit has been enhanced in several ways. The most common is to include a hysteresis in the sensing circuit so that the ON voltage threshold is higher than the OFF voltage threshold. This helps to prevent the circuit from chattering on and off. Without a separate threshold, there could be situations where the addition of the second battery to the charging circuit would immediately drop the voltage below threshold. This would, in turn, immediately cause the relay to drop out. And this would then raise the voltage. The cycle would repeat. Having some hysteresis in the control loop helps to reduce this tendency.

One of the principal advantages of using an ACR or VSR is to help isolate the electrical circuits in the boat into two distinct groups:

--battery and loads associated with engine starting

--battery for all other loads

The engine starting loads are typically just the engine, its solenoid, and its starting motor. The other loads are vessel lighting and electronics.

The reason for maintaining isolation is twofold. On engine start there are often voltage transients produced. These voltage transients can cause interference with or damage to electronic devices. Isolating the starting loads from the electronic loads will prevent starting transients from reaching the electronics. The other advantage is isolation of the starting battery from discharge by any load except the engine. This will prevent the starting battery from being drained by non-starting loads (such as lights, pumps, or electronics) when the engine is not running. Larger horsepower engines are very difficult to pull start, so it is paramount that the engine starting battery be kept fully charged and able to crank over the engine for starting.

The general design of an ACR or VSR has been intended to only connect the two battery systems when the engine is running, that is, when the engine is putting out charging current and the engine starting battery has reached a full-charge terminal voltage. In most cases this will occur only when the engine is running. However, it is possible that if both batteries are near full-charge, an ACR or VSR could be fooled by the high resting voltage of the two batteries, and the voltage on the sense line may be high enough to trigger the switch to the ON condition. This will parallel the two batteries, even though the engine is not running.

The harm in this comes when the engine is cranked over for starting. If the ACR or VSR has paralleled the two batteries, voltage transients from starting may reach the electronic loads. If this occurs, one of the main reasons for using an ACR or VST is lost: the electronic loads will not be isolated from starting transients.

To overcome this situation, Blue Sea Electronics have developed a more sophisticated version of the ACR. They call their device a starting-isolated ACR. The ACR switch is always opened during starting. This is implemented with a simple circuit which monitors the engine starter solenoid line. When the engine is cranked, voltage on the engine solenoid line is sensed and the ACR switch is immediately opened (if closed). Blue Sea says that the switch operates fast enough that it is open before any transient voltages develop during cranking.

The new ACR from Blue Sea is their model 7610, and they call it a 120 Amp SI Automatic Charging Relay (Starting Isolation). In their literature Blue Sea are a little vague about exactly how this new ACR works, but on page four of the instructions for installation the wiring is shown. The connection to the engine solenoid circuit is detailed.

Blue Sea Web Page on Starting Isolation ACR

Instructions--See Page 4 for details

The literature also mentions two sets of time constants, but, again, the details are not clear. I presume that the different time constants refer to a combination of time and voltage which must exist before the relay operates (ON) or drops out (OFF).

An additional feature is a provision to illuminate a remote indicator to show the status of the ACR switch (open or closed).

This new device from Blue Sea looks like an improvement over other ACR or VSR devices. I had been giving some thought (as one does during these long winter nights of idleness and lack of real boating) to adding an ACR to my small boat electrical system. The Blue Sea 7610 looks like the leader of the pack.

HAPPYJIM posted 02-01-2007 11:16 AM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
Don't modern electronics have voltage spike protection and if so, is the ACR just for back-up?
cwolf posted 02-01-2007 12:44 PM ET (US)     Profile for cwolf  Send Email to cwolf     
Last year I installed Blue Sea's previous ACR model. It works very well with my two battery setup. As with all their products the build quilty is very high. I did have to call their technical support to get some clarification on installation but were very helpful and patient with my questions.
cwolf posted 02-01-2007 12:59 PM ET (US)     Profile for cwolf  Send Email to cwolf     
I should add that it makes a lot of sense for them to include the 5511e switch with this as it makes for a complete package. One thing I encountered was the status light found on the last model is designed to be continuously on (at least on the last model). I questioned them on potentially battery drain during long periods of storage. They felt that the draw from the status light was so minimal that I should not have a problem. In the end I added a quick disconnect to the ground wire which I pull apart when I know it's going to be a while before I use the boat. Other then that it works flawlessly and I never have to worry about keeping both batteries topped off anymore.

stagalv posted 02-01-2007 04:59 PM ET (US)     Profile for stagalv  Send Email to stagalv     
It is great that this has been posted by JimH because it really seems like a wonderful product. It is described as being rated as submersible too. As mentioned above the Blue Sea Systems battery switch model 5511e would complete this system well. These two items (the 5511e & the 7610 automatic charging relay) are sold packaged together and called the 7650. I found that a smart shopper can purchase the 7650 package for about $125.
jimh posted 02-01-2007 08:35 PM ET (US)     Profile for jimh  Send Email to jimh     
Regarding the time constants mentioned for the relay, they work like this:


If the sense voltage is above 13.0-volts, the relay closes within 120-seconds. If the sense voltage is above 13.6-volt the relay closes within 30-seconds.


If the sense voltage is below 12.75-volts, the relay opens within 30-seconds. If the sense voltage is below 12.35-volts, the relay opens within in 10-seconds.

Regarding the Starting Isolation feature, if the relay is closed and the starting sense line goes to 12-volts, the relay will open within 0.34-seconds.

As far as elimination of the starting transients, this is not totally eliminated while the two batteries are combined. If a transient occurred in the first 0.34-seconds the ACR would not affect it.

The design of the Blue Sea ACR with Starting Isolation in regard to the starting voltage is to prevent the engine starter load from dragging the voltage on the non-starting battery and its load down to the rather low voltage which can occur while cranking. It is common that under the heavy load of engine starter motor cranking the battery terminal voltage can dip as low as 10-volts. If the electronic loads on the vessel, such as a GPS or other sophisticated devices, experience a dip in their supply voltage down to this level, they will often automatically reboot or re-initialize. Rebooting (from low supply voltage) may cause the devices to lose data. In the case of a GPS, for example, if the device reboots during engine starting, it could be several minutes before the receiver again acquires lock on the satellite signals it needs to function. Prevention of this type of voltage sag during starting is the designed purpose of the Blue Sea ACR with Starting Isolation.

It bears repeating that the ACR will generally not be closed when the engine is not running. However, under certain conditions when both the engine battery and the house battery are both floating at a full-charge level, the ACR may be fooled into operation, and the batteries will be combined. If the engine is started under these conditions, the combined batteries might still sag low enough that electronics could be affected by a low voltage.

Now, you might say, "doesn't the ACR drop out when the voltage is low?" Yes, it should, but there may be some time constants involved which prevent the ACR from dropping out immediately. These are related to the idea that there should be some hysteresis in the control loop to prevent chattering. So a normal ACR could remain in COMBINE mode for long enough to affect the electronics. In the Blue Sea Starting Isolation model, a separate sense line detects engine starting and drops the relay out (if closed) very rapidly.

jimh posted 02-01-2007 08:43 PM ET (US)     Profile for jimh  Send Email to jimh     
The Blue Sea ACR with Starting Isolation (P/N 7610) was announced back in September of 2006, but it just went into production in January of 2007. It should now be available at distributors and dealers.

As noted, it can be paired with other Blue Sea products to make a modern vessel primary battery distribution system. There is more technical literature on ACR operation on the Blue Seas website:

On ACR operation:

Blue Seas Technical Paper

There are several articles which discuss enhancements in the new ACR which were designed to help the device operate with multiple-bank battery chargers.

hoppyray posted 10-06-2007 03:08 PM ET (US)     Profile for hoppyray  Send Email to hoppyray     

Thanks for all the great info. I almost made the move to do it completely different. Do you have a recommendation on setting up a single outboard, with two batteries and a charging system so I can hook up to shore power and an extension cord at home to keep both batteries charged? I would like to find a charging system I can mount in the boat that will allow me to hook up to shore power, run lights or radio with out draining the starter battery. It would also require the ability to keep the house battery , along with the starter battery at full charge in the morning. Any product recommendations and design ideas would be appreciated!


glen e posted 10-06-2007 09:53 PM ET (US)     Profile for glen e  Send Email to glen e     
Jim and I agree... ACR's are a great addition to a boat..see here for the verado suggestion:

davej14 posted 10-10-2007 10:02 AM ET (US)     Profile for davej14  Send Email to davej14     
While this is clearly a great product, I question the claims of transient protection on engine start. A response time of 0.34 seconds is really quite long when dealing with voltage spikes that could damage electronic components. Most transient suppression devices (MOV's, transorbs, sidactors) react in microseconds <=0.000001 sec. A quality piece of marine electronics will have built in transient protection that is robust enough to handle surges in your boat's electrical system. Those of us with single battery systems can rest easy and ignore the marketing hype.
bluewaterpirate posted 10-10-2007 10:43 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Ray ......

You should also look at the BEP products. I installed a BEP 716 Distribution Cluster on my Whaler and have been very happy with its performance. It also uses a VSR technology. I've installed both systems on clients boats and have never had any failures or operational issues. They've both worked as advertized.

Here is a link to my web site that will let you view my installation.


bluewaterpirate posted 10-10-2007 10:46 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Here's a slideshow view of the install. aspx?username=bluewaterpirate&album_id=196006§ion_id=617057


jimh posted 10-10-2007 08:48 PM ET (US)     Profile for jimh  Send Email to jimh     
In regard to the time constant of 0.34-seconds in which the Blue Seas ACR will drop the connection between the house battery and the starting battery when the ignition switch is moved to the START position, I agree that this is not fast enough to occur before some voltage spike might be generated, but I believe that the intention of this feature is more along this line: when the starter motor is cranking it can tend to drag down the battery to a low voltage, perhaps as low as 10-volts. Because the normal time constant for disconnecting the ACR is quite long, 10-seconds, it is possible that the house buss would thereby be dragged down below 10-volts as well during the engine cranking. If the house bus voltage drops below a nominal 12-volts, perhaps even just as low as 11-volts, the electronic equipment connected to the house bus could be affected. It is quite typical that some devices will shut off or restart with an input voltage that falls below a nominal 12-volts.

Therefore, by dropping any connection between the house and starting battery the ACR will prevent the house bus voltage from getting dragged down during engine cranking. This is a useful feature. If not for the special starting circuit drop-out feature, the time constant of 10-seconds would apply, and the house and starting batteries might be in parallel during that time.

In this regard I think the Blue Seas ACR has a nicely engineered solution to this potential problem (pun intended).

hoppyray posted 10-14-2007 10:48 PM ET (US)     Profile for hoppyray  Send Email to hoppyray     
Nice work Tom!

[Ray then changed the topic of discussion from automatic charging relays to one of general advice on small boat electrical installation and secondary battery distribution. This portion of his article has been moved to a new discussion--jimh]

jimh posted 12-07-2008 08:26 AM ET (US)     Profile for jimh  Send Email to jimh     
{Fixed some dead links to the Blue Seas website.]

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