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Author Topic:   BLUE SEA SYSTEMS Improved ACR
jimh posted 01-16-2014 02:36 AM ET (US)   Profile for jimh   Send Email to jimh  
In early 2012 BLUE SEA SYSTEMS made a change in the design of some of their automatic combining relay (ACR) products that is worth noting. The change has been applied to some of their dual-sensing ACR products, and provides a worthwhile improvement. The voltage sensing circuitry now looks at the voltage on both batteries to see if either of the batteries is below a low voltage lockout threshold. If one of the batteries has a very low terminal voltage, the operation of the combiner relay is inhibited. This prevents a situation occurring in which a very deeply discharged battery is placed in parallel with a battery having full charge or near full charge. If this were to occur, a very high current could flow from the charged battery into the discharged battery. Although Blue Sea Systems usually recommends having fuses in the circuit which would open if a really high current were to flow, there is a good chance that many installations of these ACR products omit those fuses. By providing for an low voltage threshold, the ACR device can avoid connecting two batteries together when their voltage levels are very different. This should avoid the problem of a very high current flow between the batteries.

On these new ACR products from Blue Sea Systems the low voltage threshold in a 12-Volt system is set for 9.5-Volts. A 12-Volt battery with a terminal voltage below 9.5-Volts is a very dead 12-Volt battery.

For an example of a Blue Sea Systems ACR product with dual sensing and undervolt protection, see the literature at SI-ACR_Automatic_Charging_Relay_-_12_24V_DC_120A

It is nice to see this product evolving and under continual refinement. The operation of the undervolt threshold is described as follows:

As a safety feature, some ACRs prevent combining into a severely discharged battery. A dual-sensing ACR will monitor the voltage on both batteries and will not connect if either battery is below the undervoltage lockout level. Use caution when combining into a battery with extremely low voltage, because this might represent a faulty battery or a problem elsewhere in the system.
tmann45 posted 01-16-2014 11:23 AM ET (US)     Profile for tmann45  Send Email to tmann45     
The under voltage lockout was implemented in the prior model of the Blue Sea ACR. The change made 03/2012 was to just change the voltage at which the lockout occured, from 10.8 volts to 9.5 volts.

See for the specs before the revision. This is the version I have installed.

jimh posted 01-16-2014 01:00 PM ET (US)     Profile for jimh  Send Email to jimh     
I was under the impression, perhaps wrong, that the prior versions lacked the low-voltage threshold. If they had the feature all the time, I must have been ignorant of it. Perhaps Blue Sea Systems was not promoting it very clearly, or perhaps I didn't read their literature with enough detail.

If the change in 2012 was to make the threshold low-voltage level 9.5-Volts from 10.8-Volts, let us look at that to see what it improves. A typical 12-Volt lead-acid battery when discharged would have a potential of about 1.95-Volts per cell, or 11.7-Volts according to Wikipedia. With a load, the terminal voltage at discharge is said to be 1.75-Volts, or 10.5-Volts. In order for a 12-Volt battery to fall below that level, it would probably need to have a shorted cell, that is, become a five-cell battery. That would put its terminal voltage under load around 8.75-Volts.

I suppose if the ACR stops working at 10.8-Volts it might fail to charge a very deeply discharged battery that was under load. Perhaps that is the reason behind the change to 9.5-Volts as the low-voltage threshold for combining. A shorted cell battery would likely be below 9.5-Volts when under load, and the ACR would not operate. Thus the ACR will try to charge a deeply discharged battery but will not put a battery with a shorted cell into the circuit.

jimh posted 01-17-2014 02:37 PM ET (US)     Profile for jimh  Send Email to jimh     
I inquired with Blue Sea Systems about their ACR products and when they had a low-voltage threshold. Here is the history, as best as I can tell from the information they sent me:

The first model of ACR was a model 7600, and it was introduced at least six years ago, that is, c.2007. It had an adjustable setting for the voltage at which the relay would close. This is called the combine voltage. Once the relay closed, it stayed closed until it detected the undervoltage or the overvoltage. The undervoltage was fixed to alwasy to be six-percent below the combine voltage. On the 7600 the combine voltage was adjustable and could be set between 12 and 14-Volts. The undervoltage would be six-percent less. For example, if the combine were set to be 13.6-volts, the undervoltage would be 12.784-Volts. The overvoltage limit was adjustable from 13.5 to 15.5-Volts. The relay would open if closed and either battery exceeded the overvoltage.

The model 7600 was replaced by the 7610. In the 7610 the undervoltage became slightly different. The combine voltage was made to be fixed, but it was given weighting for voltage over time. And the voltage previously called undervoltage, that is, the voltage which would cause the relay to drop out if energized, was also given weighting for voltage and time. I would call this the uncombine voltage. This was also fixed in terms of time and voltage.

A new feature was added called the undervoltage limit. The undervoltage limit sets a low voltage absolute limit. If either battery is below this limit, the relay can never close. This is provided as a way to protect against the possible situation in which one battery is really very dead or has a shorted cell, and the other battery is very charged (that is, it is above the combine voltage or is being charged to be above the combine threshold). If you connected the two batteries, a large current would flow from the charged battery to the discharged or shorted-cell batttery, and that could be a hazard. In the original 7610 this undervoltage limit was fixed at 10.8-Volts.

The combine and uncombine time and voltage thresholds are as follows:

The relay closes or combines when:

--for at least 30-seconds there is 13.6-Volts present, or

--for at least 90-seconds there is 13.0-Volts

The relay opens or uncombines when:

--there is 12.35-Volts (or less) present for ten seconds, or

--there is 12.75-Volts (or less) present for 30-seconds

In c.2012 the 7610 was revised and the undervoltage limit changed to 9.5-Volts. To summarize the three models and their three voltages:


7600: (none) / Adjustable 12 to 14-volts;six-percent less / Adjustable 13.5 to 15.5-Volts

7610: 10.8 / (see above) / 16-Volts

7610 c.2012: 9.5-Volts / (see above) / 16-Volts

Operationally, it seems to me that in the original model 7600 ACR the relay could close if one battery were above the combine voltage even if the other battery were very deeply discharged. Once the relay closed, there could be a big drop in voltage from the combined load, which would cause the sensed voltage to drop off and go below the uncombine threshold. The relay would chatter on and off, perhaps mitigated by some delays.

With the 7610 the absolute low voltage limit was added, which prevents the relay from closing even if one of the batteries is above the combine threshold. I believe this added a new state for the device to be operating in that did not exist in the 7600.

jimh posted 01-17-2014 02:52 PM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE: It should also be mentioned that another feature, called starting isolation, was added to the ACR products some time in their evolution. The starting isolation feature forces the relay to open (or uncombine) when there is an engine start being initiated. This is another good evolutionary feature in the Blue Sea Systems ACR product. It was precisely at engine starting that there should be the most isolation of the house and cranking circuits, and it was possible that with an ACR and two fully charged batteries, the ACR could be closed at engine start, putting the circuits in common. The starting isolation feature in implemented by wiring a sense circuit to the ignition key switch START circuit from the ACR. The ACR drops off, if energized, when it senses voltage on the START circuit.

I really like the evolution and developement of the Blue Sea Systems 7610 ACR, and I would use one on my boat if I had not already gone to another approach to solving the problem by using a secondary battery charging output from my main engine alternator.

silentpardner posted 01-17-2014 06:35 PM ET (US)     Profile for silentpardner  Send Email to silentpardner     
Great research on this brand of ACR Jimh!

I currently have 2 ACR's installed on my Whaler 27, 1 for the primary starting batteries and another for the secondary house batteries. I have 5 batteries total, 2 dedicated starters, 1 for each of the 2 engines, and 3 total house batteries running off the secondary, or auxiliary, circuits of the engines. I use a lot of house capacity due to my Cannon electronic downriggers and 2 Dolphin electric deep drop reels.

I am adding another battery and ACR currently in my Windlass installation project. I have a Good (brandname) fully automatic windlass at the marina for installation at this time, should have all the installation completed by the end of next week. I am not certain we are going with the brand of ACR you are researching here in this thread, but I have my technician watching your research work here.

Currently, as this is being written, we are installing 150 gallons of added fuel capacity and completely redesigning the fuel system, but the new windlass system is next.

Thank you for your continuing exhaustive research on this subject. I have to ask...are you an electrical or electronics engineer by trade?

jimh posted 02-04-2014 03:09 PM ET (US)     Profile for jimh  Send Email to jimh     
My degree is not in E.E., but I have worked in electronics as an engineer for many decades. I can also write fairly lucidly. The goal of the website has always been to collect and organize information about boating topics.

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