Blue Sea ACR question

[nautikaljeeper]

Having a 140A alternator and 525AmpHr house bank and 100AmpHr starter bank would the Blue Sea System ACR 120A be the wrong choice or would I have to step up to the 500A model? My thinking is the alternator would likely never see 140A output or is this pushing the limit and if so is there a better suggestion.
Blue Sea Systems SI-ACR Automatc Charging Relay - 12/24V DC / 120A

 

[centerline]

do you have an external regulator?... they will get the most out of what your alternator has to offer, but its not usually the max that the alternator is capable of.... probably 100A is about the high limit for the 140A alternator, and even then only if you have dead batteries.
the external regulators are easier on the system while making the alternator work a bit harder for a bit longer and do its job more efficiently.
I wouldnt hesitate to use the 120A, on the assumption that it is rated for a working load, rather than "burnout" load....

 

[Maine Sail]

If properly wired for a large house bank and small start bank, meaning all charge source current is fed to the house bank FIRST, then the relay will never see anywhere near its 120A rating.

 

[nautikaljeeper]

If properly wired for a large house bank and small start bank, meaning all charge source current is fed to the house bank FIRST, then the relay will never see anywhere near its 120A rating.

Yes this would be the way it would be wired. So am I correct to assume you would be comfortable with using the 120A ACR in this system or would it be better off to spend the extra for the 500A? House bank does not go below 65-70% SOC before being charged. Yes it does have an external MC-614 regulator and hopefully it is getting programed correctly from all I have read from your post Maine Sail. Your information is greatly appreciated.

 

[centerline]

if you have your heart set on the 500, get it and install it, otherwise the 120 will work fine for your application....

 

[Maine Sail]

Yes this would be the way it would be wired. So am I correct to assume you would be comfortable with using the 120A ACR in this system or would it be better off to spend the extra for the 500A? House bank does not go below 65-70% SOC before being charged. Yes it does have an external MC-614 regulator and hopefully it is getting programed correctly from all I have read from your post Maine Sail. Your information is greatly appreciated.

Yes the 120SI is more than enough IF the alt hits the HOUSE bank first. The 120A rating is a continuous rating, it can handle up to 210A for 5 minutes. With a 100Ah start battery, and alt fed to house first, there is no way the ACR is carrying 120A, which would be a 1.2C charge rate of the start battery. Even if it could do this it would not last for any more than a few seconds and then only when the start battery was flat dead.

For someone who wires the ACR incorrectly, with alt feeding to the start battery first, then yes the 120SI could be marginal and you could eventually damage it. Of course you would also have other issues such as relay-cycling due to the incorrect installation..

99.5% of the time your start battery will be pulling less than 20A across the relay, most often less than 3 - 4A..... The 500ML is a great device but is overkill here...

 

[nautikaljeeper]

99.5% of the time your start battery will be pulling less than 20A across the relay, most often less than 3 - 4A..... The 500ML is a great device but is overkill here...

This is what I was thinking and just needed your confirmation. Thank you for your response.