Isolator vs. ACR?

Hey Guys,

I am going to add a start bank to our new to us boat. (Mainship 400T with twin yanmars). Current set up is 2 8D, one starts one engine, the other starts the other engine and serves as the house bank.

My plan is to parallel the 2 8D (eventually I will likely replace them with 6V, but they are in good shape so I will use them up) into a house bank and add a 4D as a start batt (I already have a really nice 4D box).

My thought was to send both alternators to the start batt and use an ACR (which I own) between the banks. My mechanic suggested using a battery isolator instead.

I'm pretty ignorant on Isolators, would you guys give me your thoughts on which is better?

Thanks

An isolator is nothing more than a pair of diodes that prevents the load on one battery (the house) from running down another battery (the starting battery). They do have a voltage drop of about 1/2 volt across them which screws up simple alternators so that you will not get a full charge (even less than you get with a simple alternator).

An ACR (automatic charge relay) is a relay or sometimes a solid state device which combines two batteries and lets the charge flow from one to the other when the voltage on one gets above a set point, usually 13.3-13.5 V. This happens when the alternator (or shorepower charger) is supplying current and one battery is charging so that there is no risk of drawing down the start battery. It has no voltage drop so doesn't interfere with alternator charging.

I recommend the ACR or its combiner equivalent. But I would suggest the opposite of what you were thinking: Hook the atlernators to the house battery bank which needs recharging most of the time and then let the ACR combine with the starting battery when the voltage gets up to the set point.

Check out yandina.com for tips, techniques and inexpensive but quality combiners.

David

Check this out. Just ordered one from them at $100 less than West Marine.

Promariner Proisocharge Battery Isolator 130amp 2-Alt 4-Bat - 12v

An ACR (VSR or "battery combiner") is a voltage sensing relay that connects both battery banks together when there is a charging voltage present (from the alternator or charger) and separates them when there is no charging voltage so excessive use of one bank doesn't discharge the other bank.

A battery isolator consists of diodes in series with the charging wire from the alternator to the batteries (one diode for each bank). The diodes keep the current from flowing from one battery to the other so excessive use of one bank doesn't discharge the other bank.

You may be thinking that they accomplish the same purpose and essentially, they do. One problem with the isolator is that there's about a half a volt voltage drop through the diode so unless the alternator can be adjusted for a higher voltage output, the batteries never get fully charged.

The trend seems to be in favor of the combiner rather than the isolator. Installation is much simpler and the end results are the same.

My boat has a combiner. If it didn't, that's what I would install.

Thanks for all the help Guys, I am thinking of using the ACR. If I wire both alternators to the same bank do I need to worry about one feeding back to the other? Should I use an isolator between the alternators and the batteries to prevent that? And still have the ACR in between the banks?

What if I have the port engine charge the start (port side) bank and the stb charge the house (stb) bank with the acr between them?

My simplistic way of thinking about it is that the charging current is sort of like water filling two buckets (batteries), it flows into the port bucket (if both are wired to the port side) until it is full, then flows through the ACR to fill the stb bucket. What if there is a "hose" going to each bucket?

Thanks.

OK, I am going to hijack my own thread here, but I just had a thought (often not a good thing).

Current Set Up:

Stb Bank (1 8D) starts Stb Engine.

Port Bank (1 8D) starts port engine AND serves as house bank.

Parallel switch in between them.

What if I just paralleled the two 8D on the port side and added the 4D to start the stb side? I know that would leave me without a dedicated start bank, but I could always parallel the two if needed, and theoretically, the 4D on the stb side would be isolated from the house bank.

So that would mean the new set up would be:

Port bank (2 8D) starts port engine and serves as house bank.

Stb side (1 4D) starts STb engine.

Parallel switch between them. And maybe the acr?

Port engine charges port bank. Stb engine charges stb bank.

Sure would be simple.

Thoughts?

You could wire one alternator to house and the other to the starter and that would work fine with a combiner to tie them together when the voltage gets up to 13+ V.

But two alternators wired to the same battery bank won't be a problem. They will both work together, but the one with the higher voltage set point will dominate. Kind of like people!!

And to your last post: Reading your first post, your ultimate objective was to replace your 8Ds with 6V golf cart batteries for the house bank. That is a good long term solution.

David