Wire sizing for battery switch and ACR hook up

Stu Jackson · Jul 12, 2009

Blitz said:
The project I have at hand is to install the blue seas Automatic charging relay and a dual circuit plus battery switch.

Please don't do that. You can use a simpler 1-2-B switch and save the $$ on the ACR with the same switch if wired correctly.

For comments on the poor concept of the dc switch, please see: http://c34.org/bbs/index.php/topic,4623.15.html

Once you've read that let's discuss further. Unless you've already bought the parts...

Maine Sail · Jul 13, 2009

3/0 not #3 wire

Blitz said:
Trying to hook up a new battery switch and checking all wire sizing. I have a Yanmar 3GM engine and not positive of the starter draw. The starting circuit from the battery, through the battery selector switch and to the starter is 14 feet then the ground back to the battery is another 4 feet. I'm coming up with AWG #3 sizing - does that make sense?

The 1kw Yanmar starter on the 3GM consumes a max of just under 225 amps at absolute peak load. Absolute peak load is usually less than a second or two at the most but the wire must be sized for the max draw. Sizing for 250 amps would be even safer but still puts you at the same wire size.

As for wire gauge size you have 19 feet round trip (14 + 4) so:

19 feet at 250 amps (sized for safety factor) and 3% voltage drop requires 3/0 wire NOT #3 wire. #3 wire is like a drinking straw compared to a 3/0 which is like a fire hose. 20 feet is a lot of distance for a battery run.

If you size it at exactly 19 feet at 225 amps, & no safety factor, with a 3% voltage drop you are at a 2/0 wire but are really pushing that envelope because at 20 feet, just 1 more foot of wire, you are at a 3/0.

Maddog · Jul 13, 2009

Blitz - PM me if you like. I installed this same system and have been very happy. I'll answer any questions I can.

Ron20324 · Jul 13, 2009

Maddog said:
Blitz - PM me if you like. I installed this same system and have been very happy. I'll answer any questions I can.

Respond on this thread and we can all learn. Might save someone from asking the same question in a few days.

Maine Sail · Jul 13, 2009

Start / House

Blitz said:
I'm curious with more than one battery in my house bank would it be better to hook up the alternator to the house or starter bank while using this set up?

Wire it directly to the house bank and let the ACR handle combining to the emergency/start bank.

Blitz said:
Thinking the starter battery so that it remains topped off and bleeds off to the house bank which is consumed while sailing or at anchor.

First try thinking of your start battery as an emergency batter unless you are making it a dedicated direct wired starter battery with no ability to start using the house bank, other than combining, which I really don't like..

There really is no reason to use a "start" battery to actually start a diesel of your size. Using one bank, the house bank, is much more simple and then your emergency bank is always at the ready. Even if you do start off a start battery you consume virtually nothiong in terms of battery capacity. I can start my 44hp Westerbeke off my house bank and not even register .1ah of consumption. That means I am burning less than .09 amp hours to start a 44hp diesel. An LED Sensibulb burns about the same current, in one hour, as it takes me to start my engine. Remember you are only talking high amperage for less than 1 or 2 seconds.

Either way the combiner will "combine" the batteries and the longer they are combined the more they will equalize. This means that if you connect a dead house bank to a fully charged start battery the start battery can transfer some current to the house bank in an "in-rush", if alternator input is insufficient to satisfy the house, until both become equal in voltage then the current will flow where it is needed.

Maddog · Jul 13, 2009

Maine - What if your start and house are two different types of batteries. A group of 6v on the house bank and a Optima blue top for the start. I have a Pro Mariner 20 amp charger that can handle 3 banks. I'm not sure it can sense each one seperatly. The charger is hooked to the house and the ACR takes care of the start bank.

I am commited to using the dual switch with the ACR. So far it has worked great for me. The only time it failed was with a broken alt wire and 5+ year old batteries and those needed to be replaced anyway. That is a different story. No battery switch will save you from that.

Stu Jackson · Jul 13, 2009

Maddog said:
Maine - What if your start and house are two different types of batteries.

Then you buy a duo charge from Balmar.

Maddog · Jul 13, 2009

Stu Jackson said:
Then you buy a duo charge from Balmar.

Thank you Stu, I appriciate your input and experience. But the question was in regards to my AC powered battery charger being able to sense mixed battery banks and weather to use the ACR to charge the the start battery from the house bank while on shore power. inreading the Duo Charge manual all the wiring diagrams show it only being hooked up to an alternator. Is that correct? or would you use it off the Pro Mariner charger as welll?

Stu Jackson · Jul 13, 2009

Maddog said:
inreading the Duo Charge manual all the wiring diagrams show it only being hooked up to an alternator. Is that correct? or would you use it off the Pro Mariner charger as welll?

Any single source (point) source of charging is applicable. One shorepower output, one alternator output, you name it, a solar cell, a wind gen, you get the drift...

Maine Sail · Jul 13, 2009

It's a boat... nothing is simple..

Maddog said:
Maine - What if your start and house are two different types of batteries. A group of 6v on the house bank and a Optima blue top for the start. I have a Pro Mariner 20 amp charger that can handle 3 banks. I'm not sure it can sense each one seperatly. The charger is hooked to the house and the ACR takes care of the start bank.
.

There are a couple of problems mixing batteries of different types:

#1 Many AGM require specific charge parameters that are not the same as wet cells. With one charging source you will likely either over or under charge one of the banks which will lead to shorter performance life. Lifeline AGM's claim that they can be charged on wet cell voltages yet some other brands don't make this claim and can't or should not be charged at wet cell parameters. Optima's are more like Lifeline in that they can be charged on a wet cell type profile.

#2 Wet cell batteries accept charge current very differently than do AGM batteries do. AGM batteries will basically accept what you can hammer them with, within reason, while wet cells accept much less over a given period of time. This term, for how much charge current a battery can absorb in a given amount of time, is called acceptance.

Take for example a boat with a 100 amp alternator and a 50% depleted wet cell bank. In one hour you might put back in about 25% of what you took out. As the state of charge rises the wet cell battery will accept less amps. It will accept the most when nearly dead. Wet cell acceptance is generally 25% of the 20 hour ah rating of the bank.

An AGM battery basically accepts what your little alt can give it. Until recently Concord battery had no acceptance limit rating for their AGM batteries. Another example is from Lifeline batteries where they explain that due to the low internal resistance design of the Lifeline battery a 100ah battery can accept an inrush charge current of up to 500 amps. That is some seriously fast acceptance!!!

With AGM's and wet cells a duo charger or Echo Charger and two different shore chargers or one that allows it to output two separate charge parameters at the same time is a much better idea than a combiner and one shore charger unless the absorption and float voltages for both battery types are the same. For shore charging you may want to add a second dedicated AGM charger for those who have voltage sensitive AGM's like those made by Trojan. It does not need to be big but should be programmable for AGM batteries. You will also want to consider a charger that senses battery temperature.

Maddog · Jul 13, 2009

Thank you Maine, my research paid off a little bit. We'll see how this goes over the next year or so. I may do as you suggest and place a switch on the ACR, then charge the start bank off the alterantor when we are under way.

I appreciate your advice.

GuyT · Jul 15, 2009

My 3GMF had #4 wire with almost your same length and NEVER had an issue with voltage drop from the wire causing starting issues. I did have a solenoid issue that was solved by adding a relay which is well documented/posted in the archives.
I always felt that the wire was slightly undersized and would have liked to have seen a #2 but this may have raised some issues with getting into the selector switch as the lug size and bend radius of wires become a little more difficult to deal with.
Certainly anything larger than a #2 is overkill IMHO based on my experience with the 3GMF.