Dedicated starting power

[jviss]

AFAIK, the typical alternator found on small engines puts out next to nothing at idle, and I would imagine nothing at all at cranking speeds. Once the engine lights, gas or diesel, I can't imagine the alternator putting any kind of significant load on the engine until it's running at speed, but by that time, it doesn't matter.

On mine, if I have both banks connected and they are discharged, at star-up, even at low RPM the belt squeals and the engine stumbles. It's a 125A alternator, driving 300AH of house batteries. After warming up, no squeal. It helps to charge one bank up a bit, even a little bit. But I still don't want the load when the engine's cold.

 

[jviss]

By the way, ...

...I agree that the 1/2/B switch and common electrical panel practice is antiquated, and could be vastly improved upon. I'd like to be able to choose what banks to power which groups of circuits, etc. As it is now, my 1/2/B switch selects which bank the loads are connected to, as in, all loads on the same bank or both banks. My starter and alternator are both on bank one, until I install the dedicated starting battery. I can charge bank 2 by enabling the Pathmaker, or just switching to "B" while charging. Shore power is a 2 channel charger.

 

[Stu Jackson]

Well T-bird when you combine the dead and fresh battery the fresh one begins to charge the dead one and at a fairly high rate (~0.30 the AH of the dead battery).

1. So my 410 AH dead house bank is being charged by my 100 AH fresh start battery at 410*.3= 123 amps. This represents a significant portion of the cold cranking amps on my start battery and also a will significantly lower the voltage avalable for powering the starter. If you switch the batteries it is not that significant however but still a pretty high current.

2. Questions to the group; how long will this high current last? what size cable would be required to safely handle that size current?

3. I'd stick with the old school 1-2-both-off switch and then you get to choose what is going on. Just switch to the house bank for overnighters or a very long daysail. Otherwise leave it on BOTH to have the largest size bank to draw from. My personal history suggests that you don't need to take it off BOTH unless you know you would be drawing down the entire system to a low SOC.

1. Thanks, good description.

2. Dunno, but rather quickly I presume. It's not so much a "flat" house bank, but a damaged one that creates the real harm when combined.

3. What bank arrangement do you have, Bill? Why use B if you have a small reserve bank with an adequate house bank, but one could follow this procedure if you have two equal but separated house banks. Again, in ye olden daze that was the normal design, as I think I noted earlier.

 

[Maine Sail]

I have just replaced my (two) batteries with Trojan group 31 deep cycle. I would like to be able to use both of them while at anchor without worrying about being unable to start the engine (Yanmar 2gm).
I have a standard battery selector, 1/2/both. Are there reasons why I shouldn't connect both batteries to '1', and a starting battery to '2'?

No that is a great solution! Simply wire your two new batteries in parallel and assign to #1 then assign your start battery to #2. If you prefer you can reverse the numbers. I simply prefer #1 as primary bank and #2 as secondary/reserve bank..

For added simplicity you can add an ACR or Echo charge to automatically keep your second bank topped up then you only ever need to use position #1 & OFF..

 

[Maine Sail]

AFAIK, the typical alternator found on small engines puts out next to nothing at idle, and I would imagine nothing at all at cranking speeds. Once the engine lights, gas or diesel, I can't imagine the alternator putting any kind of significant load on the engine until it's running at speed, but by that time, it doesn't matter.

We have a "typical" stock alternator and at 1000 engine RPM, and batts taking "bulk" this little 50A stock Mitsubishi dumb regulated alt puts out 39-41 amps. Many high performance alts can put out 60-85%+ of rated output at just above idle if the pulley ratio is good.

 

[Bill Roosa]

The full monty.
Alternator is connected to a iso diode box. Duel house bank is connected to one output and start battery is connected to the other. The hose cable goes to a 1-2-B-off switch and the house banks are divided into two seperate banks of equal size.
Normaly the house banks are combined to get the largest bank size to draw from. In the event of one battery going bad you can switch it out and continue on the remaining house 1/2 bank. Charger is connected to the C terminal of the 1-2-B switch for control and the start batt only gets charged when the engine is running.
If I ever need to start from the house bat I just short between the two output diodes of the iso diode box with jumper cables or a wrench and fire up the engine.

 

[jviss]

The full monty.
Alternator is connected to a iso diode box.

You will have to remote-sense the regulator or at least adjust it to compensate for the diode drop.

Not as good a setup as an echo charger, but still OK.

 

[Stu Jackson]

What's important in all this is to be able to describe the SYSTEM you have, in addition to the proper application of the the individual components. As Bill describes his, it sounds like a dual house bank with a separate start bank.

 

[CarlN]

While I too have "fail safe" things that are supposed to ensure I can always start the engine, I still worried. So I went down to the auto parts store and bought a spiral wound AGM battery (Optima Red Top). I then made up a set of "jumper" cables. These batteries hold their charge for years. The battery and the cable lives jammed under a berth. Once a year, I take it out and give it a charge. It cost me $150. Never had to use it, but it sure is comforting.

Carl

 

[ancientmariner]

"With 1-2-B, don't you have to have it on B to charge the 2 circuits concurrently?"

Now you've got me thinking, Scott!
My shore charger charges both batteries no matter what position the battery switch is in, including 'off'.
I guess I should be researching my wiring before I go any further.

 

[Benny17441]

Yes, your idea would work quite well. By combining the two house batteries you would have a larger capacity house bank which you would not draw down as much as a single battery and it will reward you with increased longevity measured by the number of cycles. You can have your starter battery on the #2 position on the switch to be used as a standby. I would refrain from utilizing the "Both" position as the two battery banks are quite different and you may not want power flowing unchecked from one bank to the other. Now remember that the battery switch position also determines which battery bank is getting charge from the alternator. I would allow the charge to flow to one bank or the other but not to "both". If you want to go through the trouble of switching to the starter battery to start the engine and then allow it to charge back from the alternator that is fine but the house bank will start it just as well. If it were me I would keep the starting battery only as a backup. I would keep it charged via the inboard charger but it would not see action unless the house bank became seriously depleted. My main reason for that would be to keep it simple and avoid mindless mistakes.

 

[Stu Jackson]

Now remember that the battery switch position also determines which battery bank is getting charge from the alternator.

Completely true, EXCEPT the newer systems run the alternator output to the house bank with a relay to charge the reserve bank, and AO never goes to the switch.

1. Take the alternator output off of the starter and run a new heavy gauge wire to the house bank positive - with a fuse near the battery bank.

2. Take the shorepower charger's output off the start battery and leave it only on the house battery positive - again fused near the battery. If you have solar panels or a wind gen the output also goes direct to the house bank.

3. Buy either an Echo Cjharge or an ACR (automatic charge relay) and wire it to the house +, start + and ground. For a single start battery I prefer the Echo Charge. It passes up to 15 amps to the start (aux) battery when a charge current is sensed and stops when the charge current is no longer present. At no time does it parallel the batteries.

What this does is change your 1/2/both switch to a "use" switch and not a charge switch. It still lets you decide which battery to use for any load but the charging is automatic.

The easiest way to use this system is to use the house bank for everything and keep the auxiliary battery for emergency use. When you arrive on the boat turn the switch to 1 (house I assume) and turn it to off when you leave. Engine starting is easily accomplished by the house bank.

And there is no chance of error.

Added 7/8/11: And if your echo charger or ACR breaks, then simply use the B position on the switch! Backup, backup, backup. Stu