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Change Battery Selector While Engine Running?

14K views 21 replies 11 participants last post by  maccauley123 
#1 ·
I have searched the forums here and looked at Nigel Calder's book and can't find an answer I am comfortable with.

I have a 2 battery system, one for starting and one for house. They are both wired to a single 4 position battery selector switch. Alternator is wired to switch as is distribution panel.

I just had to replace my starting battery because apparently it was not getting charged all the way. My practice was to usually use the "Both" selector on the switch when running the battery so in theory both batteries would be charged. Apparently this was not working well, alternator might not be big enough, and only one battery was being charged fully. The house battery was charged and was 100% healthy and the starting battery had 0 health.

Recommended I start the engine usually with the starting battery and run the engine so it would stay topped up. Every once in a while while engine is running switch to the house battery to keep it topped up. For the most part there is very little draw on the house battery since it is just used for radio, GPS, etc.

Is it OK to switch between 1, 2 and Both while the engine is running? West Marine said it should be OK and Nigel says "the switch needs to be of the make-before-it-breaks variety - both batteries are first brought on line (Both position) and then one disconnected". I take this to mean I can use Both to start and then switch to 1 or 2 depending on which battery I want to charge fully. Thoughts.
 
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#2 · (Edited)
It MAY be OK, but I wouldn't do it even if you have a make-before-break battery switch. The potential of a malfunction -- human or mechanical/electrical -- is always there, and the result could be catastrophic for your alternator's diodes.

It makes MUCH better sense in your situation to:

1. always leave the battery switch in the house battery position;
2. always charge the house batteries; and
3. use a voltage follower device like an EchoCharge or DuoCharge (or other) to keep the start battery topped up.

This is a relatively inexpensive upgrade (about $130 for parts), and completely avoids the need to switch anything. It's totally automatic.

If you really wanna do it right, in addition to the above you could install a simple ON-OFF battery switch in the starting circuit, and a hefty fuse (not required by ABYC standards, but a VERY GOOD idea). A good inexpensive switch is the Blue Sea Systems #6006 -- about $20.

Bill
 
#3 ·
Even with a make-before-break switch, you can fry the alternator diodes if you go through OFF. You do have to careful and think before you switch batteries with the engine running. Most batteries switches are of this type, but its best to check with a digital multimeter. Installation of a battery combiner is really the way to go as btrayfors suggests.
 
#4 ·
I understand all that but if so then you would never use the Engine Start Battery to start the engine, unless when the day comes the house is totally discharged, because if you do you cant switch back to the house. Doesn't that defeat the whole idea of a engine start battery?
 
#5 · (Edited)
On most sailboats with a 1/2/BOTH/OFF switch the house bank can be used for everything, and it makes it a lot easier. If you have a small bank then you may want to wait to fire up your instruments to avoid drop out during starting but the bank should easily start your small aux engine.

Once you have more than one battery in parallel or series/parallel as a "bank" you combine your cranking amps so most often your house bank will do a far better job at starting the motor than the "start" battery will.

I don't treat my start battery as a "starting battery" I consider it my reserve bank...

With MOST switches it is fine to move between 1, 2 & BOTH with the engine running. What you don't want to do is pass through the OFF position.

An easy test to see if your switch is make before break is to flip on the cabin lights and move between 1, 2 & BOTH slowly. If the lights dim or go off when passing between these positions you have a break before make or just a worn out switch..
 
#7 ·
My budget friendly choice was to change the master switch to an: off, house, both, switch. This is a switch that doesn't shut off a bank to go to another bank. It works perfectly for me as I use everything I can get when starting, and it keeps all batteries active.
 
#8 ·
Both batteries are Group 24 wet cell from West Marine. The starting is brand new, the old one was dead, and is 1000 MCA. The deep cycle is about 4 years old with 75ah and has 650 MCA. The deep cycle tested out as 100% health.

Without the engine running I have moved the selector between 1, 2 and both and nothing dimmed or switched off so I must have the right type. I will verify this again but it looks like I can manually control which one is charged for right now.

I do like the idea of the Duo Charge or similar charge regulator and have added this to my project list. I thought with just a simple 2 batter system using Both to charge was adequate, it worked for like 5 years. I didn't realize even that way one battery would be charged more than the other.

Thanks
 
#9 ·
Both batteries should get a full charge. The alternator is set by its regulator at a certain voltage (14.4 in bulk) and the batteries will accept what they need.

The problem with charging through the 1,2,both switch is that you have to control it. You have to be in "both" to charge both. If you forget and leave it that way you will drain both.

The advantage of an Echo Charge as mentioned by Bill is the charging becomes automatic. The charge current does not go through the switch and you never have to put the switch in the both position.
An Echo Charge is more than adequate and is less expensive than the DuoCharge.
 
#10 ·
Here is how I use my "old school" setup. The comments about more modern setups are well taken but assuming you'll be using the old 1,2,both switch for a little longer :

If the batteries have been sitting under an A/C charger so you are starting with them fully charged : When starting and under way, set the switch to 1 or 2, depending on the date. Odd dates, set to 1, even dates set to 2. That way your batteries get even usage, averaged out over the year.

The only circumstance when "both" should be used is an emergency when one battery will not start the engine. It should not be used under any other circumstances. "Both" should really be labelled "Emergency"

Obviously when overnighting, you'll want to drain the deep cycle battery (2?), keeping the starting battery (1) for reserve. In that case the next morning select 1, start the engine, run for 10 mins, then switch to 2.
 
#12 ·
Obviously when overnighting, you'll want to drain the deep cycle battery (2?), keeping the starting battery (1) for reserve. In that case the next morning select 1, start the engine, run for 10 mins, then switch to 2.
Mark,

If you need to select #1 to start the motor then you're drawing the house bank far to low and ideally need a bigger bank. You should never need to switch off the house bank. If you do need to then there is a bank size issue or the battery is getting long in the tooth..

Even 50% SOC a single group 24, 27 or 31 should easily start most small Yanmar, Universal, Beta, Nani or Westerbeke's...

A single group 24 bank leaves you with just 37.5 Ah's to use from a 100% full charge. When working off the alternator and only ever getting charged back to about 80% of full. Then you have only 22.5 Ah's to use before hitting the 50% DOD longevity threshold.

22.5 Ah's is far to little capacity for a weekend on most 25-36 +/- footers if you use the devices and lights most use...
 
#13 ·
This question comes up so many times, that I've developed an Electrical 101 thread with links to many discussions about this very topic, which includes replies from many of the folks who have chimed in here.

http://c34.org/bbs/index.php/topic,5977.0.html

See reply #2 for a very succinct summary of what to do. Get you alternator output off the CV post of the 1-2-B switch and follow btrayfors, Brian's and Maine Sail's advice.

All the best,

Stu
 
#14 ·
mac, there are many answers because there are many difference kinds of equipment out there and they are not all built to the same spec. For instance:

IF your battery switch is a make-before-break type, and IF it has not worn out so that feature fails, then yes, it is safe to switch while running. (Notice, two IFs.)

The problem is that conventional alternators are wired up (IF you have a conventional alternator and IF it is wired up conventionally, two more IFs) so that the feedback to the alternator is designed to cause more power to be generated when there is less (i.e. no) power coming back form the battery. Normally that's a good thing, low battery means give it more power. Except, if you break the connection, the alternator says "Gee, that poor battery really is dead, I'd better run flat out" and 15-30 seconds of that can be enough to burn out an alternator.

Except <G>...some of the new alternators have protection built into them, and they don't care about a momentary blink/loss in the feedback. Some of them have spike and surge protection built in too. Others will shut down if the battery voltage drops to ~10.5V and won't let you recharge the battery until someone gives you a jump to reset them.

More protections, more "brains".

Some battery switches (odds are yours isn't) don't just use "make before break" but they also use a second set of contacts for "field sense" lead to make sure that feedback wire is switched to whatever battery you are using.

Confused yet? That's why there's a lot of different advice, you need to know exactly what equipment you have, what it can accept, how it is wired, and then verify that it IS actually wired and working correctly.

The alternative is to always assume the worst, and never switch batteries while the engine is running. That way, no matter what the equipment is or whether it is malfunctioning, you can't hurt it.

As to why your starting battery isn't always getting charged...that's a whole other issue to pursue. Could be a bad diode, a diode isolator was installed, a bad cable or corrosion...I'd use and run JUST on that battery, while troubleshooting just that battery, to see if it isn't something "local" in just that part of the wiring. (Which could include the connection to the battery switch, or a thinner longer cable, or an older one that's rotting inside...all sorts of simple things.)
 
#15 ·
Not necessarily true, and that's why so many of us have been suggesting that you remove the alternator output from the switch and wire it to your house bank. Done that way, you could turn the switch OFF and still not cause any issues with the alternator.
 
#17 ·
"remove the alternator output from the switch and wire it to your house bank. Done that way,"
Stu, presumably you also mean with an echo charger or West/andina Combiner added in, so the house bank will in turn charge the starter battery.
More stuff. (Good stuff, but more stuff, for the folks who aren't running it yet.)
 
#18 ·
You can add one but it is not 100% necessary with a 1/2/BOTH switch if the owner is on a strict budget. Just use BOTH when charging and your doing the same thing as when the alt ran to the "C" post of the switch and you selected both. You will NOT however be able to charge the second bank while the switch is in that position so an ACR or Echo is very wise and an expense any owner should be talked into when moving the alt from the "C" post to the house bank..

I personally would much prefer a voltage sensitive relay or an Echo type device as it makes the system a lot easier to use. A Blue Seas ACR is about $70.00 and an Echo can be had for about $130.00..

Just had another fried diode situation a couple of weeks ago and may have one I am looking at on Friday. This is not an uncommon problem and is the reason a good battery switch like the 1/2/BOTH/OFF gets an undeserved bad rap....
 
#20 ·
I appreciate all the input and my plan is set.

1. At some point I will install an Echo or similar regulator and change up my wiring configuration. I completely rewired the boat a couple of years ago adding new wiring, fuses and a breaker panel etc and took great care to try and do it right. This never came up so will fix that.
2. In the meantime I will just adjust how I function. I have an analog voltmeter which was starting to show roughly 12.2 values. I thought it was supposed to be up near 13 but did not pay it much mind. Never again. I am on a river and have a roughly 30 minute motor to get to the end of the channel. Going down the river will start/run off battery 1 and going up the river will start/run off battery 2. This way both batteries will get full attention for at least 30 minutes per trip. Until I get the regulator this is probably the best I can do.

I will also pay more attention to the batteries in the off season and bring them in to get tested. The last thing I want is to not be able to start my engine when I have to get back to my mooring against an outgoing tide. I have SeaTow but would rather not have to call them. I am also planning to get an electric trolling motor and battery for my dinghy which will serve as a nice backup if needed.
 
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