SailNet Community

SailNet Community (
-   Electrical Systems (
-   -   Change Battery Selector While Engine Running? (

maccauley123 07-24-2011 01:25 PM

Change Battery Selector While Engine Running?
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.

btrayfors 07-24-2011 04:46 PM

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.


JimsCAL 07-24-2011 06:29 PM

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.

brokesailor 07-25-2011 06:37 AM

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?

Maine Sail 07-25-2011 08:03 AM


Originally Posted by brokesailor (Post 754055)
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?

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..

tempest 07-25-2011 09:02 AM

I have, at times, switched from bat 1 to bat 2 through the both setting with no ill effects, that I'm aware of. I just replaced my bank ( 2 batteries) after almost 5 years of reliable service.

Question. Are both your batteries of the same age and type?

Spyder 07-25-2011 09:17 AM

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.

maccauley123 07-25-2011 11:48 AM

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.


mitiempo 07-25-2011 12:25 PM

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.

MarkSF 07-25-2011 12:27 PM

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.

All times are GMT -4. The time now is 10:16 AM.

Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging v3.1.0 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
(c) LLC 2000-2012

For the best viewing experience please update your browser to Google Chrome