Battery switch operation on BOTH.

[seasick]

I am troubleshooting a problem on a boat with twin Optimaxes. While running various tests I had one of those DUH moments. The background:When coldish, and batterry switches set to 1 port and 2 starboard, neither motor will start. They crank over fine but wont fire. Some research online explained that those motor's ECUs are really picky about voltage and the ECU won't operate the injectors if it thinks voltage is a tad low.
Ok, I can buy that because I am seeing it happen.
Si when cold, I set both battery switches to BOTH and the motor fire right up.
Yesterday, I decided to set switches to 1 port and 2 starboard and as is often the case, the motors would not fire. Instead of changing both switches to BOTH, I changed switch 1 (port) only to BOTH and left switch 2 on 2.
Both motors fired up.
Last nigh while trying to understand what was going on, I had one of those "Am I really that dumb?' thoughts.

When one battery switch is set to BOTH and the other is set to 1 or 2, aren't both batteries electrically in BOTH mode?
I think they are and that could explain a lot.
The batteries may be getting a tad old or there may be a little extra voltage drop somewhere that causes a starting voltage drop that is out of range for Optimaxes. When the batteries are in BOTH mode, the voltage drop is effectively cut in half since each battery (in a perfect world) is supplying half of the starting current demand.

 

[Doc Stressor]

If your switches are wired correctly, yes switching either one of them to both should bring both batteries in parallel and double your reserve capacity. The downside is that a bad cell in one battery can drain the other one.

A common cause of voltage loss in older boats is corrosion working its way under the insolation starting at the battery connections. I would have both batteries load tested (or do it yourself since load testers are cheap) and replace the battery cables if they are over 10 years old.

 

[SkunkBoat]

Duh! Both means both.

 

[seasick]

Duh! Both means both.

It amazes me how an electrical engineer like me can miss something as obvious as that. Could be an age related issue.
For the record, battery cables from bats to switches are new. Other cables and connections were check, cleaned and re-tightened..
I haven't been able (actually not willing) to see what type of batteries are in that boat. They are not very old but also not new. The date stamps are not punched.
I have also read Mercury docs that state the Optis don't like any battery other than starting type. At the moment, I can't see what the battery models are and probably won't until next season. It's getting close to hauling season

 

[Doc Stressor]

You have it backward. Opti's require really strong starting batteries with at least 1000 MCA. I don't know of any deep cycle batteries that can produce that output. A few dual-purpose batteries such as the Deka 27M6 that Grady supplies will do that.

Opti's spec a higher MCA rating than any other outboard that I know of. That's because of their electronics (computer, pumps, etc) which require a lot of juice. If your battery is getting weak, the voltage drop caused by the starting motor current draw won't allow the electronics to start the engine. Putting your batteries in parallel supplies the necessary amperage. But it's never a good idea to start or run your engines with the switch in the "both" position.

Folks with Opti's generally have to replace batteries more frequently than with other engines.

 

[seasick]

My mistake! I meant to say Starting batteries not Deep Cycle.

Thanks Doc for keeping me honest. I edited my earlier post to specify Starting batteries just in case someone at a later day searches the forums. I don't want to put out bad info.

I also tested something yesterday related to the running in BOTH concern which I wholeheartedly agree with. I had to use the Both setting to start the motors but after 3 or 4 minutes of warming up, I shut down, switched to batt 1 for port motor and Batt2 for starboard and started the motors. They started right up.
I ran like that all day and had to start the motors many times while fishing.
I also had the stereo on, fridge, sonar and plotter and had no issues.
I do agree that the batteries are probably getting a bit weak. I don't know how old they are but they are at least 3 years old, maybe a lot more.

 

[Finest Kind]

[QUOTE........... But it's never a good idea to start or run your engines with the switch in the "both" position..............[/QUOTE]

Why?
I have been running my 2 Battery switches on BOTH since I bought the boat in 1991 and never had an issue.

 

[seasick]

The idea about not running on BOTH is that should you accidentally discharge a battery, you won't discharge both. Let's say you are overnighting and have a few things powered up, you could run down a battery but having the other battery still charged may allow you to start the motor/motors and get back home.
With the switch in BOTH, you would discharge both batteries and you would be stranded.
Another factor is that the two batteries may not age the same and may have different voltage characteristics. That can result in one battery being discharged by the other.

 

[seasick]

You have it backward. Opti's require really strong starting batteries with at least 1000 MCA. I don't know of any deep cycle batteries that can produce that output. A few dual-purpose batteries such as the Deka 27M6 that Grady supplies will do that.

Opti's spec a higher MCA rating than any other outboard that I know of. That's because of their electronics (computer, pumps, etc) which require a lot of juice. If your battery is getting weak, the voltage drop caused by the starting motor current draw won't allow the electronics to start the engine. Putting your batteries in parallel supplies the necessary amperage. But it's never a good idea to start or run your engines with the switch in the "both" position.

Folks with Opti's generally have to replace batteries more frequently than with other engines.

It looks like I have Deka 100a starting batteries size 24. I can't fit 27 in the space.
I will most likely have to replace them for next season but will also check for poor connections.

 

[Fishtales]

I thought the Duracell Group27s at battery and bulbs was a good deal (10% off if you order on line and pick up in store). I think most of the batteries are made by one or two manufacturers anyway.