Adding a Second Battery System

[Seaduced629]

I'm doing a "deck off" renovation of a Bass Buggy pontoon boat. It has a 1979, 90 hp Mercury on it. I want to add a second "house" battery system and charge both the start and house systems from the Merc. We intend to sleep on the boat for several days at a time and I hate to paddle.

The rectifier output is on a red wire, tied to the starter cable under the cowl. So, it can't be isolated from the battery, unless I cut it and then rig a way to exit the cowl. But on this motor, there is an unused white wire in the harness. I'm thinking, I can take the red wire off of the rectifier and tie it back. Then attach the white wire to the rectifier and the other end to the charge isolator at the electrical panel.

I'll have to tie the ignition switch end of the red wire back (since it is now a dead wire) and run a wire from the start circuit fuse block to the red (battery) wire on the ignition switch. Otherwise, I could connect the two batteries through an 18 gauge wire. Definite fire hazard.

So far, my concern is the current rating of the motor-to-boat harness connector. I'm working on getting the info from the manufacture.

What am I missing? Any thoughts?

 

[Scott Danforth]

dont reinvent the wheel. use a battery switch and ACR. wiring details in the electrical section.

 

[Chris1956]

A battery isolator connected between the batteries will work, but cost you a 1V drop to the second battery. A battery combiner is a better option, as there is no voltage drop, when batteries are both charged.

Be aware that a 1979 Merc only will charge about 9 Amps at high speed. That may not be enough to keep the batteries charged. Maybe some other charger type is batter, say a small shore generator?

 

[Seaduced629]

dont reinvent the wheel. use a battery switch and ACR. wiring details in the electrical section.

dont reinvent the wheel. use a battery switch and ACR. wiring details in the electrical section.

I don't have a charge circuit outside of the cowl, so the ACR would have to be between the batteries? I didn't find a diagram that show an ACR in use.

 

[Scott Danforth]

I don't have a charge circuit outside of the cowl, so the ACR would have to be between the batteries? I didn't find a diagram that show an ACR in use.

your battery cables exit the cowl to charge the batteries...... that is how the motor works.

so you simply add a second battery and ACR at the end of the cables at the battery. a simple 0.000002 second google search will pull that up, or go to the electrical sub-forum and look there.

However as Chris mentioned, you only have 9 amps of charge power to start with from the stator. you may need to simply go with two house batteries and have that circuit separate and charge them at home.

or add an inverter duty generator and a carbon monoxide detector

 

[Seaduced629]

A battery isolator connected between the batteries will work, but cost you a 1V drop to the second battery. A battery combiner is a better option, as there is no voltage drop, when batteries are both charged.

Be aware that a 1979 Merc only will charge about 9 Amps at high speed. That may not be enough to keep the batteries charged. Maybe some other charger type is batter, say a small shore generator?

I was thinking about the drop and I found the Argodiode Battery Isolator. They say the drop is .45v at rated current of 80A, .3v at low current. But, I need the charge wire to feed it. This my solution.

I appreciate the info about the Merc output, I had not been able to find it. I am also considering a solar panel to top off the battery at night.

 

[dingbat]

I don't have a charge circuit outside of the cowl, so the ACR would have to be between the batteries? I didn't find a diagram that show an ACR in use.

An ACR is useless with only 9 amp charge capacity.

Bear in mind also that the 9 amp output only occurs at high rpm. Varies by motor, but most don’t put out squat below 1,500 rpm.

So the question becomes, how many amp hours do you use in your typical outting? How many hours of run time do you put in during your trip? The answers to these two questions will answer the feasibility issue.

Guessing in reality, install a 1/2/both switch with a second battery and call it good.

 

[Seaduced629]

An ACR is useless with only 9 amp charge capacity.

Bear in mind also that the 9 amp output only occurs at high rpm. Varies by motor, but most don’t put out squat below 1,500 rpm.

So the question becomes, how many amp hours do you use in your typical outting? How many hours of run time do you put in during your trip? The answers to these two questions will answer the feasibility issue.

Guessing in reality, install a 1/2/both switch with a second battery and call it good.


View attachment 356393

I don't know what I'm going to need, we were looking to camp out on the boat for a week or so. I may have to try a couple of trips to see how the house battery holds up. And a solar panel is looking like a good idea. Thanks for the info.

 

[Chris1956]

A Battery Isolator is simply a high amperage diode that routes DC to the other battery.
A Battery Combiner is a relay that closes when the primary battery is charged up, connecting the secondary battery to the primary battery, for charging.

Both are connected to the primary battery and the secondary. No OB motor wiring changes are necessary.

Just so you know, the charging wire on your motor is the +dc post of the rectifier. It must have a battery connected to it at all times, or the rectifier will be damaged.

Check the watts of the solar panel. If it is a small one, it will not charge much. If your house battery is 65amp hours, deep cycle for example, (which is not large), you will want at least an 100Watt solar charger, which could charge your battery in ~ 8 hours of sunlight. of course, if you charge at night......you get nothing.

 

[roscoe]

I am also considering a solar panel to top off the battery at night.

now thats funny


Gonna need to take a lot of gas along to run that outboard at full throttle, long enough to keep the batteries charged.

So much cheaper and easier to buy a small, $450 to $1200 ( for the nice Honda ) portable generator. Just be sure to get the quietest one, not the cheapest one.
A generator will allow you to run just about anything you want, stereo, thermoelectric cooler, coffee pot.....