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Posts: 14
Reply with quote  #1 
I am removing twin 1995 Johnson 200's from my boat and replacing them with 2007 225 Etec's. During the removal of the old rigging I found the old motors were wired to a battery isolator. From looking in the Evninrude manual and looking over the harness for the Etec's I can't see where/why the Etec's should be connected to an isolator.

I am running a 3 battery system. 1 house battery and 2 independent starting batteries all on separate switches.

Can anyone confirm there is no need for the isolator now? If so, how is it included in the Etec wiring. Thanks!

Posts: 2,495
Reply with quote  #2 
The isolator enabled the single charging output of your previous engines charge the house battery while keeping the house battery from running down the cranking battery.
The ETEC engine has two charging outputs connected to the battery cable at the starter relay.
You can leave these connected and use the isolator or add the Evinrude aux charge kit and have two separate charging outputs from the engine, one for starting battery and one for the house battery.

Posts: 6,394
Reply with quote  #3 
The term "battery isolator" is not particularly precise. As RICK comments, in general it means a device that permits DC current to flow through it from a supply port to two output load ports, while preventing current flow between the two load ports. In the simplest implementation the device uses two diodes. The use of diodes results in a forward voltage drop across each diode. In the case of battery charging, this voltage drop has the effect of reducing the charging voltage to each battery by some amount, usually in the region of 0.5 to 1.0-Volts. If the charging source voltage is regulated (as it should be) to prevent over-charging, the reduction of the voltage in the "isolator"  typically results in chronic under-charging of the batteries.

There are more sophisticated battery isolators that do not use diodes, replacing them with better circuitry, and resulting in much lower voltage drop across the isolator, perhaps on the order of only 0.1-Volts. This reduction in charging voltage would generally not be harmful and would generally only mean a slightly longer charging time would be necessary to restore battery charge.

On that basis, the decision to retain or remove the "isolator" would be greatly influenced by the type of isolator in use in a particular installation.

There is really no basis to assume that an E-TEC engine would have an effect on the need for an isolator. The E-TEC engine in stock form provides only a single battery charging output, no different than any other outboard engines.

As RICK has already mentioned, some E-TEC engines provide for an accessory wiring harness that will split the E-TEC battery charging output into two leads, which will allow the engine to charge two batteries simultaneously. 

I recommend as follows:

--obtain the Evinrude literature  "Predelivery and Installation Guide" of your E-TEC engines. In that literature several drawings will show the recommended configuration for using TWO E-TEC engines with THREE batteries. Use one of those configurations. Note that the drawings show a particular type of battery switch. Use the type of battery switch shown in the drawings. Providing a schematic or pictorial drawing of a suggested wiring arrangement is the best method to describe it. I will not try to describe the wiring here in a narrative. See the literature and its drawings.

--if shown in the Predelivery and Installation Guide for your E-TEC engine that the Auxiliary Battery Charging Kit is to be used, then purchase and install the optional Auxiliary Battery Charging Kits for the E-TEC engines, as shown in the literature.

For advice on how to download the "Predelivery and Installation Guide", see the closed-topic article at


For advice about installation of the Auxiliary Battery Charging Kit see

EV-Diagnostic cables are back in stock.

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