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WakeWorld Discussion Board » >> Boats, Accessories & Tow Vehicles Archive » Archive through November 17, 2003 » battery isolator not charging stereo battery « Previous Next »
By salty87 (salty87) on Tuesday, October 14, 2003 - 2:27 pm:    Edit Post Delete Post
is it possible that a fully charged starting battery is preventing my isolator from charging a nearly dead stereo battery??

i never have to run thru no wake zones or anything, the stereo is always on but the engine isn't.

could the alternator be too small, its supposedly only a 35 amp (1987 vintage).

would it be better to use a battery switch in this application?


By Rod McInnis (rodmcinnis) on Tuesday, October 14, 2003 - 3:08 pm:    Edit Post Delete Post
No, if your isolator is wired and working properly, then you shouldn't ever end up with one fully charged and one dead battery

How do you have the isolator wired? The main wire from the alternator should connect to the appropriate terminal of the isolator, and then the other two ouputs of the isolator should connect to each battery. When the engine is running, you should have around 13.5-14.2 volts at the alternator. The voltage at the batteries should be just slightly less, about 0.7 volts lower than what it is at the alternator.

I would double check that the isolator is connected properly

By salty87 (salty87) on Wednesday, October 15, 2003 - 7:19 am:    Edit Post Delete Post
i've tested the output from the isolator, its closer to 12.8 when the rpm's are the batteries. i haven't measured the output at the alternator.

i need to re-do the battery connections anyway, mine are embarassing. wires all over the place.

i need to replace my batteries too but i want to get this fixed first. maybe one of them is bad already.

anyone have pics of their battery connections with an isolator?

By Rod McInnis (rodmcinnis) on Wednesday, October 15, 2003 - 11:02 am:    Edit Post Delete Post

12.8 volts is a bit lower than I would have expected unless one of the batteries was really discharged.

It is possible that your alternator output just isn't enough. How many watts are you pumping out that stereo? The disadvantage of an isolator is that it does reduce the rate at which the batteries charge. If you run the stereo battery down a lot when the engine isn't running at speed, then the reduced charging rate might not be enough to bring it back up.

The ideal setup when using an isolator is to have a voltage regulator for the alternator that has "remote sense". This allows the voltage regulator to see the voltage at the battery (instead of what the alternator is putting out) and it adjusts for the voltage drop due to the isolator. You won't find such a regulator in any stock configuration, and they are not exactly cheap. If your altnerator has an external regulator you could change it for the remote sense type. If you have an internally regulated alternator (which is the usual case these days) you're stuck.

You could replace the alternator with a higher output model. The isolator will still cut the output a bunch, but the reduced output of a 75 amp alternator will be a lot more than the reduced output of a 35 amp unit.

The cheapest and easiest solution would be to buy a "Battery Combiner" and do away with the isolator.

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