Battery Isolator with Battery Drain

[Bensigler]

Hi there,

I have a twin engine setup with 2 starter batteries and a bank of house batteries. I was having an issue that my house systems were draining my house batteries and my house batteries together. The problem stopped when I disconnected my battery isolator (the house batteries drained, but the starter batteries were just fine). So, I bought a new isolator and installed it and now the same problem. When I run the fridge and the lights, all batteries in the boat go dead.

Does anyone have any suggestions? What could this possibly be? The starter batteries are only connected to the house batteries through the isolator and a negative connection. Could a short on the house batteries cause such a malfunction?

P.S. I had all the batteries tested and replaced the one that was not functioning at optimum.

[GordMay]

Greetings and welcome aboard the CF, Bensigler.

[DeepFrz]

Sounds like you either have a fault with the isolator or things are not connected properly.

Did this start to happen after any changes or additions to your electrical system?

Double check the wiring changes you may have made just before this problem started.

You can check the diodes in your isolator with an ohmmeter. Disconnect all cables and make sure you have a high resistance in one direction and a low resistance when you reverse the leads.

Make sure that you don't have any wiring bypassing your isolator.

Your start batteries should have a cable to the isolator and a cable to the starter, and that is all (possible exception, instrument panel). Check at the isolator, the battery and the starter to make sure you don't have wires connected. If you do, make sure you know exactly what they are for and where they go. Also check the full length of your starter battery cabling to make sure there are no connections that you don't know about.

To reiterate, there should be no cables connecting the start and house battery hot leads (+12 volts). All batteries should have one common ground bus (-12 volts).

[donradcliffe]

Its time to get more familiar with your wiring. First thing to do is check the emergency cross-connect switches.

[capngeo]

Another option is to disconnect each battery lead in turn, switches all off (one at a time) and test with an amp meter back to the pos post. The one showing a load is the culprit. At least that will narrow it down for you.

[Bensigler]

Thank you all very much for your help and your nice welcome.

In regards to my current wiring set up, I have made sure that the starter batteries are hooked up to the starters and crossover solenoid (used for jumping) only.

The house batteries are connected through just one wire to the main panel in the cabin and a few smaller wires for the auto pilot and flo scan gauges. Besides for all the common ground wires, there are no other connections that I can see.

In the interim, I have found a "band-aid" type solution. I installed an on/off battery switch between the aux battery lead on my isolator and my house batteries. This allows me to swing on the hook and drain the house batteries completely without effecting the starter batteries. I have tested this set up yesterday and it worked. The house batteries drained, but the starters were ok.

This seemed to indicate that it's the isolator which is the issue here. I double checked the wiring today. 2 posts in from the alternator and 3 posted out to the batteries. 1 and 3 are the starters and 2 is the bank of house batteries (wired in parallel). This is exactly what it says in the manual.

I am confounded b/c this is a brand new isolator I bought last week. I took it strait out of the package myself.

I have discovered something else puzzling as well. Once the house batteries err drained, I ran the twin engine cruiser at 3500 rpm for an hour trip and the house batteries were not charged by the alternators at all. The starter batteries were being charged. I triple checked that my battery switch was in the right position and I am sure i wired it correctly.

I appreciate any more wisdom you might have. I going to try and test the isolator with an ohm meter over the weekend. I would really like to fix this on my own, but I think I might have to call in a wiring specialist.

P.S. This is an old problem that I am just making time to fix now. Nothing new was added to the system except the radar and chart plotter/autopilot screw years ago.

[DeepFrz]

This allows me to swing on the hook and drain the house batteries completely

I hope you didn't mean this literally.

Please tell us what brand and model number of battery isolator so we can look it up.

[Andina Marie]

An isolator works by charging the LOWEST battery first before starting to charge the next lowest. If your house battery is too low, or has a bad cell, or the alternator output is not sufficient to bring the house battery voltage up, then the starting battery will not get a charge. Disconnecting the house battery stops the charge being diverted to the house battery so then the starting battery can charge.

Definately get the alternator output checked to see if it has adequate output.

Consider using a Combiner100 instead of an isolator. Isolators have a built in voltage drop so your batteries may not get a full charge but there is no voltage drop in a Combiner100. In addition the Combiner will give priority to the starting battery and make sure it has a charge before starting to charge the house battery. They also regulate the load on the alternator so it doesn't overheat and works more efficiently.

[DeepFrz]

Time to get the ohm and voltmeter out to see what is going on. Are you sure you closed the disconnect switch? By the way, why auxiliary? I don't recall seeing that label on the battery isolator I worked on. AM brings up a good point, the alternator sense wire should connect to the battery downstream of the isolator. That will take into consideration the voltage drop across the isolator. I, personally, think that isolators are just fine and work well if they are hooked up properly.

[Andina Marie]

Originally Posted by DeepFrz

<SNIP>AM brings up a good point, the alternator sense wire should connect to the battery downstream of the isolator. That will take into consideration the voltage drop across the isolator. I, personally, think that isolators are just fine and work well if they are hooked up properly.

Therein lies the problem.

To which battery are you going to connect the sense wire?

If it is on the starting battery and it is only slightly discharged from just starting the engine, then the regulator will cut the alternator back to a minimal charging rate to top it off. But this minimal output it is producing is all going to the house battery and it will take forever to charge.

If it is on the house battery it will force the alternator into maximum output causing it to overheat and cut output as much as 40%. A Combiner100 not only isolates the batteries but provides priority to the starting battery and regulates the amount of charge going to the house battery to within the alternator capacity.

[DeepFrz]

It should always be connected to the house battery, the same as if you had an echo charge or duo charge instead of an isolator.

I think you have this thread mixed up with another one, this OP does not have an alternator overheating problem. At least not a stated one.

[Bensigler]

Hi there, and thank you again )

Ok, I'm going to check the make and model when I get back to the boat (hopefully Monday). I'm also going to use an ohm meter and take some measurements. Lastly, I'll google the combiner100 and learn more about it.

Looks like I've got some homework to do now

Thank you all again,
-Ben
P.S. As far as I can tell, the alternators are sending 13.8 volts and they are not overheating.

[David M]

Are we talking about a battery isolator (a big diode) or a battery combiner (an automated solenoid switch)?

[nigelmercier]

Originally Posted by Andina Marie

An isolator works by charging the LOWEST battery first before starting to charge the next lowest...

May I correct this (often quoted) misinformation.

A (diode) isolator is simply a diode in each leg of the charging circuit. These act as a one-way valve, so when not charging they prevent one battery draining the other. When charging, the diodes conduct (the valves open) effectively shorting the two batteries together. There is no concept of it charging one battery first, this is simply due to a low battery accepting a charge faster.

In addition, because the batteries are connected when charging, it makes no difference which battery has the "Battery Sense" lead attached, so long as it is downstream of the isolator.

[OceanPlanet]

Originally Posted by nigelmercier

May I correct this (often quoted) misinformation.

A (diode) isolator is simply a diode in each leg of the charging circuit. These act as a one-way valve, so when not charging they prevent one battery draining the other. When charging, the diodes conduct (the valves open) effectively shorting the two batteries together. There is no concept of it charging one battery first, this is simply due to a low battery accepting a charge faster.

In addition, because the batteries are connected when charging, it makes no difference which battery has the "Battery Sense" lead attached, so long as it is downstream of the isolator.

Wrong. The batteries are not "shorted" together when charging. As you say in your first sentence, the diodes are one-way valves. The voltage of the two batteries will remain different until the lower voltage (less charged) battery reaches the same voltage as the other.

So it DOES matter which battery the alternator regulator voltage sense wire is connected to (and temp sense also, for that matter), as the alt. reg. will adjust it's field output to full (bulk) until the sensed battery reaches the absorbtion voltage level.

And if the alt can't handle charging at full output for a reasonable length of time, it should be a bigger/better alt.