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Old 10-11-2008, 13:22   #1
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Battery isolator voltage drop

I'm using a Guest 30 amp battery charger when on shore power. It's old, but is putting out 13.4 volts.

This is connected to a Guest battery isolator model 2430. The voltage in is 13.4, the voltage out is 12.7. Is this drop normal?

My house bank will never charge above 12.7 volts.

When the engine alternator is running I can get the bank up to 13.2. The connection is via a Guest 2401 battery isolator.

The bank, which is made up of 4 Trojan SCS200 115 amp hour batteries, 12 volt, will settle in at 12.4 volts within a few minutes of terminating the charge. They are 1 year old.

Am I looking at normal numbers for this set up?
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Old 10-11-2008, 14:08   #2
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It is normal for the voltage to drop across the isolator diodes to be 0.6 to 0.7 volts. You are undercharging your house bank as a result. You should hook the Guest battery charger directly to the house bank and eliminate that battery isolator.

The alternator isolator may be useable if you have a external regulator on the alternator. If the regulator sensing voltage wire is between the battery and the isolator, the regulator will compensate for the voltage drop across the isolator. Your measured voltage of 13.2 volts after significant engine time is about right for a multi-stage regulator, but too low for a single stage regulator.

A resting voltage of 12.4 volts indicates that you may have some sulfating as a result of chronic undercharging, and equalizing the batteries is probably a good idea.

I assume you have some way of combining your battery banks so that you can charge all of them if you hook the shore charger directly to the house bank.
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Old 10-11-2008, 14:31   #3
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It sounds as though the Guest 2430/2401 are diode isolators. As such, they seem to be functioning as designed: they will always show about a 0.6V drop from input to output. (I think this is called the diode forward voltage drop, Vf). Not knowing the battery type (gel, AGM or wet), it could take a very long time to recharge at 12.7-13.2V.

You might consider replacing them with switching-type isolators that show no voltage drop. Blue Sea has one, I think, for these levels of current. Thinking about it, I am not sure why you need two isolators...

Alternatively, you could wire the charger and alternator directly to the house bank and reverse the isolators so that the house bank will always see the higher voltages. The start battery will then see the lower voltages but it should only require minimal charging.

It sounds like the charger is an old ferro-resounat type. I found the investment in a new 3-stage charger well worth it in terms of battery life and ease/time of charging.

- - Dave
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Old 10-11-2008, 20:09   #4
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Battery combiners can cause problems especially when the solenoid switch closes and you suddenly have a rush of current. The guy at the yard where I haul out said he has seen numerous problems with battery combiners.

What I did was put the voltage sensor terminal on my alternator on the the low voltage side of the isolator. Check with a marine electrician first.
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Old 11-11-2008, 06:09   #5
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Thanks for the advice so far... I may look at re-wiring as suggested.

We tied back up at the dock yesterday and plugged in. I let the charger run for 7 hours and got the bank up to 12.9. (flooded cells)

The bank settled to 12.5 volts with everything off, in about 20 minutes after turning the charger off, that was 9 pm. I turned the reefer back on and turned on the mast light as well. When I got up at 0330 for my anchor check, (yea, even at the dock... habits) the system was at 12.0 volts.

I replaced some very old cruddy batteries about a year ago with the Trojans. I previously had four 6v (wired for 12) golf cart batteries into two banks. (2 batts each bank, they were separate banks due to placement locations)

The cruddy old batteries seemed to hold up better then the Trojans... I just don't get it.
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Old 11-11-2008, 06:51   #6
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Quote:
Originally Posted by lannen View Post
I let the charger run for 7 hours and got the bank up to 12.9. (flooded cells)

The cruddy old batteries seemed to hold up better then the Trojans... I just don't get it.
Lead-acid batteries need 13.8 volts to get a full charge. Your batteries are not getting fully charged.

You need a three stage marine grade battery charger that has multiple terminals. One terminal goes to your start battery(s), one terminal for your house battery(s) and another terminal if you have a third bank of batteries that are isolated from the first two battery banks. Some boats have this.

As for charging the batteries fully with the engine, you need to put the voltage sensor on the alternator on the opposite side of the isolator from the hot terminal on the alternator. This way the alternator sense the voltage on the system side and not the alternator side of the isolator.

Yes, the voltage on your engine will run higher, but a half volt difference will not hurt anything. I said check with a marine electrician in case your diesel has computers (ECM's) that run it. Most smaller and older boat diesels though do not have ECM's that control the engine. I wanted to make sure that running the engine with a higher voltage does not affect them, if you have them.
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