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Old 21-06-2009, 07:14   #1
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House DC on AC Shore Power?

Our last boat had a shore power battery charger that seemed to have no problem both charging the batteries and supplying enough DC house electricity. Not so the new shore power charger I just installed in our 20s' foot sailboat. The charger, I found out too late, is the type that can only charge the batteries, but not keep up with DC house loads at the dock on shore power. I did not know about this issue until after the install, clarified when I called the company.

So, instead of buying a more expensive batter charger and wasting the one I have, can I just add an AC-powered device in addition to the battery chager that will allow the battery charger to do its thing with the batteries, but separately supply DC house loads from AC shore power directly to the DC distribution panel independantly of battery charging, preventing the batteries from being discharged to supply house loads? Am I looking for a recitfier or swithchable power supply?
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Old 21-06-2009, 09:29   #2
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Not a good idea, IMHO.

Need more info re: your boat, battery bank size/type, and anticipated 12V DC house loads, but the short answer is you should cut your losses with the batt. charger you have (put it on eBay) and get one which is properly sized to both charge the batteries and supply house loads while on shorepower.

Chargers don't have to be expensive to be good. For many boats, the Iota line of chargers (they have sizes from 15A to 90A) are excellent, sophisticated, and relatively inexpensive.

Bill
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Old 22-06-2009, 17:24   #3
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Not a good idea, IMHO.
Why? To answer your application questions: Group 24 house and group 24 engine start batteries, minimal DC requirements (cabin lights, cellphone charger, anchor light, VHF, etc.).
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Old 22-06-2009, 17:30   #4
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You are making things too complicated. Exchange or sell the underpowered DC charger you have now and replace it with one that is not underpowered...one that will allow for charging your batteries at a good rate while you are using most of your house DC loads. For a twenty something foot boat I would be looking at something no less than around 50 amps DC. That would be a typical minimal size for your sized boat.
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Old 24-06-2009, 03:11   #5
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Let me try asking this another way. What is it about "cruising" shore power battery chargers that allows them to smart-charge battery banks but also supply additional amperage to run DC appliances connected to the house battery distribution panel? What circuitry do they have that cheaper chargers do not?

The charger I got (aimed at trolling boat market) apparently does not have that capability, and merely floats the batteries with minimal amperage once the initial charging stage is done. When I flip on more than a couple of incandescent lights, the charger switches back to a higher charging mode from float, and flips back when I turn off all but one light.
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Old 24-06-2009, 04:50   #6
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clearsea,
I'm no expert but it sounds to me like your battery charger is doing exactly what it is supposed to do ...... Just curious, have you tested your batteries to find out what condition they are in?
Good luck, Bill A.
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Old 25-06-2009, 02:53   #7
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I'm no expert but it sounds to me like your battery charger is doing exactly what it is supposed to do ...... Just curious, have you tested your batteries to find out what condition they are in?
Everything's new: wiring, batteries, charger, etc. The companythat made the charger confirmed my dilemma with the charger model I bought.
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Old 25-06-2009, 07:34   #8
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The charger I got (aimed at trolling boat market) apparently does not have that capability, and merely floats the batteries with minimal amperage once the initial charging stage is done.....
Therein lies the rub. This charger is inappropriate for the application. It's a trolling motor battery float (trickle) charger, not a marine shorepower battery charger/power supply. Dump it for what you can (or keep it as an emergency spare) get the appropriate device.
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Old 25-06-2009, 10:42   #9
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Look up Iota chargers on ebay or elsewhere on the net. I was turned on to them by someone on this site. Very cost effective.
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Old 25-06-2009, 11:57   #10
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I'm not sure I fully understand your problem. You say that the charger charges your batteries and goes from absorption mode to float. Then as you turn on DC loads (which are supplied by the battery), your voltage drops, the charger senses it and reverts to absorption mode to top off the battery. That's what it is supposed to do, but it probably shouldn't do it quite as quickly as it is. Perhaps there is not enough reserve in the chargers capacity to stay in the float mode or the problem could be that the battery voltage is dropping too quickly under load. It almost seems to me that some of your problem could be insufficient house battery capacity.

Are you using deep cell batteries or battery for your house loads (starting or auto batteries are not appropriate for house loads)? What would happen if you used the loads without the charger connected? Does the charger provide enough extra DC current for the loads when it does go into the absorption mode?

If so maybe you really don't have a problem with the charger and should look harder at the batteries.

Good Luck

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Old 27-06-2009, 02:04   #11
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Quote:
Originally Posted by clearsea View Post
Let me try asking this another way. What is it about "cruising" shore power battery chargers that allows them to smart-charge battery banks but also supply additional amperage to run DC appliances connected to the house battery distribution panel? What circuitry do they have that cheaper chargers do not?

The charger I got (aimed at trolling boat market) apparently does not have that capability, and merely floats the batteries with minimal amperage once the initial charging stage is done. When I flip on more than a couple of incandescent lights, the charger switches back to a higher charging mode from float, and flips back when I turn off all but one light.
Assuming we are talking specifically about smart/switch mode chargers of the same voltage, all that sets your trolling charger apart from chargers used on cruising boats is their power output. This is best defined as the current (amps) they can supply continuously.

If your maximum onboard amperage drain exceeds the output of the charger then clearly you need a more powerful one.

As mentioned elsewhere it seems your charger is working as it should, but from your comments may not be powerful enough. All the smart charger is interested in is achieving or maintaining a specific battery voltage relevant to the mode it is in at the time. When you take power from your batteries by switching on onboard devices, the charger needs to have the reserve of power to compensate. If it can't cover this additional load, the batteries discharge to the point where the charger is obliged to go into bulk/boost mode.

Putting another smart charger in parallel with it will do no harm but working this way they may not give their individual full performance unless perfectly matched. A better solution would be a single charger capable of supplying all your needs.

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Old 27-06-2009, 08:19   #12
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Full scale smart chargers also monitor amperage load (not battery voltage) via a shunt. As it knows how many amps are being taken out, it can adjust charging appropriately.
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Old 27-06-2009, 10:40   #13
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I think the term you are looking for is Load Shedding. It is done primarily in the better Inverter/Chargers which give priority to loads rather than the charger, and only reduce the current to the charger.

Joe S
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Old 27-06-2009, 21:37   #14
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svquest2, while higher end chargers do that, that's not what I was talking about (I'm not sure if you were replying to me or not). A charger that only monitors voltage can't adjust itself to current/instant 12VDC loads like a smart charger with a shunt can.
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Old 27-06-2009, 22:03   #15
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If you're not trying to smelt aluminum, give the thing a chance. I don't think you have a problem.

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(30 years retired electronic tech, retired cruiser from a boat which never had to run the engine for electricity but had 100 ah load per day which even impressed Richard Spindler from Lattitude 38)
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