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Old 06-14-2016, 08:21 AM   #1
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Battery charging question

I want to check this out with the group... seems like there are wrinkles to everything. My battery setup is 1 group 27 starting for the genny, 1 4D starting for the main engine, 2 4D house. From memory the 4D's are about 100ah each. All are flooded lead acid. Shore power charging is a Magnum 40 amp with three legs.
My question is about charging away from shore. Generally the house batteries run down to about 50% soc in 12 hrs, mostly from refer and lights at night. I have the genny battery wired to the start battery with an ACR. The engine and house can be combined with the usual 1-2-both switch. The genny alternator is rated at 50 amps. Am I correct that running the charger off of ac while the genny is running gives me additive current from both the charger and alternator for a faster charge? I'm talking about bulk charge up to 80% or so. I keep a close eye of the water levels, especially that group 27, but so far it doesn't seem to be overcharging. Am I doing anything wrong?
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Old 06-14-2016, 08:36 AM   #2
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Your genset powered shore power charger and your engine alternator will coexist just fine, although the engine alternator won't contribute much as the batteries get close to full.


12 hours to consume 100 AH (actually I think 4Ds are higher) is a lot. I would add additional house battery capacity. Four 6v GC batteries wired in series/parallel will give you 440 AH of capacity and will fit in about the same space as two 4Ds.


You also should consider a larger shore power charger if you anchor out a lot. This will minimize genset running time.


David
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Old 06-14-2016, 08:44 AM   #3
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Probably won't get much from the generator's alternator as it lacks a smart regulator. It will probably see the high bulk charge voltage from the charger and add little of its own. I would leave the generator battery separate from everything else and let the generator alternator handle the charging. No chance of accidentally draining that battery.

Ted
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Old 06-14-2016, 10:23 AM   #4
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I would leave the generator battery separate from everything else and let the generator alternator handle the charging. No chance of accidentally draining that battery.

Ted
OK - so generally when I am underway I let the generator run for the fridge and to keep the batteries charged - my gen battery is a separate one. Any harm in letting the generator run on? (Northern Lights diesel)

Possibly over the July 4th weekend I may have a sitter for my two older dogs that don't do well on board at all - they have mobility issues. No harm in letting the gen run the AC all night?

mike
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Old 06-14-2016, 02:02 PM   #5
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No harm if it's a diesel generator and your sitter understands how to at least monitor it and shut it off if need be.
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Old 06-14-2016, 03:00 PM   #6
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100 hour 4D sounds very small. Those batteries are usually about 200 ah. You mention that the "generator alternator" is 50 amps, but i think you mean that the generator output is 50 amps? That would indicate that you have a 4.5 or 5 kw generator? The generator can run your AC all night, but you can damage your generator if you run it lightly loaded. Generators like to work.
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Old 06-14-2016, 05:06 PM   #7
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Thanks for the replies. The capacity of the 4D's might be 200ah, not at the boat but sounds good. I am going to switch to gc when these die. For clarification, genny is 8kw ac. Genny's alternator puts out 50 amps 12v dc. That 50 amp alternator output is tied into my battery bank.
My shore charger s 40amps. I'm looking for something more than the 50 amps by running both together while on the hook, trying to get everything back to about 80% soc in an hour of running the genny. My only big concern is harm to alternator or charger or batteries.
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Old 06-14-2016, 05:42 PM   #8
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When you change to gel cell batteries, you may have to change the output of your alternator and battery charger to suite the charge curve of the gell batteries. Some of them have different charge voltages and float voltages. That being said, I think you really do need a bigger battery charger. As a general guide you want to be able to charge at about 25% of the banks ampacity. The alternator may be rated at 50 amps but it will very quickly loose output as it gets warm and you are probably only seeing 20 amps or even less than that continuous. A lead acid battery bank at 50% SOC can accept a charge rate of about 25% of the battery (bank) capacity. So assuming that you have two 200 ah batteries in parallel, the batteries can accept about a 100 amp charge rate.
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Old 06-14-2016, 06:17 PM   #9
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Just fired up genny.... charger putting out 35 amps. I'll see what it is in a half hour.
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Old 06-14-2016, 08:35 PM   #10
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Half hour later...10 amps from the charger. Think Alls well, just making sure I'm not wrong in my combining charging sources. Yes all your advice with battery size, etc. is what I will do next time around. For now, just trying to make the most of what I have.
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Old 06-14-2016, 09:40 PM   #11
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OK - so generally when I am underway I let the generator run for the fridge and to keep the batteries charged - my gen battery is a separate one. Any harm in letting the generator run on? (Northern Lights diesel)

Possibly over the July 4th weekend I may have a sitter for my two older dogs that don't do well on board at all - they have mobility issues. No harm in letting the gen run the AC all night?

mike
Mike,

If you are underway, the batteries should be getting charged by the engine alternator. Is the fridge only 120 volts ac and not also 12 volts dc? If so, I would strongly recommend adding an inverter / battery charger. This would allow you to run the refrigerator when cruising without running the generator. Many inverters such as the Magnum Energy have a programmable battery charger which can be set to your type of battery. Also the maximum charge rate can be set to match the bank size up to 125 amps.

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Old 06-14-2016, 11:47 PM   #12
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What I get from your description is this:

1: You have a load that is heavy enough to deplete your batteries from fully charged to 50% in 12 hours.

2 Your charger can only put back a max of 40 amps. This means you will get an lesser output, averaging no more than about 25 amps. You haven't stated what type of charger this is, but it is unlikely to be a modern, 3 stage "smart" charger, judging only from the small size.

3: your alternator (on the genset) is rated at 50 amps. This too is unlikely to have a "smart" regulator, so will put out close to 50 amps only briefly when you start up the genset, only enough to recharge the genset's own start battery. After that burst of amps, the alternator will put out very little.

The best way to improve your charging regime is to start by reducing your loads. If refrigeration is AC only, there isn't much you can do, short of throwing it away and putting in a DC only fridge. Modern DC compressors will draw 1/3 or less than the best AC only fridge. You may be able to have your box redone with a DC compressor.

Once you have done all you can with the load, you need to improve your charging equipment.

You will want at least 100 amp charging capacity, through a "smart" regulator. You may also want an inverter, so go for the Magnum or Xantrex, combined Inverter/charger. This will get you charging capacity, "Smart" regulator, and a 2000 to 3000 watt inverter, all with an echo charger for the start batteries.

With the right equipment, you can anchor out every night and run your genset only a few hours a day.

I have a fridge and a freezer, both DC, a 2000 watt inverter/charger, as described above, and I usually run the genset an hour in the morning, making hot water for showers, and another hour later in the day, as needed. I never let the house batteries get too low with that regime. I know they are being babied, as I recently replaced them after 10 years. I am about to replace my single start battery (for all 3 engines) after 12 years, so I know it too has been babied.
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Old 06-15-2016, 07:34 PM   #13
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Mike,

If you are underway, the batteries should be getting charged by the engine alternator. Is the fridge only 120 volts ac and not also 12 volts dc? If so, I would strongly recommend adding an inverter / battery charger. This would allow you to run the refrigerator when cruising without running the generator. Many inverters such as the Magnum Energy have a programmable battery charger which can be set to your type of battery. Also the maximum charge rate can be set to match the bank size up to 125 amps.

Ted
Ted -

Inverter / Charger is Freedom Marine 2500 Watts with Heart Interface. Gen is the NL 5.5 Kw. Fridge was a PO 120 VAC item from Wally World. Between charging and a fridge load, I was thinking that isn't much of a genset load. Running the air would certainly load the system enough though.

Didn't mean to hijack the thread...
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Old 06-16-2016, 06:56 AM   #14
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"Am I correct that running the charger off of ac while the genny is running gives me additive current from both the charger and alternator for a faster charge?"

Not usually as each will charge only according to the voltage it sees.

When either see voltage the higher output V will do all the work.

Best is to figure how to split the house bank so each charge source only sees a set of batts to charge.

With 2 charge sources you may be able to charge at the highest rate for each batt bank.
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Old 06-17-2016, 09:06 PM   #15
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Yup, FF nailed it. While 2 charging sources connected to one battery or battery bank will coexist just fine, whichever puts out the most voltage will do all the work. Even if it's just .1v higher, the lower device will see the higher voltage as a fully charged battery and will do very little. That said, as also has been mentioned a typical alternator will not normally put out anywhere near its max for any length of time. A smart charger on the other hand WILL put out its max until the battery bank nears 70-80% charge at which point it will begin to taper off. So if your battery charger is a smart charger you will get a lot more out of it than a standard alternator. The one way to get significant output from more than one charging source is to have a way to separate the batteries like a switch so that you end up with different batteries connected to different charging sources. Even then you still may be better off with everything connected to the smart charger.

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Old 06-17-2016, 09:34 PM   #16
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My Genset alternator feeds my genset battery. It wont help the house unless I tie the house to the genset battery. I have a three output charger. If I run the genset the genset alternator quickly brings the genset battery to 14.4v. The battery charger does nothing because it sees the 14.4v. Not sure how the sensing works but even if one bank is low it wont charge if one bank is high. So I tie all three banks and the genset alt and the charger output are maximized.
All this is a rare occasion because the solar panel help out and I seldom run the genny.
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Old 06-18-2016, 08:18 AM   #17
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I'm glad the two sources can coexist, that was my main concern. I guess it's time for a clamp-on ammeter or a battery monitor to find out exactly how much charge I'm getting. Brandy new Sterling 40amp charger (thanks Compass Marine) indicates around 35 amps, no idea about the genny alternator output. It'll be interesting to find out. Thanks for all your thoughtful replies.
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Old 06-18-2016, 08:41 PM   #18
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Yes, clamp on ammeter is very enlightening and handy at checking all sorts of things.

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