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Old 05-12-2015, 08:30 PM   #1
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Charger advice

I need a new charger, is a 3 bank 20amp big enough?

Bank 1 - engine start - (2) GC-135 - 226AH = 542AH
Bank 2 - house - (4) 3EH - 115AH = 460AH
Bank 3 - Generator start battery - single small 12v - size unknown.

Needs.

1. Top off house batts via genset. My single significant load is a Norcold fridge, which pulls 6a 12v. We are only away from shore power for 3-4 consecutive days at a time, once per month.

2. Maintain charge on generator battery, as it does not have a alternator, and pulls a small load to run the fuel pump.

What size size charger should I get? How do I prevent a high amp charger from cooking the little generator battery?

Thanks in advance.
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Old 05-12-2015, 08:40 PM   #2
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I believe you only need a group-27 at the most as a start battery. As far as I can tell the GC-135 is a 6 volt so your total amp hrs is 226, still overkill for a start battery. The 3-EH is also a 6-volt battery, 260 total amps.
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Old 05-12-2015, 09:46 PM   #3
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Well, aside from the confusion as to the size of your house battery, let's assume that you have a 460 AH house battery. A typical trawler with your type of fridge will use about 100 AHs in 24 hours. Unless you want to run your genset 5 hours to recharge that battery bank you need a bigger charger.

Flooded cell batteries can be charged at 25% of their AH capacity or 115 AH in your case. But that is a big and expensive charger. A compromise would be a 50 AH charger that you would have to run your genset for only a couple of hours to recharge 100 AHs.

To your other point. I wouldn't worry about it. Just hook up the 3 outputs of the battery charger to the three battery banks.

But there is one other way which will limit current to the other batteries. I haven't used one, but Balmar's Duo Charge unit is said to have different voltage charging settings for the secondary battery. So You could just have one starting battery for both the genset and main engine and charge that with a Duo Charge.

But it might be cheaper to just get a small 10 AH, 2 output charger and use that for the main and genset starting batteries.

David
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Old 05-12-2015, 11:22 PM   #4
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Thanks for the details. Math fail on the amps. Corrected below.

Bank 1 - engine start - (2) 6v GC-135 - 226AH = 12v 226AH
Bank 2 - house - (4) 6v 3EH - 115AH (two groups) = 12v 230AH
Bank 3 - Generator start battery - single small 12v - size unknown.
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Old 05-13-2015, 06:51 AM   #5
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Generator run time is more expensive in the long run then additional small chargers. Do the math on getting a 20/30 amp charger for the start batteries and a 40/50 amp charger for the house bank. Cutting your run time at anchor is pleasant.
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Old 05-13-2015, 07:02 AM   #6
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The simplest way to charge the house well is with a good sized alternator and a smart voltage regulator with a temp sensor.

Most noisemakers can spin a 60-70A car alt , some have the room for a different dual front pulley and a large truck $135A alt for $135 works well.

If there is room a solar panel can make up for some of the fridge loads .

Also with room extra insulation will help the fridge need less run time.

The hassle with chargers is most ratings are BS.

They may pump a rated 50A into a dead batt at 10'5V , but most can not make their rating in a batt nearing completion at over 14V.

Run on a tiny (under 10KW) noisemaker the DC charger output gets even worse.
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Old 05-13-2015, 07:31 AM   #7
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Agree a single 12v battery could be appropriate for the engine start bank. Engine manual and starter into will specific minimum MCAs/CCAs.


Suggest charger be sized with future battery set-up in mind. If a single engine battery frees space for a larger future house bank (e.g., 4x 6v GCs, for approx. 440 Ah), a charger appropriate for the resulting lay-out will be useful.


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Old 05-13-2015, 07:56 AM   #8
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My choice would be a Newmar PT-40 or larger. Very reliable chargers, three outputs. Not cheap. I put one on my boat because I got tired of replacing batteries damaged by bargain chargers.
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Old 05-13-2015, 10:04 AM   #9
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I was in the same position a year ago before we started our 5 month cruise up the ICW. We had a Charles 30 amp 3 bank charger with a refrig that uses 150 amps daily in the summer sucking power from 4 AGM 105 AH batts (total house 420 amps). In that config I would have to run the genny almost continuously to replace those 150 amps using the 30 amp charger. Remember you won't get full amp output from your charger due to line loss and other inefficiencies and if it's a 3 bank, the other 2 banks will pull from the charger too.

My solution was overkill; a 100 amp 3 bank Charles charger that does the job in an hour and a half, twice a day. Now no worries about batt charging and the batts are topped off twice a day. The new 100 amp charger will not be on the boat when it's sold!!
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Old 05-13-2015, 10:41 AM   #10
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Speaking of battery chargers, does anyone know of a charger that will charge 3 banks and have an adjustable charge rate?

We have a TrueCharge2 40 amp charger and two house banks of 4 Trojan T 105s each and a start bank of 2 group 27s. The problem is that when we have 3 or 4 overcast days in a row I like to run the genset to top off the batts (we have solar). It will charge at 100% output for a very short time then drop to 80 for a short time, then 60, then 40 and so on. It stays the longest at 40 and almost forever when it gets to 20% output. My understanding (maybe I am wrong) is that the golf cart batteries can take a high charge to about 80% charged before needing to step down the charge rate.

I feel like I am being shortchanged buying a 40 amp charger that takes forever to charge the batts because it steps down the charge rate too quickly.
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Old 05-13-2015, 11:37 AM   #11
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"Smart" battery chargers are self limiting. They sense the state of charge and limit the amps provided to the batteries. This is designed to prevent battery damage.
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Old 05-13-2015, 11:50 AM   #12
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Quote:
Originally Posted by BlueYonder View Post
"Smart" battery chargers are self limiting. They sense the state of charge and limit the amps provided to the batteries. This is designed to prevent battery damage.
I realize this, but with 850 ah of batts I think they can take alot more than 40% output (16 amps). I wonder if having the start bank which is fully charged almost 100% of the time is messing with the (smart) charger. If I was to fully charge the batteries with the TrueCharge2 40 it takes literally 8 hours to get it to drop below 20% and 6 hours of that time it would be charging at 40% or 20% (16 or 8 amps).
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Old 05-13-2015, 01:10 PM   #13
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The TrueCharge and other multibank chargers treat each bank separately. One bank can receive most of the current while another bank receives very little.

Your charger is probably sized a little low for your battery capacity. It is reducing current based on the state of charge of the batteries, not the amount of capacity left to recharge. If your batteries have reached 80% state of charge and the charger starts reducing current, it is working properly. An undersized charger will then take longer to finish charging the battery than a more powerful charger.
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Old 05-13-2015, 03:05 PM   #14
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Don't buy a Xantrex. I bought one because the price was right ...... mistake.... there was an AC to DC leak within the unit.
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Old 05-13-2015, 03:36 PM   #15
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I got rid of my Xantrex because it was putting out a higher voltage than it was supposed to. Replaced it with a Victron Phoenix charger. The new charger seems to be of much higher quality (and cost).
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Old 05-14-2015, 06:52 AM   #16
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"If I was to fully charge the batteries with the TrueCharge2 40 it takes literally 8 hours to get it to drop below 20% and 6 hours of that time it would be charging at 40% or 20% (16 or 8 amps)."

This is the profile built into the charger that is created to extend your battery life.

A bigger charger will help , but a big alternator with smart V regulator will do the fastest job.

Chargers are great overnight dockside or living aboard , as rapid battery chargers , as you see , the have big problems.

Some of the newer large off grid chargers have adjustable charge profiles.

But there not inexpensive.
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Old 05-26-2015, 08:27 PM   #17
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I ended up buying two 25amp changers as the price was right. One charger for each large bank.

In the process of installing I discovered the DC lines which run from the charger to each bank are not fused. What size fuse and type of fuse assembly should I use? Is there a guideline on where the inline fuse should reside? My preference is within 12" of the battery charger, as the electrical locker is much easier to access underway vs the engine room battery position.

I'm not at the boat now to confirm wire gauge, but can do so in a few days.
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Old 05-26-2015, 08:36 PM   #18
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One more note... at the moment I have one 25amp charger installed. I may just hang onto the second as a backup. I've accepted reality, 8+ hours for a full charge is acceptable. We are rarely away from shorepower more than 3-4 days and the fridge is my only power hungry item. We have two 5 day coolers and the ice is holding for the duration. Limiting DC fridge use to 48 hours or when underway fits our current application.
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Old 05-26-2015, 08:53 PM   #19
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Fusing in this case it tricky since both the batteries and the charger a sources of power. I believe the really correct way to do it is with two fuses, one right at the batteries and one right at the charger. If you only did one fuse, I'd put it at the batteries. Just be sure the wire from the fuse to the charger can handle the full current that the fuse will allow.
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Old 05-26-2015, 09:39 PM   #20
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The charger should have its own fuse. I don't think it's needed at the battery, but I could be wrong.
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