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Old 12-02-2014, 08:01   #1
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Charging Help

I have a 100a mastervolt charger as one of the charging sources for my 840ah gel cell bank. I cannot get it to put its full output into the bank. On shore power with batts at about 60%, it puts in 45a. This should be near its max as we have sufficient AC available and the batteries are low enough to accept a big charge. The same thing happens with a generator. How can I tell if it is an issue with the charger or if the batteries just won't take the charge?
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Old 12-02-2014, 08:22   #2
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Re: Charging Help

Not familiar with the specifics of the Mastervolt but as I understand it is a programmable charger and has temp sensing for the batteries.

I would first check the settings on the charger to make sure the output wasn't limited by some internal program.
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Old 12-02-2014, 08:31   #3
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Re: Charging Help

Another thought. Did the same charger previously put more amps into the same batteries at the same SOC? Have you changed anything in the system? Had anyone working on the wiring?
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Old 12-02-2014, 12:28   #4
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Re: Charging Help

since it happening for both the alternator and a generator I would check wiring connections to be sure you don't have a bad connection causing a voltage drop (which is also a fire in the making)

but then if the generator just supplies the battery charger it would be the same as shore power
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Old 12-02-2014, 14:30   #5
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Re: Charging Help

When it is charging at 45 amps, measure the voltage at the battery terminals. To get an accurate voltage, calibrate your meter against a car battery that has sat overnight, but was used the previous day.

The voltage should be 14 or a bit more. If significantly less, then the charger is at fault, probably programmed too low.

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Old 13-02-2014, 23:14   #6
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Re: Charging Help

one of the dipswitches is a constant float. I have seen this bumped before during an install. and caused low output.

otherwise I've installed 10-20 mastervolt 12/100-3 and all had full output.

there is 3 dipswitches. I would check them.

one is the flooded / gel setting. one is a constant float. and I forget the 3rd.

what is the amps and voltage on the screen of the charger? and what is the voltage at the batteries? if the run is long I've seen some pretty bit voltage drops on a 100a charger. and will cause less amps too. the mastervolt should be within around 10' with 2awg.


by "one of the sources" what else do you have? do you have a big charger / inverter as well? does it put 100a into the bank? that would answer the battery question.
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Old 14-02-2014, 18:36   #7
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Re: Charging Help

Thanks for the replies. I will check the dip switches and see if they are set correctly. I have a mastervolt MICC monitor with a shunt right at the batteries. They eventually get up to 14.1 and then go to float. Its just that it takes a long time and they do not accept the full available output. I also charge with two 60A alternators, which also never seem to put 60A in but that may just be the nature of these alternators. I have 720w of solar and all of that seems to go in, but its never very much to begin with. I have two 80a 220v/50hz mastervolt chargers. (this was originally a european boat). They run off the diesel genset. (The generator I mentioned in the original post was a Honda 2000w 110v generator I borrowed to test the charger.) These two chargers act similarly - ie, when the batteries are low, their combined output initially jumps up to around 140a, but quickly drops to below 100a, even when I think the batts should take the full output. Is this how batteries act when they are getting old? They do eventually take a full charge and discharge normally.
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Old 14-02-2014, 19:05   #8
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Re: Charging Help

if the two 80's are putting in less then 100 too then I'm guessing it's either the batteries or an issue with the wiring. something lose / corroded, or under sized.

I'm not very firmilar with gel batteries. what should they take as a charge? flooded are around 20% and agm's are around 40%

I had one boat with 2 100a mastervolts and a 120a inverter / charger. and it would put 300a into agms. but it was a 1600ah bank. we were getting 400+ with the alternator running as well.
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Old 14-02-2014, 19:20   #9
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Re: Charging Help

Buy two automobile high beam headlights rated 50-55A each. See if the charger can supply enough power to light 'em both up fully at the same time. Or three low beams, typically 35? Amps each.

You can often get dual beams that have only one good filament, free, in the dumpster or at any auto parts place where they're selling new replacements, fwiw.

If it can't, then it isn't putting out 100A.
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Old 14-02-2014, 22:47   #10
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Re: Charging Help

the are 55w... which is about 5 amps. and I wouldn't power a load off a charger.

you should be able to put a large load on the batteries though. and the charger should keep up.

if the batteries won't take 100a at 50% with no loads but if you put a 50a + load on the batteries and the charger goes up to 100a. (50 to batteries and 50 to loads) then the charger is fine but the batteries won't take any more.
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