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Old 07-04-2014, 14:02   #1
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Chargers in parallel, lowish output question.

I have two brand new 100A Victron Centaur chargers and 750AH of AGM.

With SOC at 70%, I see the following:

With both chargers on, they output 50A each (total 100A)

With either one off (i.e. only one running) the remaining one will increase its output to 75A.

Why is this so?
Should I not be be seeing 75A from each?


And with SOC of of say 75%, should I not be seeing the rated 100A from each?

(actually what happens is when I first turn them on, they both output 100A but after 10 minutes or so, they ramp down to 75A. Both chargers are installed in a cabin, on a wall with good ventilation, ambient air is 81F)
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Old 07-04-2014, 14:52   #2
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Re: Chargers in parallel, lowish output question.

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Originally Posted by neelie View Post
I have two brand new 100A Victron Centaur chargers and 750AH of AGM.

With SOC at 70%, I see the following:

With both chargers on, they output 50A each (total 100A)

With either one off (i.e. only one running) the remaining one will increase its output to 75A.

Why is this so?
Should I not be be seeing 75A from each?


And with SOC of of say 75%, should I not be seeing the rated 100A from each?

(actually what happens is when I first turn them on, they both output 100A but after 10 minutes or so, they ramp down to 75A. Both chargers are installed in a cabin, on a wall with good ventilation, ambient air is 81F)
Actually, that's about right.

Your two Victrons, together, can put out about 26.7% of the battery bank's total capacity (or 200 amps into a total AH capacity of 750).

At a relatively high SOC, those AGMs will only accept about 22% of rated capacity, or 750 x .22 = 165 amps.

See the two graphs in post #49 and #50 in this thread: Battery Charger Power Draw Questions

Your situation is closest to the 25% charger, i.e., the green line in the first chart and the yellow line in the second chart.

I'd be interested to see what output you get from those two chargers at a lower SOC, say 50-60%.

Other factors to consider:

1. How are you measuring SOC?
2. How old are the batteries and what is their remaining capacity? How measured?
3. Ambient temperature.

Bill
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Old 07-04-2014, 15:45   #3
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Re: Chargers in parallel, lowish output question.

Bill is absolutely correct. Even if you had one 200 amp charger you would probably see the same thing: full output for a few minutes then ramping down to about 150 amps.

The reason is that the charger's bulk charging voltage won't go high enough to drive the full amperage output of the chargers into the batteries at that SOC. If it could go high enough (and it might take 15+ volts) it would create too much heat in the batteries.

David
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Old 07-04-2014, 16:03   #4
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Re: Chargers in parallel, lowish output question.

Hi Bill

Many Thanks.. informative and instructive as usual!.

To answer your questions:

a).. 6 x Lifeline GPL-31XT with a claimed Lithium-like charge acceptance of 2.5C ! well, according to their brochure anyhow.

b)...Age : 5 months old.

c)...Never tested for capacity on the basis they are almost new and nearly always floated to 100% .... should I get them tested?

d) SOC is measured using a Victron Battery Monitor BMV-602S.

e) Ambient Temp is 81 degrees .. in the Georgetown, Bahamas.. but I hasten to add that they are mounted in the aft cabin on a wall with excellent ventilation - they are in the breezes coming down the companionway. I was expecting overheating issues.

More info.. : I only ever use the genset to drive these chargers and my plan was only to charge them up to about 80% if needed - I have 700W of solar to float them up to the 100% on a daily basis. I have so far only used these chargers once a week - just to run the genset.


More questions....

What about my setting the dip switches to the "Other" battery type which has an absorption voltage of 14.85 .. approximately 0.5V above the AGM recommendation? Seeing as how I will only use these chargers on the gen set and only up to 80% SOC... I don't think I will hurt anything, but may be able to get a higher charge rate (and work the genny harder) for longer.

How come a single charger does 75A and when the 2nd one is turned on they both settle at 50A?

Thanks

Niels
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Old 07-04-2014, 16:11   #5
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Re: Chargers in parallel, lowish output question.

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Originally Posted by djmarchand View Post
Bill is absolutely correct. Even if you had one 200 amp charger you would probably see the same thing: full output for a few minutes then ramping down to about 150 amps.

The reason is that the charger's bulk charging voltage won't go high enough to drive the full amperage output of the chargers into the batteries at that SOC. If it could go high enough (and it might take 15+ volts) it would create too much heat in the batteries.

David

Hi Dave

Many thanks. The brochures for the AGMs state that a charge rate of 2.5C is possible, so on that basis, I assumed that less than 0.3C would be a walk in the park.

Perhaps the clue is that I have never got down below 70%SOC and its starting to slow down its acceptance?

Do you have an explanation as to why a single charger puts out 75A and when the 2nd one is turned on they both settle on 50A instead of 75A?

The answer is probably all in the Lead Acid chemistry.

Next time its Lithiums.!
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Old 08-04-2014, 00:42   #6
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Re: Chargers in parallel, lowish output question.

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Originally Posted by neelie View Post
Do you have an explanation as to why a single charger puts out 75A and when the 2nd one is turned on they both settle on 50A instead of 75A?
This may be because the voltage drop in the wiring between the charger and battery is less with two chargers (and therefore two output wires).

The 14.85 is read at the battery charger the actual voltage at the battery will be less. A small boost in the voltage at the battery will give a big increase in output and may account for the extra 25A.
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Old 08-04-2014, 06:46   #7
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Re: Chargers in parallel, lowish output question.

Hi Noelex.
Reduced voltage drop? Certainly possible but the cable run is only a total of 14ft (7+7) and it's one size up from ABYC minimums according to the guy who installed them.

Also, the 14.85V I was referring to was the highest voltage I could select using the dip switches. I have not done so yet, being worried that I might do some damage to the batteries.

I re-checked the Lifeline brochure and they state a maximum voltage of 14.6 for bulk/absorption.

My gut feeling is the extra 0.25V will be unlikely to do any damage if the chargers are only used for less than an hour, and up to about a maximum of say 85% SOC.

Unfortunately, I have an oversized gen set and only want to use it to push in the amps at a high rate. I'll leave the solar to top them off.

(Btw, I took your recommendation and got the Midnite Solar Controller - an excellent piece of kit, easy to use and the control panel can be detached and mounted remotely in a convenient place. Thanks!..great advice!)
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Old 08-04-2014, 16:02   #8
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Re: Chargers in parallel, lowish output question.

In case anyone is interested, I changed the dip switch setting to reflect an Absorption Voltage of 14.85V. This has markedly improved the the longevity of the higher output. And my gennie is working harder.

So maybe there was a significant voltage drop in the 14 round trip feet of AWG2.!

As an aside, anyone planning on installing 2 x Victron 100A chargers, don't do what I did.

I mounted them one above the other in an area although with great ventilation. Problem was, the hot air being expelled by the lower unit gets sucked into the upper unit resulting in it initially ramping down and then finally shutting down as it got too hot. Who would have thought?

The cure: I have stuck on a couple of bits of plastic to divert the flow of the hot air. This seems to work.

In the end though, I feel my answer lies in Lithium.
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Old 11-04-2014, 15:56   #9
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Re: Chargers in parallel, lowish output question.

Lead is dead! Long live LFP!
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Old 11-04-2014, 16:03   #10
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Re: Chargers in parallel, lowish output question.

Moribund, maybe!

But, before we get too excited about Lithium-Ion batteries....in any of their formulations....you might want to read the excellent piece in the latest Seaworthy from Boat U.S.

Bottom line: not yet ready for Prime Time :-)

Bill
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Old 11-04-2014, 16:19   #11
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Re: Chargers in parallel, lowish output question.

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Moribund, maybe!

But, before we get too excited about Lithium-Ion batteries....in any of their formulations....you might want to read the excellent piece in the latest Seaworthy from Boat U.S.

Bottom line: not yet ready for Prime Time :-)

Bill

I just read that piece by mr, Charles Fort. I characterise it as , generic, badly informed, and populist. Li technology is in use all around you. It's different from LA, that's all.

Yes it's not a drop and play solution , well it is of you want to drop many dollars, but on a roll your own basis , you just have to understand the differences.

Not prime time , but more then suitable for after the watershed , adults only.

Dave


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Old 11-04-2014, 16:41   #12
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Re: Chargers in parallel, lowish output question.

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Originally Posted by btrayfors View Post
Moribund, maybe!

But, before we get too excited about Lithium-Ion batteries....in any of their formulations....you might want to read the excellent piece in the latest Seaworthy from Boat U.S.

Bottom line: not yet ready for Prime Time :-)

Bill
I'm of two minds on the article. Of course you're correct, lithium is not yet ready for the masses. But there was a bit of FUD (Fear, Uncertainty, and Doubt) being pushed in that article and my concern is that when it *is* ready for prime time, the average boater is going to remember that article and other badly made references to the Boeing thing and decide to not move on to a better and arguably more responsible choice for batteries. The article does claim that lithium is the future, but will average Joe boater remember that or the (false) image presented of cells spontaneously erupting in flames?
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Old 11-04-2014, 18:35   #13
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Re: Chargers in parallel, lowish output question.

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Lead is dead! Long live LFP!
Bubba, you preaching to the converted here.

The only reason I went down the dreary conventional Pb-H2SO4 route was that I was time constrained due to personal circumstances. With not enough time to debug it in RI before I was buried under 10 feet of snow.

When I was living in Hong Kong, I assembled a 100AH pack and did my best to abuse it to see how it behaved at extremes (4.2 and 2.5V per cell ) and shock/horror I'm still alive to tell the tale - I have to admit I was sitting watching the Junsi like hawk as I pushed the limits. Those 4 cells are still here with me powering my hookah. No fire, not even overheating, merely a rapidly accelerating voltage trend on one cell, and weirdly not always the same one.

From a dumb ass end users (i.e.. Someone like me) point of view, the only difference is that one has to monitor cell voltages and have some protections in place. Cell / pack protection is easy but simultaneous with charger and alternator protection, a tad more complicated. Not even sure the high end, super expensive products do both.

If I had Lithiums, the question that started this thread would be moot. One doesn't have to second guess the arcane chemistry of LA., where charge acceptance is a function of temperature, SOC and the color of ones under pants on the day the measurements are taken.
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Old 12-04-2014, 10:34   #14
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Re: Chargers in parallel, lowish output question.

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But, before we get too excited about Lithium-Ion batteries....in any of their formulations....you might want to read the excellent piece in the latest Seaworthy from Boat U.S.
This article is dated January 2012!! That is ancient history in the fast improving state of the art of LFP battery bank deployment. And I love that the picture is of burned batteries…not LFP mind you…but conventional FLA.

I have been accused of being an LFP "fan boy" but this article was essentially FUD and I am very disappointed that BoatUS chose to publish it without some significant vetting.
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