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Old 23-10-2017, 08:44   #16
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Re: Battery charger...the "correct" size

The AGM makers did not acknowledge the importance of high (30/40/50%) charge rates for decades. Maybe didn't know about them!

Unexpected data comes out of customer testing and usage over time, and when the results go against vendor profitability they get disseminated slower rather than faster.

Like the fact LFP banks may be able to last multiple decades without much capacity loss, but only if charged at voltages far lower than what manufacturers recommend.
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Originally Posted by john61ct View Post
Note unless it were a critical longevity factor, mfg would not state minimum charge rates, since that implies their product is not suitable for millions of potential customers.
Above should read "mfg would not state **high** minimum charge rates"

A mfg recommending a *lower* charge rate can be considered a marketing feature.
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Old 23-10-2017, 08:56   #17
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Re: Battery charger...the "correct" size

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I have to question the 10% to be the minimum for charging.
For FLA I don't think 10% is a minimum as a general rule.

But if someone is asking what size charge source to buy relative to their bank capacity, that (to me 15-30%) rule of thumb is a good one for FLA.

No one says going lower is *bad* for an FLA bank, as the millions of low-amp solar installs attest, that's not a significant issue.

But if you are cycling down near 50%, you definitely want to get back up to 100% Full as soon as possible.

An inadequate rate, well below the chemistry's ability to accept current at that SoC, will extend the time overall that bank is sitting discharged.

Which is proven to reduce lifetime.

Plus in a solar context, the charging window is often only 4-5 hours, which is already often less than enough time for FLA to get to Full no matter how high the current.

Finally for noobs to deep cycling, we are usually battling experience and assumptions learned from the world of automotive starter batts, dumb garage chargers and cheap trickle charging "battery tenders" on the one hand and "6 stage pulsing magic desulphating" widgets on the other. Many need convincing it's important to invest almost as much in good charge sources as they would in a small bank.

So in general, defaulting to a higher recommendation rather than lower makes sense.
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Old 23-10-2017, 09:16   #18
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Re: Battery charger...the "correct" size

A good start would be to measure, or at least estimate, your house load of an evening.

A point I had not previously considered : My Pro Mariner has a really quiet fan, and a "shore power" mode, so when you are on the boat using power, its operation is really unintrusive.

The previous Xantrex had a very loud fan which was constantly cycling on and off. I often ended up switching it off until I left the boat.
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Old 23-10-2017, 09:26   #19
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Re: Battery charger...the "correct" size

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Originally Posted by john61ct View Post
>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Equalization protocol, which I only do manually I believe benefits from higher rather than very low current rate.
john, my 75A charger on my Freedom 15 I/C reverts to 15A charge rate when equalizing. Runs up to 15.35V.
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Old 23-10-2017, 09:45   #20
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Re: Battery charger...the "correct" size

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I actually have never seen the 10% number actually stated in any manufacturer spec sheet for Fla charging.
Trojan do -
though looks like could be based around actually having a chance of getting the poor neglected batteries back to 100% rather than what the battery would prefer in an ideal world.
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In applications where cycling is infrequent, such as weekend RV users, or infrequent or seasonal trolling motor usage, a charger with an output current rating between 10 and 13% of the battery's rated 20-hour capacity will suffice. In applications where battery recharge must be accomplished within 8 to 10 hours, a three stage, automatic charger, rated at 20% of the battery capacity, may be required.
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Old 23-10-2017, 10:05   #21
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Re: Battery charger...the "correct" size

Bingo.

Equalization doesn't start until the bank is already past the 100% Full point, so obviously acceptance will be very low.

Dunno about pro/con of "trying to" push higher than CAR at that point.

I do know charger's automated equalization routines should be disabled, only equalize manually, protocols are spec'd differently across mfgs.
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Old 23-10-2017, 10:33   #22
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Re: Battery charger...the "correct" size

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Originally Posted by conachair View Post
Trojan do -
though looks like could be based around actually having a chance of getting the poor neglected batteries back to 100% rather than what the battery would prefer in an ideal world.


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The last sentence says it all and seems to be the same for most manufacturers. Top temperature for charging 120℉ any more and they say damage is likely .
So the bigger the better for most of seems but you must monitor ( either manually or automatically) the battery temperature. ( exact temperature seems to vary a bit between manufacturers)
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Old 23-10-2017, 10:54   #23
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Re: Battery charger...the "correct" size

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So the bigger the better for most of seems .....
Most boats leave the marina a couple of times a year if that

Tiny charger would do them..

Does seem like the charger rate (non agm) is more to do with practicalities rather than physics/chemistry.
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Old 24-10-2017, 08:31   #24
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Re: Battery charger...the "correct" size

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I have to question the 10% to be the minimum for charging.
Question away. I'm not going to try to defend that number. It is just something that I have read from more than one source in the past.

Note also that I did not say that 10% is the minimum that you CAN use. Certainly you can use a lot less, if you want to. There will be trade-offs.

What I said is that 10% is generally recommended as the minimum for "effective" charging. That means balancing all of the trade-offs concerning the cost of the charger, the time to reach full charge, and all of that. A bigger charger will charge faster, but cost more. A smaller charge will cost less, but charge more slowly. And so on.

So, again, as I said before, you pays your money and takes your choice.
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Old 25-10-2017, 04:46   #25
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Re: Battery charger...the "correct" size

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...because any charger between say 35 to 50 Amp capacity will work fine recharging my two 8Ds - the only real difference is that the 35 Amp charger will possibly take just a bit longer than the 50 Amp charger to recharge the batt bank back to 100%....correct?
Essentially correct.

Beyond that, there's a lot of math and physics involved (see all the posts, above.)

But in the end, your question is really "is it worth the extra $$ to get a higher-capacity charger?

My answer, having lived with everything from a 15A to a 100A charger, is YES!

My reasoning goes like this: at anchor, I try to minimize the time running the noisemaker to no more than one hour, twice a day. True, my 100A charger ramps down from maybe 90A for the first few minutes, to just 15A or less toward the end of that hour.

But I get a lot of charging done in that first 15 to 30 minutes that I'd never get with a 40 or 50A charger. I basically want to be sure I'm getting the most out of that one hour genset run.

As always, YMMV if you use your boat differently.
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Old 25-10-2017, 08:34   #26
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Re: Battery charger...the "correct" size

In our own boat we have a 25A charger onto a 110Ah bank (la plain). In a boat I used to work for the charger was 100A unit onto a 600+Ah bank (la deep).

Both systems seem to work fine.

I would personally avoid too small chargers and have no complaint with too big ones. Sort of hard to find a too big one anyways once your tank bank is above 400Ah.

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Old 25-10-2017, 09:03   #27
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Re: Battery charger...the "correct" size

No issues paralleling charge sources onto the same terminals to get the amps you want
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