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Old 10-12-2018, 08:00   #1
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Charging Lead-Acid Batts with Very Small Charger

My Victron Multiplus blew up again , and I don't have time to replace it before going to the U.S. for Christmas

So I'm charging with my emergency spare charger, which is 24v x 17 amps, a cheap Chinese thing but allegedly four stage.

My house batteries consist of two banks of 4 each 105 amp/hour 12v Trojans, permanently wired together.

So 420 amp/hours nominally @ 24v. The 17 amp charger obviously struggles with this, but has gradually gotten the batts up to about 29 volts over about 36 hours.

I am somewhat worried that the charger will never kick over into float, and will overcharge and ruin the batteries. I'm thinking about taking four of them offline by removing the interlinks between two pairs, so that the charger is only dealing with 210 amp/hours of batteries, so the charger will be nearly 0.1C. I will need to leave the boat for a couple of weeks while I'm in the States, and could just disconnect the charger, but I think I would prefer to have the charger on in case there is a lot of rain and the bilge pumps are working.

Anyone have experience with this kind of situation? What is the minimum size of a charger to more or less work and go through a normal charging profile?
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Old 10-12-2018, 08:07   #2
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Re: Charging Lead-Acid Batts with Very Small Charger

watching... i have very limited and dilapidated shore power systems (i will be improving it) but was going to keep it very simple with just a smaller victron charger in the 15-30 amp range https://www.invertersupply.com/index...iABEgIpTvD_BwE
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Old 10-12-2018, 08:21   #3
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Re: Charging Lead-Acid Batts with Very Small Charger

FLA?

In any case no harm from too low amps, just very slow charging.

Long as the SoC does actually continue to climb.

Concurrent load amps?

Stop charging (or float) when amps accepted at Absorb V drops to

420 * .005C = 2A

or when the drop rate goes below .1A per hour.

No harm will come from "overcharging" at such low current past that point for a few hours, or even overnight IMO.

Now leaving it going unattended is a different question.

At least confirm the charger's operating acceptably with separate instruments before you go.

Even with top gear that's a lot of trust.

Can't you just leave the main bank isolated, run loads from the charger through a cheap starter type bank?
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Old 10-12-2018, 08:38   #4
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Re: Charging Lead-Acid Batts with Very Small Charger

I use a 100 AH FLA battery for my dinghy motor. I've used it for maybe 10 to 20 cycles of 30-50 AH. The last couple cycles were after the battery tipped over and dumped most or all of it's acid. I used pure distilled water because I couldn't find anyone offline to sell me electrolyte. My charger is a 2A job I paid $20 for, so likely quite dumb by your standards.

Through all this abuse, the battery has run flawlessly. Maybe not helpful for analyzing chargers, but FLA batteries can take significant abuse.
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Old 10-12-2018, 09:01   #5
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Re: Charging Lead-Acid Batts with Very Small Charger

Well when the normal expectation is over a thousand deep cycles over maybe a decade, there's lots of room for subjective definitions of "abuse" vs "proper care" and " damage" vs "reduced longevity".

An owner may be inadvertantly "murdering" their battery, but not even be aware of the harm done until long after a more knowledgeable and diligent owner would have scrapped that battery.

Needs and preferences vary, but informed decisions can't really be made without a reasonable understanding.

Most owners just buy replacement banks much more often than they need to, think that's normal and fine. Which it is, for their needs, might only be talking a few hundred bucks a year or even less.

Another owner living aboard off grid and travelling to primitive locations, investing in a $4000 battery bank, will want to take better care.
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Old 10-12-2018, 12:01   #6
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Re: Charging Lead-Acid Batts with Very Small Charger

On a previous boat, I had a 12 volt, 420 amp hour bank of 4 T105 batteries, that, for years, I ran off a 10 amp smart charger. It worked fine. In fact, since bulk didn't shift to absorption until really late in the charging process (charging current was only.025 C, more or less, I actually thought I was avoiding the common problem of going from absorption to float to early. My batteries went to absorption when they were at about 98% SOC, then did a couple of hours of absorption, and then would float.



I always thought that the only disadvantage was that it took a very long time to recharge from any significant level of discharge, but that didn't matter when I had shore power, and underway I had a smart alternator, and also solar.


I would be surprised if your set-up didn't work, although if there are significant loads operating, that might change things.
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Old 10-12-2018, 12:15   #7
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Re: Charging Lead-Acid Batts with Very Small Charger

What is your load when unattended?
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Old 10-12-2018, 12:48   #8
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Re: Charging Lead-Acid Batts with Very Small Charger

You don't give any information about the charge rate or the SOC when you started. 17a is a C25 charge rate so even flat out there is no problem. 19v is spot on the gassing voltage for a 24v battery so I suspect this is in the absorption phase. If the voltage does not increase above 29v for 12hr I would be happy to leave it.
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Old 10-12-2018, 12:55   #9
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Re: Charging Lead-Acid Batts with Very Small Charger

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Originally Posted by roland stockham View Post
You don't give any information about the charge rate or the SOC when you started. 17a is a C25 charge rate so even flat out there is no problem. 19v is spot on the gassing voltage for a 24v battery so I suspect this is in the absorption phase. If the voltage does not increase above 29v for 12hr I would be happy to leave it.

The batts were at about 50% when I started, and that was 48 hours ago.


I have not seen more than 29v, and got up to there only today in the morning for the first time. Obviously some DC loads -- particularly refrigeration -- are but also some use of the Eberspacher, and lighting -- have been slowing down the process. But is it harmful to hold the system voltage at 29v for such a long time, even with very little current? That's the question I think.
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Old 10-12-2018, 14:10   #10
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Re: Charging Lead-Acid Batts with Very Small Charger

Dockhead, very sorry that happened again. Could you repurpose an existing Victron BMV-712 to sense the trailing amps and open a connection to open a relay that turns off the charger?


Or is it too risky to trust that, so you can go?

Eat the food and turn of the refrig.!
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Old 10-12-2018, 14:43   #11
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Re: Charging Lead-Acid Batts with Very Small Charger

I used a 20A charger for 4 golf cart batts connected to produce 400+ AH of 12V for many years with no issue but will replace with 50A charger during upcoming refit for similar concerns and because charger is 10+ years old and got wet during storm.
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Old 10-12-2018, 15:51   #12
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Charging Lead-Acid Batts with Very Small Charger

As long as your continuous house loads arenít close to 17 amps, youíll be fine. A looong slow charge isnít going to hurt at all, maybe over a very long time as in months it wouldnít be good, but with the boat unoccupied and plugged in, just so long as it gets to full charge, it can then maintain it forever
You have had more than one failure of your charger? Isnít that unusual?

Iíd permanently install a second quality charger so that in the future itís just turn on the second and drive on, but then I like back-ups. I have a Pro Charge Ultra that is of course just a Charger, then my Inverter / Charger is a second charger.

On edit, crank the motor and let the school bus alternator fully charge them, then you will know if the cheap charger will drop to float or not, I bet it will, but Iíd feel better seeing it too.
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Old 10-12-2018, 16:34   #13
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Re: Charging Lead-Acid Batts with Very Small Charger

Here's another approach. How much rain water intrusion could you have? How often could the bilge pump run? Worst case scenario. I can't imagine any problem here.

My boat does not even have a bilge pump.
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Old 10-12-2018, 16:57   #14
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Re: Charging Lead-Acid Batts with Very Small Charger

Quote:
Originally Posted by Dockhead View Post
The batts were at about 50% when I started, and that was 48 hours ago.


I have not seen more than 29v, and got up to there only today in the morning for the first time. Obviously some DC loads -- particularly refrigeration -- are but also some use of the Eberspacher, and lighting -- have been slowing down the process. But is it harmful to hold the system voltage at 29v for such a long time, even with very little current? That's the question I think.
No, no harm as long as the endAmps Full point has not been reached.

Again, need to know FLA or AGM to know just how strong vs qualified that answer needs to be.

Of course if loads exceed charging amps, the the batts are getting "mini-cycled", so not ideal for longevity.

Cycles lasting longer than a day are getting into PSOC territory.
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Old 10-12-2018, 18:45   #15
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Re: Charging Lead-Acid Batts with Very Small Charger

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Originally Posted by MarkusK View Post
My boat does not even have a bilge pump.
Now you have done it; let's see what people think of this post.
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