Maximum 12v amp pull of a 0% SOC lead acid battery

smok

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Sep 17, 2021
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Hi, first time poster. Hopefully I'm following the rules here.

I have a question about the maximum pull of 0% SoC flooded lead acid battery when charging, assuming the power source is capable of 700+ amps

How can you know (and thus successfully guage/breaker/fuse your system) the maximum current a dead flooded lead acid battery will pull when charging.

Example -> In my design. I have a vehicle alternator (capable of 130 amps by manufacturer specs), a starter battery (capable of well over 700+ amp, for the starter obviously) that goes to an isolator -> so it doesn't drain the starter battery -> that goes to a battery bank with 4 deep cycle flooded lead acid batteries with about 400 amp hour rating total wired in parallel

My concern is that when this battery bank is at 0% SOC, it will try to pull/draw WAY too many amps from the starter battery, even for a short period of time and blow fuses/toggle switches and break breakers like crazy. Less concerned with the alternator since it's simply not capable of outputting as much amperage as the starter battery. I'm being told by many reliable sources it won't, but I'm trying to figure out why and how that works. For the life of me I can't find any literature on the subject or even someone to give me a straight answer. I've been googling and cold calling electricians, mechanics, RV stores for days. And been told many times not to worry and how to fuse/gauge/breaker/ my system safely.

But I would really like to understand this better. Please correct me if I'm using any terms incorrectly or I have misunderstood something, I'm a newbie and I can take criticism and corrections. And also thank you for reading
 
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smok

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Sep 17, 2021
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To answer my own question succinctly (although incompletely), the answer is voltage differences and ohms law. This will stop the battery bank from pulling a ton of amperage from the starter battery. This is probably as far as most people need to go, although I would like to know more and I'll expand as much as I can.

1. A 0% SOC flooded lead acid battery still has significant voltage. Also there's the fact it will likely never (barring destruction or some other catastrophic failure) truly be at 0% SOC. Caveat aside, a 'dead' or unusable but somewhat healthy automotive lead acid battery will still be at around 11 volts

2. . A 100% (also unlikely) lead acid battery will be around 12.6 volts.

When those two batteries are connected in parallel, the voltage will start to equalize fairly quickly. And there's an internal resistance in those batteries (although my research has told me it's more complex than that, due the chemistry of lead acid batteries). There's also a resistance in your entire system or circuit loop.

Ohms law calculator here https://www.omnicalculator.com/physics/ohms-law

In order to approach worrying amperages (in excess of 150+ in my particular case) for my battery bank and safety system the resistances will have to be pretty low in order to accommodate that much current with a voltage difference of only 1.6. Which as I understand it, given the conditions I described is unlikely.


What I'm missing

How do I calculate all this? How do I calculate all this in a practical way to properly make my system safe.

If there are any electric/physics guys out there, I'd really like to understand this at the highest possible level. It's been bothering me for quite some time. Please let me know if I've made any errors thus far
 

time2roll

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Mar 20, 2021
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At zero state of charge the amp draw will actually be very low because of high internal resistance due to the lack of ions in the electrolyte solution.
Charge rate will increase as the state of charge increases from zero to maybe 30%. Then during the last 20% the current tapers off again due to the rising internal voltage.
 

smok

New Member
Joined
Sep 17, 2021
Messages
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At zero state of charge the amp draw will actually be very low because of high internal resistance due to the lack of ions in the electrolyte solution.
Charge rate will increase as the state of charge increases from zero to maybe 30%. Then during the last 20% the current tapers off again due to the rising internal voltage.

Is there a way to calculate maximum current it pulls from that 30-80% mark?
 

time2roll

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Mar 20, 2021
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What voltage, what capacity? Typical GC2 pair will go 60-70 amps at 14.6 volts. Most chargers will fold the voltage down as the amp limit is reached to avoid overload. I think you will be fine. There will also be some voltage drop in the wire that will self regulate at high amperage. Thousands of motorhomes do this routine every day without issue.

If the charging cable has a fuse all should be protected so give it a go. Put a DC clamp-on ammeter on the charge wire if you are concerned.
 
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Substrate

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Apr 28, 2021
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Even easier: Most flooded lead acid battery specifications tell you not to exceed 0.1 to 0.15C, where "C" is the rated AH capacity.

Therefore, don't attach a charge source capable of delivering more than that to a floody. Ie, for a 100ah rated FLA, don't attach a charge source capable of more than 10 to 15A.
 
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