Max charging amps damage battery?

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Project_X

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Hey everyone, 

I recently installed solar and it's working well but I wanted to install a continuous duty solenoid to top off my battery on cloudy days. My house battery is a 100amp deep cycle that states the max charging amps is 30. I'm using 4 guage copper wire and two 150a fuses so it's possible that much more than 30 amps will run through the battery. I was hoping to ask those of you that know much more than I do on electricity if it would be a bad idea to use the alternator to help charge my batery? Thanks in advance!

-Chris
 

SternWake

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I doubt the 30 amps would last really long.  It really depends on the voltage the vehicle allows.

The danger is heating of the battery, and a 100Ah battery limited to 30 amps sounds like a less inexpensive AGM.

I'd personally only worry about it if the well depleted battery was located in a hot engine compartment, and I were going to drive through the desert in August.

If you really want to limit amperage use 8awg instead of 4, but I would use the 4 and not worry about it unless it is a super hot day.  Wire up a switch on the solenoid trigger wire so you can opt out of alternator charging if you desire.

KNowing how many amps will actually flow is also a wise idea.

The clamp on AC/DC multimeter is a great tool to own>

https://www.amazon.com/Digital-Clamp-Meter-Uni-Trend-UT203/dp/B005HOPRRK

clamp it over a single wire coming from solenoid/ alternator.  The 30 amps would likely only be exceeded when thebattery is well depleted, and for the first ~15 minutes after starting the engine, but that is a guess not knowing the variables. The tool above  can say for sure.
 

John61CT

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No worries, battery resistance will slow things down right quick anyway.

Measure for yourself with the batt at 50%, bring up RPM in the first few minutes you'll see a peak and then dropping. Lead batt right?

Just make sure the infrastructure in between can handle it, get a cheap infrared temp reader look for hot spots.

And of course upsizing the bank will have benefits all 'round, can't do too much with only 50AH usable storage, that implies only around 25AH use per day.
 

SternWake

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I only have 90Ah total battery capacity, and use 30 to 65Ah each night.

If 100Ah capacity is woefully inadequate, you will know it quite soon, unlike somebody who has NO operating system or has ever made any observations of actual data of a system in actual use.
 

tripper

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I have been wondering about this same question except more extreme. I am hooking up a 1000 watt solar system on my trailer, but I have only a single 12v 105 AH Trojan wet cell battery for now. When this battery finally dies (which I do not think it ever will, it has been great) I will replace it with 4x AGM's. Will my solar cook this battery (~70 amps at peak)? Or will it just kick right into absorb mode and start limiting current fast enough to not fry the battery? I am running a Victron MPPT controller, and the max current can be set on the controller.
 

SternWake

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Yes, as soon as absorption voltage is attained the battery requires less and less amps to be held at absorption voltage.


a group 27 Trojan depleted to 50%, at 70 amps instantly applied, would likely reach absorption voltage in under 5 minutes, and take 6 more hours for amps to taper to about 1, at which point it will be near full charge.

But Solar is not instant max output, and 70 amps worth of solar at its max, will likely have battery achieve absorption voltage at a much slower rate well before the solar is cpable of 70 amps at noon.

I'd not bother limiting current to it.

Trojan recommends a 10 to 13% rate, which would be 10 to 13 amps for 100AH of battery capacity, but this assumes a plug in charger and 12 hours to recharge minimum before next discharge cycle is to begin.

manufacturer recommendations are blanket recommendations basically to minimize warranty returns, not an optimized recommendation for a specific usage recharge regimen.

You could certainly choose to limit current into that battery via your controller, but the odds of the battery ever accepting 70 amps  via solar is minimal, and as long as the battery is not already 105F depleted to 50% or less, and you drive out from under a carport, at noon  on june 21, there is no issue.

Would I apply 70 amps instantly to a 50% depleted  105AH battery every day, no.  But solar is not instant max output.  Do not worry about it.  Achieving absorption voltage early in the day is a good thing for the battery.  the earlier in the day it occurs the better the chance that by sundown the battery is indeed fully charged. 

Most people assume a charge controller dropping to float voltage means the battery is full.  A hydrometer would prove this assumption wrong in most cases, but people prefer to put their faith in a light rather than an actual tool.
 

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