Maximum Charging Current for 12V AGM

matssundinsmom

New member
*Posted this in the wrong sub-forum, thought I would move it here.

I'm working on a fairly simple electrical circuit in my small camper and I'm confused about battery charging current limits.

The camper has 120V 15A shore cable which is connected by a breaker to 3 standard 15A wall outlets. There is also a 12V circuit powered by a 35AH AGM battery used for some LEDS and a fan. I have a 50W (I think) solar panel charging the 12V battery right now.

I would like to tie the 12V battery into my alternator, and potentially a battery charger, in order to give me some extra charge when needed. My problem is that the battery spec sheet lists the maximum charging current as 10.5A. My solar charge controller is rated for 30A, my alternator I believe is in the range of 60A, and a charger would likely be around 10A. These alone exceed or come close to exceeding the max charging current, not to mention times where the alternator and solar are charging. I feel like I must be missing something here, I suspect the actual current exerted on the battery is less but how do I know what that current is in order to figure out if it exceeds the max charging current. Sorry if this is a dumb question, I'm a bit new to this.

This is the battery I have: https://www.chromebattery.com/12v-35ah-sealed-lead-acid-sla-nut-and-bolt-connector.html


Thanks
 

dwh

Tail-End Charlie
No problem.

First of all, 50w of solar is only around 3.5a at 14.5v, so it doesn't matter that your charge controller could handle 30a - if you've only got 3a of solar connected to it...

The alternator amp rating doesn't matter. It will only produce however much is being drawn out. You could have a million amp alternator, but if your battery is full and only allowing 1a to flow through it, the alternator only produces 1a. The amps shouldn't be a problem, the battery is a resistor and will limit the amp flow.

What matters is the voltage. As long as the voltage regulator holds the voltage at 14v or above, the AGM will eventually get charged.

Get yourself a 10a or 15a shore charger and you're good to go.
 

john61ct

Adventurer
Don't worry about any lead battery wrt charging **current**.

As long as the voltage is kept within mfg specs, the battery will self-regulate.

Think 350A truck alts with piddly starter batts, their regulators just keep voltage under control, it's the bank's chemistry that determines the SoC vs Amps acceptance.

Now your wiring infrastructure is another matter.

I think legal liability issues are what prompts vendors to cite arbitrary max charging amps specs, even if their batt is in practice happy to bulk charge at .8+C for a while when 50% depleted.
 

john61ct

Adventurer
With a sealed battery (why? not ideal unless necessary) you should make sure charging at Absorb stops once Amps declines below say half an amp.

Also your tiny panel will rarely exceed a couple amps anyway, no worries there.

You don't want much outgassing when you can't top them up.

I would advise getting a 30-40A shore charger so when you increase the size and quality of your bank you don't need to upgrade later.

Sam's Club Duracell 6V golf car FLA are a far better value, when that time comes.

99% charge sources have the opposite problem, going to Float **way** too soon, so unlikely to be a concern in real life.
 

DaveInDenver

Middle Income Semi-Redneck
Don't worry about any lead battery wrt charging **current**.

As long as the voltage is kept within mfg specs, the battery will self-regulate.
You need to make sure to add the caveat that this assumes you also monitor the battery temperature and voltage. Depending on the design of the battery (e.g. thin-plate cells are more tolerant of fast charging than thick-plate) it's basically true that you can supply current more or less unlimited as long as you don't overheat the battery and never exceed the gassing voltage. The general recommendation given by R. F. Nelson in "Charging Techniques for VRLA Batteries" is if you set an inrush current limit of ~5C and employ ohmic-compensation feedback for applied voltage then greater than 1C rates are successful and potentially beneficial to the life of the battery.
 

matssundinsmom

New member
Thanks guys, I really appreciate the response. It looks like I have nothing to worry about.

Will the solar and alternator be able to charge together, or will the charge controller assume the battery is charged when it sees the voltage coming from the regulator? If I understand correctly, the shore charger and solar should be able to work reasonably well together as the charging scheme will be relatively similar?
 

john61ct

Adventurer
Yes to temperature issues.

Good charge sources already do temp monitoring for compensation voltage adjustments, but a safety cutoff would require a dedicated temp sensor right on the batt, which is normal on only high-end gear.

I did already speak to proper voltage regulation, if that's not working your device is broken
 

john61ct

Adventurer
Thanks guys, I really appreciate the response. It looks like I have nothing to worry about.

Will the solar and alternator be able to charge together, or will the charge controller assume the battery is charged when it sees the voltage coming from the regulator? If I understand correctly, the shore charger and solar should be able to work reasonably well together as the charging scheme will be relatively similar?
Again no problem.

Ideally similar setpoints and charging algorithm, but no harm even if not.
 

dwh

Tail-End Charlie
Will the solar and alternator be able to charge together, or will the charge controller assume the battery is charged when it sees the voltage coming from the regulator?

It will assume the battery is charged when the battery voltage rises to a certain point. If the battery is low, that might take a while. During that time power *could* theoretically flow from both alternator and solar to the battery, but in reality most if not all will come from the alternator.

The solar will essentially be irrelevant while the engine is running - unless the solar is set to take the battery to a higher voltage than the alternator/voltage regulator. In which case the alternator will be the primary source until it hits its voltage limit, then the solar will keep pushing the battery voltage up a while longer until the battery reaches the solar charge controller's set point.

Or...it would if you had enough solar to do the job. But you don't. So forget the solar while the engine is running - it won't be contributing enough to matter.

Same when the shore charger is running.


If I understand correctly, the shore charger and solar should be able to work reasonably well together as the charging scheme will be relatively similar?

They won't interfere with each other, but the shore charger will be doing all the real work.
 

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