Limiting current to Aux battery?

[SPL_Tech]

I have a campervan with a 90A OEM alternator. I am using a standard deep cycle lead acid flooded marine battery under the hood as my aux battery. I use a 100A relay to parallel the starting and aux batteries when the car is running, and it separates them when the car is off. It's cheap and it works fine.

What I want to know is how to limit the charging current to the aux battery so I dont smoke my alternator or the battery trying to charge a depleted aux battery. I want to limit it to about 30A which I think is a good trade off between charging the battery reasonably fast but not so fast the alternator overheats.

[JamesJudasPries]

Put it this way:

My van has a 120 amp alternator and thick #2 AWG wiring that is short and a mechanical isolation solenoid. 14.4 v volts cold output from alternator into my 2 GC2 golf cart batteries (232 AH) when at 40 % charge. I've measured 67 amps with a DC clamp meter under those conditions. Your alternator will be fine, and chances are at is heats up the voltage regulator will sag that back to 13.8 volts and current will go down to maybe 40 amps or less. So for your single battery, I can almost guarantee you won't see that level of current, since your wire probably isn't that thick and your single battery is NOT capable of accepting any more current.

I should add that I also run my fridge on dc under theses conditions, and I have never blown the 100 amp fuse I have installed.

I realize that it is a starting/ marine battery so in theory it could accept maybe more amps, but Ill bet it will not.

One time in my ranger truck I had a flat battery and I jump started it and immediately put a clamp meter on the charging lead. It drank 80 amps. But over 30 seconds the amps were dropping slowly. I Checked later after a 30 min drive, and it was charging at less than 20 amps.

The story is that as the alternator heats up, it output capability goes down, and the regulated voltage tends to sag too, causing the charge to taper down. The high load on the alternator is temporary and controlled, so I would not worry in my opinion.

Been doing this kind of thing for years, and no troubles.

[time2roll]

I think your alternator will be fine.

[RoyB]

SPL TECH - You was talking about charging a battery not loading a battery...
You are way over my head with your thought process...

Try this yourself - apply 14.4VDC charging voltage to any deep cycle battery (even a hybrid deep cycle battery with cranks specs) and measure how many DC AMPS it demands from the charging source. You should find it at 17-20AMPS DC Current from the charging source. I have done this routine too numerous times to count when camping off the power grid with my 255AH battery bank... The only way to get my batteries to demand more DC Current is to increase the charging DC Voltage... If I increase the charging more than the 14.4VDC level I stand the risk of boiling out battery fluids over the charge time. I also stand the risk of overheating my battery case which may lead to other problems.

You certainly can load down a battery with some pretty high DC Current load for a short period of time... we are talking two different things here...

Your inline Battery cable protection can be valued at any number you want to put in there. Pick 30AMPS if you are only going to be charging ONE battery... In my case I would blow the 30AMP Fuse trying to charge my batteries. I have an 130AMP fuse between my battery bank and the converter/charger connection... Both the converter DC output and Battery Bank switch feed my 12VDC Distribution panel supplying all my 12VDC needs in my trailer setup...

If we are talking about two different things than just ignore all I said and maybe the better experts on here can help you here...

Discuss this point with some real experts please... I only know what to expect on my three 12VDC battery bank with the capacity of 255AHs... When my charger is applying 14.4VDC I see an initial current demand of 53AMPS when first hit with the 14.4VDC. Then this starts tapering back and in about 15-20 minutes will be down to around 8AMPS if the batteries are good. After two hours my smart mode charger then drops to 13.6VDC and the charge demand drops even lower for any additional hour. At the end of this three hour period the battery bank is at its 50% charge state...

Thats all I know with my setup...

Just saying

If you measure your DC output level on your truck alternator system you find these same basic levels being used... Must be for a reason perhaps...

I don't think I could demand an additional 53 AMPS DC current from my present Truck Alternator setup when it is already being demanded with 17-20AMPs from the truck Start Battery.

I am going to be faced with a very similar circumstance soon as I am splitting up my battery bank between the trcuk and the trailer tongue ares. Will have 300AH capacity in both location. I may want my Truck alternator setup to charge one 300AH capacity in certain disconnected events when only using my truck and I believe my alternator is a 110AMP system... Normally my PD9260C Converter/charger unit will be charging the two 300AH battery banks...

This is my changes coming up for me to do something similar with you are wanting to do...

My Deep Cycle batteries have been increased to 150AH since this drawing was made... Trojans T-1275

Maybe some bigger experts will dime in here with you thought process of your Truck Alternator Setup loading down your single battery to the 650AMPS Current draw... The most I see will be 17A to 20A when your alternator goes into its bulk charging mode state of around 14.4VDC for the ONE battery. The Truck Start battery will also be demanding the 17A-20Amps so total demand from the Alternator will be 34A to 40Amps when it is putting out 14.4VDC.

What does the Alternator do when you are cranking your starter drawing all of that possible 650 AMPS... Also your AUX battery will not even be connected to the truck start system unless your alternator is doing the bulk charging mode and is putting out at least 12.2VDC switching in the BLUE SEA SI-ACR Smart Relay combining the AUX battery to the Truck Start Battery... This means the truck engine will be running then...

Like I said over my pay grade haha...

Roy Ken

[SPL_Tech]

RoyB wrote:
Battery science says if you hit a depleted wet cell deep cycle battery with 14.4VDC it will only demand 17-20AMPS of DC charge current and it will bring this up to its 90% charge state in a three hour period.

I am not sure that's an accurate statement. That might be true with some small battery that you can fit in your hand, but not with something larger. I am talking about a dual purpose standard marine battery you can find at Wal-Mart. They are used for trolling motors on boats and starting boat engines. They can crank over an engine, so they can supply 600A. If they can supply 600A, then they can accept it too. You cant really design a battery that can supply a large amount of current without also being able to accept the same amount. In other words, if a particular load were to drop a fully charged battery from 12.8v to 11.8v under load and that amounted to 100A of current to support that load, then charging the same battery discharged to 11.8v at a voltage of 12.8v would also require 100A (briefly anyway).

Fast forward to real world, my battery can provide 650 marine cranking amps and 80AH @ 1A for 80 hours. The standard for marine cranking amps is the max load the battery can support for 30 seconds and maintain a voltage of at least 7.2v. So if the battery can supply 650A for 30 seconds before falling to 7.2v, that would also mean that if the battery were discharged to 7.2v, it would take 650 amps for the voltage to rise to 12.8-12.9v over the course of 30 seconds.

I know that 7.2v is beyond what anyone should discharge a battery to, but it illustrates my point that the battery can both discharge and charge at a rate of far faster than 20 amps, and I am trying to prevent that from happening.

[RoyB]

Battery science says if you hit a depleted wet cell deep cycle battery with 14.4VDC it will only demand 17-20AMPS of DC charge current and it will bring this up to its 90% charge state in a three hour period. It will bring it up to its 100% charge state in about 12 hours of total charging... If you want to charge faster then you have to increase the charge DC Voltage and of course it will demand more DC current doing this. It will also start boiling out your battery fluids and the case will get hot and it may explode on you...

If you have a shorted cell in a battery your inline fuse between the battery and the charge source would open the circuit if the batteries is demanding very high DC current...

The bottom line here is the batteries demand how much current you are using. i.e. you could have 300AMPs available to us and it will only demand 17-20AMPS per battery that is being charged together with 14.4VDC charging Dc Voltage. If you have two batteries to be charged it will demand 34AMPS to 40AMPs DC current when using 14.4VDC charging DC Voltage. If you don't have that much DC current available it will still charge your batteries but will take much longer to do it...

If you install one of the BLUE SEA SI-ACR smart relays for charging batteries it will only connect your aux battery when the alternator is producing 14.4VDC charging voltage. I think the relay trip is set to 14.2VDC. This is the only time the two batteries (Truck Start Battery and AUX Battery) will be demanding current together from the 90A Alternator setup...

It would be alot better of course if you had a bigger ALternator.

I know what I would in your situation and that is to monitor the status of the AUX battery and when it got down to its 50% charge state I would fire up my 2kHonda Generator and recharge it back up to it 90% charge state in three hours using a smart mode battery charger or perhaps a PD9200 series converter/charger that runs off of 120VAC...

Always choices

I am one of those to never do anything to the truck start system setup. This is the only way you get back home on haha...

Just some of my thoughts here...

Roy Ken