Using a high output alternator to rapidly charge house battery bank

[adam88]

Hi all,

Let me start by saying I am a total newbie when it comes to wiring/electrical. I am trying to wrap my head around the best way to charge a battery bank FAST. I'll keep this post short so as not to waste anyone's time.

My house battery system will have a 1020ah battery bank (4x255ah lifeline AGM batteries). I won't have any solar to begin with, or a generator, and will be relying solely on my truck's alternator for charging.

I want to upgrade to a 300 amp alternator, which will put out about 225 amps at idle I believe (XPS 300).

This is where I get lost. To me, how would I pull a dedicated amount of amps off of that alternator at all times to the house battery bank. Like if I wanted to hook up directly to the alternator and pull off 100 amps at all times. That would be 1200w (or 1440? assuming 14.4 volts?) theoretically I guess. Now I understand that power would be lost from the wire running from the truck to the back of the cab (camper), but I would use thick gauge wire for that.

Realistically, assuming this scenario, if I pulled 100 amps off the alternator, with a 6 foot run of wire through the cab/chassis to the camper (whatever gauge), how many amps could be pumped into the battery bank? Roughly how many, let's say, at 50% discharge, 70% discharge and 90% discharge? How can I learn this math?

Am I looking at this logically/correctly? And what would I need between the battery bank and the alternator to make this work to pull this off the alternator, a 100 amp charger?

Educate me please .... I am naïve.

 

[calicamper]

Wow your getting into serious power systems. I only see stuff like that on big ocean racing sailboats. I sail and raced for a long time. That type of set up we typically had 3 belts on the alternator given 1 would just slip and burn up and 2 would slip or sheer the pully off, not to mention the engine would stall if we kicked on the house charger without having the diesel warmed up and running above idle. In your case you need a very highend charger and battery management system or its pointless to generate big power given the system wont manage it well. That and even highend AGMs have their limits to how they want to be charged. AGMs were all the rage in the big cruising boats 15yrs ago but many people have migrated back to old lead acid deep cycles due to finicky AGMs and costly complicated charging systems. For a vehicle thats not spending months or yrs off grid with little to no fuel sources I dont really see an advantage to doing this From a cost vs performance perspective.

 

[dwh]

The voltage regulator will keep the alternator switched on as long as the voltage is below the regulator's set point. How many amps flow depends on the resistance of the battery+wire. There is no way to guarantee any particular amp flow with a setup like that. It's a "constant voltage" system.

The battery will have a high resistance when it is very low, and a high resistance when it's nearly full. So the big amps flow will happen when the battery is between say 20% full and 80% full. As it approaches full, it will be harder and harder for power to flow through it, and the amps flowing will be less and less.

That's going to happen even if you have a "multi-stage" charger. The charger will operate in "constant current" mode try to push the full rated amps - say 100a - and will keep doing that until the battery voltage hits the "bulk stage" set point - say 14.4v. At that point, the battery is maybe 80-85% full. Then the charger will switch to constant voltage mode, hold the voltage at some value and let the battery absorb whatever it can and the amp flow will taper off as the battery resistance rises. That's the "absorb stage".


For what you're doing, there are a few options. Mostly it would be done with either a custom voltage regulator to control the alternator, or a DC-DC charger that takes power from the alternator and runs it through a multi-stage charger. You can see some of the various options here:

http://www.sterling-power-usa.com/

 

[adam88]

The voltage regulator will keep the alternator switched on as long as the voltage is below the regulator's set point. How many amps flow depends on the resistance of the battery+wire. There is no way to guarantee any particular amp flow with a setup like that. It's a "constant voltage" system.

The battery will have a high resistance when it is very low, and a high resistance when it's nearly full. So the big amps flow will happen when the battery is between say 20% full and 80% full. As it approaches full, it will be harder and harder for power to flow through it, and the amps flowing will be less and less.

That's going to happen even if you have a "multi-stage" charger. The charger will operate in "constant current" mode try to push the full rated amps - say 100a - and will keep doing that until the battery voltage hits the "bulk stage" set point - say 14.4v. At that point, the battery is maybe 80-85% full. Then the charger will switch to constant voltage mode, hold the voltage at some value and let the battery absorb whatever it can and the amp flow will taper off as the battery resistance rises. That's the "absorb stage".


For what you're doing, there are a few options. Mostly it would be done with either a custom voltage regulator to control the alternator, or a DC-DC charger that takes power from the alternator and runs it through a multi-stage charger. You can see some of the various options here:

http://www.sterling-power-usa.com/

Thanks for the explanation. Makes sense. While you wrote that I had actually found my way to sterling's site, and I am glad to know I am on the right track.

What I am confused about is their "DC to DC" systems, for instance, would have the charger hook up to the truck battery, and then the house batteries would hook up to the charger. But the truck batteries won't be the same type or quality as my house batteries. So if I were to use, say, a 100 amp charger this way, wouldn't it be pumping 100 amps through the truck battery, or does it not work like that?

Sterling also offers a unique "alternator to battery" system. I think this may be what I need. Can you take a look and tell me what you think? http://www.sterling-power-usa.com/sterlingpowerusa12volt-210ampalternator-to-batterycharger.aspx - Seems almost perfect actually... is this the best way to go?

 

[dwh]

Doesn't work like that. When the engine is running, the voltage regulator is going to keep the alternator switched on to hold voltage at whatever. That's the "chassis bus". The DC-DC charger is going to suck power from the chassis bus and step it up to do a multi-stage charge on the "aux bus" side.

The power to feed the DC-DC charger is going to come from the alternator. The engine battery doesn't really have anything to do with it, except as a convenient place to make the big wire connections.

 

[dwh]

As for the A2B - it depends. DiploStrat tried that, and found it wouldn't work for him because (IIRC) his truck has a computer controlled voltage regulator which can raise the chassis bus voltage higher than the A2B can handle.

If you had a separate, dedicated 300a alternator just for the aux battery charging, then the A2B would probably be a good way to go.

 

[adam88]

As for the A2B - it depends. DiploStrat tried that, and found it wouldn't work for him because (IIRC) his truck has a computer controlled voltage regulator which can raise the chassis bus voltage higher than the A2B can handle.

If you had a separate, dedicated 300a alternator just for the aux battery charging, then the A2B would probably be a good way to go.

Thanks again for the info, saves me a lot of learning. Ive looked into a 2nd dedicated alternator but its not a feasible option.

The B2B charger sounds like the way to go, but I then run into a problem with a lack of charging power. Sterling only seems to go up to 50 amps on the B2B charging. Maybe 100....

What did "DiploStrat" end up doing?

 

[adam88]

Did a bit more searching, saw DiploStrat posted this in March:

Again, if your truck's charging system doesn't routinely reach 14+v or if you want to play with lithium batteries, these could be a great product.

If your truck routinely runs between 14 and 15v, then go with a split charge relay. You will save (a lot) of money and achieve much higher charge rates.

My truck with the new alternator will definitely be putting out a proper voltage between 14v and 15v, so I wonder then if the "split charge relay" is simply the best way to go.

Now... I just need to find out what that is

 

[DiploStrat]

Diplostrat is rolling across Texas charging his 600Ah battery bank with the factory alternators connected with an intelligent relay and 100mm2 of cable. I get charge rates well into the 200A range, but you need to read and understand dwh's post on why you won't see that rate all the time or for very long.

i have posted way to much on this, but basically for a rig like yours I would recommend:

-- Big wires
-- Intelligent relay, aka "split charge relay. I like Blue Sea.
-- Solar to assure that you actually complete the charge.

More info in my posts in this forum and on my website.

All the best.

 

[adam88]

Diplostrat is rolling across Texas charging his 600Ah battery bank with the factory alternators connected with an intelligent relay and 100mm2 of cable. I get charge rates well into the 200A range, but you need to read and understand dwh's post on why you won't see that rate all the time or for very long.

i have posted way to much on this, but basically for a rig like yours I would recommend:

-- Big wires
-- Intelligent relay, aka "split charge relay. I like Blue Sea.
-- Solar to assure that you actually complete the charge.

More info in my posts in this forum and on my website.

All the best.

Thanks for stopping by. I think I was over complicating this big time.

One last question, I checked out the blue sea site, I assume you are referring to the "automatic charging relays". Is there any particular one that you'd recommend? Do you combine your batteries when charging, or do they remain isolated at all times? https://www.bluesea.com/products/category/Automatic_Charging_Relays

Edit: NVM. I found the one you are using in the other thread, answered my question! (https://www.bluesea.com/products/76...rging_Relay_with_Manual_Control_-_12V_DC_500A)

 

[DiploStrat]

Long boring story, but I am actually using a Magnum SBC controlling a regular Blue Sea relay. I also have a manual override switch. I much prefer the Blue Sea ACR as it has everything you need in one neat package.

At one time I was obsessing over the fact that my Chevrolet charged at as much as 15.5v and I wanted a cut out. Then I realized that AGM batteries want that voltage below freezing.

Get the Blue Sea ACR of your choice; easier all around. And do put at least 200w of solar up there. I have 500w and would love more.

All the best from Palo Duro canyon.

 

[unseenone]

Should have popped by for drinks..

This mega alternator theory may only work with some vehicles, so what is the vehicle you plan on doing it on. For example, on my vehicle, the ECU controls when the alternator will charge and I assume how much.... There is a neat company I found a while back that makes custom and semi custom alternators for military vehicles and such, that might be what you are looking for.

There might also be some limits as to how many amps you want to put into your batteries at once... depending on the battery(ies).

Just some thing to kick around. Maybe a better strategy would be to keep them charged as much as possible with solar, etc. what are you trying to run a welder?

 

[DiploStrat]

Y'all Come

Should have popped by for drinks..

Just finished making cappuccinos - cost = 14Ah. But Palo Duro is a bit of a hike from Austin, even for a Texan.

Depending on how you want to look at it, a properly wired second battery looks to the vehicle charging system like either one big battery, or one big load. As dwh has posted many times:

-- Almost no matter how sophisticated your alternator/regulator/ECM/etc. is, if it sees that the voltage of the starting battery has dropped, it will output current until that battery is charged.

-- A second battery (house/domestic/camper), connected to the starter battery with a properly sized cable, will charge at the same time. If the cable is too small, then it will be unable to pass high amperage and the second battery will charge much more slowly. If the cable is small enough, then the charging system will barely "see" the second battery and the charge rate will be very low. (This is the origin of the emphatic statement, heard on many RV fora, that "you can't charge your camper battery with your alternator.") In the worst case, you will shut off the engine before your second battery is completely charged. (Hence my great advocacy for solar chargers.)

-- A lead acid battery's ability to take a charge depends on the internal resistance of the battery and the difference in voltage between the charge source and the battery. A good charger will get the voltage up fairly quickly, with the perverse side effect that the charge rate will drop. Does this mean that large cables are not worth it? Not at all, but it does mean that even with a big alternator you are likely to spend a lot of hours at a charge rate of around 50A.

A 500w solar kit can produce a charge rate of between 25 and 30Ah in good sun. This happens to be quite good for the absorb stage of a large battery bank. The nice thing about solar is that it is always working when there is any light at all. The bad thing about solar is that charge rates are really low in the rain.

 

[adam88]

Should have popped by for drinks..

This mega alternator theory may only work with some vehicles, so what is the vehicle you plan on doing it on. For example, on my vehicle, the ECU controls when the alternator will charge and I assume how much.... There is a neat company I found a while back that makes custom and semi custom alternators for military vehicles and such, that might be what you are looking for.

There might also be some limits as to how many amps you want to put into your batteries at once... depending on the battery(ies).

Just some thing to kick around. Maybe a better strategy would be to keep them charged as much as possible with solar, etc. what are you trying to run a welder?

hey there. The truck is a 2006 F350 diesel. I don't think there's going to be an issue with an ECU but it's something to look into. The batteries will be as mentioned deep cycle AGM lifelines which can be charged up to 250% of their rated amperage (in other words, extremely quick). So no worries there. No, I am not using a welder, I am wanting to run am air conditioner overnight without draining the batteries below 50% (along with all the other appliances). Also heavy electronic use and limited space for solar panels (maybe 300w max). No generator on board, but we will be doing lots of daily driving (average 4 hours a day). So logically, the conclusion is to make use of my 325hp engine to charge the batteries while driving as much as is feasibly possible.

 

[DiploStrat]

...

The batteries will be as mentioned deep cycle AGM lifelines which can be charged up to 250% of their rated amperage (in other words, extremely quick). So no worries there.
...
but we will be doing lots of daily driving (average 4 hours a day). So logically, the conclusion is to make use of my 325hp engine to charge the batteries while driving as much as is feasibly possible.

I, too, have AGM batteries. The problem is not that your Ford cannot put out enough current; it certainly can. It is unlikely that your ECU will care, as long as you connect your camper batteries to the positive terminal of your starter battery. The problem is that unless your AGM's are discharged to 50%, they simply won't take that great a charge for that long. (All of this assumes that your cabling is adequate, say a minimum of 100mm2.) dwh has posted often and accurately on this subject.

With deeply discharged batteries you will see something like this:

First hour: 300A
Second hour: 150A
Third hour: 75A
Fourth hour: 35A

Actually, you will probably get a bit less. Real world numbers are probably closer to 100A per hour, max, with most of the charging in the first hour. This is way solar is so very valuable, it keeps charging after the engine is shut down. I have 500w, but 300 will still get you 15A or so.

N.B. Depending on insulation, temperatures, etc., you can expect your air conditioner to take about 25-50A per hour.