I have a van that I'm working on as a small camper project. I want to leave the stock battery for vehicle startup, and then add an additional battery in the cargo area to power various things while the truck is not running. Some of my questions and concerns are these:

1) I do not plan to run the batteries in parallel, so do both batteries need to be the same make/model and specs to work properly? Can I just leave the stock battery as it is and then buy a new deep cycle battery for the cargo area and then not have to worry about it?

2) The way I intended to wire them up was to leave the stock battery as it is, and then run the positive of the extra battery to the alternator and then give the extra battery it's own ground in the cargo area. Then I would wire and power whatever I needed to power from that cargo area battery and not have my startup battery touched at all. Is that going to work well just as I've explained? If not, what am I missing?

3) Is my stock van alternator going to have any problems with the added battery and keeping both batteries charged? Do I need to upgrade the alternator so that there are no issues?

4) I've heard some people talk about the need for a battery isolator when running dual batteries, is this a case where that would be needed? If so, please explain.

If you need any other information from me, feel free to ask. I have done a bit of research on this, but I am still not entirely sure of if my ideas about this is correct.

I can use some guidance from those who know about this stuff, so thanks to those who reply.

 

A dual battery system is perfect for what you want to do. Yes.....you do need an isolator. There is only one terminal at the alternator so that would mean technically both batteries would be in parallel.

Both batteries do not have to be the same model or type. A heavy duty alternator is not a must but charging two batteries will certainly put a load on it.

I personally install a continuous duty solenoid. The positive from your new battery goes to one large terminal and the positive from your existing battery or a wire from the alternator will go to the other large terminal. Then there are two control terminals..... one goes to ground and the other goes to a 12 volt circuit that is only live when the key is in the run position.

This way when the key is off.....the solenoid is open and the batteries are not connected. When the engine is running the solenoid is closed charging the aux. battery.

In the link is the exact solenoid I would use.
Cole Hersee 24059 Continuous Duty Solenoid : Amazon Automotive





You will also need two heavy duty fuse holders to protect your new battery wiring. Somewhere around 100 or 150 amps would be ok. They are called castrophe fuses in case the battery wiring shorts out. ANL is just one type of HD fuse.

 

I see, and just for clarification, if I did want to run the batteries in parallel, I would just install the batteries as I've previously described and just add some in-line fuses? Nothing else would be needed? Both batteries would share the load and lose power roughly at the same rate, or one would lose charge more quickly over the other? Would the batteries then need to have the same specs to not conflict?

 

The main drawback to parallel batteries: You run the risk of draining the starting battery along with the house battery. (You WILL do this -- it's only a matter of time. ) The isolator prevents this from happening.

Worse, if one is a deep cycle and the other is a starting battery, the starting batt's life will be reduced from the heavy charge/discharge cycling.

 

There's one last bit of information I need to find out about: A typical laptop charger is going to draw how many Amps? And, if I run that laptop for 12 hours, what size battery am I going to need to power that laptop for 12 hours and still have roughly a 50% charge? I'm not talking about running anything else here, just this one device.

Thanks for all the help so far, I'm almost ready to start moving on this project.

 

A laptop power supply is going to have variable power needs. If the battery is discharged it will suck more power than if it was not. And running on A/C it will draw different current depending on hard disk usage, DVD-ROM usage, etc. There's no way anyone here can give you a good answer to that. Certainly not and be able to tell you the state of charge afterwards of your particular battery. You can look at what's printed on the power supply but that's the absolute maximum and virtually meaningless.

Planning on going RV'ing? If you're going to be living on batteries and such, pick up a Kill-A-Watt energy monitor and that will tell you exactly what you need to know. It's less than $20. Absolutely essential in my opinion if you're going to be using inverters. That of course is assuming you'll be using the 120VAC charger. If you'll be using a 12VDC setup for your laptop then disregard that. I'd recommend putting an ammeter on your 12V cable in that case, while your laptop is running.

 

Take the worst case scenario, I'd really like to have some kind of a number. I won't hold it against you if the battery(s) dies as a result. So, worst case scenario of running the laptop for 12 hours. How many amp hours would you need to be confident that it's going to last?

 

Okie doke, but I guarantee you won't like it, nor will it be all that meaningful.

Right now I'm looking at my laptop power supply. It's an OLD laptop, mind you. It says:

Input: 100-240V ~ 1.6A-0.8A 50-60Hz
Output: 16V (==... (DC symbol)) 4.5A

This one apparently is fine with foreign 240V, and the lower input amperage is due to that. But at 120V it would be the higher one, 1.6A in this case. 1.6A x 120V = 192 VA (we'll just call that 192 "Watts" for simplicity here.)

You want to run this old laptop for 12 hours. That requires 192W x 12h = 2304 Wh. That's the energy it requires, regardless of the source.

You, however, are wanting to run it from a 12V battery, so we need amp-hours.
2304 Wh / 12V = 192 Ah.

But wait, your inverter cannot be 100% efficient. Let's use 85% efficiency. It will be FAR less if you are running a huge inverter with only a small laptop as load:

192 Ah / 0.85 (efficiency) = 225.9 Ah

I've got a crawful of Walmart deep cycle marine batteries (BIG ones) in my living room rated for 110Ah. I'm guessing they were optimistic there. Even if they weren't, that's fine too. We'll proceed.

Assume you never want to deplete your deep cycles below 25% SOC. 110Ah (walmart deep cycle) x 0.75 = 82.50 Ah per battery.

This means your little laptop, worst case for 12 hours, is:
225.9 Ah (requirement for 12 hours) / 82.50 (useable Ah per BIG battery) = 2.74 (three batts)

So you'd need THREE HUGE batteries to power my laptop at max draw for just 12 hours. Probably not the answer you were looking for. In real life you'd get by longer.

As a person who flips off their main breaker for days at a time just for fun/preparedness, I can tell you one battery won't last long, no matter how huge it is. Your laptop I think would fare far better than worst case. As long as you do not need light to see nor water for the toilet.

 

You know after further thought, I should have given you the answer that you wanted rather than the answer you asked for. Only thing is, my Kill-A-Watt meter is buried upstairs in box at the moment. If you don't get any better answers by Monday, remind me and I'll dig it out and do a test.

But I seem to remember 80W being one of "those common numbers" cited time after time for laptops. I could be wrong, but that's surely better for what you're asking than the worst case.

At 80W, you're going to need 94Ah to run it for 12 hours. Even a 110Ah battery you can't run that far into the ground, so you're looking at two batteries minimum to get you through 12h @ 80W. And again, this 80W figure is just from "those" tables. Not mine. For 40W you can halve everything. Depends on your machine.

 

Thanks for taking the time to do that. If you remember to look for your Kill-A-Watt meter later on, I'd be interested in knowing what you find.

 

Indeed I will plug that in, later on, if nothing surfaces before then. I have a feeling there are many people on this subforum that know exactly what their stuff draws.

I think everyone can agree on 40-80W though, so base your estimates on that for now.

What kind of deep cycle are you running? Meaning what is the rated Ah? I'd rather work with real numbers than hypotheticals since you're probably not going to have 2 house batts. There's a big difference between 110Ah and 65Ah.

Don't feel bad. I paid a couple hundred bucks for an Optima which I like, but it doesn't have much capacity. As they say, "capacity isn't everything". It's how one uses it.

 

I actually haven't bought anything yet as far as electronics goes. I'm almost ready to start installing my radiant barrier, then followed by the insulation. I've still got a lot of work to do yet before I start to finalize the power aspect, but I wanted to get a head start in my mind and get an idea of what I'm going to be doing and what I would need to accomplish that. I did see a few Optima Yellow Tops for a few hundred dollars that were rated at 55Ah. I'd be looking at two of those just to get me to the 6 hour mark (if we're using your estimates), but I haven't decided on anything yet. I might be okay with just the 6 hours for the laptop to be running, but we'll see.

 

55Ah <- screw that. Not worth the price. I've got a blue top, and I'm reasonably happy with it. It has NOTHING on the Walmart batts except tolerance for abuse. And I haven't abused it yet. For your van, though, don't even think about it. Walmart 110Ah for $90-100, or surplus batteries from forklift operations, etc. I walked into NAPA Auto Parts the other day and bought and c!@(load of batteries for cheap. Mama, mia!

 

You should also consider having as many of your loads as possible run directly off DC. Buy a car adapter for your laptop, buy a 12v TV, etc. Running straight off DC is MUCH more efficient than running off an inverter.

 

Good point. I can probably find a car-type charger for the laptop and avoid the loss of power caused by the inverter. I think the car charger version of the laptop power supply even draws less power than the normal one. All of that factored in, that should save me a decent amount of amp hours overall.

 

No, it's not less, it's going to be a little more than the AC, but it still won't be as much as the AC+inverter. Laptops (unless it's small like a Netbook) generally run off higher voltage than 12v, most in the 19-22v range. So inside a laptop car adapter is a circuit called a boost converter (also known as a 'Joule Thief'), which 'steals' extra current and uses it to step up the voltage.

 

My Acer netbook says the DC power is 19 volts at 1.58 amps (that is also the rating on the AC "walwort" output). You can get switching power supplies on Ebay pretty cheap and the efficancy is usally 90 to 95%.

 

Exactly my point. While it requires 1.58A@19VDC from the AC adapter, if you were to use a DC-DC adapter, in order to provide that 1.58A@19VDC from 12VDC the adapter will have to draw about 3A from the 12v source. Likewise for a normal sized laptop which normally draws 2-4A from the AC adapter, the draw from a DC adapter will be 3-8A, depending on how far above 12V the output needs to be boosted to. Also keep in mind those ratings on the laptop charger are what is needed to simultaneously rapid-charge the battery and run the computer. The power draw from the AC adapter drops after the internal battery has been charged to 90%, because then it goes into trickle mode. So it is not drawing that much all the time.