Hi everyone. I work in the oil and gas industry and I'm currently setting up my truck shell as a camper. I would like to wire a battery bank in parallel with my car battery to receive a charge while the engine is on and a solar solution as a backup or alternate charge source in order to power lights/electronics in the field. Right now I carry numerous backup batteries for my laptop and can charge via a built in inverter when my truck engine is running.

I have a basic understanding of circuitry from college, but I lack the ability to design a system, pick out appropriate parts, etc.

This project is 100% for fun and as a practical circuitry learning experience. I know there are other options (i.e. a small generator), but I'd really like to stick to my engine + alternator and solar if possible!

These are the setups that I currently have in mind:

&

Any help that you can provide to get me closer to my goal will be greatly appreciated!

 

Is it really necessary to keep the batteries separated, for instance by the switch in your first diagram? That's a problem when the batteries are not in perfect balance, there will be a sudden large current when they are connected. You could mitigate this by placing a current-limiting light bulb in series with the switch, but personally I think it would be better to keep all the batteries in parallel.

Perhaps a diode between them to only allow current to flow towards the car battery, never out. But then the alternator could not charge the other bank.

 

I don't think a switch is required, but the separation would be useful to ensure I don't kill my truck battery on accident. Some of these locations are pretty deep in the field, and a dead truck battery would be bad! Also, I read somewhere that primary car batteries shouldn't be drained/recharged constantly - tho, this could just be hearsay.

Perhaps a diode between them to only allow current to flow towards the car battery, never out. But then the alternator could not charge the other bank.

Would a diode work the other way, so the alternator could only charge the battery bank, but current could not flow in the opposite direction?

 

Yes, they're like check valves. There is one difference - they also drop about 0.7V across themselves. So for instance if the charging system is at 14.5V, the battery downstream from the diode will see only 13.8V. Depending on what you're doing, that might be OK. Also, the diode must be rated for the current it might see, and you don't want to skimp. If you expect 10A, get a diode rated for 15A or more. Big diodes may not be cheap or easy to find.

I agree you do NOT want to run down your primary battery. They are meant for large currents in short time followed by an immediate recharge. Use a deep cycle battery if you need more normal battery functions (lights, radio, etc.).

 

...Also, I read somewhere that primary car batteries shouldn't be drained/recharged constantly - tho, this could just be hearsay...

Not hearsay! An automotive starting battery is severely degraded every time it is discharged below ~85% of its normal capacity. During car starting, only ~ the top 5% of capacity is used on any given start. The rest of the time, the battery is kept slightly overcharged by the car's alternator. Do the D.S. thing just once (run your battery totally flat by leaving the lights on), and you will be replacing that battery within a few months.

This is because the plates of a starting battery are made of spongy lead with lots of voids to increase surface area. If you discharge the battery and recharge it, the lead that plates back is smooth, so you have greatly reduced the plate area.

 

Pretend it's an RV with old solid state isolator, 2 big germanium diodes anode to anode with a separatr field connection so alternator out is raised one V drop so each batterie receive same charge cutoff V. The other version uses a high current relay to charge house battery. Details have faded.

 

Good advice above.
Battery isolators have been around for decades; these are big diodes that allow current to flow into your battery bank, but do not allow your primary battery to discharge into your load.
It is quite true that your primary auto battery does not like deep discharges; repeated discharges lead to lower capacity and a shorter life. They are designed for brief high current events like starting the engine. These are defined by a Cold-cranking-amp rating.
For your battery bank you want deep-cycle/RV/Marine batteries that are designed for long & deep discharges. These are defined by an Amp-hour capacity rating; generally the rating is based on the product of amps times hours at a current draw that will allow the battery to last 20 hours. Higher current draw means the battery will not last 20 hours and the actual capacity will be less than the rating value.
You may need to install an alternator with greater output since most have been cost-reduced to the barest minimum needed to run the car. Be sure that the wiring is up to the increased current as well. Having an auto technician do the job should ensure that all is done safely.

 

Here is a new schematic ...

1. Alternator: Might require an upgrade. I'll have to do some research on this. My truck is a 2014 Silverado, I'm not sure what it comes equipped with from the factory.

2. Wiring: Batteries in parallel. Completed by or under supervision of mechanic/electrician.

3. Diodes: Something like this? I haven't looked at sizing yet.

http://www.americanrvcompany.com/Roadmaster-794-Hy-Power-Diodes-Camper-Trailer-RV

4. Separator Field: Not really sure what this is.

5. Battery: Marine/RV batteries should be easy enough to find locally.

6. Inverter: A standard one, but I'll wire it in directly and build a housing for the marine batteries, inverter, and wiring connections.

http://www.amazon.com/Power-Bright-...UTF8&qid=1383712785&sr=8-11&keywords=inverter

(external power charge and solar to come later)

 

With a single diode, houseattery might not charge fully. Might check with RV dealer to see what is now used. I'd go for relay.

 

The way I would approach it is to use a dual battery isolator wired directly off the main output stud of the alternator then branching out to the two batteries.

Reason being vehicles have their alternator's voltage referenced to a point closer to the battery and not from an internal point inside the alternator to compensate for the voltage drops that happen in their wiring harness between the alternator and the battery.

On the old Delco SI series there where the two input leads usually marked 'F' and 'S' or '1' and '2'. 'S' or '2' was the voltage sensing lead for the regulator and that usually tied into the main wiring harness close to the battery.

Anyway because of this remote voltage sensing having a .7 volt drop from a battery isolator between the alternator and battery sets won't make much difference being the regulator will just bump the alternators output voltage a bit more to compensate for it.

Now relating to your stock alternator I would really be tempted to upgrade it to a heavy duty aftermarket one. I am guessing the 2014 Chevys have the typical undersized overrated ones that don't have enough reserve capacity to do anything other than charge their normal stock battery without burning up.

BTW any good deep cycle marine battery will generally work just fine as a primary vehicle starting battery. Over the years when I have had to put new batteries in my vehicles I usually try and find the largest deep cycle or dual purpose marine battery I can physically fit in the vehicle. Even with our -30 F winters here I have not had any issues with not having enough cranking power from one.

 

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