Adding a parallel battery + alternator to my solar setup

bpm255

Hi all,

First time poster, long time lurker. I have a campervan solar setup that looks like:

2x100 watt panels -> charge controller -> 155 AH VMAX 12V AGM battery -> fusebox -> DC/AC loads

The battery is about 2 years old and has been plugged into the solar setup for its entire life. The battery has been cycled maybe 50 to 100 times in this period with discharge averaging ~30% (down to 70% SOC that is), maybe a dozen or so times of discharging a little deeper.

I would like to add my alternator and an additional battery to this setup. I have been looking at battery isolators but they all just seem like glorified switches, so I'm wondering - can I just hook + to + and - to - from my "house" battery to my "car" battery (w/ appropriate cables) with a switch on the + side and plan on manually enabling it when the vehicle is running? Is there a reason (other than forgetfulness) why this would be a poor choice?

Also, I know that my car battery is "grounded" to the vehicle chassis. My "house" battery is NOT grounded to the chassis, each load has a dedicated negative return wire. My inverter is also not "grounded" to the chassis. Would connecting my "house" and "car" batteries present any problems if one of them is "grounded" and the other is not?

Next question: I know that mixing batteries of different age is acceptable but discouraged - can anybody weigh in on the likely consequences of adding a new, identical battery to my setup in parallel? Given that these batteries are rated to ~1000 cycles at my average DoD and my battery has only seen ~100, is this age difference really so consequential?

Thanks for your time!

mike95490

Adding a simple switch for your house/starting battery won't "hurt" anything, EXCEPT the big 300A surge as the full battery tries to charge the emptier battery.  Boats have these sorts of switches, rated in hundreds of amps.

The other issue is - Do you have an inverter ?  If you ground the inverter power supply (battery) to chassis, that can be a major change, depending on the inverter.

manzanita

I assume that you've checked your alternator's maximum output in amps and that a depleted battery plus loads won't burn out your alternator?

I burned up a medium sized alternator once doing that.

mcgivor

The two batteries should be isolated when not running to prevent loads draining the start battery, the house battery should also be isolated when starting, to prevent it from being used as the start circuit. In order to achieve this there are various methods, switches, solinoids, diodes etcetera, but by far the simplest is to utilize a diode isolator designed for the purpose, see attached pdf. Nothing worse than having the start battery dead or worse still damage caused by component failure, inverters and starters alike draw significant amounts of current.

Conectecting both battery negatives to the chassis should not be a problem but the inverter should have a dedicated negative conductor, don't use the chassis. If the battery is connected negative to chassis then the inverter input negative  would be likewise connected, output wise it's best to float the neutral, especially a modified sine wave type. Using an older battery in parallel with a new one? You seem to already know, it's not ideal.

mike95490

Diode isolators are a poor choice, as they introduce a half volt drop in house/RV voltage from the alternator.   here's a link to a bit of a off site discussion about several types of isolators, and of course, they sell them.

http://rvlife.com/tech-tips-battery-isolators-in-three-flavors/

 (looks to be a smart FET switch)

The Victron isolator in the above data sheet, will likely not work with new vehicles that have the voltage regulator embedded in the Engine Computer. 

mcgivor

@mike95490 Being that the house battery is AGM and the start battery is most likely flooded the slight drop in voltage may be actually advantageous, not trying to be argumentative in any way, comments welcome.

Estragon

OP said the plan was to hook up an identical battery, so voltage drop would be bad.

IMHO, if the existing battery is a starting type, it would be better to add a true deep cycle for house loads (or add a starting battery if existing is true deep cycle).

I have a 1-2-both-none switch on my boat, with a starting battery and 2x6v deep cycle for house loads, which works well.

bpm255

Thanks for the replies y'all! To clarify my situation:

I have a solar setup in my van that uses a dedicated AGM battery. My car came stock with an AGM starter battery. I'd like to wire the two of them together with a switch so that I can charge the solar battery from the alternator, AND I'd like to add a new, parallel AGM battery (no switch, just a new battery) to my existing solar battery. The new battery will be of the exact same make/model/capacity, only difference is age.

I know that it isn't ideal, my question is one of economics. I want to add battery capacity, so of course I could buy two brand new batteries but I literally live in my car and would have zero use for the old one. But the old one is far from "dead" and so it'd be a waste just to recycle it - I tried selling it and haven't been successful. So I'm trying to get a better picture of just what consequences I'd have from mixing the old batt. with a new one. It seems like the new battery might just wear out a bit faster than it would have on its own - if this is the case, I think it still makes economic sense to mix them?

mike95490

You will need to be sure your alternator will stand up to the heavy load that 2 low house batteries can present, as others have said, they can burn up.   If that happens, look at a company called BALMAR which makes super HD alternators for charging batteries, isolators, dual channel voltage regulators, and all sorts of stuff in this realm.  http://www.balmar.net/products/

If you can manage a manual battery switch, use a marine style rotary switch, if not, get an automatic one, but please, don't rely on the simple diode style, it will undercharge the house battery.  And it may not be sufficient to power 2 house batteries, which can easily exceed 150A if they are low when you start the engine and the switch engages.

Simply adding a new battery parallel to a year old battery, will quickly degrade the new battery.   A special problem with AGM batteries, is that their internal resistance is so low, that the wiring differences cause one of the batteries to be slightly undercharged.  

My suggestion is to live with the battery you have, till it fails, and replace it with a pair of 6V -180ah Golf Cart AGM batteries wired in series to make a 12V bank.  The series connection insures equal charging, of the fresh pair of batteries.

Anyway a bit more reading about isolators

https://www.jamestowndistributors.com/userportal/document.do?docId=149

http://www.boats.com/resources/marine-battery-isolators-old-types-versus-new/

 marine rated gear is typically MUCH more well made and reliable, than RV gear. And it costs more.

Estragon

Yes, the main risk is lifespan of the new battery.

With a flooded battery, you could better assess the health of the old battery by checking SG, and catch a developing problem early. With AGM, all you can do is monitor voltage, and if you have a DC clamp meter, ensure equal current. With a non deep cycle battery used as a deep cycle, it has probably "aged" more than if had been used for a year in a starting-only application.

Also, anytime batteries are paralleled, even if new, there is a risk of some not getting charged properly because of varying resistance in wiring, etc. More batteries = more risk. As a rule, we advise against more than two in parallel, but two is okay providing wiring and connections are good.

The bottom line is the new battery will soon appear to be the same "age" as the old one. If the old one is in good shape, it can be worth adding another. If not, it may be cheaper to wait for the old one to die and replace with two new.

ILFE

I would suggest finding a local "auto-electric" shop, where they install multiple battery systems and such, on a regular basis. I had a local place (just looked and noticed they are still in business), from more than 30 years ago, who did all of my auto-electric work back then. Dual battery systems, heavy (high-output) alternators, heavy diodes for battery isolation, and switchable alternators that could be turned on and off from the dash of your vehicle, when the additional output wasn't needed. Then, you could discuss, directly, what you want done. 

Regarding running a new deep cycle with an old one, that - I just wouldn't do. Batteries should be purchased at the same time, being about the same age.

My two cents. 

oil pan 4

I have tried charging g additional batteries with a vehicle alternator.
If you are only going to idle the alternator then it's probably only going to put out 10 to 15 amps.
What I would do is use a medium duty truck alternator driven by a small gasoline engine. That will provide a hundred amps easy.