dual battery question

svynx

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Dover, pa
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looking around
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unsure on what
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i want to lift
Real quick, and most likely real easy question. I'm installing a dual battery system. Main battery is an Optima red top. Second battery will most likely be an Optima blue top deep cycle. I've been told that I can directly wire the two batteries together (neg to neg, positive to positive). Makes sense to me, and would install a 100 amp fuse as well. Here's the question. Do I need to run the negative terminals together, or can I ground the second battery to the chassis in the same manner as the main battery? I'm not going to worry about charging the batteries separately since they will both be connected to the alternator. The batteries will be wired in parallel (doing so to get the bump in amperage) using 4 gauge wiring.

Besides the grounding question, do any of you see a problem with this setup?
 
I think that it's a bit more complicated than that, SV, as with the way you are proposing, the charging circuit will "see" the combined internal resistance of both batteries, and stop charging when this hits the programmed point, regardless of the state of charge of the other battery.

Some kind of isolator is needed to ensure that each battery is properly charged at all times.

Do bear in mind that I have already told you more than I know about these things ... :poke: :iconwink: :rotfl:.
 
You haven't actually said why you feel the need for an auxiliary battery. Accessories such as sound system, car fridge, ???, or just for extra cold start cranking capability?

You will note that the standard battery is grounded by a nominal size cable to the engine bay skirt near the main fuse box, and by a decent sized cable to the engine block near the starter motor. If you ground the auxiliary battery to the chassis, any contribution it makes to the starter load will reach the starter (i.e. block) via that main battery small earth and the numerous tweety-sized earth bonding wires, none of which are designed to carry that sort of current. This will certainly come into play for a longer time if you plan on fitting a winch.

Assuming the aux is in the engine bay, I would personally fit a ground cable to the BLOCK, and if at all feasible I'd get that point as close as I could to the starter location and the existing block earth of the main battery. An additional earth to the chassis adjacent to the aux battery location (as is done for the main) would also be recommended.

In the positive, I would have a fuse as you suggested, sized to recognise what discharge current it may see (i.e. share of starter/winch). I would also fit some sort of isolator/relay if planned accessories are likely to risk deep discharge of one battery, so that you retain cranking ability.

As far as charging is concerned, the alternator (unless it is one of those "smart" ECU-controlled types) will charge both satisfactorily when parallelled. If it IS one of those pestilences, then you will need a proper DC-DC charger to counteract the behaviour of the charging system if you want your auxiliary to be properly charged. More on this if it affects you.
 
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Rat: interesting thought on the isolator. I will have to talk with my electrical guys about it, and see what they have to say about them. I do remember him saying something along the line of not needing one since the voltage won't change, but it can't hurt to ask different sources for opinions.

Zip: reasons for the aux battery: winch, 400w inverter, lights (led and hid). Plans down the road are for a small compressor. I'm building the car for slight off roading, with a main focus on camping. Having a charging station (ie the inverter) would be a nice touch. The compressor is self explanatory, as are the lights and winch.
I suspected that grounding the aux battery in the same manner as the main battery wouldn't be a problem. I was considering just going from terminal to terminal, so two wires total, battery to battery. But I feel that having more grounding points is always a bonus. I will be adding another ground going from the alternator to both batteries, along with a new ground going from both sides of the motor and heads using 4 gauge wire. I feel that that will in essence eliminate the crappy factory grounds.

Thank you both for your thoughts. I have a little more research to do.
 
I have a dual battery system that I used to use in the XV and it is currently awaiting for a decision on the fate of the Forester whether or not it gets installed.

If you have a smart alternator then you can't use the old style solenoid type isolaters that simply puts the battery in parallel when activated. A smart alternator senses when the start battery is charged then goes into a trickle charge mode. If you have been using your auxiliary battery for a camping fridge and other accessories then it will never charge. The local Subaru stealership couldn't tell me if the XV was fitted with a smart alternator or not so I went with using a Ctek DC to DC charger, which is suitable for both smart and non-smart alternators.

https://www.ctek.com/au/en/chargers/D250S DUAL

Basically it will charger at 14.4V with and input voltage down to 12. It has an input so my 80W solar panel can be connected and it's inbuilt with an MPPT tracker for it. And solar is a good idea if you stay at a campsite for a little while.
 
I am not sure about the need for two batteries anywhere in the lower 48, let alone in the east, though I understand that Australia or Alaska may be another story.

I live in the Southwest, the region where extra battery power is most needed. I upgraded to an AGM main battery. I also purchased a DieHard 1150 portable power unit from Sears at a good discount on Thanksgiving (online). The portable power uses AGM battery, too, and in addition to jump starter features a good air compressor, 2xDC outlets, 2xUSD outlets, and a 400W AC converter with two outlets.

My battery is Duracell branded, made by East Penn in PA, it is deep cycle and it costs the same as the Red Top, over which I chose it. A battery like this should be able to handle any extra LED lights. I am not sold on HID conversions making any real difference in terms of function.

I am not familiar with winches, but presumably that will be used once every few years, for which the AGM main battery should be more than enough, especially when backed by a solid portable unit. More importantly, the question is why bother with a winch on a vehicle not meant for tough off-road use. A come-along will cost a fraction of that. I bought a 2 ton one for a friend: sure, it still needs rope, but the main part is quite compact and easy to pack and carry. He does actual rock-crawling in a heavily modified Legacy with a steel front end in place of the factory plastic, but even he does not have a winch. Generally, you never want to be in a situation when an actual powered winch is really necessary, like a Wrangler or a truck being winched over a rock wall. The come-along should do, should one misjudge a moderate mud spot or something.

In short, the problem I see with your setup is that it reads as a huge overkill to me, both in terms of having two excellent batteries instead of one + a good portable power unit with powerful jump starter, and in terms of winch. But, I have been wrong before...
 
I tossed the idea of a second battery around a bit but in the end just went with a potable jump pack. It is not the largest of batteries but it has a few nifty attachment that are useful in the field.

https://www.jaycar.com.au/productView.asp?ID=MB3694

It is more than enough to get me out of trouble, plus the compressor is strong enough to pump up the tyres (bit slower than I would like but meh).

I like it as I can just pop it in the shed when I dont need it rather than lugging around a spare battery all the time.
 
^ That's what I ended up getting too, Doug.

I found that the compressor was next to useless, but if I connect one of my twin cylinder compressors across the jumper leads (it draws about 20+ amps ... ), it works fine :biggrin:.
 
Yeah, it is not easy to find a portable power that is worth paying for. I do not know about Australia, but in the US I only found two products worth buying: the Die Hard series and something else from Northern. I was surprised that the air compressor works as well as it does.
 
If you do end up going for the dual battery setup make sure you put a breaker switch as close as possible to each battery.

In the event of a crash both circuit breakers should trip out and thus stop a potential fire.

Cheers

Bennie
 
svynx said:
Real quick, and most likely real easy question. I'm installing a dual battery system. Main battery is an Optima red top. Second battery will most likely be an Optima blue top deep cycle. I've been told that I can directly wire the two batteries together (neg to neg, positive to positive). Makes sense to me, and would install a 100 amp fuse as well. Here's the question. Do I need to run the negative terminals together, or can I ground the second battery to the chassis in the same manner as the main battery? I'm not going to worry about charging the batteries separately since they will both be connected to the alternator. The batteries will be wired in parallel (doing so to get the bump in amperage) using 4 gauge wiring.

Besides the grounding question, do any of you see a problem with this setup?
Click to expand...

What are you trying to achieve? Are you adding a second battery for car audio or for a winch/fridge, etc?

I ask because how you arrange the batteries will really determine the answer.

Yes, you can just run the 2 batteries in parallel, effectively creating one big battery, however you will need to use two new batteries of the same kind, ie 2 deep cycle agm, or 2 lead acid batteries. The reason is that the 2 batteries won't be charged correctly. You can and should add an isolator, meaning that the batteries are charged and power drawn from independently of one another. You can find isolator's that combine when the vehicle is first started so that the engine receive's the maximum voltage and amperage possible.

The benefit of an isolator is that you can run accessories to each battery and each battery being on it's own circuit won't get over charged by the alternator. You can leave the stock battery stock, and then put all of your accessories on the second battery, so that if you pull too much amperage with the accessories, you can still start and run the engine and basic functions of the car.

I was looking into a Hellroaring Isolator, and spoke with the owner several times. He suggests putting all the accessories, lights, winches, etc on the primary battery, then using the backup battery for starting the car. But to me that seems backwards, it's also backwards how it's recommended on other online 4x4 forums, such as pirate 4x4 and expedition portal.

Another benefit of the isolator is that you can run different batteries, like you had mentioned, wanting to run a normal battery as well as a deep cycle. I will say with your grounds, while you can run them both to the same ground, you don't need to. Just grind back to bare metal and insert a bolt.

As well if it's easier for you to mount a battery in the trunk, or wherever, an agm battery is sealed and won't off gas like a normal battery, and you could get the added benefit of better weight distribution over the axles.

If you have any more questions on the subject I can find good links to the subject that I have bookmarked on pirate 4x4 and the such.

Andrew
 
Btw, all other things aside, where in the engine bay does one put the second battery?
 
I've seen them fitted near the ABS unit on NA vehicles that have the airbox/ filter at the rear of the engine.

harder to do on turbo's with TMIC's though
 
Yeah, I see no room in the H6 either.
 
AGM batteries do not like heat so it's better to keep them out of the engine bay. My Foz aux battery sits in the rear passenger footwell; my Triton has one each mounted on the rear wheel arches.

For what it's worth in ideas here's what I did with the Triton; I ran zero gauge cable under the car with a 250 amp fuse on each end. A 4 position switch sits in front of each battery; with these switches I can select (Normal) = passes through a CTEK DC DC charger to both rear batteries; isolate the CTEK and connect both rear batteries (for rear winching) or directly connect all 3 batteries for heavy winching or cranking the car if the primary battery is flat or to provide power from primary if secondary batteries go flat (keeps the fridge with the beer cold). There are several aux circuits in the rear, cig plugs, Anderson plugs etc and all are protected with appropriate fuses.

With this power I can easily run my 3000w inverter from which a 800W inductive cooktop and 2 slice toaster can connect and once a year at a charity event I run a 1000W microwave for short bursts.

This is also all supported by 2 x 100W solar panels on my rooftop camper.

Power to the people! :-)
 
Kevin said:
AGM batteries do not like heat so it's better to keep them out of the engine bay.
Click to expand...
^^^^^ This, definitely.
Kevin said:
My Foz aux battery sits in the rear passenger footwell.
Click to expand...
My solution - which was predicated upon by the AGM I had and the transient nature of the installation - is written up at https://ozfoz.com/forum/viewtopic.php?f=46&t=47673
It was for a trip of 15,000km, and is now about to go back in for another one. But between times this is my daily so I wanted it easily removed. Maybe some useful ideas there.
 
Agree - my Foz has its battery in a canvas tool bag so is easily removed.
 
What battery likes heat? This is the first time I hear of AGM being worse than regular in heat. It is extremely hot here much of the summer anyway. It can be 70C in a vehicle parked outside...

Every time I have to start my car over the summer, which lacks remote start (for now), I feel like I am toast.
 
None like heat, but flooded cell types aka your "normal" cranking battery tolerate it far better than SLA/AGM types.
 
zippo said:
None like heat, but flooded cell types aka your "normal" cranking battery tolerate it far better than SLA/AGM types.
Click to expand...

How so? If you read the various articles on the subject, new stories, etc. The automakers are going from lead acid batteries to agm because of the heat tolerance and other factors. Their are alot of personal opinions on the subject of agm's not tolerating heat, but no actual data to backup such claims.

Heat hurts all batteries, shortening their lifespan, not just an agm, lithium, or lead acid.
 
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