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Dual Battery Questions

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In a couple of weeks, my Jeep will be 5 years old (going by my purchase date). I am still running a stock battery, which at this time is still functioning normally. But, of course the clock is ticking on it and the cc's are a little too low for what my winch needs. As I am startingto plan out a replacement,

I was startingto think about switching to a dual battery setup. I would add a new battery and still run the stock. When the stock finally goes, I would upgrade that battery then. 

I don't necessarily plan on going nuts with electronic upgrades. No lightbars here. Just want a little extra for remote trips, longer camps, and charging phones, cameras, etc.

So, is it possible to do this utilizing the stock battery? Is it even worth the effort?

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Easy route?  A quality AGM battery with a NOCO jumper like this https://no.co/gb40

With the old lead acid batteries, I think there was more of an imperative to go dual. Less so with modern AGM.

I agree that your current battery is long in the tooth and it would be good to get ahead of it before a cold winter's night finishes it off.

Edited by kobbs_77

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A high-capacity AGM like the Odyssey PC1500 (135 minute reserve capacity, $290) or Optima Yellow Top (120 minute reserve capacity, $280) has almost twice the reserve capacity of your stock battery, can be discharged to 50% and recharged thousands of cycles (deep cycle), and still provides CCA's to start your engine and run your winch.  If you're not running a refrigerator or several 12V fans all day long, this is probably sufficient for your needs by itself.

When you go to dual batteries you have four components to consider: 1.) the alternator, 2.) the batteries, 3.) the battery isolator/controller system, and 4.) compatibility with your snorkel.  Let's address these in reverse order.

Snorkel compatibility

Some snorkel systems require modifications to stock battery compartment or intrude into the space that a dual-battery tray needs.  AEV (and Chinese copies) are fine.  River Raider is not.  I don't kow about the rest.

Battery Isolation

You can just hook two batteries in parallel.  As long as they are identical in every way, nothing else is needed.  Identical means same manufacturer, model and age. Otherwise you can overcharge and ruin your batteries (or even have them catch fire when excess hydrogen is off-gassed).  The danger here (aside from mis-matched batteries) is if you don't pay attention you can run both batteries down so low that you can't start the engine.  So an isolator is recommended.

An isolator is really just a solenoid, which itself is really a high-current relay.  There are "coil" terminals that go to ground and a switched +12 source that's only on when the engine is on, such as the seat heater fuse in the TIPM.  There are load terminals that get connected to each battery's positive terminal.  When the engine is on, both batteries are connected so the alternator will charge them, and the winch can draw off them.  Because a winch can draw up to 550A under full load, you need at least a 300A solenoid (500A would provide an additional margin of safety) and use 0 AWG wire and 300A fuses or circuit breakers between the solenoid and each battery.  You should put a 1N4001 diode on the lead to the TIPM to prevent current spikes from the coil when it switches off, and will also act as a 1A fuse.  All this will run you around $100, or you can buy a kit from Painless Performance for around $200. 

The challenge here is wiring the devices and power outlets you want to use while camping to the second battery.  So you'd also need an auxiliary fuse block and power outlets. 

A variation on the above is to connect the solenoid coil +12V lead to an always-on +12V source, with a low-voltage disconnect set at 12.2V.  This way both batteries are connected together at all times until the battery voltage falls below around 12.2V (60% remaining capacity).  Then the solenoid will open so your starting battery capacity has enough power left to crank the engine, but your devices hooked to the second battery still get power.  Note that lead-acid batteries should not be discharged below 50% of capacity.  You'll either need to manually monitor and disconnect the second battery or add a second low-power disconnect and relay/solenoid to automatically protect that battery from excess discharge.  (AGM batteries can actually be discharged to 20%, but this reduces the lifespan). 


Starting batteries are designed for quick discharge/charge of high current but very little battery capacity, and are rated in Cold Cranking Amps (CCA).  Deep cycle batteries are designed for slow discharge/charge of low current but high battery capacity, and are rated in Reserve Capacity (RC).  There are some AGM batteries that can do both, and these are preferred when using a winch because they can provide high current for extended periods of time.  (Under full load, a winch can draw up to 550A.)  Examples are Odyssey PC1500 (125aH RC, 850 CCA) and Optima G34 Yellow-Top (120aH RC, 750 CCA).  This is roughly 2.5X the deep-cycle capacity of a starting battery, though at 3X the price.  The other advantages of AGM is they are truly maintenance-free (never need to add water), are leak resistant (can be mounted on their side), are more immune to damage from vibration, are less affected by cold temperature, and have a low self-discharge rate (won't loose charge quickly in storage). 

If all you are going to do is run a few LED lights, a 12V fan and some phone / camera chargers a few hours a day, a single Odyssey or Optima battery will do the bill.  Even if you don't start your engine for a few days.  To play it safe, you might want to add a low-voltage disconnect between the battery and a sub-fuseblock for those devices so the battery won't get discharged below 60% (about 12.2 volts).  Blue Sea makes an excellent 65A low-voltage disconnect that can be found on Amazon for around $80.  You can also find some 30A LVD's on Amazon for around $20, plus a few bucks for an enclosure.

If you decide you want to run a 12V refrigerator or an inverter to power a microwave, toaster oven or coffee maker, then a dual-battery setup makes more sense.  You can hook the batteries directly in parallel or use an isolator as described above.  If you don't use an isolator you must use two identical batteries (brand, model and age) to avoid overcharging and damaging the batteries.  If you use an isolator you can use different capacity batteries but they must be the same design (e.g., Flooded Cell, SLA or AGM) or you can overcharge and damage the batteries. 


Your Jeep's alternator (140-160A, depending on year and model) is sufficient to power the vehicle, run the stock or upgraded head unit and some lights, and still charge two batteries.  About 60A of the alternator output is used to run the engine and essential electronics, leaving 80-100A for charging and accessories.  Unless your Jeep is lit-up like something out of Close Encounters, or you've got a ghetto-thumpin 800W sound system always on max volume, the stock alternator will recharge the batteries as expected.  I know of one higher-output alternator, from Mean Green, that offers 220A output.  This won't necessarily charge your batteries faster; it only makes sense if you are already close to the limit of your stock alternator (but it can reduce the amount of battery power used when winching).  You can also get a local alternator/electric motor repair shop to rewind your alternator for higher output capacity, and probably for half the price of a Mean Green unit.

It can take several hours to charge off the alternator.  You might want to get a NOCO battery charger so you can fully recharge your batteries at home, without running the engine.


Unless you need to run higher-wattage devices for extended periods between charging, a single Odyssey PC1500 AGM battery with a low-voltage disconnect to a sub-fuseblock is the way to go.  You'll spend around $400 total and need to run some new wires.  I suggest running a 4AWG primary wire from the battery to a 40A in-line fuse under the hood, and then to a Blue Sea (http://a.co/cRuckY6) or Docooler (http://a.co/izh6p2F) LVD installed in the rear cargo area.  Then go from the LVD to a Blue Sea fuse block to a couple sets of 12V and USB power outlets.  Or, if you want fancy, get this DROK meter/relay (http://a.co/1uTOnhO) instead of the Blue Sea LVD, program it for 12.2V low voltage disconnect, 13.2V reconnect, 40A high current disconnect (I'm using the DROK on my trailer solar system). 


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