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Old 09-28-2012, 09:48 AM   #1
Lomez OP
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Cage- wiring up a 2nd battery in parallel

Want to upgrade to two batteries in the cage, for winter. To start, I was just going to install a second battery, and wire it in parallel (obviously).....end of story. The result should be nothing different than a single battery but with double the Ah capacity. However, I've heard others say there could be problems with this setup. ?? I can think of no "problems". Some have said it will tax the alternator too much. ?? But, if that were the case, then driving with the headlights on would be a problem because they are "taxing the alternator". As long as both batteries are in decent condition, I can't see any truth to that claim.

Ultimately, I'm sure I'll wire/isolate the #2 batter to charge only, thus becoming an emergency "backup" source of power. But for now, I just want to double my Ah capacity.

Thoughts?
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Old 09-28-2012, 10:03 AM   #2
miguelitro
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hard winters in pasadena lol!

i had dual batteries wired in my old tacoma for baja trips using a sure power isolater made for rv use, battery cut off switches for each battery as the batteries were in the bed and the 0 gauge welding cable ra under the cab. battery 1 was start duty only and the optima deep cycle was stereo, off road lights, aux fuse box and inverter. need be i could always run one jumper from pos to pos and double my starting power. i considered running cable and putting in a switch just for this but carrying jumper cables was easier.
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Old 09-28-2012, 11:30 AM   #3
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Gotta run an isolator.
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Old 09-28-2012, 11:58 AM   #4
victor441
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FWIW dual battery setups are common on boats and often have a switch to use either battery or both....might be a good option for your cage, a switch is $40 or so IIRC...could use the batteries alternately and only switch to "both" if starting was a problem


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Old 09-28-2012, 12:06 PM   #5
gn77b
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I would not do it.

a lead-acid battery has a voltage of ~12.8 volts when new and fully charged. the discharge characteristic is very non-linear, 12.4V meaning just about fully discharged.

batteries of different makes, types and "backgrounds" (usage periods and types) "die" at different rates. the battery that gives off a higher voltage will "try" to charge the other one. batteries need to be charged in a certain way, failing to do that means a shorter life. not only the better battery will discharge itself trying to charge the other one, but the "bad" one will never fully receive the energy and will dissipate it as heat.

I would only do it with NEW, absolutely identical batteries. otherwise, forget it.
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Old 09-28-2012, 12:15 PM   #6
ragtoplvr
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Use 2 new batteries, made the same month, fully charge before installing. Millions of over the road trucks, farm tractors, even diesel locomotives, have 3 or more batteries in parallel, works great for them. Just do not mix brands and amount of charge.

You do want a OE alternator, much of the china crap do not have high enough temp magnet wire to charge hard for long times.

Rod
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Old 09-28-2012, 12:20 PM   #7
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Multiple parallel batteries have been used for years in stationary power systems for telecommunications. Should not be a problem.
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Old 09-28-2012, 12:21 PM   #8
gn77b
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Multiple identical parallel batteries have been used for years in stationary power systems for telecommunications. Should not be a problem.
Ben
fixed it for you.

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Old 09-28-2012, 12:25 PM   #9
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fix it for you.
I've seen lots that used different manufacturers and even different capacities.
No fix required.
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Old 09-28-2012, 12:28 PM   #10
gn77b
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I've seen lots that used different manufacturers and even different capacities.
No fix required.
Ben
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Old 09-28-2012, 01:03 PM   #11
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I worked in telecom power and just retired this year. The central office power batteries are identical within a string and are wired in series, typically 24 and are floated at 52 volts, and stings of various battery sizes and brands are often paralleled as was said...however vehicle/engine applications are very different. When we changed start batteries for the central office diesel generators (sometimes as many as 8 in series/parallel combination for a big 1 MW engine) the industry standard was to replace all at once and all had to be identical

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Old 09-28-2012, 01:20 PM   #12
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All (most?) Diesel pickup trucks in the US run 2 12v batteries. My Dodge has two under the hood. Nothing special - no isolator, no gizmos, but I do replace them in pairs.

It also has a huge-ass alternator.
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Old 09-28-2012, 02:16 PM   #13
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IMHO, this is a situation where classroom theory and real life are a tiny bit different.

I have seen many many dual battery setups with non-identical batteries. As long as both batteries are initially in good shape there is not a problem.

In other words, if you put battery A in a vehicle and operate it without a problem for a week and then you take that out and put battery B into the vehicle and operate it with that one for a week without any issues, you can tie them together and run AB in parallel with each other with no issues either.

The whole "battery too big for the alternator" wives tale used to actually be an issue so there is validity to it, but that was back when everything ran off generators instead of alternators. What would happen is that a car might need 20 minutes of running to get recharged enough to replenish the current it used to start the engine. If you double the AH capacity of the battery now it needs 30-40 minutes to recharge. This was especially important with 6 volt systems but dang, it hasn't been that way for 50 years now and still the legend lives on.

Anyway, in a perfect situation you would use 2 brand new identical batteries but as I mentioned as long as they are both in good shape you'll be fine.
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Old 09-28-2012, 02:35 PM   #14
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So, did some extra research and think I've got the answer. You don't need identical (voltage yes....but that's it) batteries while the engine is running, and they are being charged/maintained. However, when the engine/charging system is shut off, dis-similar batteries COULD in theory self-discharge each other due to the varying potentials, and other factors. So.....an isolator is needed when the engine is shut off. That's my take, but as always, open to new facts
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Old 09-28-2012, 05:50 PM   #15
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I've got 5 (yes, five) in my Scion xB, and a custom fitted 270A GM alternator in my Scion. It has something to do with my other hobby, car audio (Four 15"s on 7kW, working on getting 1kW to the front stage. Its ridiculous, yes.). Generally, as long as the batteries are the same age and chemistry, you're okay. Problems occur when you throw a new battery in with an old one. The old one will constantly draw the new one down to its resting voltage when the engine is off, reducing it's lifespan. No issues with the alternator, though if you consistently take the batteries to deep discharge, the alternator will work much harder for long periods of time to recharge both, I suppose that could kill off an alternator on its last legs. If you're using it for starting and rarely draw the batteries down (using electronics with the engine off), I wouldn't worry about it. You'll want to make sure the batteries get fully charged though, if you have a absurdly small alternator and only go for short drives, its possible they won't recharge fully because of the added capacity. Most modern vehicles should have adequate capacity to handle it though.

Battery isolators come in a couple different flavors, diode and solenoid based. Diode based have no moving parts, but being based on diodes, drop the voltage from the charging side to the secondary battery (not so good for batteries). Mechanical ones have the issue of having moving parts, but are otherwise the idea choice if you must run an isolator. The neat thing about running two batteries though, is that, although you spend more money on batteries, they should in theory last longer because the discharge is split between the two. Instead of draining a single battery 10% doing whatever, you drain each battery 5%. The shallower the discharge, the longer batteries tend to last. I'm not sure if the longer life is enough to offset the additional cost of batteries, but just something cool to note. My suggestion is to skip the isolator, and run a digital voltmeter directly to a battery (or one to each, if you're extra OCD). You'll share the load between the batteries, they'll fully charge with no voltage drop and no solenoids to fail, and the voltmeter will allow you monitor the condition of the charging system better than the idiot light on the dash (they don't come on until things are very wrong). If you're doing things like running an portable freezer or something, the isolator might still be a good idea. Also, take a look at DieHard Platinum and Platinum Marine batteries. They're simply rebranded Odyssey batteries with a much less painful pricetag, and far better warranty network (Google if you don't believe, Odyssey has confirmed it). If you have any questions about my particular setup, ask away
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