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Old 10-18-2015, 11:44 PM   #1
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Adding a second house battery for a trip

Hi everyone!

I have a 2004 Damon Challenger 348w that I'm driving to Florida next week. The coach has a 900A deep marine battery as the house battery. I'm planning to replace this with either Trojan's or Crown's in January.

I was wondering if I could hook up an old 750CCA car battery in parallel to my existing coach battery for the trip to get a little extra from my 12v system. I can see from my planned route that I will have to do some dry camping and I wouldn't mind a little extra juice.

Thanks

zog
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Old 10-18-2015, 11:58 PM   #2
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It's ok to mix battery sizes in parallel. The car battery is not designed for deep discharge, as I'm sure you know, so depending on the discharge it's subjected to, it might not be as good for starting again, but you probably already know that, too.

Try not to draw the car battery down past 50-60%, and you'll help it last longer.
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Old 10-19-2015, 01:12 PM   #3
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Hooking up an old battery to a newer one can easily suck the newer one down to the older ones charge level....
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Old 10-19-2015, 03:25 PM   #4
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Quote:
Originally Posted by zogthegreat View Post
Hi everyone!

I have a 2004 Damon Challenger 348w that I'm driving to Florida next week. The coach has a 900A deep marine battery as the house battery. I'm planning to replace this with either Trojan's or Crown's in January.

I was wondering if I could hook up an old 750CCA car battery in parallel to my existing coach battery for the trip to get a little extra from my 12v system. I can see from my planned route that I will have to do some dry camping and I wouldn't mind a little extra juice.

Thanks

zog
Why not buy 2 new batteries now for your journey? Seems like that would be the best choice out there.
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Old 10-19-2015, 04:13 PM   #5
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Why not buy 2 new batteries now for your journey? Seems like that would be the best choice out there.
MONEY!

I read somewhere:

"Owning an RV is a lot like owning a boat. As long as you can write a check, you won't have any problems!"

I know I got a great deal on my rig, (I paid $10,000 below NADA), but it seems that everytime I turn around I'm having to buy something else! That's why I'm trying to go cheap and use an existing battery that I'm about to recycle. I'm going to be in an RV park down in Florida this year, so I won't need the house batteries for the winter. My trip will take a max of 5 days and I will mostly need the juice when I'm stopping for the night to sleep. The furnace and an MP3 player will probably be all that I run and once I get south of Washington DC I probably won't even need the furnace, (It was -5 c/23 f last night and I slept with the window open).

I'm planning to go solar by the summer and I am planning my budget so that I can afford to go with higher end gear, (MPPT vs PWM), this way I can expand the system later. I only have my VA disability to cover my bills, so I have to be careful about my spending. So for now, a used battery is a cheap solution to give me some extra amps if I need them.
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Old 10-19-2015, 05:37 PM   #6
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It is not a good idea to mix old and new batteries in the same bank. It's even a worse idea to mix different size batteries in the same bank. It is just plain nuts to mix old, new, and different size batteries in the same bank.
Just my opinion but don't do it.
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Old 10-19-2015, 05:51 PM   #7
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It is not a good idea to mix old and new batteries in the same bank. It's even a worse idea to mix different size batteries in the same bank. It is just plain nuts to mix old, new, and different size batteries in the same bank.
Just my opinion but don't do it.

I agree. Especially since you describe it as "old" and something you were going to "recycle"....this weak old battery will drag the good battery down, and possibly cause your charging system to overcharge both batteries - ruining your good one as well.

What you can do is bring along the old battery, but don't hook it up. If you run the good battery down, then swap the cables from the new to the old battery.
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Old 10-19-2015, 06:13 PM   #8
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What you can do is bring along the old battery, but don't hook it up. If you run the good battery down, then swap the cables from the new to the old battery.
That's a good idea, actually. Thanks! BTW, is there anyway to connect it to the charging system without messing with the other battery? I.e. both batteries are charging but only one is hooked up to the house system?
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Old 10-19-2015, 06:18 PM   #9
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This is cut from Yandina.com FAQs. Experts in battery managment.

Although the conventional recommendations are not to mix old and new, good and bad, big and small, etc. in parallel, those misconceptions actually only relate to batteries in SERIES. You should understand the reasons and make up your own mind. The truth is that you can parallel just about anything. The downside is, if they are not matched, one battery (the newer) will tend to carry more of the load and the total capacity will be slightly less than the sum of the individual capacities. If your new battery has a capacity of 100AH and the old one still had 45AH left, then in parallel you will get something around 145AH, where had they both been new 200AH would fit in the space, but it will help and it will work.
When the old one eventually dies, it will drag the new one down but this happens whenever two batteries are in parallel and one dies, the only difference here is it is going to happen sooner than it would had they both been brand new. So big deal, you gained some residual use out of the old one and the new one will not be permanently damaged - just charge it up again.
The bottom line is the batteries in parallel provide more power - never less - than either one on its own - it just may not be the "ideal" way to do it.

A** The whole process of charging batteries in parallel is naturally self regulating, naturally governed by terminal voltage and current flowing through the internal equivalent resistances. For simplification, each battery can be considered as a perfect battery that holds the charge combined with a series resistor representing the internal resistance, and a parallel resistance representing the self-discharge leakage current.

Lets say you have a charging source dumping out say 100 amps. If you have 4 batteries in parallel sharing this source the distribution of the charge current is governed by those internal voltages and internal resistances. From Ohm's law, the charging current that flows into each battery is the voltage divided by the resistance. In this case the voltage is the charging voltage arriving at the terminals minus the actual internal voltage divided by the series resistance of the battery. A discharged battery is going to put a larger voltage differential over the internal resistance so more current will flow to it. A charged battery is going to put less voltage drop across the internal resistance so it will take less or none of the available charging current. A large plate battery which has a correspondingly lower internal resistance will demand more current because of this lower resistance. A small motorbike battery in parallel will have a high internal resistance and demand very little current.

All these demands for current result in a common voltage on the parallel circuit which is what the alternator or charger sees. None of the batteries will ever be "overcharged" as a result because the charging voltage is controlled. Even the smallest battery will only be seeing the same voltage so it is like the other batteries are not there. If you suddenly removed all the big ones and left just the tiny bike battery on the 100 amp alternator, the regulator in the alternator cuts the current back to a level that limits charging to a safe level.
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Old 10-19-2015, 06:19 PM   #10
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Adding a second house battery for a trip

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Originally Posted by zogthegreat View Post
That's a good idea, actually. Thanks! BTW, is there anyway to connect it to the charging system without messing with the other battery? I.e. both batteries are charging but only one is hooked up to the house system?

Something like this will isolate the two batteries, but allow them both to be charged. The trick-l-start feeds a controlled amount of charge current from one battery to the other, in only one direction. It would keep the good battery from draining into the older battery when not being charged from shore power or the engine alternator.

Instead of a starting battery, you would just connect it between your two house batteries.

The best scenario is still to buy 2 new matching batteries....then there is no worries.

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