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Old 04-13-2021, 03:49 PM   #1
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Default Battery hookup

I want to hook up a second battery. i understand that the "proper" way is to connect the batteries together in parallel, and then connect the bank to the inverter/charger. My problem is that where my current battery is mounted, there is no room i can mount the 2nd battery. But there is room next to the inverter. What problems, if any, if i were to connect the second battery straight to the inverter just like how the first one is connected with its own circuit breaker/fuse. Basically the inverter is drawing from separate inputs from the two batteries. Both batteries are identical, same brand and size.
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Old 04-13-2021, 04:28 PM   #2
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I am not certain I understand what you are proposing, but:

You can still wire the two batteries in parallel, even if they are not physically adjacent, just using longer wires to interconnect. Best practice says that the two wires between the batteries be the same length and gauge, and that ground be taken from one battery and +12 from the other one. This keeps everything nice and balanced.
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Old 04-13-2021, 04:44 PM   #3
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I am not certain I understand what you are proposing, but:

You can still wire the two batteries in parallel, even if they are not physically adjacent, just using longer wires to interconnect. Best practice says that the two wires between the batteries be the same length and gauge, and that ground be taken from one battery and +12 from the other one. This keeps everything nice and balanced.
Yea i know i can still connect them in parallel. I just wanted to avoid running maybe 8 feet of wire to the other battery when i can connect it to inverter which would be right next to it.
i have a Carrado Axion. The coach battery is mounted underneath in front of the rear axle. I want to mount the 2nd battery inside next to the inverter.
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Old 04-13-2021, 04:49 PM   #4
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Yea i know i can still connect them in parallel. I just wanted to avoid running maybe 8 feet of wire to the other battery when i can connect it to inverter which is right next to it.
OK. Then, could you draw a simple diagram, or at least describe your proposal in more detail. I am having trouble picturing how both batteries would be charged if they are not connected together.
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Old 04-13-2021, 05:46 PM   #5
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OK. Then, could you draw a simple diagram, or at least describe your proposal in more detail. I am having trouble picturing how both batteries would be charged if they are not connected together.
Its been a while since i posted photos on a forum. I used to use photobucket, but I guess its no longer free. Let me figure out a way to post a diagram.

But basically right now, the 1st battery is connected to the inverter/charger. Pos of battery to pos of inverter and same with negative. The second battery i would also connect it to the inverter the same way. So both Positives of the batteries are connected to the Positive of the inverter... and both negatives of the batteries connect to the negative of the inverter.

In theory, they are in parallel because both positives are together (connected at the inverter) and both negatives are together
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Old 04-13-2021, 05:59 PM   #6
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Its been a while since i posted photos on a forum. I used to use photobucket, but I guess its no longer free. Let me figure out a way to post a diagram.

But basically right now, the 1st battery is connected to the inverter/charger. Pos of battery to pos of inverter and same with negative. The second battery i would also connect it to the inverter the same way. So both Positives of the batteries are connected to the Positive of the inverter... and both negatives of the batteries connect to the negative of the inverter.

In theory, they are in parallel because both positives are together (connected at the inverter) and both negatives are together
From your description, it sounds like the batteries would be in parallel in practice as well as in theory. Should be fine, with the provisos listed above. If you ignore these rules, you risk having the batteries charge unevenly. How much it matters in practice, I can't say--guess it depends on length and wire size.

P.S. -- to post a picture, use the "paper clip" button on the top of the message edit screen. It is a two-step process: (1) upload the image; and (2) insert the image in the message. Oddly, they both use the same paperclip button.
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Old 04-13-2021, 06:34 PM   #7
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If you're planning on adding a (wet) lead-acid battery inside, make sure it's properly vented to the outside. Otherwise, you'll need to replace the batteries with AGM. Avanti's advice on wiring is spot on. Different length wires, especially 8 feet each way (16 feet total) will lead to a charging imbalance due to wire resistance and possibly a shorter battery life.
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Old 04-13-2021, 08:32 PM   #8
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If you're planning on adding a (wet) lead-acid battery inside, make sure it's properly vented to the outside. Otherwise, you'll need to replace the batteries with AGM. Avanti's advice on wiring is spot on. Different length wires, especially 8 feet each way (16 feet total) will lead to a charging imbalance due to wire resistance and possibly a shorter battery life.
Batteries are Lithium. Yea i figured the different wiring length would be an issue. The 1st battery is about 8ft from the inverter. The second battery is about 2 ft of wire to the inverter
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Old 04-13-2021, 08:39 PM   #9
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From your description, it sounds like the batteries would be in parallel in practice as well as in theory. Should be fine, with the provisos listed above. If you ignore these rules, you risk having the batteries charge unevenly. How much it matters in practice, I can't say--guess it depends on length and wire size.

P.S. -- to post a picture, use the "paper clip" button on the top of the message edit screen. It is a two-step process: (1) upload the image; and (2) insert the image in the message. Oddly, they both use the same paperclip button.
here's a quick sketch. also keep in mind one battery will have longer wires going to the inverter. So as mentioned, they would charge/discharge uneven. But other than that, what other issues would there be? Both batteries are 100AH Lithiums. Connecting this way, would my total still be 200ah as if they were connected parallel correctly?
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Old 04-13-2021, 08:43 PM   #10
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Batteries are Lithium. Yea i figured the different wiring length would be an issue. The 1st battery is about 8ft from the inverter. The second battery is about 2 ft of wire to the inverter
This doesn't sound bad to me.

1) Connect the two batterys' + and - terminals together with two identical lengths of wire.
2) Use a 2' wire to connect the + terminal of battery 2 to the inverter. (n.b.: it doesn't matter how long this wire is--it is only the battery interconnects that need to be balanced).
3) Bond the inverter's negative terminal to the chassis
4) Bond battery 1's negative terminal to the chassis.

Done!
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Old 04-13-2021, 09:43 PM   #11
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Someone else might have mentioned this, but you can always balance the batteries by using a cable that is about as long as the one that connects to your other battery but just keep it coiled up next to it. As long as the parallel runs are equal you will be fine.
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Old 04-14-2021, 09:39 PM   #12
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One quick additional thought. Coiling wire carrying a high current will produce a magnetic field. I did a quick calculation using this calculator (https://calculator.swiftutors.com/ma...alculator.html) and it looks like 8' of wire, coiled 6" in diameter for 5 turns would produce a 0.004T magnetic field at 200A. According to Wikipedia this is about the strength of a refrigerator magnet, so probably not a big deal.
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Old 04-14-2021, 09:48 PM   #13
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One quick additional thought. Coiling wire carrying a high current will produce a magnetic field. I did a quick calculation using this calculator (https://calculator.swiftutors.com/ma...alculator.html) and it looks like 8' of wire, coiled 6" in diameter for 5 turns would produce a 0.004T magnetic field at 200A. According to Wikipedia this is about the strength of a refrigerator magnet, so probably not a big deal.

Of more worry in a coiled up wire is that they can overheat in some conditions. We even had our pull out extension cord reels taken away at our last job because they allowed you to plug in without the cord fully extended and that broke the safety rules for hot cords. Would that be an issue in this case, hard to tell, but probably not.
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Old 04-18-2021, 04:14 PM   #14
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Batteries are Lithium. Yea i figured the different wiring length would be an issue. The 1st battery is about 8ft from the inverter. The second battery is about 2 ft of wire to the inverter
The 6 foot difference in length can create a different resistance to each battery, but using a larger gauge wire on the longer run can minimize it. You can use an online resistance calculator to determine what wire size will have the same resistance as your existing cables.

If the resistance is not matched, one battery will provide more current while the other is somewhat current limited by the wire. It's not the end of the world, but it does mean one battery is doing more work. Also when charging, one battery may not get as much charge. This second issue is more of a problem with RVs where the house batteries are located far from the isolator or charging relay and charged by the alternator.

Do you have a DC to DC converter with a lithium charging profile to properly charge the lithium battery set off your alternator? Two different battery chemistries should not be run off the same charger.

Bottom line is to balance the DC resistance from each battery, not the cable length.
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Old 04-18-2021, 05:01 PM   #15
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The 6 foot difference in length can create a different resistance to each battery, but using a larger gauge wire on the longer run can minimize it.
If you use matched interconnect wires and tap the positive and negative feeds to the load on opposite batteries, this should not occur.
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Old 04-18-2021, 05:59 PM   #16
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His home run method is actually better, because it does not double the IR loss (I=current x R resistance) in the cable to the inverter/ charger. Each battery has its own path.

Tieing the batteries in a string means the resistance from the charger/inverter getting to the second battery is not the same as the first battery. First battery sees R1, the first cable set, and second battery sees R1 + R2 of both cable sets.

It's a simple current divider to share current. Balancing each battery cable resistance to each battery equalizes both charge and discharge pathways.

That all said, if the second battery is added to augment longer run time at low currents rather than peak current capacity, the IR losses are not as much and balancing is not as critical.
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Old 04-18-2021, 06:28 PM   #17
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His home run method is actually better, because it does not double the IR loss (I=current x R resistance) in the cable to the inverter/ charger. Each battery has its own path.
It is NOT "better". The described "loss" is irrelevant given proper wire sizing.
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Tieing the batteries in a string means the resistance from the charger/inverter getting to the second battery is not the same as the first battery. First battery sees R1, the first cable set, and second battery sees R1 + R2 of both cable sets.
Incorrect, assuming you take off positive and negative from opposite batteries. This is completely standard practice and it is a bit silly to argue otherwise.

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Old 04-18-2021, 06:37 PM   #18
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I'll run some numbers and show you the math.

My assumptions:

Typical battery cable wire. I can run several sizes for comparison. Maybe he can tell us what his first set gauge size is.

Inverter is probably the largest load, such as when running a microwave. Knowing the peak currents here helps.

Charge currents from inverter/chargers are usually smaller than peak load currents, (like the 100+ Amps needed for a microwave). But knowing his charger can also help.

I'm also assuming it's a 12 volt system.
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Old 04-18-2021, 06:42 PM   #19
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I'll run some numbers and show you the math.
Wouldn't it be easier just to draw a picture showing us the asymmetric current path that you claim exists?
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Old 04-18-2021, 06:48 PM   #20
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I'll agree with your diagram, since that does balance the two battery cable resistances if it's wired that way.

But the location of the first battery may be difficult running three big wires to the second battery if the charger is somewhere between the two batteries.

The electrons don't care how you balance the resistance. Either way can work.

But if he decides to add a third battery, that's when matching cable sets to balance batteries won't work.
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