Author Topic: Question for the electrical knowledgeable  (Read 235 times)

Offline somewhereinusa

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Question for the electrical knowledgeable
« on: August 13, 2019, 04:28:08 AM »
I'm upgrading my house bank from 4, 6V lead acid golf cart to 4, 12V lithium batteries in parallel .
My inverter has a max charge rate of 150 amps and each battery has a max continuous discharge rate of 150A.
According to the various cable size charts I have looked at 4 gauge is good for 200A at 4 ft with 2% loss. My cables will be less that 2 ft.
I can get a really good deal on some 4 gauge wire. Does that wire size sound right with a safety margin?
Is there any good electrical reason why I can't simply connect all four batteries to the inverter at one stud instead of using some kind of buss bar? All connections from stud to each battery will be the same length.
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Offline muldoonman

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Re: Question for the electrical knowledgeable
« Reply #1 on: August 13, 2019, 04:34:55 AM »
I would call the Inverter Manufacturer and run it by tech. I imagine they have seen everything or almost.

Offline somewhereinusa

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Re: Question for the electrical knowledgeable
« Reply #2 on: August 13, 2019, 05:31:04 AM »
I'm not talking about the wire to the inverter it is 4/0 I'm talking about the battery connections from the stud to the individual batteries wired in parallel. Inverter is 3000 W.
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Online richard5933

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Re: Question for the electrical knowledgeable
« Reply #3 on: August 13, 2019, 05:48:00 AM »
If you have four batteries wired in parallel, I've seen where the recommendation is to connect one lead from the charging source to a post on battery #1, and the other to the opposite post on battery #4. The theory is to equalize the charge across the four batteries as much as possible. The site on saw this on showed a breakdown of the difference that was present if both were connected to the same battery, but I can't recall right now the name of the site.

Does your inverter also serve as your charger? If so, then the charge should go over the same cables used to feed the inverter from the batteries.

You have a 3000w inverter, so the cables need to be sized to power that. Doesn't that mean you need something like 4/0 battery cables?
« Last Edit: August 13, 2019, 05:51:35 AM by richard5933 »
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Offline bevans6

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Re: Question for the electrical knowledgeable
« Reply #4 on: August 13, 2019, 08:01:15 AM »
four batteries in parallel, I'm assuming a 12 volt system.  Four wires on one stud is electrically identical to four wires on a bus bar, but mechanically a bus bar is far superior.  I would have the bus bar, personally.  It's bad practice to stack power cables on one stud, but it does happen.  You can size cables in two ways - for the maximum potential source current or for the maximum potential load current.  Most systems, including house wiring, motor wiring, etc, is sized based on load.  Your load is your inverter, nominally 3000 watts, which translates to 240 amps at 12.6 volts if you max out the inverter, which you probably won't do.  You have four batteries, if you wire them in parallel with a common bus bar as the connection point, you will need to have cable worthy of 1/4 of 240 A, or 60 amps each.  Size 4 cable is nominally rated at around 90 amps, so it would be fine between the bus bar and each battery, likewise between each battery and ground, and you would use 0000 (4/0) cable nominally rated at 260 amps between the bus bar and the inverter, and ground for the inverter.  Which all sounds exactly like what you plan to do, so I think you are fine with your plan.  If you change loads, or reduce the number of batteries with the same load, your current per battery will change, as will the ampacity requirement of the individual battery cables.  So don't go to 2 batteries instead of 4, for example.  Three batteries would be OK (80 amps per battery).

The other way to connect batteries in parallel is to daisy-chain them and as noted, connect positive from battery one and ground from battery four.  In this case all the cables have to be sized for all the current.  It's possible this is theoretically better for something, but I like the bus bar and individual cables to each battery method.  Each battery needs sufficient cable for only the current that it is supplying.

Brian
« Last Edit: August 13, 2019, 08:10:13 AM by bevans6 »
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Offline somewhereinusa

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Re: Question for the electrical knowledgeable
« Reply #5 on: August 14, 2019, 05:07:24 AM »
Thanks Brian, I needed someone to verify my math and method of obtaining the end result.
Many have recommended this site http://www.smartgauge.co.uk/batt_con.html for combining batteries, method #3 eliminates the problems associated with #1 and #2. I'm not sure I understand why #4 is better.
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Offline bevans6

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Re: Question for the electrical knowledgeable
« Reply #6 on: August 14, 2019, 06:04:35 AM »
You are planning method 3, I believe, with your bus bar setup.  Method 4 is overly complex and achieves nothing over 3.  The issues are two-fold - the internal resistance of the batteries, and the resistance of the cables.  At the end of the day, what you are doing is best.
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Offline chessie4905

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Re: Question for the electrical knowledgeable
« Reply #7 on: August 14, 2019, 06:08:42 AM »
As the last two provide perfect balance, number 4 makes all the connection points at the batteries, not at an off battery connection point. Either should be fine. Whichever works for you.
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Offline bevans6

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Re: Question for the electrical knowledgeable
« Reply #8 on: August 14, 2019, 10:26:45 AM »
scenario 3 and 4 are not the same electrically, even if they both balance current draw from each battery.  In the #4 scenario, the two cables that connect two batteries directly will carry 1/4 of the total current, the two cables that connect two pairs of batteries will carry 1/2 the total current, and of course the two cables that connect to the load will carry 100% of the current.  In #3, all the battery cables carry the same current, 1/4 of the total.  For the OP, wanting to use #4 cable to wire all the batteries the third scenario works fine but the fourth scenario requires #2 cable for the middle pair of cables that carry 1/2 the current.
1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Allison MT-647
Tatamagouche, Nova Scotia