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Old 03-26-2019, 08:03 AM   #1
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How to connect 24v to 12v step down?

Trying to figure out where to connect the step down converter to my system and also if i need any type of fuse going into the converter? Do i need some type of terminal before the inverter, do go directly to the batteries?

Any help greatly appreciated.
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Old 03-26-2019, 08:18 AM   #2
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I would run a 12V distribution panel and/or multiples of them in strategic locations.

All depends on amps required and where.

Every wire should be protected, close as possible to the source side.

Blue Sea makes great stuff.
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Old 03-26-2019, 08:35 AM   #3
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I would tap into 24V between battery and fuse for inverter with the correct size fuse for the converter.
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Old 03-26-2019, 09:34 AM   #4
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Agreed.

I would also recommend leveraging 24v directly as much as you can.

I am running 24v house batteries and, so far, my only loads that require 12v are my Webasto and my smoke/CO detector.

Lights, USB chargers and water pump are all 24v.
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Old 03-26-2019, 09:42 AM   #5
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Thanks,

by tap into what does that mean?
Can i come directly off the battery posts for both and just fuse the +?
I have a 30amp converter, what size wire/fuse should that dictate?

unfortunately it was a little last minute that i decided to go 24v so i had already purchased 12v items.
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Old 03-26-2019, 10:04 AM   #6
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Connect to the battery. Fuse at the battery with a fuse sized to protect the wire.
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Old 03-26-2019, 10:05 AM   #7
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I mounted our 24 to 12 volt converter very close to our webasto. It is dedicated to only the webasto. The webasto is sensitive to voltage drop during the start up phase when the glow plug takes a lot of current.

I would also use a fuse in each separate battery string so that a broken / shorted cell cannot cause an internal dangerous current inside your battery bank.

Our labtops and TV screen ( LCD monitor) are ran of a separate 24 to 19 volt converter that we switch off when not in use as to reduce parasitic losses. All other users run of 24 volt directly... Fridge freezer lights waterpump.

Good luck j
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Old 03-26-2019, 11:34 AM   #8
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Originally Posted by joeblack5 View Post
I mounted our 24 to 12 volt converter very close to our webasto. It is dedicated to only the webasto. The webasto is sensitive to voltage drop during the start up phase when the glow plug takes a lot of current.

I would also use a fuse in each separate battery string so that a broken / shorted cell cannot cause an internal dangerous current inside your battery bank.


Good luck j
Good point.

I would recommend not having any parallel battery strings. That eliminates a number of potential issues.

2v,4v and 6v batteries are readily available up to 1000 a/h. Four 400 a/h L-16's turned out to be a good fit for my install. 24v - 400a/h and no parallel strings.
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Old 03-26-2019, 12:04 PM   #9
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I would recommend not having any parallel battery strings. That eliminates a number of potential issues.
j
I understand this, but already have the batteries unfortunately
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Old 03-26-2019, 12:05 PM   #10
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Originally Posted by joeblack5 View Post
I would also use a fuse in each separate battery string so that a broken / shorted cell cannot cause an internal dangerous current inside your battery bank.

j
Thanks,
I have not heard of this.
How large of a fuse do I need here, is there a specific connector to use?
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Old 03-26-2019, 12:09 PM   #11
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Current bank setup
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Old 03-26-2019, 03:14 PM   #12
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To keep the currents in each battery string the same the resistances in the cables need to be the same. At very large inverter current that is complicated since it depends on the quality of the crimps and the cleanliness and flatness of the eye connections. Personally I would run separate wires from each battery string with a 100 amp fuse at each 24v positive terminal. So 4 wires total and make the connection at the inverter. From there I would take one conductor towards your load center.
You can test the quality of your setup with a clamp on DC amp meter ( hall sensor) to check if the current in both parallel strings are the same.

If you want to go better then insert a shunt resistor that can be connected to an Ah gauge setup. If you want to be at the " best" level then you need a separate shunt in each battery string with each a separate Ah gauge to monitor each string separately.
You can find used quality Ah gauges and shunts on fleabay for about $50 to $75.

Good luck. Johan
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Old 03-26-2019, 06:16 PM   #13
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Fusing within the bank just adds complication and resistance, one master CP at the bank output (within 7" is ABYC spec) is fine, also an emergency cutoff.

Paralleling is fine to boost Ah capacity, also helps with redundancy and keeping weight per unit down, just best to keep the strings below 3 to avoid imbalance issues.

Set up a 24V loads buss and distribution boxen, then a 24-12V converter between there and each 12V distribution point.

Victron makes Orion converters, as well as other more fancy one good to use as a DCDC charger also if you need a dedicated 12V battery e.g for nav electronics or something high current, big-amp converters get pricey.

Mastervolt, Samlex all the usual marine suspects.

Watch out for cheap Chinese gear, derate by like 80% and carry spares, best to just avoid IMO.

And btw all these complications and expense can be eliminated by only going higher than 12 V if you have a compelling reason to do so.

Saving on wiring gauge is not that.
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Old 03-26-2019, 06:19 PM   #14
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this is canonical http://www.smartgauge.co.uk/batt_con.html
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Old 03-26-2019, 07:57 PM   #15
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Quote:
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Fusing within the bank just adds complication and resistance, one master CP at the bank output

I am sorry John but I have to disagree. Each series string is its own bank. Two series strings in parallel ..two fuses.

The higher resistance argument does not hold a lot of water since the fuses currents are in theory half the total current. With Thevenin theorem the combined fuses resistance is also half. Using two half amperage fuses gives you the added benefit that if one string has failed ( cable damage. Cell damage...acid loss..) That the whole inverter current is now on a single string and on a single half amperage fuse. That should fail and so bring the problem / defect to the surface so that it can be repaired.
With respect Johan
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Old 02-21-2021, 12:16 AM   #16
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I'm just trying to use 12v brake lights and marker lights in my mci-9 not 24v
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Old 02-21-2021, 06:30 AM   #17
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I'm just trying to use 12v brake lights and marker lights in my mci-9 not 24v
Hi Frank, you'll certainly get overall info here...but if you want specifics on the MCI coaches you ought to check out the Bus Conversion Magazine forum over here: https://www.busconversionmagazine.com/forum/index.php

We have an MC-7, but haven't converted the lights yet. I have read some great discussions of that process over at BCM.
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Old 02-21-2021, 09:42 AM   #18
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Originally Posted by rossvtaylor View Post
Hi Frank, you'll certainly get overall info here...but if you want specifics on the MCI coaches you ought to check out the Bus Conversion Magazine forum over here: https://www.busconversionmagazine.com/forum/index.php

We have an MC-7, but haven't converted the lights yet. I have read some great discussions of that process over at BCM.
Thanks for the info I plan on using all of the 24v and just need 12v for the lights and utilitie trailer Thanks again
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Old 02-21-2021, 10:20 AM   #19
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Wiring for balanced battery bank.

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Thank you for this site, really helpful! Spoiler alert - diagram for recommended balanced load on multiple batteries.

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