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Small Cabin Forum / Off-Grid Living / Battery disconnect
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# Posted: 28 May 2019 19:31

So I have a question about the location of my battery disconnect.. when I set up my little system I put the battery disconnect between the busbar and the battery. This isolated my battery but left my inverter connected to my charge controller

When I purchased an additional battery and decided to integrate it in parallel I reread my original instructions and they clearly said to put the disconnect between the charge controller and the busbar. This leaves the inverter connected to the battery at all times

Now I'm wondering if I should go back to isolating the battery with the switch and possibly putting another switch to the additional battery.

Any thoughts about the pros and cons?

# Posted: 28 May 2019 23:19

There should be two disconnects using a circuit breaker or a fuse.

Between the solar controller and the battery its quite typical to use a circuit breaker. Use a cb when the load will be switched more often.

A fuse is more likely between the inverter and the battery. The switch is on the inverter. (And there's probably already a fuse inside the inverter.)

Why no circuit breaker to the inverter? Because someone might use it to disconnect the power to the inverter.

It is always safest to use the power switch on your inverter to turn off power from the battery. There's a little computer in the inverter that makes sure you don't switch off during the fridge starting or while the inverter is working hard, thereby damaging a valuable piece of equipment. The inverter shuts itself down softly, as it were.

So that's why you can use a circuit breaker between your solar controller and the battery (which is probably also fused); and a fuse between the inverter and battery.

Now. Many people are starting to put circuit breakers on their battery modules when using lithium. This allows you to easily add or subtract a module. Important if you move your system. It also means your battery is isolated from the system until you flip the cb on. And if a power surge hits your cb will also protect the battery.

I use the QO series of load centers and cbs as they are DC rated to 48v officially and to 125v by Schneider. Note: only QO and I think QUO are DC rated.

Does this help?

# Posted: 29 May 2019 06:13 - Edited by: Wilbour

I guess it helps. If you see the attached drawing....

When I had one battery I had the disconnect in position #2

My original schematic had directed me to put it in position #1 which works well if you have more than one battery going to the bus bar.

I do see many people linking battery to battery in parallel but I worry about the charge/discharge being out of sync from one battery to another. Therefore using the bus bar seems to be a better way.

If I revert back to my original setup, isolating the battery on it's own, then I would need an additional disconnect.

I see creeky what is saying about leaving the connection from the battery(ies) to the inverter live and just using the toggle on the inverter itself to isolate the inverter when needed.

# Posted: 29 May 2019 06:30

OOOO, me like this

# Posted: 29 May 2019 07:59

Creeky.... do you have a link to Schneider to show their QO breakers will take as much as 125v DC? Or some other document? I only ask as I'm afraid the electrical inspector might bulk at using these ac breakers (they say right on them rated to 48v DC) when charging volts will be higher than 48v dc.

Thank you!

# Posted: 29 May 2019 08:07 - Edited by: Steve_S

I have a Blue Sea E-Series switch on the battery bank to allow me to entirely isolate the battery bank (2 strings of 4 for 24v) from the DC Panel. Blue Sea switches are great !

2 Parallel strings is not a problem and pretty much accepted. But when you want 3 or more strings, every battery maker will tell you to go with a Bus Bar setup and there's no problem if you want to do that with 2 strings either.

My System diagram with 3kw inverter

I chose to do it this way to ensure that when I want DC in OFF then it is OFF everywhere ! There a lot of amps waiting to fry anything at any opportunity and I don't want to be that "anything" .

Now I am switching inverters and so a few things that go with it to a new Samlex EVO-4024 and my diagrams will be adjusted accordingly. One of the "Big Changes" is that I am now adding two AC Breakers into the mix, as recommended by both Samlex & MidNite (my controller is a Classic 200). Because I have Genny Power (L5-30) coming in to the Inverter/Charger, I am installing a Square D 40A single pole Breaker between the Generator & the Charger input. On the AC Output side I'm installing a Square D 45A Single Pole Breaker also in a Square D QO4L-100S (was the cheapest) (believe it or not, the two hole units were $20 more ! so.... room for expansion later I guess, IF needed. And the US prices are 400% LESS ! can get a 2 hole subpanel for $15 there … ugh !

A note of caution for folks on Inverters, Wilbour likely already knows this so this is a General Note. Inverters have fair sized capacitors in them that HOLD quite a bit of juice. Even if the unit is powered off and DC is disconnected, there is residual power in there and it can deliver quite a huge jolt !

Hope it helps,

QO140 - Square D 40 Amp Single Pole Circuit Breaker
QO145 - Square D 45 Amp Single Pole Circuit Breaker * not std stock in stores, uncommon.
QO4L100S 120/240 AC Sub-Panel
Schneider Dual Rated AC/DC Circuit Breakers info
Are QOU breakers listed for use on a DC voltage system ? FAQ, more info there in FAQ Section

# Posted: 29 May 2019 09:21

You got it right there Wilbour. Perfecto.

QO has a wide range of boxes as shown above.
Pick the one that matches your use/loads. Remember, the ratings are continuous, so go by your max continuous draw. Not surge. W=VxA
So a 1000w inverter uses 24vxXa. A = 40. A QO145 would be good.

Nobadays. 12, 24 and 48 are classes. So a 12v rating is actually 13.6. etc. I know. Its very weird. That's DC. So 48 class covers 36-66v. Even 72.

In Steve's system, he's wiring for lead. He has two parallel strings and I would like to see both strings on breakers/fuses. Separately.

For lithium, especially systems that are portable like Wilbour's, it's easier to wire to a bus bar. Makes adding and subtracting devices easier.

And that's what breakers, as shown in wilbours diagram are good for. Safer and easier to add battery modules.

Note that wilbour's system also has a 24v device use. Lights for sure. Not sure about a pump. But see how much easier it is to add that into the system with a bus bar.

Blue seas has really lovely DC fused bus bars for those loads. Watch the Amps rating on your use.

Summer time! Folks are building! Huzzah!

# Posted: 29 May 2019 09:28

"Nobadays. 12, 24 and 48 are classes. So a 12v rating is actually 13.6. etc. I know. Its very weird. That's DC. So 48 class covers 36-66v. Even 72."

Got it! I saw on the Schneider site that the QO breakers are made for DC voltages between 48v - 125v DC....double pole breakers for higher than 48v DC.

I suspected that the 48v rating was conservative of what it is capable of doing. Hopefully my electrical inspector won't question using these breakers.

Thank you!

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