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Battery Disconnect Switch OR Resistor?

TedH

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Jun 5, 2020
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In my current RV solar power wall, I use a 600amp blue sea rotary switch as a quick connect between the Lifepo4 battery and the rest of the 24vdc system (inverter charger, solar charge controller and secondary 24vdc charger). I turn the blue sea switch "ON" before I switch the Aims inverter "ON" and switch the PV breakers on that feed solar to the solar charge controller. When I upgraded from 24vdc/200ah to 24vdc/600ah, my 300amp blue sea switch would overheat and fail. The 600amp switch appears to handle the 'rush'. It still makes a hot sound when the amps rush through it initially. I then turn on the Aims inverter.

Q: Before I switch to my Victron Multiplus II 24v 2x120 inverter/charger, do I need to add a resistor to bypass the power switch and pre-charge my Victron inverter capacitors? Does my blue sea switch act as a sufficient resistor between the battery and inverter, rest of the 24vdc systems?

NOTE: I 'assumed' the switch provides the resistance/protection given it has 'ignition protection' built in. My Aims inverter/charger has lived happily in this configuration since 2021.

From the AI on Google search: "The Blue Sea Systems E-Series Master Battery Switch Single Circuit is typically used between a battery bank and an inverter. It is slightly larger and has a higher current rating than the High Current On/Off Mini Switch (6006). It is recommended to use this switch with 3000VA inverters."
 
The amount of batteries have no effect on the size of wiring or the on/off switch. Amp hour is different than amp.

Now you have more batteries the recharge load is more spread out and might be fine without a resistor. Not sure if the bluesea has anything to do with the precharge.

I've always turned on my solar and let it do the precharge then turn on the batteries.
 
The amount of batteries have no effect on the size of wiring or the on/off switch. Amp hour is different than amp.

Now you have more batteries the recharge load is more spread out and might be fine without a resistor. Not sure if the bluesea has anything to do with the precharge.

I've always turned on my solar and let it do the precharge then turn on the batteries.
My Aims solar charge controller installation manual requires battery connection before PV breakers turn on "to protect the solar charger from damage". It appears to require the battery power to operate properly. It shuts down when/if my battery goes offline (I assume to protect the solar charge controller). BMS shuts battery down if protection parameters are exceeded.
 
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My Aims solar charge controller installation manual requires battery connection before PV breakers turn on. It appears to require the battery power to operate properly. It shuts down when/if my battery goes offline (I assume to protect the solar charge controller) due to protection imposed by the BMS.
How does it know? If you turn on your batteries and it's sunny out then you turn off the batteries I'd assume the mppt wouldn't know the batteries are off as it's outputting the voltage.

I've ran my batteries empty and the solar has turned my system back on but now I'm not sure if the batteries went to 0 or just below 5% where the inverter shuts off.
 
In my current RV solar power wall, I use a 600amp blue sea rotary switch as a quick connect between the Lifepo4 battery and the rest of the 24vdc system (inverter charger, solar charge controller and secondary 24vdc charger). I turn the blue sea switch "ON" before I switch the Aims inverter "ON" and switch the PV breakers on that feed solar to the solar charge controller. When I upgraded from 24vdc/200ah to 24vdc/600ah, my 300amp blue sea switch would overheat and fail. The 600amp switch appears to handle the 'rush'. It still makes a hot sound when the amps rush through it initially. I then turn on the Aims inverter.

Q: Before I switch to my Victron Multiplus II 24v 2x120 inverter/charger, do I need to add a resistor to bypass the power switch and pre-charge my Victron inverter capacitors? Does my blue sea switch act as a sufficient resistor between the battery and inverter, rest of the 24vdc systems?

NOTE: I 'assumed' the switch provides the resistance/protection given it has 'ignition protection' built in. My Aims inverter/charger has lived happily in this configuration since 2021.

From the AI on Google search: "The Blue Sea Systems E-Series Master Battery Switch Single Circuit is typically used between a battery bank and an inverter. It is slightly larger and has a higher current rating than the High Current On/Off Mini Switch (6006). It is recommended to use this switch with 3000VA inverters."
It’s not just about the switch being able to handle the inrush, it’s about not blowing up the capacitors inside chargers, inverters or the FETS in a bms. Always precharge before closing a DC battery circuit or follow the directions for batteries equipped with pre charge. Unless it’s an emergency, reduce load or turn off the equipment before opening the disconnect.
 
It’s not just about the switch being able to handle the inrush, it’s about not blowing up the capacitors inside chargers, inverters or the FETS in a bms. Always precharge before closing a DC battery circuit or follow the directions for batteries equipped with pre charge. Unless it’s an emergency, reduce load or turn off the equipment before opening the disconnect.
How does that work specifically? Inverters are charging capacitors so one shouldn't be able to negatively affect one in another device, right? The FETS and such are specifically designed to protect the batteries so should be able handle any inrush
 
$280 solves all these issues. I got tired of BMS's shutting down so installed this beast. It has been 100% reliable.


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How does that work specifically? Inverters are charging capacitors so one shouldn't be able to negatively affect one in another device, right? The FETS and such are specifically designed to protect the batteries so should be able handle any inrush
The FETS are there to shut down as a last resort and the inrush is massive and so fast that it’s usually waaay more than they are rated for. Hold down a momentary switch for 5 seconds to a 25 watt 30 ohm resistor right before you close the switch is cheap insurance. There’s just no need to beat up your system. When a charger powers up from the AC side, you may hear a groan from an inrush but the power line and transformer greatly ease this in comparison to the potential of a battery. IMG_1370.jpeg
 
The FETS are there to shut down as a last resort and the inrush is massive and so fast that it’s usually waaay more than they are rated for. Hold down a momentary switch for 5 seconds to a 25 watt 30 ohm resistor right before you close the switch is cheap insurance. There’s just no need to beat up your system. When a charger powers up from the AC side, you may hear a groan from an inrush but the power line and transformer greatly ease this in comparison to the potential of a battery. View attachment 210294
The precharge works excellent and is a LOT cheaper than that current limiter. I had one in my camper but it was not wife proof so I had to replace it with the $280 wife proof version. Mine was in a little project box with a pretty red button that said "Push First". But she didn't. ANd both batteries shut down and I was off doing something else and she had no power and she asked me why we spent $56k on a trailer that didn't work.
 
$280 solves all these issues. I got tired of BMS's shutting down so installed this beast. It has been 100% reliable.


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Is this always connected? If so, are similar 'DC surge limiters/battery protectors' from Victron and others sufficient for less than $280-300?
 
The precharge works excellent and is a LOT cheaper than that current limiter. I had one in my camper but it was not wife proof so I had to replace it with the $280 wife proof version. Mine was in a little project box with a pretty red button that said "Push First". But she didn't. ANd both batteries shut down and I was off doing something else and she had no power and she asked me why we spent $56k on a trailer that didn't work.
Haven’t found anything wife proof or ease the pain of a verbal dig. Yup got the button on the box too.IMG_1371.jpeg
 
Is this always connected? If so, are similar 'DC surge limiters/battery protectors' from Victron and others sufficient for less than $280-300?
This is always connected. It has a lot of electronic bits sandwiched between the giant aluminum thingies.

Don't know about other options. I found this one, bought it, it works so I stopped looking.
 
The FETS are there to shut down as a last resort and the inrush is massive and so fast that it’s usually waaay more than they are rated for. Hold down a momentary switch for 5 seconds to a 25 watt 30 ohm resistor right before you close the switch is cheap insurance. There’s just no need to beat up your system. When a charger powers up from the AC side, you may hear a groan from an inrush but the power line and transformer greatly ease this in comparison to the potential of a battery. View attachment 210294
This appears wired to bypass the Battery on/off switch and to be activated only when the button is pushed to precharge. I assume I can set up with permanent connections by connecting to the in and out studs on my battery switch and put in a small plastic box/container on my power wall next to my battery on/off switch.

I see a similar resistor listed on Amazon: https://www.amazon.com/uxcell-Aluminum-Resistor-Wirewound-Resistors/dp/B07D1ZTKDS

Will this on/off button switch work: https://www.amazon.com/DIYhz-Momentary-Pushbutton-Switches-Normally/dp/B08B1PWJ21/ref=sr_1_15?crid=DO7O5I269Q4V&dib=eyJ2IjoiMSJ9.DuY35t9xrPEV4SUfXOIzPsWbXFsMQ7EguDu-VeN3RKTpX9oJS-knnp8kFOm1Nkl8Ex-1qdOwocTj2CVQbkWDASSw1rs0w94uxtbW4Rh1A2ZMGFNYjox3TGASpWt6jlfhvPpSMKJQVyvZ-pF3PskRvcO3MmWu9ArMyM6P4LqXCD-XCoMYzClVFrn7a13GQ1c1ub9vAy9qaeuXlj0TuoCYiZVPCKmx6c_PLCOezRoVdcVdfC2SqdzxFu8hHEePMOJJLfPupU6ej8xpgRO8a97uLH03sc8JQ0ctWhYp-yi5yoU.p2fI8DoVI_8BR0O75emHN3yIGlgZoakahxFNqAaBx4I&dib_tag=se&keywords=25+watt+button+switch&qid=1713548004&s=industrial&sprefix=25watt+button+switch,industrial,122&sr=1-15
 
Is this wired to bypass the Battery on/off switch and to be activated only when the button is pushed to precharge? I assume I can set up with permanent connections by connecting to the in and out studs on my battery switch and put in a small plastic box/container on my power wall next to my battery on/off switch.

I see a similar resistor listed on Amazon: https://www.amazon.com/uxcell-Aluminum-Resistor-Wirewound-Resistors/dp/B07D1ZTKDS

Will this on/off button switch work: https://www.amazon.com/DIYhz-Moment...x=25watt+button+switch,industrial,122&sr=1-15
That's what I used. I had it wired to my battery on/off switch. It's not a blue smoke hazard because the amps are so low. Just don't hold the switch in for a long time. One second does the trick.
 
Is this wired to bypass the Battery on/off switch and to be activated only when the button is pushed to precharge? I assume I can set up with permanent connections by connecting to the in and out studs on my battery switch and put in a small plastic box/container on my power wall next to my battery on/off switch.

I see a similar resistor listed on Amazon: https://www.amazon.com/uxcell-Aluminum-Resistor-Wirewound-Resistors/dp/B07D1ZTKDS

Will this on/off button switch work: https://www.amazon.com/DIYhz-Moment...x=25watt+button+switch,industrial,122&sr=1-15
Correct on all accounts. Even though the switch isn’t rated for DC, the resistor limits the arc within.
 
Just for giggles;
After installing the discharge circuit, put a volt meter across the terminals of the open disconnect switch and note the voltage. Now push the momentary switch and see how fast voltage drops. Release and notice how slowly the voltage climbs back up. This gives you an idea of how much time you have to work with.
 
Do make sure the switches you get are momentary, normally open. It isn’t perfect, it’s Amazon 😂
 
Correct on all accounts. Even though the switch isn’t rated for DC, the resistor limits the arc within.
I don't understand this precharge. Can you explain why, and what is going on? Is this needed for all inverters, or just a certain size? Thanks.
 
I don't understand this precharge. Can you explain why, and what is going on? Is this needed for all inverters, or just a certain size? Thanks.
The input capacitors on inverters can draw a lot of current really fast when they are charging. LiFePO4 batteries can provide a lot of current really fast. The BMS might see this as a fault and put the BMS into protect. Now you have to find a way to wake up the BMS which is not as simple as it sounds.

My SOK batteries had no issues with a 1200W or 2000W Xantrex inverter. They really did not like my Victron MPII 3000. They went into protect every time. The precharge circuit charges the inverter caps before you connect the batteries. A simple, elegant and cheap solution.

If you forget to precharge, you will have the pleasure of tracking down a 15V DC power source to wake them up. My solar wouldn't wake them up. My truck charge line wouldn't wake them up. A smart 120V battery charger wouldn't wake them up. I made a small battery pack with a buck converter to wake them up. What a giant PITA.
 
A-B-off battery switch could be wired up with the resistor.
That is a great option, too. I had it that way and my wife went to B instead of A first. Arrrrrgggghhhhh!!!! I guess I needed an interference method so it had to go through A before A+B. I could see her just switching it to A and leaving it there and then telling me the power didn't work again.
 
A downside to the precharge circuit is if the inverter decides it needs to reset for some reason. When it powers back on, it may send the BMS(s) into protect. That happened to my wife as well.
 
The input capacitors on inverters can draw a lot of current really fast when they are charging. LiFePO4 batteries can provide a lot of current really fast. The BMS might see this as a fault and put the BMS into protect. Now you have to find a way to wake up the BMS which is not as simple as it sounds.

My SOK batteries had no issues with a 1200W or 2000W Xantrex inverter. They really did not like my Victron MPII 3000. They went into protect every time. The precharge circuit charges the inverter caps before you connect the batteries. A simple, elegant and cheap solution.

If you forget to precharge, you will have the pleasure of tracking down a 15V DC power source to wake them up. My solar wouldn't wake them up. My truck charge line wouldn't wake them up. A smart 120V battery charger wouldn't wake them up. I made a small battery pack with a buck converter to wake them up. What a giant PITA.
Is the 15V DC power source required for a 12v Multiplus II and not the 24V Multi?
 
Is the 15V DC power source required for a 12v Multiplus II and not the 24V Multi?
It has to do with the battery BMS. My 12V SOK batteries would not wake up until I applied 14.8V or more. I used a bench power supply to find out what it took to wake them up.

I'm sure all BMS's are different. Maybe not. You would have to get your BMS to go into protect then put a power supply on them to see what it takes to wake them up.
 
A downside to the precharge circuit is if the inverter decides it needs to reset for some reason. When it powers back on, it may send the BMS(s) into protect. That happened to my wife as well.
When the inverter takes a break, ya gotta remove the loads immediately before it restarts.
Is wife proofing even possible?
I highly recommend one of these to keep an eye on your load. You can configure a contact to operate a warning device at a selected current level
 
When the inverter takes a break, ya gotta remove the loads immediately before it restarts.
Is wife proofing even possible?
I highly recommend one of these to keep an eye on your load. You can configure a contact to operate a warning device at a selected current level
Wife proofing is expensive. But I really want her to like glamping so she will go on the road with me.
 
And just to be clear, not all BMS's have issues with all inverters. Had I stopped at the Xantrex 2000W inverter, I would have never known about precharge circuits or sleeping BMS's.
 
It has to do with the battery BMS. My 12V SOK batteries would not wake up until I applied 14.8V or more. I used a bench power supply to find out what it took to wake them up.

I'm sure all BMS's are different. Maybe not. You would have to get your BMS to go into protect then put a power supply on them to see what it takes to wake them up.
Wow that’s a lot of voltage to wake up. I’ve heard of some that need this.
I friend got a bunch that the previous owner thought were dead but they just needed waking up.
 
It has to do with the battery BMS. My 12V SOK batteries would not wake up until I applied 14.8V or more. I used a bench power supply to find out what it took to wake them up.

I'm sure all BMS's are different. Maybe not. You would have to get your BMS to go into protect then put a power supply on them to see what it takes to wake them up.
Gotcha. I have a wake-up button (I have to swap it in after removing UART Bluetooth dongle - v2 BMS limitation) on my Daly BMS for that situation.
 
Haven’t found anything wife proof or ease the pain of a verbal dig. Yup got the button on the box too.View attachment 210296
Can you share what is inside with parts list? VERY clean solution! I like the indicator light to show the voltage difference. Does the light go out once the voltage is equalized/capacitors are charged?
 
The FETS are there to shut down as a last resort and the inrush is massive and so fast that it’s usually waaay more than they are rated for. Hold down a momentary switch for 5 seconds to a 25 watt 30 ohm resistor right before you close the switch is cheap insurance. There’s just no need to beat up your system. When a charger powers up from the AC side, you may hear a groan from an inrush but the power line and transformer greatly ease this in comparison to the potential of a battery. View attachment 210294
Instead of using a 25 watt resistor, I use an incandescent automotive lamp. You push the switch button and the light turns on an then immediately starts to dim as the voltages even out. When sufficiently dim, let go of the switch. The big functional advantage of the light bulb, versus a resistor, is the visual feedback that confirms the pre-charge circuit is working and knowing when the voltages are sufficiently even. You can get automotive bulbs in 12V (obviously), 24 V, and 48V. The 48V bulbs are sold on Amazon as "forklift bulbs"
 
In my current RV solar power wall, I use a 600amp blue sea rotary switch as a quick connect between the Lifepo4 battery and the rest of the 24vdc system (inverter charger, solar charge controller and secondary 24vdc charger). I turn the blue sea switch "ON" before I switch the Aims inverter "ON" and switch the PV breakers on that feed solar to the solar charge controller. When I upgraded from 24vdc/200ah to 24vdc/600ah, my 300amp blue sea switch would overheat and fail. The 600amp switch appears to handle the 'rush'. It still makes a hot sound when the amps rush through it initially. I then turn on the Aims inverter.

Q: Before I switch to my Victron Multiplus II 24v 2x120 inverter/charger, do I need to add a resistor to bypass the power switch and pre-charge my Victron inverter capacitors? Does my blue sea switch act as a sufficient resistor between the battery and inverter, rest of the 24vdc systems?

As others have pointed out, you really need to precharge (for SECONDS) with the resistor to prevent catastrophic damage to the inverter's circuit boards, wiring, and the switch contacts from massive overcurrents as the giant capacitors are suddenly charged from giant batteries. When you connect a capacitor to a hard voltage source (without inductance between - which causes other problems), you dissipate as much energy in the wiring as you store or remove from the capacitor, and the lower the resistance, the higher the current (up to near infinity if the resistance is very low). You want to dissipate that energy in a resistor, which is DESIGNED to turn electrical energy into heat without slagging down in the process, rather than in the circuit board traces, the capacitor "plate" foils, and the tiny area of the switch contacts' initial touch, and do it slowly enough that the resistor can handle the job.

Since you're using a rotary battery switch for shutdown, here's a hack. (It's cheap, except that you may need to buy and swap in a different model of the switch.):

* Use a two-battery selector/combiner version of the switch.
* Hook the battery to the "battery two" connection.
* Hook the precharge resistor between the battery one and battery two connections.

So setting 1 is precharge, setting 1+2 and 2 are battery. When turning on switch from off to 1, wait a few seconds, switch to 1+2 (then onward to 2 if you feel like it). Turning off: switch to 2 and then off (so you don't switch to "resistor in circuit" when there is high current or charging current running through the battery line.)

If your "resistor" is an incandescent bulb, I'd wait until it was too dark to see for at least as long as it had been visibly lit after you started precharging. The precharge goes slower as it approaches even, and switching to zero resistance even when the voltage mismatch is small may still produce damagingly high currents.

NOTE: I 'assumed' the switch provides the resistance/protection given it has 'ignition protection' built in. My Aims inverter/charger has lived happily in this configuration since 2021.

Not sure, but I think "ignition protector" is a metal oxide varistor from the output terminal to ground, to keep the alternator voltage from spiking and burning out your electronics (including maybe the alternator's regulator) if you switch the batteries off when the engine is running (in a typical boat installation). The battery load normally clamps the voltage so that it doesn't go too high, but disconnecting it suddenly while the alternator is charging the battery will cause the voltage to go 'way up for much of a second while the regulator tries to reduce the excitation (which takes a while because of the inductance of the field winding).
 
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If possible, my preference is to have the solar always charging the battery pack - 100% of the time.

If you just turn the inverter on / off by itself, then you don't have to worry about the pre-charge.
 
If possible, my preference is to have the solar always charging the battery pack - 100% of the time.

If you just turn the inverter on / off by itself, then you don't have to worry about the pre-charge.
What about night time?
 
If possible, my preference is to have the solar always charging the battery pack - 100% of the time.

If you just turn the inverter on / off by itself, then you don't have to worry about the pre-charge.
Really? So, as long as the inverter switch is OFF when I turn the battery switch on, there is no risk to damaging the inverter? Then, I just turn the inverter on after turning the battery on? I thought purpose of precharger was to prevent a surge when turning the inverter on.

My current system using Aims inverter/charger and Aims solar charge controller is on 24x7. The only time it is 'down' is if my BMS shuts bank down due to protection fault (RARE) or if I shut the system down for maintenance. I am preparing to add the Multiplus II and Smart Solar MPPT in place of the Aims to get VRM monitoring of AC IN/OUT, Solar PV and DC load.
 
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Really? So, as long as the inverter switch is OFF when I turn the battery switch on, there is no risk to damaging the inverter? Then, I just turn the inverter on after turning the battery on? I thought purpose of precharger was to prevent a surge when turning the inverter on.

My system is on 24x7. The only time it is 'down' is if my BMS shuts bank down due to protection fault (RARE) or if I shut the system down for maintenance.
It is. Won't help to have the inverter off before turning on batteries and then turn the inverter on. At least not with my MPII. I tried that.
 
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