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Questions about fuses for inverter and converter

Oranjoose

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RV application. Currently I have a 280ah 12v battery and 120a Overkill bms, as well as a WFCO "power center" that distributes AC and DC power, along with breakers. I want to be able to grow the system in the future, so I'm planning to buy a 3000 watt inverter, and later build another 12v 280ah battery with another 120a Overkill bms.

My questions surround fuses for my situation now and where I intend the system to go. Each Overkill has a max amp cut-off of 130 amps, and so I won't expect to draw more than around 1200 watts per battery. Part of my confusion lies in that the power draw is across both the inverter and the WFCO 12v power distribution. The WFCO can supply around 900 watts from 12v DC (for the 12v appliances and lights).

Do I connect each battery positive and BMS negative to a separate fuse? Here's a terrible diagram of that:
1618185881222.png
Or can (or should) I join each respective battery positive and BMS negative to respective fuses? A terrible diagram of that example is below:

1618185993892.png
Do either of these even properly wire these batteries in parallel? The RV's wiring already has leads to go to the battery for 12v power, which run through busbars and the power center. Do I even need to add fuses then, since I presume the wiring has already set something up adequately?

I suppose I'm not exactly sure the best way to parallel the BMSes, and so I might have done that wrong. I'd also love to add solar with an MPPT controller, which further complicates my understanding of how the wires and fuses hook up.

I'd appreciate any guidance here.
 
I did not do the fuse rating calculation, but quick notes for the fuse on 12V 3000W Inverter will be about 300A fuse due to conversion loss. You do not need to have fuses on the negative.
 
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What size wire? The WFCO has fuses and you should have same rating near the battery. No fuse needed on the negative side.
 
Thanks for the tip about fuse only on positive. I'm still confused about a couple things.

1. Where all the wires go. With an inverter, DC power center, MPPT solar controller, and at least two negatives coming out of each BMS, that's a lot of wires to attach.

Should I get two busbars, one for positive and one for negative? This way, I'd attach the positives from each battery, the inverter, the DC power center, and the MPPT to one busbar, and all the respective negatives to the other busbar? Would that parallel the batteries effectively and get everything to work properly?

2. Where the fuses go. Considering busbars in item 1 above, should I connect one fuse out of each battery to the positive busbar at the capacity that adds up the draw from the inverter and DC power center? Or should I have no fuse from batteries to busbar, but rather one 400 amp fuse from busbar to inverter, etc.?

@time2roll That is correct that the WFCO power center has its fuses for the various appliances it powers. Maybe that means I can get away with no fuse from the busbar (as asked about above) to the power center? I haven't checked the wire gauge of wires running from the WFCO power center to the battery. If I remember correctly, they look to be around AWG 4?
 
Thanks for the tip about fuse only on positive. I'm still confused about a couple things.

1. Where all the wires go. With an inverter, DC power center, MPPT solar controller, and at least two negatives coming out of each BMS, that's a lot of wires to attach.

Should I get two busbars, one for positive and one for negative? This way, I'd attach the positives from each battery, the inverter, the DC power center, and the MPPT to one busbar, and all the respective negatives to the other busbar? Would that parallel the batteries effectively and get everything to work properly?

2. Where the fuses go. Considering busbars in item 1 above, should I connect one fuse out of each battery to the positive busbar at the capacity that adds up the draw from the inverter and DC power center? Or should I have no fuse from batteries to busbar, but rather one 400 amp fuse from busbar to inverter, etc.?

@time2roll That is correct that the WFCO power center has its fuses for the various appliances it powers. Maybe that means I can get away with no fuse from the busbar (as asked about above) to the power center? I haven't checked the wire gauge of wires running from the WFCO power center to the battery. If I remember correctly, they look to be around AWG 4?
yes, they look to be around AWG 4
 
@orangejoose-

Everybody here will help, but I think you need to regroup and lets us know what you actually want to grow.
Specifics make a big difference when fusing. Distance matters

At the minimum, your RV provides 3600 watts AC on shore power.
Adding a 3000 watt inverter does not add to that, it's a substitute for when no shore power is available.
You need two Overkill BMS batteries to operate a 3000 watt inverter, so build the second battery then get the inverter.
If you plan on using the 3000 Watt inverter in a limited capacity now, you would fuse it much differently than when you use two batteries.

Your RV is wired and fused correctly for whatever 12V battery you want to put in it, lead or lithium.
Does the "power center" have a lithium compatible converter?
If it does, then it doesn't even come into the picture unless we're talking about feeding it through a transfer switch from the inverter.

When it comes to wire selection and fusing, there are calculations for planning and calculations of practical use.
The two don't always get the same result.

Solar too? What RV are we talking about?
 
You can use the fuse holder terminal to combine wire connections.
Typical 55 amp WFCO would come with #6 wire to the battery.
 
@Oranjoose Is it a high frequency or low frequency inverter?
Is it an inverter/charger or just an inverter?
Either way your 2x 120 amp bms are under spec for the application.

for high frequency
3000 ac watts / .85 conversion factor / 12 volts = 294 dc amps
294 dc amps / .8 fuse headroom = ~368 fuse amps.
Put a 200 amp fuse as close as possible to the positive terminal on each battery.
Use 2/0 awg cable to connect each battery to a busbar rated for at least 400 amps.
Use 2x 2/0 awg each with its own fuse to connect the inverter to the busbar.

for low frequency
3000 ac watts * 1.5 low frequency fudge factor / 12 volts = 375 dc amps
375 dc amps / .8 fuse headroom = ~468.75 fuse amps.
Put a 250 amp fuse as close as possible to the positive terminal on each battery.
Use 4/0 awg cable to connect each battery to a busbar rated for at least 500 amps.
Use 2x 4/0 awg each with its own fuse to connect the inverter to the busbar.

 
Thanks for the input everyone. I'm very grateful.

@deeuubee
To supply more information, the batteries, inverter, and MPPT are designed to be located within the same compartment where no wire needs to be longer than 3ft. I haven't designed the exact layout, but I'd try to ensure that the highest amp wires would be shortest.

The 12v power center (WFCO 8955) was not designed for lithium, but is compatible with it, so I plan to stick with it for now, and then swap it with a lithium-oriented converter later if necessary.

The RV is 34 ft long and has a 30 amp 120v input shore power connection, with plenty of rooftop space for panels.

I plan to install my system in phases.

Phase 1: connect my single 280ah battery and 120a BMS to the 12v power center, swapping out the old FLA battery. I'm hoping this is mostly plug and play, but the BMS has at least two negative cables coming from it, and the power center only has one negative cable currently in place. Connect all three cables to a busbar?

Phase 2: add an inverter that will give me some room to scale up my system. I'm currently looking to get this Renogy 3000W inverter: https://smile.amazon.com/dp/B07MKN7...abc_A98BDWJ3Y52TJQYR6FWQ?_encoding=UTF8&psc=1

I would start by wiring the inverter to a 30a 120v outlet, where I can connect my RV shore cable, and flip the converter's breaker so as to prevent a charging loop.

Phase 3: add around 600 watts of solar to the roof and install an MPPT charge controller to the system. Since this is part of the plan, I want to design phase 1 and 2 to consider this and choose parts accordingly. I'm trying to understand how they come together though. MPPT, inverter, and power center all connected to the same busbars? Or will that create a big problem?

Phase 4: add another 280ah battery to the system, paralleling another BMS. I don't know how to parallel the BMSs. Connect the two negative wires of each BMS to a busbar, shared with the MPPT, inverter, and power center? Does this terminal block then have like 8 inputs? @time2roll said I can share fuse holders with multiple wires. Maybe I can do that instead of a busbar in some place on the positive side?

@smoothJoey
I really appreciate the specifics. When you say to use 2x 4/0 cables with their own fuses between busbar and inverter, do you mean to have two positive cables attached to the positive input terminal of the inverter? Would this be functionally creating somewhat of a single larger-than-4/0 cable from busbar to inverter? 500 amp fuse on each of those wires between busbar and inverter?

I recognize that the system is under spec with only one battery. I'm hoping to give myself some room to grow. Will says in his videos to buy the biggest inverter you can afford, which seems like good advice. Therefore, I'm basically getting an inverter to match the shore power capabilities and not the initial single battery.

I think the Renogy 3000w inverter (linked above in this reply) is low frequency.

Is it okay that I attach the MPPT, inverter, and power center all to that positive busbar? And then would I also attach all the negative cables to another busbar?

Are the negative cables also 4/0 to the inverter, somewhat mirroring the positive busbar but with no fuses?

I think I'm starting to grasp this a bit better.
 
@smoothJoey
I really appreciate the specifics. When you say to use 2x 4/0 cables with their own fuses between busbar and inverter, do you mean to have two positive cables attached to the positive input terminal of the inverter?
Yes
Would this be functionally creating somewhat of a single larger-than-4/0 cable from busbar to inverter?
Yes
500 amp fuse on each of those wires between busbar and inverter?
250 amp fuse as close as possible to the positive terminal of the battery.
You should also have 250 amp fuse each each of the positive wires going to the inverter.
This is for the low frequency inverter.
If its a high frequency unit then its 200 amp fuses and thinner wires.
I recognize that the system is under spec with only one battery.
The BMSs are under spec even with 2.
Its only 240 amps combined.
Tiny differences in the path resistance means the won't perfectly split the current between them.

I'm hoping to give myself some room to grow.
Suggest you go to 24 volts.
Will says in his videos to buy the biggest inverter you can afford, which seems like good advice. Therefore, I'm basically getting an inverter to match the shore power capabilities and not the initial single battery.

I think the Renogy 3000w inverter (linked above in this reply) is low frequency.
Its high frequency.
3000 watt low frequency inverters weigh >50 pounds.
Is it okay that I attach the MPPT, inverter, and power center all to that positive busbar? And then would I also attach all the negative cables to another busbar?
Yep that is a great way to proceed.
Are the negative cables also 4/0 to the inverter, somewhat mirroring the positive busbar but with no fuses?
Yes
I think I'm starting to grasp this a bit better.
Sounds like you are getting the hang of it.
 
One thing to look out for.
You will need to reconfigure the wiring for the ac2dc converter.
When you start powering you ac distribution panel with an inverter it makes a power loop back to the inverter.
 
If you will be using shore power or a generator I suggest you get a 2000 watt inverter/charger with built in transfer switch.
Then just disable your converter.
2000 watts is much more manageable at 12 volts.

I will do the math on spec.
2000 ac watts / .85 conversion factor / 12 = ~196 dc amps
~196 dc amps / .8 fuse headroom = ~245 fuse amps
You can use a single 4/0 awg cable instead of doubling up.
And a single 250 amp fuse.
Here is one example.
 
Its high frequency.
3000 watt low frequency inverters weigh >50 pounds.

Gotcha. The reason I thought it was low frequency is that a couple of people on the Amazon questions and answers for the product said it was low frequency. Here's a screenshot:



I'm more inclined to trust your identification on this than Amazon questions and answers though.

I have updated my diagram, along with wire sizes, based on my understanding so far.

1618263867733.png

I put 6 AWG wires between the battery and BMS because my understanding is that the BMS is already preventing overcurrent with its own disconnect to protect the wires between the battery and BMS. With the overcurrent protection at 130 amps, I didn't think very big wires were necessary there.

With that being said, it makes me wonder about the positive wire sizes too. I recognize that the inverter is capable of 3000 watts with 6000 watts surge, but as you said here
The BMSs are under spec even with 2.
Its only 240 amps combined.
The batteries through those BMSs won't even be able to produce the output the inverter is capable of. Does that not make a difference for wire sizing?
2000 watts is much more manageable at 12 volts.
It seems for two batteries, 2000 watts suffices. However, I'd consider getting a third 12v battery down the road with another BMS in parallel. Across those three batteries, the combined 360 amps of BMS should perform above a 2000 watt inverter. What can using a 3000 watt inverter hurt if it is never pushed above 2000 watts?

Going back to wire sizes, would I still be using 2/0 AWG positive wires before getting the second battery? Or does that wire size need to go up since the second battery isn't there to offset the load? If that's the case, then if there are three batteries, then is 2/0 AWG bigger than needed, since the load is distributed across three batteries?

Or is 2/0 AWG too big as it is, since the batteries can only provide up to 2000 watts because of the 120a BMSs, and therefore should be sized as if it's a 2000 watt inverter? Or do I still need the 2/0 AWG wires in case the RV tries to consume a load above 2000 watts, and so even though the BMSs can't handle it, the wires around the bus bar need to be protected before the BMS? If that's the case, then if it were an actual 2000 watt inverter, then the inverter would disconnect above 2000 watts, protecting the wires?

Suggest you go to 24 volts.

My reason for trying to stick with 12 volts is that I'm starting with only 4 cells, and can therefore only go to 12 volts as it is. I'd have to get a 12v inverter for phase 2, and wire my solar in 12v for phase 3 (phase planning mentioned in an earlier reply). It's not until phase 4 that I'd have enough cells to consider 24v, and at that point, I'd already have most things purchased, sized, and installed for 12v. It would be easier, I think, to just drop in the second battery and BMS, with very little modification, particularly if I planned for it.

Having the batteries modular like that can have its advantages too, if for example, a cell or BMS fails. The system would continue working with hardly an inconvenience while I replaced the broken part.
If you will be using shore power or a generator I suggest you get a 2000 watt inverter/charger with built in transfer switch.
Then just disable your converter.

Getting an inverter/charger does sound like a good direction to go. However, and I may just have to chock this up partially to laziness, but the existing charger in the WFCO power center and wiring is located a bit away from the battery compartment, and so I figure that it would be more complicated to wire an inverter/charger, instead of just an inverter and use my existing charger. Right? Or would it be just as easy as using those existing cables from within the battery compartment on the inverter/charger? I'm guessing no, because power is only going through those cables when the converter is turned on in the power center, right?

As it is, I have a breaker for the existing charger, so my plan was to just flip that off when off-grid/off-generator, and turn on the inverter. Then when on-shore/on-generator, I'd turn off the inverter (probably keep the inverter off most of the time anyways) and flip back on the charger. Seems easy enough, but if the inverter/charger doesn't require a lot of additional wiring around the power center and between compartments, then maybe you're right and I should just go with that.

Sorry for the long reply. I am very appreciative of all the help you've provided so far, and if you or anyone else has more input, then I'd be even more grateful!
 

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I put 6 AWG wires between the battery and BMS because my understanding is that the BMS is already preventing overcurrent with its own disconnect to protect the wires between the battery and BMS. With the overcurrent protection at 130 amps, I didn't think very big wires were necessary there.
The double 6 awg wires usually come with the bms and they usually have silcone insualation that is rated for much higher temperature than the welding cable that is typically used the rest of the system core.
With that being said, it makes me wonder about the positive wire sizes too. I recognize that the inverter is capable of 3000 watts with 6000 watts surge, but as you said here

The batteries through those BMSs won't even be able to produce the output the inverter is capable of. Does that not make a difference for wire sizing?
If you push the bms to its limit the fets will open under 130 amps load.
I would not expect them to last more than a handful of times doing that.
The thing about fets is they tend to fail closed.
That means you won't know that they failed until the fail to protect your battery.
It seems for two batteries, 2000 watts suffices. However, I'd consider getting a third 12v battery down the road with another BMS in parallel. Across those three batteries, the combined 360 amps of BMS should perform above a 2000 watt inverter. What can using a 3000 watt inverter hurt if it is never pushed above 2000 watts?
Should be fine.
The only penalty is slightly higher idle consumption.
A 15amp breaker in the master position would be sufficient to guarantee this I think.
Going back to wire sizes, would I still be using 2/0 AWG positive wires before getting the second battery?
You don't want to have to replace wiring I would think.
And for matched resistance you need to use the same wiring for each pack.
Since you can only pull 120 amps through the bms you could go as low as 0 awg but I wouldn't. I don't want my wires getting to 90c which will burn fingers right smartly.
A single bms can deliver ~1224 ac watts full bore through the inverter.
120 / .8 = 150 amps.
Or does that wire size need to go up since the second battery isn't there to offset the load? If that's the case, then if there are three batteries, then is 2/0 AWG bigger than needed, since the load is distributed across three batteries?
If you have 3 batteries each could get by with 0awg wire.
Together they can deliver ~3672 ac watts through the inverter.
but you should de-rate by .8 to account for asymmetries in the current path.
Which is 2937.6 watts.
Or is 2/0 AWG too big as it is, since the batteries can only provide up to 2000 watts because of the 120a BMSs, and therefore should be sized as if it's a 2000 watt inverter? Or do I still need the 2/0 AWG wires in case the RV tries to consume a load above 2000 watts, and so even though the BMSs can't handle it, the wires around the bus bar need to be protected before the BMS? If that's the case, then if it were an actual 2000 watt inverter, then the inverter would disconnect above 2000 watts, protecting the wires?
The only limit on wire size is how much you wish to pay and your ability to physically manipulate it.
4/0 is a practical limit for our application though.
You can get wire up to 1000mcm which is just crazy.
I have a chunk 2 meter chunk of 250mcm sitting by my desk.
It would make a brutal weapon for self defense.
Getting an inverter/charger does sound like a good direction to go. However, and I may just have to chock this up partially to laziness, but the existing charger in the WFCO power center and wiring is located a bit away from the battery compartment, and so I figure that it would be more complicated to wire an inverter/charger, instead of just an inverter and use my existing charger. Right?
Wrong.
If you get the inverter/charger you should put it as close to the batteries as possible.
Then you just disable the converter in the ac/dc distribution panel.
If you use and inverter without charger you will need to rewire the converter to avoid a power loop and you will need to buy a discrete transfer switch so that you can use shore power. If you wish to do that it can be done but the real reason for getting an inverter charger is that they are very configurable.
They also massively simplify the typical usage model.
With a converter you are basically stuck with the charge profile that it comes with and they usually kind of suck(some more than others).
The discrete inverters almost never have a configurable low voltage disconnect which is a right pita.
Or would it be just as easy as using those existing cables from within the battery compartment on the inverter/charger? I'm guessing no, because power is only going through those cables when the converter is turned on in the power center, right?
I don't know what is there but If its the original stuff its likely crappy.
I also don't know how your rv is laid out but with lifepo4 it generally makes sense to have the batteries close to the core system.
Because lifepo4 they don't off gas its normal to have them in the living space so that they don't get to hot(heat shortens the life of a battery) or to cold(if you charge a lifepo4 battery below 32F its almost certainly immediately dead).
As it is, I have a breaker for the existing charger, so my plan was to just flip that off when off-grid/off-generator, and turn on the inverter.
Human error is largely preventable with proper engineering.
You strategy is an accident begging to happen.
Now I see how very different our design philosophies are. :)
UPDATE TO FIX SOME MATH ERRORS.
 
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For the next iteration its time to start computing for voltage drop.
That means estimating the round-trip length of each circuit.
voltage drop is a significant consideration at 12 volts.
 
Going back to post #9

Yes for all the high amperage wires being short.

The WFCO power system if ok for lithium but will be a little slow charging.

Phase 1...you may want to add a DC/DC charger for the tow vehicle, the lithium can overwhelm you truck alternator. Use a bussbar for the negative connections and you can use a bussbar/fuseblock combo for the positive. All wires leaving the positive bussbar require a fuse or breaker, you can try something like this....

Phase 2....lots of inverter choice depending on what you want to power, as suggested sometimes larger inverters have a high idle consumption. Wiring the inverter to an outlet is the easiest way to install, but you need to plug in when you get to camp. That's up to you, I have done 2 trailers that way and am working on 2 more-they all like it...just remember to turn off that breaker you mentioned. With that system you don't need a transfer switch or charger in the inverter and don't have to try to get 2 #10 romex between the converter and inverter. If you want to upgrade to a transfer switch in the future you can use an external one, it requires only 1 Romex with the gauge depending on the wattage of the inverter

Phase 3...yup they can all be connected to the same bussbar/fuseblock

Phase 4...yes 2 bms and both get connected to the bussbar, some suggest using a class T fuse for each battery on the positive feed to the bussbar.

The inverter manufacturer (if brand name) will specify fuse size and wire gauge generally a 4/0 is good up to 3000w inverter. It's a good idea to add some type of precharge switch to reduce arcing when powering up the inverter.

Positive and negative cabling should be the same size on a given circuit.

I don't think Renogy inverters have a great track record for dependability....
 
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@smoothJoey @Whinny

Thanks for the awesome guidance. I think I'll go with a 2000 watt inverter for now, based on what has been said. Across all four phases I described, there's no need for something higher than a 2000 watt inverter. It would only come into play once a third battery and BMS is installed, which is a ways away. I hadn't thought of the higher idle power consumption either.

My thought would be that it still should be relatively easy to upgrade to a 3000 watt inverter down the road. If I get busbars with enough terminals, then if I started with smaller wire, I could double the wire of the exact same length for the larger inverter later (or just get bigger wire and replace the wire I guess).

Human error is largely preventable with proper engineering.
You strategy is an accident begging to happen.

I believe you were saying this in reference to not using an automatic transfer switch with something like an inverter/charger. I generally do want to go with smart and safe design choices. For the manual converter switching, my take is "what's the worst that could happen with that?" If I forgot to flip the converter switch, the resulting charge loop, if I understand correctly just creates lots of loss of energy with the battery effectively trying to charge itself. Not any serious risk of major damage or danger. Or at least I hope.

It's reassuring to hear that @Whinny has had success with creating systems where the converter is manually switched on and off. I looked into installation for inverter/chargers and it definitely looks a bit more involved, and certainly more expensive.

Before making any final decisions with system designs, I'd like to ask a question I'm hoping to get some input on.
I opened a metal electrical box in my battery compartment, which is adjacent to the currently-vacant on-board generator compartment. This is what I found in that metal electrical box:

1618370644189.png

I'd like help figuring out what this is for. I figure it's either

1. A box allowing for creating an outlet to supply electrical from shore power in order to power some kind of electrical appliance in these compartments.

2. An incomplete, but ready-to-go setup for an on-board generator to supply electric input to the RV, kind of like a second shore power input. My RV had the option for an on-board generator, but that option was declined. Maybe this is a remnant of the RV being designed for an on-board generator?

How would I check what this is for? The wiring kinda goes behind the walls, and so it's a bit difficult to follow it. I'd like some advice on it, because this is an "accident begging to happen" :) .

Thanks again for your hugely helpful replies.
 
My bet would be behind door #2.....
Look for the other end behind the power centre...it might be to go into a transfer switch.
Is this a Motorhome, 5th wheel or Travel trailer?
My bet now is Motorhome......
Nothing will blow up if you leave the converter on-it will just use up a lot of battery.
 
Is this a Motorhome, 5th wheel or Travel trailer?
My bet now is Motorhome......
That's a good guess, but strangely enough it's a toy hauler with a fuel tank designed to fuel the "toys" brought along. Since it has a fuel tank, they also include the option to have an on-board generator, where there is actually already a fuel hose run to the generator compartment, but the hose is tied off and capped right before going in the compartment. This is what made me think that electrical box might be generator related.

Look for the other end behind the power centre...it might be to go into a transfer switch.
I'm not near the RV right now to check, though I'm not sure I'd immediately know which wires behind the panel were coming from that box.

I did take a picture of the power center panel earlier, if that might offer any clues.

IMG_20210407_032004.jpg

I'm guessing that the breaker labeled "refer/gen" has to do with the [non-existent] on-board generator. But maybe not? I'm guessing "refer" refers to refrigerator? Not sure what the heck "6F1" means if I'm reading that handwriting for the breaker below correctly.

Not that I won't take out the power center to check behind it, but I'm curious if there's a way I can confirm any suspicions before doing that.

Thanks for the helpful response.
 
I believe you were saying this in reference to not using an automatic transfer switch with something like an inverter/charger. I generally do want to go with smart and safe design choices. For the manual converter switching, my take is "what's the worst that could happen with that?" If I forgot to flip the converter switch, the resulting charge loop, if I understand correctly just creates lots of loss of energy with the battery effectively trying to charge itself.
You have not understood.
The scenario I was concerned is not about a transfer switch.
Its about a power loop between your inverter and your converter.
Your converter will end up drawing its full rated power plus overhead from the inverter.
that is ~75 amps.
The inverter will try to draw that power from your converter and your battery.
You will drain your battery to low voltage disconnect in <4 hours.
While you are draining the battery you will only have 45 amps of 130 free.
Its likely that you will trip the bms on over-current in this case.
Tripping a bms over-current will dramatically decrease its lifespan.
Think of your bms as a reserve parachute.
Those are the worst scenarios that comes to mind on .5 cups of coffee.
 
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