First Build: (3) 7s16p (7s48p); design and planning

Howdy; I've been in energy industry since early 2000s, and I believe energy storage has the potential to radically transform power markets.I have been wanting to build an energy storage system for the past few years to teach myself about the technology. It was through this process that I found Jehu's YouTube videos, which sped things up a bit for me. So I finally pulled the trigger on 2 cases of Panasonic NCR18650BD cells (qty 380, typical capacity 3.18 A, Nominal Voltage 3.6 V,Max Charging Voltage 4.2 V) and then (60)of Jehu's PCBs and (4) of his BMS PCBs.

My goal is to build a nominal 24 V 7s16p system with (3) banks, each with its own BMS PCB--so I suppose this technically would be a 7s48p battery. Nominal Voltage will be 25.2 V and typical Amp-Hrs should be 152.6 Ah, which then translates out to 3,847 Wh. Charging voltage will be 29.4 V. This equates to a 12V-equivalent 320.5 Ah, which is basically (3) lead acid camper batteries (ish).

Each of the three banks of (16) PCBs will have a Jehu BMS to keep the cells balanced within each bank.

I know people are going to light me up on this, but I am going to install this system in our camper, which I'm basically going to treat as my little test lab. The Camper is 12V. I am going to install a large 24V-12V buck converter, which I fully understand sucks and is imperfect andi is going to be a failure point and is going to cut down on round trip efficiency. But my goal is not roundtrip efficiency. My goal is learning how batteries work, and this application will be just fine for my purposes. My ultimate goal will be to install this system in a new office ran exclusively by solar and this battery, but that won't be for a couple years. So for now, I learn.

I'm only going to charge the unit via 110V shore power at this point.

I am going to install an inverter that I can use to power the AC side of my camper's breaker to power AC outlets in the unit (microwave, coffee pot, laptops, etc). Max AC load can be assumed to be <2000W (and probably only 1350 W, which is size of my microwave--but I'd like some buffer room).

The system will be installed within the unit, which can be assumed to be dry.

Next summer, I may opt to add roof mounted solar panels, so that I can charge the system when we're out and about. But for now, only assume shore power charging.

I'm posting this here because I feel like I have a pretty good idea on how to build up each of the (3) 7s16p banks--along with their BMS units--from Jehu's videos. I need help with:

1. Selecting a DC-DC buck converter, which I will just home run right into the DC side of camper's DC panel.
2. Selecting a 110V charger that can deliver 29.2 V and enough Amps to safely charge my system, as well as accommodate future solar panels (although I am fine running solar panels right into a MPPT and then right into the batteries if that is better).
3. Selecting an inverter somewhere in the range of 2000-3000W.

I'm sure that I will have other questions as I go, but, as the old proverb goes, "A journey of a 1,000 miles begins with the first step"--and this is my first step.

Thanks a ton for your comments, questions and feedback. I REALLY appreciate them.

aventeren said:

1. Selecting a DC-DC buck converter, which I will just home run right into the DC side of camper's DC pane

Click to expand...

No experience here.

aventeren said:

1. Selecting a 110V charger that can deliver 29.2 V and enough Amps to safely charge my system, as well as accommodate future solar panels (although I am fine running solar panels right into a MPPT and then right into the batteries if that is better).

Click to expand...

I use 3 x 14s chargers from YZPOWER @ 15a each for a total of 2100w of charging power from 110v.
Here's a 7s version @ 10a (250watts)- you could do more than 1 in parallel.
https://www.ebay.com/itm/283842642080

See AIMS inverter PLUS charger below..

aventeren said:

[*]Selecting an inverter somewhere in the range of 2000-3000W.

Click to expand...

[*]Reliable 3000w is a cheap but not bad for cheap inverter - several youtubes on it - no surge.https://www.amazon.com/Reliable-Inverter-Voltage-Converter-Display/dp/B01M5L0A5L/ref=sr_1_1?dchild=1&keywords=Reliable+3000w&qid=1592439981&sr=8-1

Next up (for me) is AIMS inverter + charger (25a). Has 3 x surge and 3 x the price. https://theinverterstore.com/product/3000-watt-pure-sine-inverter-charger-24-vdc-to-120-vac/

I used Reliable in the early days but use AIMS (ETL listed)for my home system now - been happy with AIMS.



Notpushing these products,just sharing what worked for me

Thanks for your advance, @OffGridInTheCity.

I ended up ordering this 24V-to-12V buck converter. Should do everything I need.

I suppose the biggest question now is whether to buy stand alone charger & inverter--or whether to buy an integrated unit (per your email and options). I like the ~$60 or so for the charger--that seems like a no brainer...but I'm not sure that I'll be able to go the integrated route at this point due to cost. So I just bought the YZPower 29.2V 10A charger, which should be just fine for my purposes at this point.Heck, I may decide to never add an inverter to this particular system...we'll see. So thanks for the tip on the charger!

So now I have a buck converter and a charger--and TBD on inverter.

What is the best way to split power from the battery into the buck converter and charger? Should I use a little busbar for that?

Also, I have a question on the system. So if I home run my 12V buck converter output into the 12V camper bus bar, then presumably a portion of my 24V battery will be used to charge the 12V battery. So I suppose that I should install some sort of battery switch between the camper's panel and the 12V battery so that I don't end up using my24V battery to charge the camper's 12V battery. Is that the right way to think about this?

Also, when the camper is plugged into the truck, the truck's alternator sends a small amount of 12V power to the camper's 12V system. I emailed Jayco, and got a 12Velectric schematic here. It doesn't look like the camper has a wire that homeruns back to the 12V control center, which means that it must connect right into the 12V battery. I'll investigate that and report back.

I ordered a digital current and voltage power energy meter from Amazon. I think there are some YouTube videos out there that describe how to install these, so I think I'm good there.

Can you think of anything else that I'm going to need? I ordered XT60 connectors, which I plan to use for battery output connectors. What else am I missing here? Fusing? Are there any protective things that I should consider adding?

Thanks again for your thoughts and inputs on helping me figure this out. I REALLY appreciate it.

>What is the best way to split power from the battery into the buck converter and charger? Should I use a little busbar for that?
I would start with some 4awg (or larger)wire + lugs (and crimper). This will give you something you can put a nut+bolt thru to make connections. Busbar wise - many justbolt several lugs togetheror get a flat piece of metal (or make one by pounding a copper pipe flat) and drilling holes to bolt the lugs to. You want the bus to be mounted to something non-conductive and then covered once wire/lugs are attached to avoid possibilities of shorting things. @LithiumSolar just posted a youtube - see 4:57 for example of nice busbar setup -https://youtu.be/RunwcEkFqWc

>So if I home run my 12V buck converter output into the 12V camper bus bar, then presumably a portion of my 24V battery
>truck's alternator sends a small amount of 12V power to the camper's 12V system.
I would not hook these to 2 systems together. To my mind you need an isolation switch so you can choose either 1) 12v sub-system is energized from the12v from camper battery or 2) 12v from the 24v battery.... but not at the same time. **Maybe someone know of a piece of equipment (other than a manual switch) to manage the 2 power sources - but that's outside my exeperience. A manual switch would work.

If you're thinking "Could I charge the 24v battery from the alternator"... There's some efficiency loss but you could do a 12v -> 110v inverter and then run the YZPOWER charger to the battery. The 2 systems would remain isolatedfrom each other but you'd need 12v@25a to get 300w inverter to work with the YZPOWER. You could also buy a less-amps charger and maybe do 100w inverter to 120v 24v@4acharger. Anyway - you get thatidea.
There are also 'systems' that handle dual battery / camper / alternator situations but typically they are 12v and 12v - not 12v and 24v. Maybe someone knows of something.


>What else am I missing here? Fusing?
- Fusing and/or circuit breakers - between battery and inverter, battery side of DC-DC, 12v side of DC-DC (maybe multiple fuses for multiple circuits such as lights, refrig, etc).
- You need a BMS on the battery that can cutoff the 24v battery if something is wrong
**If you work up a wiring diagram (or document what you wire up) - then folks here could more directly answer certain questions such as 'where should I use a fuse'.




[size=small]>What else am I missing here?[/size]
Big picture? So you have a camper with 120v incoming power? A typical scenario is to have an ATS (automatic transfer switch) so that when the camper is plugged in to external power, it can run on that (and perhaps charge your battery while its running on external power). Then when you turn on the Inverter, the ATS will switch over to 120v from the inverter instead of outside. This let's you turn on/off your inverter (e.g. use or don't use your battery)seamlessly when you have external power. This may not be useful to you - just a thought. Here's an example of 120v@30a ATS...https://www.amazon.com/gp/product/B00153EYTO/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 Also that AIMS (above) not only has charger built-in but also an ATS built-in - so its charge, inverter, and ATS 'all in one'.

Saw this youtube on ATS (as I mentioned above) but gives detailed 'plan' to use itto charge your batteries while on external power. It might apply to your situation and the detail is good

Thanks a ton. So I just ordered a battery switch. I'm going to install that on the 12V side of the buck converter, and tie the switch into the camper cable between the camper's battery and the camper's 12V bus bar. That way, the 12V bus bar will be either seeing my battery's 12V output or the camper's 12V battery's output. Goal will be to eventually replace the existing 12V battery with nothing--but I'll keep it there like training wheels for the time being (plus, I'll be able to then isolate my battery from parasitic camper loads when camper not in use).

In terms of ATS, the camper has an ATS now, so I'll just use that. I'll then plug the camper into shore power, and then plug my charger into the camper's 110V system to power the charger and therefore charge the battery. So I'm going to see how that goes first. I may need to install a dedicated circuit due if the camper's existing 110V breakers can't handle the charger's requirements--but we'll see. I can always just run a dang extension cord to the charger if need be.

I'm leaving for the weekend now, so I'll pick this back up next week. Next week I'll plan on pulling together a rudimentary wiring diagram and post that here--so that I can get some feedback on safety, fusing, etc.

Ah, one last thing. BMS. So I bought (4) ofJehu's BMS units. I'm going to have (3) 7s16p battery banks wired in parallel to make my 7s48p battery. Each 7s16p bank will have one of Jeu's 60A BMS units, which I am under the impression should work for battery management (cell balancing during charging, high voltage shutoff during charging, low voltage shutoff during discharge). Is that a safe assumption?

aventeren said:

Ah, one last thing. BMS. So I bought (4) ofJehu's BMS units. I'm going to have (3) 7s16p battery banks wired in parallel to make my 7s48p battery. Each 7s16p bank will have one of Jeu's 60A BMS units, which I am under the impression should work for battery management (cell balancing during charging, high voltage shutoff during charging, low voltage shutoff during discharge). Is that a safe assumption?

Click to expand...

Nothing wrong with your assumptions. Some BMSs also do a temp cutt-off. You want to be aware of tempssince this is an (outdoor) camperand avoid temp extremes.
Here's some typical Lithium ion numbers forguidance:
Charge limits: 0C to 45C (32F to 113F)
Discharge limits: 20C to 60C (4F to 140F)

At a practical level, just cool or heat the camper to something 'livable' before putting any kind of load on the batteries

Right on! Thanks for the temp range. The upper end should be fine, but the low end may be an issue when I take camper elk hunting (Montana = cold during elk season!). So Ill have to think of a way to keep the battery system warm on those trips. Maybe Ill create an insulated box that I can keep warm with a heat lamp or some other heat source.

Have you ever seen a low temp solution in any other posts? Ill have to do some searching on that this week.

>Maybe Ill create an insulated box that I can keep warm with a heat lamp or some other heat source.
Exactly.

[size=small]>Have you ever seen a low temp solution in any other posts?[/size]
I've not seen youtubes on this. I presume you plan to heat the trailer (maybe propane ordiesel)andjust blow some of that heat over thebattery? A lot of BMSs will have a temp probe to help ensure the battery is warm (or cool) enough if you're looking for protection.

From what I read,you can discharge'a trickle of current' below specs - say -10F. For example, maybe turn on a 5wLED light to assist in turning onthe heat. This would not benearly as risky/destructive to the batteries as turning on significant power - such as1000w inverter - before the warm up.

So after doing a bit of research, I have discovered "Positive Temperature Coefficient" (PTC) heaters, which may work for me. I found one on ebay that is 24V, 4-20W and 60C (140F) for $8 or so. So the idea here, I suppose, would be that I would place the battery within an insulated box, and on the inside I would place one or more of these PTC heaters that would be controlled by a thermostat of some sort that had a low temp on trigger (maybe something like 40F) and then a high temp off trigger (maybe something like 50F).

So I did a bit of research, and here is a 24V digital temp controller thermostat for $9 that may do the trick--plus I can then mount it next to my other digital energy displays to show me what the battery box temp is at all times--not just when it was cold. It would also be interesting to see how the battery temp varied throughout the summer months.

Shifting gears to solar, if I was to add 24V solar panels to my system, would I just home run those leads into a MPPT charge controller and then land the MPPT leads into the 24V bus bar?

I'm going to work this week on a wiring diagram so that I can start noodling on fusing and other safety measures.

Thanks again for your help! I REALLY appreciate you feedback and comments. This is fun.

>Shifting gears to solar, if I was to add 24V solar panels to my system, would I just home run those leads into a MPPT charge controller and then land the MPPT leads into the 24V bus bar?
Yes, Solar PV-> MPPT ---> bus. The bus connects the charge controller,battery, inverter, and the 24vDC->12vDC step-down together. There might be a shunt in there between the battery negativeand the bus to measure amps going in/out of the battery. You would fuse (or use circuit breakers) between the bus and the things connected + a fuse between PV and Charge Controller. For inverter output and 12v output you do standard distribution box/fusing for 120v and 12v.

Agree its fun! My home is running smoothly now (on solar) so my next project is a cargo trailer conversion to a camper... of course to include solar + 18650 battery bank! In fact the plan is park the trailer next the house and tie it into the house battery bank under the house So I'm really interested to learn what you do and how it works for you.

Ok, it turns out that I suck at drawing electrical diagrams. With that caveat, here is my electrical diagram.

I know that I have zero fusing or breakers or protection in here at all...largely because I'm not sure if the protection should be on the + or - side of the circuits. So I sure would appreciate any help anyone has for me.

Thanks!

---

I also have (4) digital shunt meters that I'd like to install to measure battery charging, battery discharging and the 12V load side. I'll reserve a 4th for the solar charging circuit when I add that. But I'm not real sure how to add these digital shunt meters to the circuit, either.

Excellent diagram. I've circled places for fusing / circuit breakers. Typically, you have a central 'box' that contains the busbars / a lot of DC wiring converges - and this is a great place to put fusing. Fuses protect against SHORTs!! (and overloads) so fire doesn't erupt. You might do circuit breakers as they not only fuse - but give you on/off switches so you can isolatethings to work on the system. Not sure of your requirements but you might get by with a simple 12v(24v) fuse box (same as 12v power circuits)- and you can pull the fuses to turn things off.

I would fuse/circuit-breaker the positive wires. I've made some quick notes. Again - you might get some additional help/comments here in case I missed somthing




"ANL" fuses are true inline fuses and look like this (overview link on Amazon) -https://www.amazon.com/s?k=ANL+fuse&i=automotive&ref=nb_sb_noss_2

Here are some cheap inline'circuit breaker' type fuses -https://www.amazon.com/ANJOSHI-Hold...4&sr=1-1-70f7c15d-07d8-466a-b325-4be35d7258cc I found out that you can only turn these on/off maybe 50 times before they stop working - they are not ment as daily on/off switches.

Here are high quality DC circuit breakers (5, 10, 15, ... amp)that mounton a DIN rail in the control box https://www.amazon.com/MidNite-Sola...motive&sprefix=Midnite+,automotive,226&sr=1-2

This is DIN rail - for Midnite breakers and other things use this standard to mount things inside a control box -https://www.amazon.com/Pieces-Slotted-Aluminum-Inches-7-5mm/dp/B079TX7WDQ/ref=sxin_8_ac_d_pm?ac_md=1-0-VW5kZXIgJDEw-ac_d_pm&cv_ct_cx=DIN+rail&dchild=1&keywords=DIN+rail&pd_rd_i=B079TX7WDQ&pd_rd_r=342cfe49-dfe8-49f7-bdfd-bd82ced94d15&pd_rd_w=gJVRD&pd_rd_wg=X4P31&pf_rd_p=65218b5e-9d60-4e42-83ae-a40c4223301c&pf_rd_r=5PR3WJBBWR57WP6Q275H&psc=1&qid=1592948260&s=automotive&sr=1-1-22d05c05-1231-4126-b7c4-3e7a9c0027d0


For 120v output - you can use standard distribution box / with standard 120v circuit breakers (same as your house)

For 12v output - folks tend to use a 12v fuse box like this -https://www.amazon.com/WUPP-Blade-W...tion+block&qid=1592948008&s=automotive&sr=1-8

All the above are examples, not pushing these exact products except for the DC Midnite Breakers (DIN rail mount) I use a lot and I also useOutback DC breakers as they mount to a metal box with on/off togglesticking out (instead of DIN rail stylebeing inside the box) -https://www.amazon.com/OutBack-AC-D...DC+breaker&qid=1592948169&s=automotive&sr=1-3

Well that was an amazing post! Tons of info! Thanks!

Here is my updated Battery System Diagram with fusing shown, as well as showing the 12V and 110V fuse/breaker boxes and their loads.

I also took my best guess at how the ATS works--but I'm just not sure how it's wired until I get into it. It is my belief that it is integrated into the 12V camper panel/110V breaker panel because I know the camper currently knows what to do when it is given 110V and then what to do when the 110V is taken away.

Thanks for the tip on the 12-24V fuse block. To accommodate future heater needs, I may need more than a 6-way blade unit--plus I think that I'm going to need more than 30A per circuit and 100A total. I found this 12-way unit, but still each circuit is limited to 30A and the overall limit is 100A--so again, unless I'm missing something, that isn't going to be enough. So the $25 solution appears to be lacking for my use case---unless I just bought several of these...and loaded each to below 100A with all of the attached circuits. But that seems weird. So I need to think on this for a bit. The Midnight stuff that you posted up is pricey. Damn. That appears to be the solution, but dang it those are expensive.

My MPPT solar charger is sounding like it will be either 45A or 60A.

The DC 29.2V charger is 10A

The 24-12V buck converter is 60A.

The battery has a C-Rate of 3.14 and a Max Continuous Discharge Current of 10.0A. Given my newness here, I'm not even sure how to determine the amp rating of that circuit.

The little PTC heaters I'm looking at are something like 10W, which would be 10W/24V = 0.42A--so not much on those--and I may have a few of them bussed together somehow depending on how many W it takes to keep the batteries above 50F in the winter when charging.

I did read that my 18650 cells have a charge range of 10-45C (50-113F), discharge range of -20-60C (-68-140F) and storage range of -20-50C (-68-122F). So I should be ok from a discharge and storage temp perspective so long as I don't charge them when they are <50F (hence the PTC heaters and insulated box).

I haven't looked at 24-110V inverters yet, but I have to think those are going to be in the range of 2000-4000W on the AC side--and I'm not sure on how that will translate to amps on the 24V...but a lot.

So I need to think a bit on the fusing plan. I could run with a mixture of 6- or 12-way blade types (keeping attached circuits to below 100A), and then some purpose driven in-line fuses for the larger loads (inverter, buck converter, MPPT, etc). This kind of makes me wish that I would have designed around a 48V battery now!

Looks OK except the ATS. The ATS (Automatic Transfer Switch) is 110v only ... you have it 'connected' to 12v in the diagram. For 12v - you indicate you have amanual switch, to switch between the vehicle 12v battery or the 12v from the 18650 battery system. This manual switch (12v) would be completely unrelated to ATS (110v).

An ATS is a "Y" device where one input is Shore(ExternalPower) 110v and the other input is Generator(Inverter)110v and the box chooses one or the other - for a single 110v output. The box has a pair of relays that are mechanically connected - e.g. when 1 turns 'on' the other turns 'off' and vice versa.

The Generator(Inverter) input takes precedence. Ifthe ATS detects 110v from the inverter, the internal relays will click over to letthe inverter110vthru to the output. If the Generator(Inverter)turns off, then the relays goes back to let Shore power thru to the output ifShore is energized. If both theShore and Inverter are0v then things just stay as they are as there's no power to cause the pair of relays to switch. When you power up next time - if its Shore, then the ATS will let it thru to the output. If its Inverter, then that will go thru to the output. If its both - then the Inverter will win and go thru to the output.

The ATS typically has a 10-20sec declay on the Generator side - e.g. once it detects power on that input, it will delay switching to it for 10-20seconds... to give the generator time to fully run it's motor. This is nice for an Inverter when used instead of a generator -it give the inverter a few seconds to fully turn on/power up. This also avoids a cascading relay switch back/forth in rapid succession... so the system stays under control.

FOR YOUR ATS - you should find in your box 2 inputs and 1 output. So 1 set of 110v wires from shore going to a relay. Another set of 110v wires that would come from the Inverter's output to another relay (right beside the 1st relay). Then an output set of 110v wires - this is what goes to110v sockets around in the camper to power things. You may find a small circuit board nearby the relays - that does the 10-20sec delay.

Ok, trying to get the fusing dialed in. I think that I am going to use these terminal post style in line fuses, which are available in 50A, 60A, 80A, 100A, 150A, 200A, 250A and 300A. I was curious if anyone could help me with what size fuses I need per circuit. Here are my questions:

1.My MPPT solar charger is sounding like it will be either 45A or 60A. So should I buy an in-line fuse rated at 50A (if 45A MPPT) or 60A (if 60A MPPT)--should I upsize the fuse and buy a higher amprated fuse...and then if upsize, how much much upsized?


2. The DC 29.2V charger is 10A. So would buying a 50A fuse be overkill here? Should I be running this through a blade type 10A or upsized fuse?

3. The 24-12V buck converter is 60A. Again, do I run a fuse right at 60A or upsized?

4. The little PTC heaters I'm looking at are something like 10W, which would be 10W/24V = 0.42A--so not much on those--and I may have a few of them bussed together somehow depending on how many W it takes to keep the batteries above 50F in the winter when charging. Should I just run these through a small fuse block--and then do I run 1A or upsized fuses per PTC heater circuit?

5. Inverter. I have not planned for an inverter at this point, but our microwave is 1,350W, so we would likely do either a 2,000W or 3,000W AC inverter. I Googled around, and found that these have an internal efficiency of something like 90%. So a 3000W inverter converting to 120V-AC would be 25A-AC at 100% efficiency and 22.5A-AC at 90% efficiency. On the DC side, 3000W at 25.2V-DV would be 119A-DC at 100%. A 4000W peak burst at 25.2V-DV would be 158A-DC. So how would I fuse this circuit? What is the rule of thumb here?


6. 24V battery to 24V busbar. The 18650 cells I bought have a C-Rate of 3.14 and Max Continuous Discharge Current of 10A. My battery is (3) 7s16p banks for a total of 7s48p with a nominal voltage of 25.2V and a max charge voltage of 29.4V--although the DC charger I bought is only rated at 29.2V and 10A. The cells have a Typical rating of 3.18Ah, which makes my 7s48p battery 152.6Ah. So how would I size this fuse based on the above 24V circuits?

Thanks a ton. I REALLY appreciate your questions and comments.

Let me try to move the conversation forward on fusing... there are couple of reasons to fuse:

1) No 1 is to protect the wire itself. You don't want currentto burn up the wire causing meltdown, shorts, fire etc. So a fuse/circuit-breaker should 'pop' before the wire burns.

2) Mo 2is to protect a devices. A lot of devices (such as an inverter) will have their own protection built-in so you don't need to do anything, but some devices do not such as the DC-DC converter. In this case, its OK to fuse the device as long as the fuse is <= what's needed to protect the wire.

3) Other - there are other reasons but the above is primarily what I use as guidance.


Let me try to give some guidance to your questions:

>1. My MPPT solar charger is sounding like it will be either 45A or 60A. So should I buy an in-line fuse rated at 50A (if 45A MPPT) or 60A (if 60A MPPT)--should I upsize the fuse and buy a higher amp rated fuse...and then if upsize, how much much upsized?

Solar panels 'max out' in the current they deliver - even if you short them. So you don't have to worry (like you do with grid power or battery bank) about 'huge current inrush' With proper size wire, it will never burn up - so fusing is not required. I would get something to allow max amps to the Charge Controller (so you can add more panels later)but mainly its about on/off so you can disconnect the current flowing into the charge controller when you want towork on the wiring.
*Lightning surge - I don't have good data on this but a thread a while back indicated that one shouldn't worry about rubber tire vehicles with panels on top.


>2. The DC 29.2V charger is 10A. So would buying a 50A fuse be overkill here? Should I be running this through a blade type 10A or upsized fuse?

On the DC side, this is like the panels above - the DC going into the battery is 10a max - it won't surge to 100a if it shorts - so this is not needed as long as wire is big enough to handle 10a. Its OK to put a 10a fuse on this in case the wire's short or the device shorts internally? - but this is not a high priority in my mind. I don't fuse the output of my chargers. On the 110v AC side - the 110vplug (the charger is plugged into)should have a circuit-breaker at the distribution box for the AC wiring.


>4. The little PTC heaters I'm looking at are something like 10W, which would be 10W/24V = 0.42A--so not much on those--and I may have a few of them bussed together somehow depending on how many W it takes to keep the batteries above 50F in the winter when charging. Should I just run these through a small fuse block--and then do I run 1A or upsized fuses per PTC heater circuit?

Sounds good. In this case, my thought is you want to fuse the wire from the battery to buss (rather than each individual PTC) so that if any one of them 'short out' the fuse will pop before the wire burns up.


>5. Inverter. I have not planned for an inverter at this point, but our microwave is 1,350W, so we would likely do either a 2,000W or 3,000W AC inverter. I Googled around, and found that these have an internal efficiency of something like 90%. So a 3000W inverter converting to 120V-AC would be 25A-AC at 100% efficiency and 22.5A-AC at 90% efficiency. On the DC side, 3000W at 25.2V-DV would be 119A-DC at 100%. A 4000W peak burst at 25.2V-DV would be 158A-DC. So how would I fuse this circuit? What is the rule of thumb here?

Make sure the wire from battery to the inverter is large enough - e.g. 3000w inverter / 24v battery = 125a could flow. So 125a or 150a fuse with wiring to match. The inverter itself should have internal fuse - so I wouldn't worry about the inverter. This is a case of fusing the wire in case of a short - e.g. I've had an inverter cause a dead short when it blew up and trip the breaker between the battery and the inverter.


>6 24V battery to 24V busbar. The 18650 cells I bought have a C-Rate of 3.14 and Max Continuous Discharge Current of 10A. My battery is (3) 7s16p banks for a total of 7s48p with a nominal voltage of 25.2V and a max charge voltage of 29.4V--although the DC charger I bought is only rated at 29.2V and 10A. The cells have a Typical rating of 3.18Ah, which makes my 7s48p battery 152.6Ah. So how would I size this fuse based on the above 24V circuits?

NOTE: When you say "....typical rating of 3.18ah...' your mixing capacity with amps. Capacity is overall power in the cell. TheC rating is the spec that says how fast (how many amps) you can draw at any giving second. A 1 C = 3.18a max draw/cell A 2 C would be 6.36a max draw.

Fuse to wire and max you want to draw out of the battery. Let's say that'in theory' you your battery could deliver1.5C (4.77a/cell). This would be 4.77a * 48cells = 229a @ 24v. However... unless these are high drain (high amp) cells - the voltage typically drops dramatically if you try to pull 4.77a/cell and you'll find the battery reaches 3.0vor 2.8v cut-off within a few minutes. You should be prepared to do some tests. I would go with 0.3C (1a/cell) or less a more realistic guide. 1a * 48cells = 48a @ 24v. So might only need a 50a circuit breaker. This would be 24v * 50a = 1200w inverter.

You can wire for a 100a / 2400w inverter or 200a / 4800w inverter - no problem in terms of fusing/circuit-breaker. But if you were to draw that much power from the battery it could exceed what you want it to handle.

So just match things up between 1) what the battery is capable of handling + 2) wire size to the inverter.

FYI....
In my home system, my battery 'could' deliver 1,500a - however I have it shunt-triped (fused) at 400a as this is the max the overall system (inverter discharge, solar charging) need. Its also the max the wiring can handle and its also the max I wanted to spend on a circuit breaker

Howdy, just a quick update. I'm currently testing my 18650 cells, which is taking a LONG time. I plan on getting back to wiring and fuses when the testing is further along.

I was able to score a used free 60A MPPT from a jobsite--and also a bunch of free solar panels. So I may be looking at adding solar sooner rather than later.

By far my biggest issue remaining is wire size and fuse sizing per circuit. I have a colleague that can help me with that later if need be.

Ok, back to charging and testing and charging and testing and charging and testing...