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First Build: (3) 7s16p (7s48p); design and planning
#1
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.
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#2
(06-17-2020, 10:32 PM)aventeren Wrote:
  1. Selecting a DC-DC buck converter, which I will just home run right into the DC side of camper's DC pane
No experience here.

(06-17-2020, 10:32 PM)aventeren Wrote:
  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).
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..

(06-17-2020, 10:32 PM)aventeren Wrote: [*]Selecting an inverter somewhere in the range of 2000-3000W.
[*]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/300...o-120-vac/

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



Not pushing these products, just sharing what worked for me Smile
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#3
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 my 24V 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 12V electric 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.
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#4
>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 just bolt several lugs together or 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 the 12v 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 isolated from 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@4a charger.    Anyway - you get that idea.      
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'.




>What else am I missing here? 
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/B00153...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'.
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#5
Saw this youtube on ATS (as I mentioned above) but gives detailed 'plan' to use it to charge your batteries while on external power.   It might apply to your situation and the detail is good Smile        https://youtu.be/iASOOzUCRto
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#6
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) of Jehu'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?
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#7
(06-19-2020, 06:59 PM)aventeren Wrote: Ah, one last thing. BMS. So I bought (4) of Jehu'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?
Nothing wrong with your assumptions.    Some BMSs also do a temp cutt-off.   You want to be aware of temps since this is an (outdoor) camper and avoid temp extremes.
Here's some typical Lithium ion numbers for guidance:   
Charge limits:   0°C to 45°C (32°F to 113°F)
Discharge limits:   –20°C to 60°C (–4°F to 140°F)


At a practical level, just cool or heat the camper to something 'livable' before putting any kind of load on the batteries Smile
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#8
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 I’ll have to think of a way to keep the battery system warm on those trips. Maybe I’ll 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? I’ll have to do some searching on that this week.
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#9
>Maybe I’ll create an insulated box that I can keep warm with a heat lamp or some other heat source.
Exactly.    

>Have you ever seen a low temp solution in any other posts? 
I've not seen youtubes on this.     I presume you plan to heat the trailer (maybe propane or diesel) and just blow some of that heat over the battery?    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 5w LED light to assist in turning on the heat.  This would not be nearly as risky/destructive to the batteries as turning on significant power - such as 1000w inverter - before the warm up.  
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#10
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 60°C (140°F) 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 40°F) and then a high temp off trigger (maybe something like 50°F). 

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.
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