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6s vs 7s for 24v vanlife setup
#1
Hey guys this is my second post on here. I've been drinking the 18650 koolaid the last few weeks eating up youtube content and i am building a ~400 cell powerwall that i want to integrate into a vanlife build that i am doing. The problems arise in that i bought this cheapish charge controller from ali express (https://www.aliexpress.com/item/40008329...0832946950)

And i'm wondering whether i should go 6s or 7s to make everything work right.

I am doing a 24v system because of the higher wattage solar panels available that i want to mount on my pre existing roof racks (it should be easy with the z brackets or bars.) My problems arise because i can't seem to find a good inexpensive inverter that will work with 24v nominal in actuality it's 4.1x7s=~28.7 volts. Ok i was going to buy this one: (https://www.amazon.com/LVYUAN-1500W-Inve...54&sr=8-11)

It says its good for 18-30v and yet it has a over voltage shutdown of 27.5 which obviously wouldn't work with 7s. So it would work with 6s apparently, but then would going with 6s cause problems with the charge controller? Ok it looks like i am better off going with 6s because the low voltage detect and float voltage are adjustable on the charge controller, and can be set for 6s's charging parameters. I also think 6s would allow me to use one of these 12v to 24v boost converters (https://www.amazon.com/Cllena-Converter-...ics&sr=1-5)
And tie it directlyinto the charge controller off of a dc switch, to charge off the alternator.
I basically answered my own question here i think, but i would really appreciate peoples advice here as it seems more people recomend 7s vs 6s, and i was wondering what the down sides are for 6s.

Oh yeah and the cells that i got so far if anyone cares are the ones from batteryhookup with 20 cells for .17cents a cell. I've opened about 17 packs and most are the red sanyos from 2006 (sadface, but atleast they dont overheat like the first one i did that had the purple coller, those are legit heaters and the ones with the red on red are cool in comparison) Also there were 3 or 4 packs of the pink samsungs (they all test at 2000+ma, and are from 6 years ago) one pack with the lg's The red sanyos seem to be trash as the best ones test at 1500ma, and most are around 1000ma. I don't know definitevely as i have a knock off charge controller called the bc-3000 whitch doesnt seem to discharge consitently if that makes sense with the red sanyos, i don't know if the red sanyos are really that bad or if the charge controller is just junk. I've got a load of laptop batteries on the way so hopefully i will find some decent cells.
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#2
In general, I'd recommend 7s for lithium-ion - this mirrors traditional 24v almost exactly.   You'll find plenty of equipment / options for that range.  

If you want a cheap inverter - Reliable - https://www.amazon.com/WZRELB-Reliable-Frequency-Generator-Converter/dp/B01EML562E/ref=sr_1_3?dchild=1&keywords=Reliable+inverter+24v&qid=1590085895&sr=8-3  is OK but it has no 'overload capability' at all - will blow.  
(I've had several of these - blew 2 of them up and use a 1500w in my 7s DIY portable solar generator)

If you want a next level inverter with grid charge / ATS / Surge capabilities) - AIMS is good - https://theinverterstore.com/product/200...charger-3/
(I use AIMs as my main home inverter - The 2000w @ 48v and 12,000w @ 48v models)

Not pushing either of these, just sharing that they have worked for me.  There are many others that are equally good.
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#3
I have an AIMS 1500w 48v inverter that works well. They make it in a 12v and 24v model as well I believe. The only complaint that I have is that it has some sort of interference that messes with my bluetooth ear buds. Otherwise works great! Fan on demand and it does what is says. It was under $300 so I would put that in the cheap category. I also have some Victron inverters. Those are great! The phoenix ve.direct 1000watt is about $400 which is reasonable. Both have a wide voltage range that will work with a 7s setup really well.

You didn't mention your requirements. How much power do you need?
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#4
What are you planning on running with your battery pack? If you just plan on running a fridge and smaller devices, a 3s will simplify everything and you shouldn't need any boost converter for most 12 volt devices including inverters, I never needed one. Right now I have a 220ah 4s lifepo4 in my van, but previously i used a 94ah 3s li-ion and still use a smaller 31ah 3s16p pack as a backup. Right now I'm collecting more cells to build a larger 3s battery pack in the 100ah range. 

Any solar panel 21 volt or 36 volt will charge a 3s, the solar converter will drop the voltage to what you need. The mppt from alixexpress I was reading the reviews (in spanish) and someone mention its not an mppt controller, its pwm. It will still work but won't give you any increase amps like a true mppt would. A true mppt would be in the 100 dollar range for the cheapest one.

Since you still haven't built your battery, I highly recommend you get y1030 IR tester(I payed 57 dollars) , very useful for old batteries. I just got my tester about 6 months ago and its one of the best purchases I've done. I been finding alot of cells that I would have used in a pack that had high IR. Last thing you want is to build a pack and then have problems later on. Use every tool available to weed out the bad cells. With this tester its quick to test the cells, any with high IR you don't have to waste time doing capacity tests. It won't take long to test 400 cells and seperate your good ones from the bad. I have used red sanyos before in my battery packs with no problems, now I check them with my iR tester, they check between 50 and 60 milliohm which isn't bad for laptop batteries. I notice cells with high IR always get hotter then other cells when tested at 1 amp.

As soon as you connect a load to the 7s or any pack the voltage will drop, so the 24 inverter should be able to work with a fully charge 7s.

yr1030 IR tester
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#5
RE:”Any solar panel 21 volt or 36 volt will charge a 3s, the solar converter will drop the voltage to what you need.”

What solar charge controller are you using for 3s Li-ion. Almost all 12v solar charge controllers are not comparable with 12v Li ion

I have seen 24v Li ion with step down converter as the most common way to get to 12v
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#6
Not everyone gets it right the first time... you may have to re-work some of your setup.

Last thing you want to do is go cheap on the charge controller... Why not get a nice $120 Victron Smart MPPT?

Not only will it work better than what you listed, it's fully programmable making it 100% compatible with lithium batteries.  It also  has Bluetooth connection for data and statistics logging.  Its perfect for your use as you can see how much solar the van is generating from your phone.

This is what I use in my off-road truck, and it provides great insight into what the system is doing without extra components, meters, or gauges. Also has a low voltage disconnect which you can use to control a relay for "always on loads" like USB sockets.

These days, at least for small projects, I think buying anything else is a waste of money.  But that's just my opinion.

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