My Nitecore UMS4 USB had inconsistent tests on internal resistance from the start, then the display pooped out on me, went to a basic charger from Night Owl i think…
But im now getting into 21700 batteries and wanting a good, powerful charger that can give me more info, check for capacity and possibly battery health..
Needs to be able to run on 12v D.C. or HP USB..
Im still searching now but thought id ask here as well.
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My charger of choice is the Vapcell S4 Plus.
I asked ChatGPT this question…
Can a Vapcell S4 Plus run on 12v D.C. or HP USB?
Here’s the answer… 
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Vapcell S4 Plus V3.0
I have it about 3 years and im very pleased with it ! (Especially for it’s money !) (And the powerbank function it works perfectly, i test mime 2 times without any problems !)
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Everything about it sounds right, definitely the 12v D.C. input, my whole place runs on that from a solar system, inverter for the A.C. and fridge tho..
I like that you can set current manually, in auto mode does it push the batteries hard for a faster charge or just enough for them to get warm ?
I do have to question the numbers on their batteries tho.. 4000 mah 18650s !! Hmmm
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I bought 6 of the 3800 mAh cells from them. They ranged from 3760 to 3810 . 1 amp discharge on the Mc3000.
If the pattern holds, I would guess that the 4000 mAh rated cells should be pretty close to that.
I was expecting to be disappointed, but I was not.
They are low CDR though… 10 or 12 amps IIRC
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Didnt think a 18650 could be more than 2600 mah..
I was gifted some that said 9000 mah but was half the weight of my Samsung 2600 mah..
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Miracles of modern technology… 
Yes there are crap cells from several places that just lie about capacity. I was skeptical too, until I tested them.
Genuine 3000 mAh and even 3500 mAh cells from the big brands have gotten pretty common.
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Nice..
Need to get my hands on some and make a power bank, had planned on doing one up with 21700s to make a 12v for camping..
Thanks for all the info..
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21700s would be better if bulk is not a problem. The top Vapcell (I think) is the F60 with a rated 6200 mAh.
But there are quite a few rated at 5000 mAh and high CDR as well. Check into the recent tab less cells.
Im leery of the cheap BMS, DIY power bank units, will need a 3s unit that i can get 12v, 5v USB and a type C charge..
The charge in, usb out and high and low cutoff is normal, but if i wanted to pull 12v is it possible to still have low cutoff ?
Ive not seen that so thats what is holding me back..
I use a small 7ah agm 12v for camping, got direct 12v and a box with an on/off switched twin 5v usb.. run a small fan and charge lights ..
Would love for it to be smaller and not weigh as much..
3s 3p would give me the 12v and 15ah..
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Is there a reason you’d avoid the likes of Ecoflow type “powerbanks”? Most seem to offer 12V output and are fairly well reviewed.
Failing that, a few of us have the ZY-Q6-100W DIY Powerbank thing. You could pick up a 12V USB C PD trigger as long as your power draw is less than 36W (3A).
LiFePO4 cells seem like a better voltage match if you’re trying to substitute Pb-acid, but (last I looked) the availability of BMS etc parts is not as good as the options for NMC.
I would imagine, if you’re using any devices with their own Pb-acid 12v battery low voltage cut off, that you might not be able to get the full capacity of a 3S NMC arrangement as this will be 9-12.6V.
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Good question, but I think it is possible. The low voltage cutoff is on the battery side. The 12 volts on the output.
It would seem like you would need a boost for the output. I am not sure I have seen anything that you could DIY that does both in a single circuit… But I haven’t really looked.
I second this. I have a little River 3+ unit from them. It has USB A and C outputs (100 watts on the USB C), A 10 amp 12v volts output, and AC outlets that will give 600 watts. There is an expansion battery pack for it that doubles the capacity and has a 140 watt USB C port on it. Uses liFe batteries. It covers most of the bases… except if you need replaceable cylindrical cells.
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I got the charger ordered, thanks guys.
Seems ill do a multi circuit, one for charging and usb output, a second one for 12v output and safty cut off. And im wanting to build it myself for the fun of it..
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This is a big ol chager, dang.. already doing a capacity test on one of my oldest 18650s, funny, advertised new as 2600mah… its at 2624mah and still 18% to go on the discharge side, will know more on recharge.. not bad for an 8 year old cell.
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I have quickly scanned/read this topic, so if my message below isn’t fully on topic, I apologize.
It is not a DIY powerbank, but it is a ‘power station’ which also has a 12V 10A out. A while ago I managed to pick up the Newsmy P72 from Aliexpress for ‘cheap’ and so far I really like it. I can’t find it anymore on AE at the moment, but here is a link (and price) to Newsmy their own website:
It is a 270Wh (11.25V/24Ah) based on LiMnFePO4 chemistry (Mn = manganese), instead of ‘regular’ LiFePO4 cells, which has almost all of the benefits of LiFePO4 cells, but has a higher energy density (per weight), due to the higher nominal voltage.
Besides 2x 100W USB-C (in/out) ports and 2 USB-A ports, it also has a 12V/10A output. Charging can be done via USB-C, it can handle 2x 100W input with that, but it also can be done via the cars 12V output or via a solar panel (MPPT max 100W).
Also comes with a bunch of adapters:
I managed to pick it up for € 60,- (with the current exchange rate ~$68,50) and so far I really like it. It’s not the most light weight power station with 2,3kg, but that is to be expected due to its 270Wh capacity. Still, its size is relatively compact for the capacity.
The only downsides I’ve found so far, is that it doesn’t show battery %, but only a battery symbol with 5 squares. The other downside is that, when not charging/discharging, it doesn’t seem to have an automatic turn-off function, as far as I can tell.
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What is the safety cut-off you refer to. Is this protecting the cells? If so, on a BMS it is commonly referred to as Low Voltage Protection (LVP).
If you mean something else, please explain. Your usage of the phrase in seemingly different ways has me confused. 
Yes LVP
I concur with Mandrake50, it’s not quite clear what you’re trying to build.
Typically a battery pack will have a Battery Management System (BMS) which will protect (normally) against over voltage, under voltage, over current.
You will have the outputs of the BMS available to provide power to whatever load you wish within the limits of the BMS.
You cannot safely tap the battery directly.
Your proposed pack will be between 8.4V and 12.6V assuming you use typical NMC cells and depending on state of charge.
If you’re using a 12v appliance with it’s own “battery saver” low voltage cutoff, these are normally configured for Pb-Acid “12V” batteries and will cut out at c.11V, which leaves about half of your DIY battery capacity unused.
Unless you find a BMS-Charger solution (which I’ve not yet seen), you will need a charger of some sort to follow the correct Constant Current- Constant Voltage profile to recharge this battery. You will want a BMS which balances the 3 parallel strings of cells.
To expand a bit more on DIY battery building, spot welders are quite affordable now, there are “bolt together” cell holders which I’ve considered in the past, and some of the LiFeP04 cells have threaded connectors so would not need welding.
This comment is not to put you off completely, just be aware that Pb-Acid and lithium batteries are different beasts.
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Yeah i got a PB in use now and why im looking into stuff to properly use lithium.
Most of the units ive seen will have a type C charge port, std usb power out port, but not seen one that also has a 12v output. So i was going to get a separate unit that will cut off the power for the 12v output if the voltage drops low enough, and im talking 3s lithium protocols..
Unless the units ive already looked at has a way to get all of the 3s power with protection AND the 5v usb..
Ill show one here in a second.