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diy solar

diy solar

My DIY battery thread

robbob2112

Doing more research, mosty harmless
Joined
Nov 19, 2023
Messages
5,486
Location
Colorado
I have been bitten by the bug and the only cure is to build a couple.

I have a wattcycle 300ah battery that turns out to be a 280ah battery. And from Will's review it doesn't have any over-current protection until you get well up around 800amps. So I am replacing that BMS with a JBD BMS SP04S060 - the 300amp variety with all the ports on it. I don't need heat, but doesn't mean I can't use the port for something else.


Of course since I got bitten by that bug I ordered 8 of the 314ah MB31 EVE cells from 18650battery.com . They are currently backordered, but come with bus bars. ETA is Feb 21st so it will be a month wait.


So I ordered 2 more of the BMS.... so I will have 2 of them from Aliexpress and one from their official store... so I can compare all three and see if the aliexpress version is inferior in some way and get a refund if it is. If it fell off a truck or was a QA reject that might not show until hooked up.

Funny thing on the MB31 EVE cell data sheets - some say the continuous discharge rate is 1C and some say it is 0.5C ... so I am not sure what is right - will see what the paperwork that comes with them says.

I will be looking to see if they are factory EVE cells with the terminals laser welded on or not - what I can find says the factory cells with terminals have countersunk dots on the lower part with a red and blue dot of paint in them and that the weld depth is 2mm. The other option would be for 18650 to have contracted with someone that buys the raw cells and adds the terminal after the fact - not sure which it will be. So long as the QR codes are untampered with and check out I think it will be fine. I do have a bench PSU and a load tester so I can check that they have the correct capacity when I get them.


Does anybody know what the ardiuno board with voltage measurements is? I saw it used in the MRBF explosive testing, but was thinking it might be a good way to watch/record/verify the capacity test independant of the meter I have which is the purple board with 4 resistors and is capable of 11 amps.


Beyond that I have some 3mm poron on order to put between the cells as insulators and am still deciding on either aluminum or steel or wood end plates. Presumably if I go to the local metal fab place with a drawing they can just use a laser and cut out the plates for me from either cold rolled steel or 5052 or 6061 - depending on what they stock. I only need 4 of the endcaps. I have 10ft of 1/2in threaded rod already and would just need to put springs over it to compress the cells.
 
Well, I got the BMS and it is pristine -

Installed it and all seems good - it reminded me I needed to connect the SW wire and short the ends -- i.e. discharge wouldn't come on.

I am going to print a tray that raises the lid and provides a spot for the BMS to live with a fan that is temperature controlled.... i.e. put a space under the heatsink on the bottom and feed outside air in from one end while it exhausts out the other or the sides...

I need to add at least 20mm in height to the lid to hold the new BMS - the heatsink is HUGE compared to the old on and the

This is what I see in the overkill app - - I tried the JBD apop, but it won't ever let me complete registration so I can't sign in and use it ... just a spinning circle waiting on a code forever....

This is connected to the wattcycle 300ah 280ah battery - so the capacity is 3840 * 1000 = 3840000 -- So this is what I am trying to set the capacity to --

but when I try to write it to the BMS it just stis on waiting for BMS.... What is the solution to this?

Are there any other changes that need to be made beyond capacity?

I am looking at your Mr Blue @sunshine_eggo


Screenshot_20250202_114253_Overkill Solar.jpgScreenshot_20250202_115047_Overkill Solar.jpg


20250201_160842.jpg


Old BMS vs new BMS
20250201_140507.jpg20250201_140558.jpg
 
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This is connected to the wattcycle 300ah 280ah battery - so the capacity is 3840 * 1000 = 3840000 -- So this is what I am trying to set the capacity to --

Huh?

280Ah * 1000mAh/Ah = 280,000mAh

If you're really trying to set 3,840,000mAh, maybe that's too high?

Cell full voltage for 3400 is too low. 3500 is more typical. You need a LONG absorption time at 3.40V to hit 100%. It doesn't matter that it doesn't match the 100% cap voltage value. Those are just for BMS SoC guesses when it loses track of SoC.
 
Huh?

280Ah * 1000mAh/Ah = 280,000mAh

If you're really trying to set 3,840,000mAh, maybe that's too high?

Cell full voltage for 3400 is too low. 3500 is more typical. You need a LONG absorption time at 3.40V to hit 100%. It doesn't matter that it doesn't match the 100% cap voltage value. Those are just for BMS SoC guesses when it loses track of SoC.


AH, I am an idiot - I was reading the side of the battery

trying 280000

and it wrote -

I set to 3500 for absorbtion

I am being de-virginized where it comes to batteries and BMS - one of the subjects I never did a deeeeeep dive on yet
 
NOTE - first lesson learned - when disconnecting things - tape all the exposed battery metal first... then as each connection gets disconnected tape or otherwise insulate it. I thought I was being careful enough until I heard the pop and smelled the ozone... Just a little black mark on the old BMS.

No harm done fortunately.
 
Mos time was one of them. It only found under parameter veiw.
There was some stuff I found online and on this forum about it. There was something else, but I forgot.
Both seem to work fine other wise
 

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Interesting thing -

I am testing with both a heat gun and a ceramic heater with a fan.

The heat gun maxes out at 11 amps and is fairly stable - turn it on and it climbs to that and stays, turn it on low and it is 65 amps....

The ceramic heater is different - turn it on and it climbs to 115 amps and is pretty stable at that on hight... BUT if I adjust the temperature knob on the heater to turn it off it drops to just 0.5amps ... but when I turn it back on the current spikes to 215 amps for a few readings then drops back to 115 amps and stays...

That seems like an inductive kick of some sort?? I assume the nicrome wire is wrapped in circles around the ceramic core, but I didn't realize it would cause a current kick like starting a saw...
 
Yep all good now -- how the heck did it know my battery size I wonder - or is it possible there is a max limit and it hasn't been run into until cell size has grown?

Yeah. Was thinking a 999,999Ah limit or some such. You exceeded this, so it was giving you a passive "eff you and your effort to write bogus data"

I wish battery heating upper temperature was adjustable. Seem to shut off at 15C

why?

And showed when the heater is on.

agreed.

This old app is still the better one. As far as I am concerned....
A few things need to be set on their app instead of the overkill app.
But I use both.


I've used both the overkill and xiaoxiang. Concur that the old xiaoxiang is infinitely better, but I still prefer the Overkill.

NOTE - first lesson learned - when disconnecting things - tape all the exposed battery metal first... then as each connection gets disconnected tape or otherwise insulate it. I thought I was being careful enough until I heard the pop and smelled the ozone... Just a little black mark on the old BMS.

No harm done fortunately.

If you don't make sparks at some point, you're not maximizing your learning experience.

:P
 
Probably not super critical but
My 12v pack, sits in my trailer. And gets charged during driving. And the low current through the 7flat rv plug, doesn't leave much for actual charging. 2 amps max. If the heater shutoff a bit sooner it would charge more, approx 5amps. When it is -15/-10C outside by the time I get anywhere to work it's done no charging just mainly heating. If I plug into shore power it's not a big deal....

I kinda underestimated how long it would take to warm the batteries.

I really should have a DC/DC charger in the trailer to help it. Or run bigger wires.

but since we are talking about this.
have you noticed that at low charge current the bms doesn't cut out the charging when below low temp cutoff?
seems to need a good 4-5amps for it to kick off charging...
in my case the heat mat is on and drawing and not turning off the charging so it inputting 1.5amps ish...

Can the cells handle a tiny amount? Or is it really bad.
Eve mb31 314ah
 
Ah... that makes sense.

Yeah, they can, but it needs to be VERY low... probably so low, it wouldn't make a noticeable difference in your situation.
Is there a % number? Because I assume bigger the battery the higher very low becomes...

I have ordered a delay relay for the heater and I hope that will allow it to trip out the charging, before the heater comes on.
 
Showing when the heater is on seems like a very simple add-on... LED and a resistor across the heater wires... Just limit the current to the minimum...
 
Is there a % number? Because I assume bigger the battery the higher very low becomes...

I have ordered a delay relay for the heater and I hope that will allow it to trip out the charging, before the heater comes on.

Numbers I've 0.005-0.01C, so 0.5 to 1A for a 100Ah battery.

You would need to bypass the BMS.
 
Interesting that when I hooked it up originally the cells were all around 3.3v ~ 3.5... since then they have all floated up to 3.6v except the one cell and it has constantly lagged the others. Makes me think that Wattcycle's BMS was using a lower setting for max charge than I have on the JBD



Screenshot_20250203_090758_Overkill Solar.jpg
 
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