Built my first pack, was going great until thing got saggy.

[Pete84]

Hi all just joined in a hope for some advice, I’ve tried to do this solo (YouTube)so far and just about pulled it off. If hindsight was real eh always something happens!
I got an ebike a while back and have been playing ofc, it came with a 24v battery zzzz slow so I joined two 18v makita tool battery’s and it was great at 36v, 5ah.

I then decided to build a 36v pack because I needed more like 7.5 ah to be safe, so anyway I managed to get 28 Samsung 26h battery’s and 2 25rs, (10s36v 7.5ah) they’re used but not badly atleast I was told that.

I built it all and went for ride combined about 12 stone weight on bike, it was going great until the batt got to about 37v and the hills combined with high throttle made the lvc kick in, dropped to 30v fast but then recovered to 70%, this never happened with my makita battery’s I could bleed em dry

my bike is bafang 250w 24v hub with a 350w controller rating 16-18 amps, I’m still learning about these but know a lot more then I did a month ago, do I need to throw this battery pack in the bin? I don’t think it can supply the amps can it? Or could it be my spot welding say not being perfectly flat as used batteries?.or just crap batteries?

Also you think I could 7s the 26h and use in a 250 w ebike or will it also sag to death.

[Pete84]

Guess was trying to say anything I could of missed that causes this type of drop like wiring? I’m using household electric cable but seems fine with my other batteries.? I’m Hoping I’ve messed up now because somewhere making it because it’s
gone from bad to worse , bms seem to be asleep nothing when turn key and won’t give more then 0.3v on c- or p- ,plugged into charge nothing happening.
took apart battery is still up at 36.3v cross terminals, voltage comes back when I unplug the balance lead but not right up, gives 32.7 on p- and 32.3 on c-. When I plug in voltage goes off again.

Got another query here as I’m paranoid now my battery’s aren’t very good and likely will make another pack at some point anyway , is nkok or cell supply.co.uk trust worthy, as after some actual reasearch this time into the battery’s and not just how to make packs I’m looking at some 21700 reclaimed Samsung 40t 35a thinking of 10s2p them for 8ah, is this a smart idea? also looking at some 30q in 10p3s but cost more.
And are reclaimed battery’s any good, it says never charged or discharged,

edit quickly took bms apart and looks like white oxide or something on balance resistors close to negative side, I scraped with needle and voltage has come back, battery now charging, could the sag be this bms?

[stts]

Yea, you have a spaghetti mess going on here. Your motor is being overloaded. 250watt at 24v is only good for 10 amps. Its no good for up hills. So the more hills you have the worse it is on your motor. The Makitta batteries work because they dont have a BMS in them. But they are hard on your bike motor.

You are a newbie and already building your own battery with used cells. Not good. You are using 2 different kinds of cells in the same pack. Not good. You dont have a clue what the condition of the used batteries is. Not good. You are not sure that your spot welds are right. Not Good. You have a BMS and it keeps shutting off. That says that the BMS wont allow you do do what the Makitta batteries have been letting you do. It could be you have the wrong BMS or its now damaged or the wiring is wrong.

You are still a newbie without enough skills for what you are doing. I would say to tear your pack apart and start all over again. Using a file to grind off the rough welds on the batteries. Then practice making good flat welds till you get it right. Practice at building a whole pack until you get it to work. Practice until the entire pack is very strong and doesnt move around when you pick it up. Learn how to make solder joints that you cant pull apart. It takes practice to do all that right. So learn how to do all this properly with what you already have before you decide to buy more stuff.

And if your using used batteries, you need to buy a testor that can test the capacity of these batteries. Because a reading of 3.7 volts doesnt mean its a good battery.

[Retrorockit]

He's in UK so 250W may be all he's allowed. 36V @ 10A=360W. I'm afraid though that 250-350W is what most bikes even with 1000-1500W motors spend most of their time at.
36V x 7Ah = 252Wh / 20Wh/m = 12.5 miles range. That would be flat and no wind miles. There's a reason 12Ah batteries are "normal" for OEM Ebikes.
Lower Voltage needs bigger wires due to more Amperage per Watt. So household lamp cord may not be suitable.
1- A small motor needs all the help it can get
2- Small battery packs are limited at what they can provide in the best circumstances.
3- Sketchy wiring, and used batteries can only reduce the results from ideal.
It's good that you have the skill and equipment to build your own pack.
Here is my advice- The only thing that costs more than doing it right the first time, is doing it right the 2nd time.
Save up for new matching cells, build a proper sized battery, and don't save money on wiring.
OR
The simple truth is the battery is the most expensive part of an Ebike. If you buy a decent sized battery it should last about 3 years. Compare riding your Ebike 3 years with constantly spending time and money on stuff that doesn't work.

[Pete84]

Thanks I think it was the bms playing up, I’ve cleaned it of what looked like water damage on the negative and first balance pin on the circuit board and the resistors near them, eBay for ya.
full charge was 40.8 the other day and now it took 42v, is this normal for voltage within certain ranges or was the bms having an issue undercharging?.

The battery now seems within my expectations for range vs a 5ah makita 36v.
I got 7 miles with some nasty hills, all hills here with me and lots of fishing tackle and never hit lvc before home, I throttle only level or downhill road, it will do me that!
Theyre good welds after inspect and it’s wired good, possibly needs better wires though, my head just went and questioned everything I did as I couldn’t work out why??
I take onboard what you’ve said and will put that into my next attempts for sure, I guess this is my prototype and now I feel like they’re easy to make once you’ve done one right? You learn the extra stuff you missed which would give it that extra life.

I only bought all the stuff to make them because I think long term it will pay off , now I’m thinking of putting 20x Samsung 35e inside two old makita battery’s then I could charge twice as fast and use my makita chargers as it will be two separate 18v 2p5s 7ah, this one takes ages with one charger and slotting those makitas into the battery box I built is actually quite satisfying, plus I could hammer my actually makitas and just fix them when needed.
kinda wish I’d done that first but oh well.

[AZguy]

42V is normal maximum [full charge] voltage for 10p, 37V is "nominal" - 30V is about as low as you want to go, a volt or so higher might even be better