RC Lipo pack inspection, series/parallel charging, and lifespan/safety tips

Got an e-bike for cheap at a garage sale...however...no battery. I've been doing RC stuff for years so I figured I could handle it. The bike says "48V", but I have no idea if that is "nominal" (3.7v) or max. I decided to get a total of 15 Lipo cells (5 x 3S)
Almost certainly it means nominal, and almost always 48v means 13s (52v means 14s). Fully charged it probably only takes 54v or so.

It may operate normally at higher voltage, but it's LVC (and any display/battery meter it may have) is probably setup for the lower voltage pack, with controller shutoff probably around 41-42v, so if you have no BMS on the pack to prevent overdischarge, you may need to monitor at least pack voltage manually and stop discharging above about 45-49v depending on how low the cells you have can safely go without stressing them.

I had a variac and some big caps laying around and obviously a rectifier. I set that to about 65 volts out. I charge it through a watt meter that shows me the voltage. Now if I let it go forever it will overcharge them a bit, so I'm always watching it. I go to about 63 volts (4.2v per cell).
What did you use for current limiting to prevent cell damage from overcurrent?

What did you use for individual cell monitoring to ensure every cell remained under maximum voltage?

So now I'm thinking I could "split" the packs into 2 6S packs and a single 3S. My charger has dual 6S. So I could balance charge the two 6S packs at the same time and charge the other 3S with another charger or wait and do it on the same charger.

If any of that make sense or not, please advise. I'd like to charge it like I have been as one 15S pack, but it clearly needs a balancer to keep it in check.
If it is going out of balance, the cells are not all the same characteristics (capacity, internal resistance, etc) and will never behave the same as each other. Using matched cells with the same characteristics will keep the pack balanced (all cells behaving the same) without an external balancer until the cells age enough to become different in characteristics.

Balancing unmatched cells will make them all the same *voltage* at some point in the charge or discharge curve, but doesn't make them the same capacity, etc. so the pack will still only ever perform as well as it's worst cell.

If you just want a whole-pack (15s) balancer, there are balancers for various chemistries, that can be used while charging or even left on the battery all the time if desired (like a BMS would be); you just have to pick one that is either programmable or is preset to the balance voltage you want all the cells to end up at. Each balancer channel also needs to be able to shunt the amount of current your power supply setup provides when the pack is at full charge. If it can't, then cells that are already full will continue to charge, albeit at a slower rate (by the amount of current the balancer *can* shunt) than the others.

Otherwise, splitting the pack and using the RC chargers in balance mode will work, with the complications of unplugging and plugging things and the potential risk of misconnections or damaged / failed / poor reconnections.
With the 65v output from my variac setup, it would charge at about 2 amp and slow down nearing full charge. I originally balance charged all the 3S packs to 4.2. But I would pull the pack out and apart once in a while to balance charge them again. I guess for now I'll do that at least every other charge since the cells seem a bit unequal now (different ir?)

It would be nice to charge it all at once, but I really need to get a balancer or BMS to do that. I've been looking for a balancer but they all seem to be from China and some of the shipping times go into August, uhg.
2 amps isn't bad.

You should still cycle those batteries to see what they're at, a $100 iCharger is very cheap insurance against a lipo fire ( price range is $2000-$1,000,000 ) :mrgreen:
Update on my Jerry rigged pack. I have replaced one of the bad 3S packs with a decent 2S pack from racing onroad (so now it's 14S). It seems 2 of the 3S packs are pretty bad, they heat up during a ride and those same packs take a really long time to charge up (albeit, about the same mAh).

Yes, they were HK packs and only 20C at that.

I have my little watt meter (Astro Flight, it must be 20 years old) hooked up in series on the bike and boy does the voltage drop under load. Even on a fresh charge it goes from 58v down to 45-48v at 12 amps. So really I only have 2 decent 3S packs now. Surprisingly I've seen upwards of 600 watts on a full charge.

Any suggestions for a reasonably cost efficient cell/packs? I'm thinking I should probably retire at least those 2 packs that get hot. And maybe just redo the whole thing with new cells at this point.

I had some experience with round 18650 cells, but as I recall they only delivered 10 amps or so. Not looking for huge capacity, my rear end seems to be my limit on riding time.

Oh and any advice on a BMS? Probably the most important thing, ;).

Oh and any advice on a BMS? Probably the most important thing, ;).

You mean battery murdering system? Sounds like you know enough to be able to do your own monitoring and avoid the possible problems a BMS can cause. NEVER trust a BMS.
Hi, I'm planning my first ebike. I have a 26 in steel frame bike and I'm thinking of a 350-500w front hub. My commute is a flat 5 miles. I have years of flying rc planes with lipos. These batteries from hk seem like a good fit:

Huge capacity, light weight and a screaming deal. They are 10C, which is really low compared to my airplane lipos, but at 10Ah should be capable of 100 amps. The bike load should be more like 10 amps or only 1C, right?
I've been using Multistar 6S X 10Ah (the big boys) for years. I bought a whole bunch when Hobby King was almost giving them away. 10C is plentry for an Ebike and I don't think they would catch fire even if they were vigoriously rubbed together.
What I have learn during the last 10 years of powering my ebikes w/ LiPoly;
1) Forget the BMS.
2) Don't parallel, if more capacity is needed, get bigger bricks!
3) Bulk charge and avoid "breaking the string". I use a very nice sealed MeanWell w/ adj. Voltage (HGL series, I think).
4) Don't go over 4.10V or below 3.65V and keep them at a storage Voltage of 3.90V if not using that day.
5) If they do start to stray, balance w/ Battery Medics (the blue ones w/ access. lights to discharge. Not that you will need to fast discharge, but these are the better BMs.).
6) Keep waiting packs in Refridge. and they will store forever.
This is the way I have been doing it for years and it's become second nature. A hr. before I intend to go out, I bring the pack (Usually just two 6S's at 12S/1P for 10Ah.) from storage Voltage to 4.10V and ride. When I come back, I bring them up to a little over 3.90V and use a pr. of Battery Medics to drop them down to 3.90V exactly. The nice thing about BM's is they can be left alone. This means they are at perfect balance @ 3.90V and if they stray some on their way up to 4.10V to get them ready for the next ride, unless one is greater than .005V over/under, I don't worry about it.
If I don't over-discharge, the Multistars are good for scores, if not 100's of cycles.
PS I did use MS 4S bricks for a while, as they less expensive. But they took up so much room in my batt. container as opposed to the 6S's.
PSS I cut the XT's off and use bullit connectors. I find they are easier to work with and take up less space inside the batt. container.
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Ok, Battery Guru's...

1.) I have accumulated far too many lipo cells from various sources - mostly 'pre-owned' singles. Time to unload'em, but how?
Seems many would suggest to gently discharging them, then transport to the nearest recycler. But now I'm reading tidbits stating NOT to completely discharge - potentially initiating an explosive fire hazard. So what say you?

2.) I'm on the threshold of a fresh install of (new) LifePo4 solar storage batteries. So, my understanding is (and please correct me if I'm wrong), to avoid charging or discharging these puppies when temps drop near 30f. I got that, but what about safe temps during static (disconnected) storage? Will LifePo4 survive below freezing?

Appreciate the wisdom All.
2.The accepted temperature that will damage a LiFePO4 cell when charging is 0 ºC 32ºF what does the data sheet on your cells say? should give storage temperature too. if you build a insulated box to install your solar batteries in that would help place a low wattage light/heating pad into insulated box on a thermostat set to come on at 5ºC 41ºF that could work. from what I understand is charging below 0ºC is harmful. Some LiFePO4 can be discharged down to -20ºC -4ºF. It is best to have a plan heat pad/light bulb, etc for winter if you are in areas where it dips below freezing. But first check with datasheets on the cells
Later floyd
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Thanks floyd,
Your numbers closely mirror what the manual states. Just preferred to confirm it from multiple, reliable sources.
Yes, I'm mid-ship assembling an insulated storage container. Still undecided what to employ for a heater. I'm far enough north to see occasional negative (F) temps.
From what i understand, discharging to 0v makes them contain less power ( therefore less flammable ). The problem is that, if charged after, they're damaged, so could go kaboom.
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Nice variety... thank you. I'm off-grid, so I need to be mindful of Watts (and reliability) of the heater.