Seeking custom pack builder + advice on power for a 24v Oset

B

BikePilot

1 mW
Joined
May 29, 2016
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13
Could anyone recommend a high quality pack builder that could put together a 6s4p NCR18650GA pack with BMS for me?

I'm interested in using a 18650 pack to power my son's Oset 12.5 (a very small kids trials bike--rider is 2yrs old). It comes standard with 2s1p 12v 10ah SLAs. My primary motivation is reducing weight as the bike is really heavy for him at almost 50lbs. If I can get extra run time and less voltage drop under a load that'd be awesome. Based on what little I know and what I've read, a quality 18650 pack, like maybe the Sanyo NCR18650GAs in 6S4P, should give me a 10lbs weight reduction, more range, and more consistent voltage as compared to the current SLAs. An alternative, and the most popular solution I've seen for these bikes, are 6s Lipo RC packs (the 10ah multistar from HK is very popular). I'd love something a bit more durable and less fussy than RC lipos if I can get it done at similar weight. To keep things simple I'd like to retain the stock controller and motor so need to say close to the voltage range of the SLAs.



 
Thanks so much! Location is Tampa Florida in the USA. Size is pretty flexible. My current SLA battery pack is 150mm long by 125mm wide by 110mm tall and the bike has padded steel battery tray and hold-down (very much like you'd find in a car) to fit these dimensions. There is space to go up to 170mm long, 125mm wide (so no extra room there), and probably 140mm tall (a bit tricky to measure, but there is quite a bit of space above the current battery pack).
 
Ive dealt some with 24V system wheelchairs and lithium batteries, and I prefer using 7s li-ion. The 6s holds a lower voltage on average than 24v lead acid.
The battery "energy gauge" shows almost empty when there is plenty of battery left. By using cells that fall some in voltage over the discharge cycle, I have had better real charge state representation on the gauges from 7s. If the bike has no such instruments, it would only affect power and speed.

For applications like this, I would use 18650 holders, which means roughly 20mm per cell, and roughly 70mm total height of the pack alone.¨
The main reason being rigidity, and air around the cells. Cells in holders are formstable enough to be clamped down, if wrapped with the approperiate liner.
Another advantage is that balancewires can be run inside the pack rather than on the outside if desired.
7s4p would be something like 140x80x70 (LxWxH) mm. Or 120mm lenght for 6s. So plenty of room.
Shouldnt be hard to find a builder for this in the US.
 
Thanks so much, that sounds like a perfect approach, I'll research the battery holder option. I'm not sure what the upper limit in voltage is for my controller so was thinking of playing it safe with 6s, but maybe that's taking things a bit too far. 7s would max out around 29.4v, right?

Has anyone ordered from batterybro.com? They are able to build a pack to spec and appear to use quality grade-A cells.
 
24V led acid battery on charger reaches at least 28V, so, only 1.4V difference is not problem.
Most of 24VDC equipment has declared maximum voltage 30V.
Two choices are:
1. LiIon/LiPo 7S (max charging voltage 29.4V)
2. LiFe 8S (max charging voltage 28-29.2V)

LiFe with proper BMS can be easily charged to 99% when it is charged up to 3.5V/cell.
While testing my BMS, I tried with Headway 38120S and charged 8S pack to 28V and let it to balance to 3.5V/cell.
When the pack has been well balanced (cell voltage difference <10mV), have changed balance threshold from 3.5V to 3.75V and started charger again; the 10Ah battery took only ~0.1Ah when reached 29.4V with all cells balanced to 3.75V. 8)
Charging battery to bit lower voltage will extend it's life, as well as avoiding deep discharging. For "cost" of ~10% of battery capacity you can extend battery life ten times or even more...
 
The funny thing is if you look at the spec sheets on the SLA/AGM 12v 8Ah batteries such as this UPG: http://upgi.com/Themes/leanandgreen/images/UPG/ProductDownloads/D5743.pdf
You will see these batteries only put out 4.8Ah @ a 4.8A draw. You'd be drawing at least double that maybe 4x that. So the actual output capacity may only be around 4Ah!
So to get similar runtime all you'd need as a 7s2p pack (14 cells total!) That weighs about 1.5 pounds vs 10 pounds for the SLA's.

I just made a 7s6p pack using NCR18650BE's for a 300W electric scooter (to replace 2x 8Ah SLAs). The 7s6p pack I made is quite a bit smaller and ligher than the SLA's but stores like 3x+ more energy! I used BE's because they were a fair amount cheaper than the GA's. I do agree that 7s is a better choice for a 24v replacement, but might put a tad more stress on the motor and controller, but the overall voltage isn't much different. 25.2 or 25.9v nominal vs 24v. But the voltage sag will be a lot less with several paralleled Li-ion cells. I measured 25 mili ohms on new 8Ah SLA's. You'd be at like ~8 miliohms from 4p GA's.
 
are you planning on getting stock packs from china or would you rather invest in quality cells with a custom made pack to size?
people willing to make a pack in florida will be difficult to find. if you can find someone it will probably not be near you with cheap shipping costs.
i am capable of building custom packs but i live in europe making shipping prehaps a bit cost prohibative if you have a tight budget.

using standard cina packs will probably be cheaper but the lower quality cells/packs and lower capacity might make it a bit of a waste of money when compaired to a high end pack.
 
Thanks! I much prefer to use a quality pack and am not terribly concerned with cost. Thanks everyone for the advice. I'm not opposed to building my own pack also if there are no reasonable options for buying one ready to go. I'll need to learn best practices but am generally comfortable with most mechanical things, soldering, etc. What is the consensus regarding using BMS in the pack vs an RC style balance charger like a powerlab 6 or something?
 
BikePilot said:
Thanks! I much prefer to use a quality pack and am not terribly concerned with cost. Thanks everyone for the advice. I'm not opposed to building my own pack also if there are no reasonable options for buying one ready to go. I'll need to learn best practices but am generally comfortable with most mechanical things, soldering, etc. What is the consensus regarding using BMS in the pack vs an RC style balance charger like a powerlab 6 or something?
Click to expand...

If you install a fuse + low voltage alarm/volt meter and a balancing plug you may be able to get away with not using a BMS.

Soldering is not really advisable because it can damage the cells. You can either build or buy a cheap battery tab spot welder for around $100-150 though, and you could always sell it later if you don't have a further use for it.
 
bms is WAY safer. using a RC charger requires you u have a RC charger on hand and limits you in choice, quality is also an issue.
having a BMS with balance built in only requires you to have a charger that can get the voltage needed and the BMS will do the rest. much more independent solotion then getting a new RC charger if it dies.
BMS also limits current and watches the voltages, if a cell gets too low it will disable the pack until it's charged again protecting your expensive pack from killing it with discharging it too far.

i would never build a pack in something like this without a BMS. soldering is also not recommended, the heat needed to solder is not good for the cells and might even pop the protection on top of the cell (if present). spot welding and proper cell holders are the only real choice for packs in stuff that gets trown around a lot.
 
Its a child that will ride the bike -> The system has to be very user friendly.
I would say a bms is mandatory for the low voltage cutoff.

There's a consensus that soldering "hurts" cells, and I believe it does, but I have never seen any data that quantifies the bad effects.
I have a pack out of 180cells (12s15p) that has been used for the past 3 years mainly during winters, that I soldered before I had spotwelders.
It doesnt have a big amount of cycles, maybe 150, but I havent noticed any damage or abnormal capacity loss at all.

With this Oset pack, its only 25ish cells, so the batterypack will be fairly cheap if you build it yourself.
If the alternatives are expensive or takes alot of time, I would seriously consider soldering with a 100w+ iron as a last option.

Batteries for spare time hobby products usually sees less cycles than daily transportation packs.
How many cycles does your child put on the current battery in a year?
 
Soldering can be especially bad on cells equipped with a PTC. But a lot of high drain cells don't have PTC's. The heating up of the PTC causes it to change phases and disable the cell, once it cools the cell will work again. However just one instance of tripping the PTC can cause PTC's resistance to increase for days, weeks or months if not longer.

I don't know any data regarding damage from soldering, but the other reason why most advise against it is because Li-ions can become unstable at temperatures in excess of ~80C+
 
i have popped the protection on quite a lot of recoverd cells when i tried to solder them with my 85W iron.

soldering is very not recommended and a incredible waste of high end cells (and a LOT of work!) if you kill them because of that.
 
Thanks! So no soldering :) I'm not against buying a spot welder and going that route if it's the best approach.

He just started riding a few weeks ago, but has really taken to it. Probably will average 5-6 rides a week so maybe ~300 cycles a year. Most rides don't come close to fully draining the battery, guessing maybe 20% most of the time. 90% of his riding is at a fast walk/slow jog making tight turns and stuff in the yard. I would guess that he'll ride 'till the battery is done or nearly done no more than once or twice a month.
 
Find out the amp draw of your controller. The controller will kill a battery if not matched with quality cells in high enough of a parallel of cells. The higher the amp draw the more cells needed in each parallel string. Plus use a bms lvc,hvc and cell balancing to make it plug and play. It is not best to run to cutoff. Leave some juice in the tank. You kill lead by draining to low. This kills lead over short time.
 
Thought I would follow up: A fellow ES member provided a 6s4p pack of Tesla cells (he built it, all welded and wrapped). I charge it with an RC balance charger (icharger 3010b, no BMS in the pack so far). We've been using it since July and it's working wonderfully. It knocked 11lbs off the Oset (nearly 1/4th the bike's total weight!) and has far more range than my 2yr old has attention span. The longest ride he's done was 11 miles and voltage was 3.76/cell at the end of the ride (started at 4.1). It certainly requires adult supervision, but most anything powered does when it comes to toddlers! I plug in a LVC alarm when he's riding for good measure and unplug it and the battery when not in use. BMS might be a good addition in the future, but for now it all seems to work well and I struggle to locate a simple, known-reliable BMS.

Thanks for all the help!
 
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