Buying first battery pack, what should I look out for?

[Loker]

Hello! I inherited a recumbent bike recently and I bought this 750W motor and speed controller: https://www.ebay.com/itm/142505890334

I'm in the market for a battery pack now and I've compiled a list of parameters that are relevant to my search. I'd like your opinion on the list - am I missing or underestimating anything? Here's my list so far (I know a few of things are pretty obvious):

• Price - looking to minimize this as much as possible while maintaining quality :wink:
• Pack voltage - looking for a 36V pack per my motor/speed controller voltage
• Capacity - Would like a minimum of 12Ah for my commute
• Cell brand - probably will buy something with Sony/Samsung/LG or Panasonic cells
• Connector type - Not a stranger to soldering and crimping stuff so I'm optimistic I can accommodate any connector.
• Waterproof - I live in a pretty rainy city so waterproofing is a must for this build.
• Seller - Inexperienced in this field so I'm not sure which marketplaces or websites sell reliable/quality packs. I kept seeing the names Luna and em3ev so I checked them out. Luna does not carry a 36V pack and em3ev has a few but they seem a bit expensive compared to ebay/alibaba. https://em3ev.com/shop/em3ev-36v-10s4p-super-shark-ebike-battery/ Is this due to the quality/reliability of the cells?

Are there cheaper alternatives than em3ev and Luna that are still reliable? Anything else I should be considering in my list?

Thanks in advance!

 

[amberwolf]

Much of what is on ali* is not very good, even garbage (sometimes literally recycled garbage cells). Some of them aren't bad cells, but they lie about what it can do; if you don't know enough about cells, batteries, etc., you can get ripped off. (plenty of threads about that )

Some of it is alright, but it's hard to tell what is and isn't until you have it in your hands and test it's capabilities over time.

If you're looking for specific cell brands, ali* and ebay / etc are probably not a great place to look. It's easy to rewrap cells with whatever markings one wishes, so counterfeits are common. And in a sealed pack you can't see them so you don't even know if they've used those, much less real ones, until you test it. Sellers will say their pack has in it whatever will make someone buy it, so unless they are a known-trustworthy seller, with stuff verified by other people that bought from them *recently*, people who do know their battery stuff and have tested it, etc., it's tough to know you are getting what they say it is. Some sellers are honest...many have no idea what they're selling, and some are just plain scammers.

Also, the cheaper it is for the same thing, the less likely it is to be what you want.

You generally pay more for knowing the stuff you get is what they say it is, and what it can do. EM3EV and Grin Tech are two that if anything probably underrate their stuff, but you can generally expect it to do what they say it can.

Some companies that might otherwise be good overrate their stuff; Luna and other places have been known to do that sometimes--I have an old pack here that *can* do what they say, but it is very hard on it, and it shoudln't be rated for that. So you have to be careful and check out what the cells in it are really capable of, not on the spec sheet under laboratory conditions, but in real life in a pack, without being hard on them (unless you don't care about longevity, voltage sag, etc).


Some other stuff you may or may not have thought about:

A lower voltage system has to supply higher amps to get the same wattage. So this is harder on the battery, and takes a better battery to do it, than if you used a higher voltage system (like 48v or 52v).

Those higher voltage batteries are probably a bit cheaper for the same watthours (range), because they're probably more common nowadays, in the higher quality packs.

For your system, they don't specify the actual current limit of the controller, only the presumably average power capability of the system as a whole. 750w / 36v = about 21a, so you can guess that the battery must be able to supply *at least* 21a continuous. But you can't know until you use a wattmeter to test the controller under load, which requires a battery.

The system you have is only 25 km/h top speed. That's 15mph. Presumably that is on a full battery charge, so it'll get slower as your battery discharges, so you may lose a couple mph or more over a trip (depends on whether that is top speed at 36v average voltage, or if it is top speed at 42v full charge.

750w should be enough to maintain that speed on at least some hills, depending on how much you and the bike and anything else on it weighs, and how steep they are (a 5% hill will probably start slowing you down, a 10% probably would overheat the motor and you'd likely be down to a fast walking speed), and against headwinds, depending on how strong they are. Is 15mph fast enough for you?


36v * 12Ah is about 420wh. Depending on the aero of your 'bent and the conditions you ride under, on completely flat roads with no wind, you'll probably take about 10wh/mile with no pedalling at 15mph, so you'd get up to 40+ miles out of it. If you have hills, let's say there's a lot and it's 5% average, it could take 25wh/mile or more, bringing range down to only 15-16 miles or so. If there are headwinds, it also brings range down.


As your pack ages, it will lose capacity. So if you get a pack that can *just* do the trip you want, then within a certain time it will not be able to do it unless you ride slower, etc. Similarly, if you ever have to detour around something, or have a bad headwind day, etc., or some worst-case combination of the above, it'll also take more capacity than it will for just the trip. So adding at least say, 20% to your desired capacity is usually a good idea.


Batteries in general are not built waterproof; some might be, but it's probalby going to require putting it in something like a Pelican case or the like to make it really waterproof, or potentially a fair bit of DIY to make the existing casing truly waterproof. Most of the cases they come in (if it's in a case at all, and not just shrinkwrapped or duct taped), would be better served by putting drain holes in them at the lowest points of the mounted-on-bike casing so the water taht does get in can get out.