4s Motorcycle Battery

1xAA

New member
Joined
Sep 27, 2020
Messages
27
Building a small pack for an ATV that charges at 14.5v <1000mah. Some are building packs w/o a BMS which I am not keen on. Can a safe 4s really be safely used in this situation. I ordered BMS, but wanted to see what the trend is in the community.

Thanks again for all the help.
AA
 

Korishan

Moderator
Staff member
Joined
Jan 7, 2017
Messages
6,401
It depends on what bms you have ordered and how many amps you'll be pulling.
 

1xAA

New member
Joined
Sep 27, 2020
Messages
27
I bought a 10a and a 20 for testing, but do I even need a BMS? I can;t imagine no, but some are doing this. Found one guy who put the balance cable on then removes for annual balance charge. lol
 

daromer

Moderator
Staff member
Joined
Oct 8, 2016
Messages
5,572
Yes you need One. You also need to understand current. 10a bms wont start your atv.

Best is to do LiFePo4.. also note that just because its doable doesnt justify that its the right way. Me for instance Still run la in most of My vehicles as starter battery... Or LiFePo4. I dont have anyone with liion :)
Neither less. I would never run a system. Without bms that doesnt do balanced charging and protection
 

1xAA

New member
Joined
Sep 27, 2020
Messages
27
Little pack just can't do it; cranked some but acted like a "low" battery barely turning the engine over. By the time I was done trying the cells were starting to warm up. Connectors were good/clean. Guessing the cells are not rated for this current. I think that green samsung is high current but I don't think the other cells are.
SOC: 14.9v.
90cc engine, max cranking current observed with good battery was 22amp. Looks like a 30amp BMS will be needed.
Charge voltage 14.5volts.

Maybe a 3s9p (yes BMS) configuration would be better with these leftover 8650s?


image_ajazyr.jpg
 

Korishan

Moderator
Staff member
Joined
Jan 7, 2017
Messages
6,401
Little pack probably had bad voltage drop due to only 2p. Jump to 4p and it'll probably handle it a lot better.
 

1xAA

New member
Joined
Sep 27, 2020
Messages
27
Thanks Korishan...I did a 3s3p (does need 4p) config since I have a 3s 25a BMS. It cranks OK now, but has limited stamina. I think I can live with the limited stamina since it has a kick start too. This battery compartment is very tight; designed for 5.5"x1.5"x3.25".

Note: This time I think I saw 27amps during cranking so 30amp BMS would be minimum.
 

daromer

Moderator
Staff member
Joined
Oct 8, 2016
Messages
5,572
The crank current is this low since the voltage most likely sags ALOT. You should design the battery to cope with 50A easy and 75A BMS if you ask me. Dont go to small here!
18650 aint made for this in first hand unless you buy high power cells like cells that do 30A. It looks like you have laptop cells.. They aint made for doing more than 2A at most. You are pushing the limits alot.

Please redo that asap if you want it to last more than a couple of cranks and potentiall lithium fire. A BMS dont protect against such a problem that you have.

Im sorry if this crushes your idea here but trust me on this on :)
 

1xAA

New member
Joined
Sep 27, 2020
Messages
27
Daromer, I thoroughly believe you on this one. :) I need the drill batteries like that one Samsung I referenced earlier. Just trying to do something with these leftover cells. Prob be better off just buying another Pb for around $22usd, but before I do that will just twist out the kick pedal and kick start this thing.:)
 

daromer

Moderator
Staff member
Joined
Oct 8, 2016
Messages
5,572
:)
Just beware that pushing cells could cause Them to cause issues later on like when charging :)

I use leftovers too btw :)
 

pjones

New member
Joined
Oct 17, 2020
Messages
5

He isn't charging it with the alternator, but the 26650s can deliver the amps needed. By the time you add a BMS to this, you might not have the room to fit this on your bike.
 

rebelrider.mike

Active member
Joined
May 25, 2017
Messages
382
Hey,
I hope your battery works out. When I set out to build one for my lawn mower, I started with a car battery, and an amp meter. The idea is that a car battery will have more amps available than the mower could ever use but the same Voltage, so the starter will take whatever it wants. Then the amp meter tells me how many Amps the starter actually wants. In my case with a small engine, the starter wanted 22A. So I was able to plan all the cells and hardware accordingly.

Also, my simple mower engine does not charge the battery when in use. It's meant to get charged externally once or twice a year. So that simplifies things a lot. Maybe that info will help? Good luck on your project. :)
 
Top