Looking for a reliable 14S 40A bms

[emaayan]

hi..

i'm looking for a reliable (meaning folks who tried ,and had good experience with) 14S 40A bms, while i plan to run my battery at 30 continuous, i'd like to to use a bms which is rated slightly higher then that, just in case.

the problem is that most of the 14S bms i saw (like the one from greentime) are rated for 30A, and some bms's that are rated for 40A, and some even only have 6 FETS. (like suppower)

i'm aware that best tech power are considered one of the best, but it's not like an order online from them, so i'm checking all my options..

 

[Sunder]

You can't just put 2 x 30A BMSes in parallel for a 60A BMS? Or you don't know how/don't want to do the extra wiring?

If you're running 30A continuous, surely your peak will exceed 40a up a heavy hill, or on start?

 

[emaayan]

double the components ,double the things that can go wrong, i'd rather have one piece thanks

my controller is a 9FET infineon controller, set to 28 ah max, are you saying the peek will be higher?

 

[amberwolf]

Another way to look at it is you ahve redundancy--if one dies you can cut it out and still keep going with the other one, albeit at reduced power.

 

[emaayan]

yea i thought about, it if i talk an example from my software job, we have many mini software heartbeats to check on our software and they keep in the way creating false positives , so i'd like one good one. my current pack is from em3ev (if i was sure 100% i would be able to order from them again, i'd do it in heartbeat) and it has one bms. so i'd rather have one good one.

btw this guy says their bms has 14 FETS of HY 3410 fets, are you familiar with them?
https://www.aliexpress.com/item/58-8V-lithium-ion-battery-PCB-and-BMS-48V-with-40A-constant-charge-and-discharge-current/32652290266.html?spm=2114.01010208.3.2.9asIiw&ws_ab_test=searchweb0_0,searchweb201602_2_10065_10068_10000009_10084_10083_10080_10082_10081_10060_10062_10056_10055_10054_10059_10099_10078_10079_10000012_10103_10073_10102_10000015_10096_10052_10053_10050_10107_10051_10106,searchweb201603_9,afswitch_5,single_sort_0_default&btsid=3d5f02e0-f213-4f66-874e-6c43a8afe09d


i'm also talking to best tech and they wanted to know how long would i want the max contiunous discharge.

 

[emaayan]

if i'm using 18650pf cells would it be safe to specify 4 second peak discharge duration?

 

[Sunder]

Another way to look at it is you ahve redundancy--if one dies you can cut it out and still keep going with the other one, albeit at reduced power.

I wouldn't say "another way". I'd say the only way. You're in software? Think about this as RAID 1 - It's twice as fast when it's all working, and twice as reliable for half the rating. Since you've set your controller to peak at 28A, then you should in theory suffer no loss of performance at all, if one goes down.

In fact, reliability should be more than double, because each BMS is handling less current, and therefore less heat is generated, less electromigration, and a longer life.

 

[emaayan]

i believe raid are designed to work this way.

anyway as this is my first pack build, first spot welding, and all, and because i haven't read anything yet double bms's, i'd like to try to stick to a standard configuration, , as i already have a history of messing up, first time builds.

anyway the folks at best tech power recommend this board. (i'm guessing they have a 14s version)
http://www.bestechpower.com/96v3spcmbmspcbforlifepo4batterypack/PCB-D166.html

and are asking for the duration for maximum discharge current i believe it should be 4 seconds, what do you think?

 

[amberwolf]

What can your pack take?

What is the longest you need that max current for?

That's how long your BMS has to be able to supply it.

 

[emaayan]

it's gonna be a 14s9p pack, the controller's max amp is set to 30.

 

[amberwolf]

Neither of those answers the questions; you have to have the answers to those questions before you can answer the BMS question.

 

[emaayan]

well as i said it's gonna 14S9P 18650 Panasonic pf cells.

the only data i have about current discharge of those cells is that it's 10A , but i was recommended it should 5. so 9* 5 should put it around 45 amps. but controller itself is limited to 30 amps.

the problem is that while the amount of amps is specified, the DURATION is not, (or it is , and i just don't understand it) further more i never saw this type of parameter being specfied in any other bms's, so i don't even have any reference or example to ah... draw from (no pun intended)

 

[Sunder]

There's another thread on discharge "magic numbers". I didn't read the discussion, but the opening thread was definitely worth reading. Basically, if cells are rated at 5C, they don't magically last their full life time at 4.9C, and die at 5.0C after 1 second. It's always a curve, and the curve doesn't always fall off a cliff.

There is no hard and fast rule for what you should draw from a battery, but the advice is if you are using it daily, and want it to last 3 years, then half the manufacturer's recommendation is your continuous, and double that is your burst - for 10-30 seconds - enough for all accelerations, and most short steep hills. Of course, you might need to consider your route (e.g. do you have 10 x 15 second hills back to back in your ride?), your behaviour (are you full throttle from every stop?), your climate (Is it warm there most the year), your build (Are you laying it flat and actively cooling, or all bunched up inside a plastic box with foam insulation).

The answer is, there is no one answer, hence why others are trying to get more information about you, your battery and your ride. But the harder the battery is expected to work, the lower the limit I would personally set on it - especially if I knew I was prone to being throttle heavy. But then, if I knew I was going to be throttle heavy, I would build a bigger battery that could cope, not limit myself.

 

[emaayan]

well this is the fairly recent trip i've made with current battery i have
http://www.ebikes.ca/tools/trip-analyzer.html?trip=aeEutY

it's 14s9P samsung inr 18650 29e pack... i understand some say the pf is it's sort successor, as this is all new to me, i'm willing to providing any information i can

do you have a link to that thread? i'll add it do to my reading list on keep. next to the one about fuses (cause em3ev terminates his sense wires with pptc fuses but i have no idea the values i should get)

 

[vk4bxi]

Hi
I'm running the pf cells as well in a 13S4P configuration with a 20 Amp current limit on my BBS 02 750. 5amps per cell for 30 seconds is definitely as much as I would want to pull from the pack. When the pack is fully charged things are OK and "sag" is good. But the "sag" seems to increase badly as the pack discharges. So much so that I have added an extra booster cell pack to take me to 14S so that I can make full use of packs Ah capacity. A 5 amp max usable rating on the pf cells is about right. If you can set it lower to 3~4 amps even better.

Bob

 

[emaayan]

5 amps is what i've heard is recommended, since i'm planning on 9p , it's expected to have 3.3 amps per cell.

 

[Sunder]

Thread here: https://endless-sphere.com/forums/viewtopic.php?f=14&t=85770

He's advocating looking for graphs, but Internal resistance meters and contactless thermometers are dirt cheap. If you want the best results, do it yourself.