Low voltage cut off 1500w motor
 

Hi. I’ve successfully installed three different 1000w 48v conversion kits without any problems. I recently installed two 1500w 48v kits on a pair of fat tire bikes. I’m experiencing this same problem with both: When I accelerate with the throttle and volts decrease at or below 40v, the motor stops.
I’m using new LiFePO4 48v 18ah batteries. My hunch is the controllers are the problem but thought maybe the battery BMS may also be affecting the cut off. Any comments are appreciated. Thanks

I would say that you are experiencing battery sag and the BMS is protecting the battery by cutting out. You probably need a Li- ion PF type of cell to pull more amps with a small battery or much higher voltage.

The low voltage cut off on my controller is adjustable and set at 43v. This is with a 52V battery.

My battery might have a low volt cut off, I'm not sure.

I will say if your voltage is actually dropping to 40V than sounds like the system is working correctly by cutting off.

Why it is dropping that low is another question. If the battery is fully charged and that is happening then there could be several possibilities: high resistance wiring, poor connection, cheap battery. Cold weather could also be causing a greater than normal sag.

Most 48V controllers are set at 40-41V. Look at your label. If you have a meter on your bikes and it is showing 40V, the bike controller is probably tripping, as it should, Your batteries are sagging badly if that 40V is at fresh charge. LiFePO4 is only 3.6V per cell at full charge. Are they 15S (54 V)?

Most LiPo 18650 batteries will have a BMS set for 2.5-2.8V per cell, which can take them down to under 36V. I don't know what they do for LiFePO4 off hand.

Li=FePO4 has a low discharge rate. It takes a big battery in amp hours to pull many amps continuous. They are not as affected by temperature. They have many advantages to Li-Mn Cobalt batteries. Disadvantages are lower energy density and lower amperage draw. He needs about a 36 amp hour battery to do what he is doing. The battery won't keep up.

Thanks to all for the comments. I believe it would be safe to run two of my 48v 18ah LiFePO4 batteries in parallel (not series) to overcome (test?) the “sag” scenario. Does anyone feel this is unsafe or otherwise, unwise? I would ensure both batteries are at the same charge level. My intention is simply to rule out the low voltage cut off as a condition of the controller and this would not be my long term solution. If the temporary parallel battery set-up solves my cut off problem, then my long term solution seems to be a new 52v 20+ah li-ion battery. I’d then KICK myself for buying these LiFePO4 batteries for the “safety and life cycle” advantages.

The controller has the following specs:
Rated DC48V
Rated current 22A
LV protection DC40
Maximum current 45+/-1A
Speed set 1-4-2V

try it. Your sag will be at least halved. It still might be undesized.

Thank you

A 22 amp controller seems puny for a 48V, 1000w system (to me). My 48V, 1000w has a 30 amp one.

I’m still learning so I still don’t quite understand the difference between “rated current” (listed as 22A) and “maximum current” (listed as 45A)

I'm a Chemist, not too conversant about electrics and hopefully one of the engineering types will amplify (maybe need to correct this), but basically the controller coordinates the performance of the "e"-system by regulation and protection (things like torque, speed, temperature and low voltage cut off). A 22 amp controller with a 52V battery provides 52V X 22 ah = 1144w while a 30 ah controller 1560w. This is simplistic and there's much more involved. I haven't looked in awhile but maybe the tutorials at ebikesca can help.

Thanks for that. Do you know how “rated” vs “maximum” current works within the controller?

I have general knowledge, but there are others who have already attacked this (Look at KPRENS posts, probably others too).

Watts = Volts * current
1500 watts = 48volts * 31.25amps

Not even including efficiency losses, your controllers are seriously under rated for what you are asking it to do by a ridiculous amount. Even it was rated at 35 or more amps...as a minimum, you are asking for a nearly 100% duty out of your batteries.

I don't do electric bikes, but if I were an engineering man, I'd stick to a minimum of 25% overhead in "continuous duty" capacity in the controller & minimum of 50% overhead capacity in the batteries. So a 40 amp continuous duty rated controller & 72 amp hours (4x18 amp/hr in parallel) of battery to avoid stress & encourage lifespan.

You can, of course, court destroyed batteries & smoked controllers as much as you'd like.

A 20A controller that can burst to 45A isn't too shabby. It should deliver 900-1000 W in steady state mode. Not that many batteries can provide +30A though, Yes, try two in parallel.

Rated is continuous duty. It is predominantly determined by how quickly heat can be removed from susceptible components. An equalibrium sets up between cooling fans, heatsinks, & load to keep even the most stressed/susceptible components at a safe level.

Maximum is momentary & instantaneous & determined by how much heat individual components are able to withstand in "x" amount of time before you let the smoke out of the stressed component.

I missed the part about the 22 amp controller. I was thinking 40 amps. Controller is badly undersized but my question is. You say 1,500 watts. What motor kit do you have? Are you giving up peak or rated.
It's difficult to build a true 1,500 watt nominal bike and have it feel or look like a bicycle. You are into Sur Ron territory and everything must be beefier or you will break things regularly.

The controller may be cutting out to protect itself. What amps do you have set in the control parameters? The batteries are still undersized for that chemistry. Try running your stuff through the E bike Simulator over in the tools section on Grin Technologies website. You will get a better idea of what you are up against.

Probably something like this. Just your typical big direct drive motor, mounted in a 26x4" rim. Comes with matching controller.

https://www.ebay.com/itm/In-USA-Stoc...4AAOSwTQxgNgEq

Here's a 48V18aH LiFePO4 that should be able to run the above. 30A continuous discharge, Seems typical of what you find on ebay, and reasonable to me.

https://www.ebay.com/itm/48V-18Ah-Li....c101195.m1851

I haven't bothered about theoretical considerations since a Luna 52V, 14 ah battery keeps the 48V, 1000w motor humming along at up to 35 mph (or so) with nothing ever getting even warm (except on 100 degree days when everything is warm), and similar situation with a BBS92 off road and that battery. Going on six years with both systems, but had a 52V, 10 ah battery for the first five which I gave away when the new one arrived; it was still functioning adequately.

All: I'm going nuts on the learning curve of this forum. I've tried sending this via 'reply' a few times only to learn the forum wouldn't let me for one reason or another (links, jpg images, etc.) The it deleted all my text and I started from scratch...3x. Being a newbie causes some grief, but I shall overcome! I was also limited to five replies yesterday ( another newbie thing). I'm a spry 56-year old who feels like a 16-year old itching to get back into the garage and tinker with my new found knowledge. I'm also an honorably retired police detective (LAPD) and commend Doc-Wui for decoding my exact conversion kit and battery combo; much respect Doc!

So far my two 1500w fat bike builds pretty much only match my two other 1000w builds in performance, including hill climbing (those are Voilamart eBay kits on typical 21-speed mountain bakes), with the exception of the low voltage cut off situation; very underwhelming. Thankfully, I believe we are narrowing in on the reason/s for this sub-par performance. I know, "You get what you pay for..." right? But this was somewhat anticipated. This was always meant to be part hobby (the riding) and part project (upgrade tinkering). I knew I wanted four (4) family ebikes and didn't want to pay the big $$$ for four good+ quality retail bikes, with plans to upgrade each over time.

My plan remains that I will run the dual battery in parallel test ASAP, hopefully later today (I'm in PST). If the controller behaves I may consider upgrading the 63V caps to 100V, but not sure if it's worth it as I don't know if this controller can handle being upgraded. The other components may not be able to carry more load, right. I love to tinker, tweak and learn new things so the exercise is part of the journey for me. I had a link to add here, then an image for ease of view, but I couldn't share them (newbie - most have 10 posts before I can add such things). I'm not versed on reading the spec sheet for mosfets but can say the chips (all 18) are labeled: TK150E09NE

KPREN thank you, too! Agreed: the bikes must be well fortified for the 1500w motor. I will play it safe and inspect before and after every ride. I have two torque arms, each bike, installed on the back axles. I don't plan to use the bikes for anything aggressive off-road but did want the added power/torque to help lug my 240lbs up some reasonable grades (sorry, I can't easily define "reasonable"). I was very intrigued to learn of the distribution differences you mentioned between the Li-ion and LiFePO4 cells. I will be exploring another project where I dissect three of these LiFePO4 batts (yeah I bought 3 thinking I was getting a good deal) and creating from them two new 52V ??Ah 18S6P packs with new 40A BMS....Yeah, I need to learn the math. I also want to thank you for the tool lead via Grin Technologies. I've seen a few of their YouTube videos. I'll be visiting that page soon.

2old and base2, thanks for your comments guys. I sincerely appreciate you all!
Cheers, Rick CA

Not an expert on this, but 100V can be dangerous (realizing that current is a factor too) which is why I've stayed at 52 max (of course 58.8 off the charger) for now.

I wouldn't go above 58V with any off the shelf kit Voilamart or similar 48V kit controller. You can change the filter capacitors to as high as you want, but the voltage reducing chip that they're feeding is only rated for about 60V.. You can buy 72V and higher voltage controllers that address that issue with a more robust circuit, .

100V DC is too risky. You get careless and grab power and ground at the same time because it's an ebike and you been doing it for 5 years. Probaly kill a toddler. Might kill an adult if fingers are wet.

Newbies can't post pictures until they get 10 posts and can't post more than I think 5 posts in a day.

Anything above 60 volts is legally no longer a low voltage system. Different rules apply above that level.

Rather than run the risk of running two batteries in parallel I would seriously consider getting a decent battery. the luna wolf I'm running claims 50 amp continuous and has no problem powering my BBSHD.

Whose rules?
I am not questioining the statement, I am just interested, as it seems like an important point.

Found this article https://www.electricbike.com/52v-battery-3077-fet/ talking about voltage limits.

BTW, the OP should find the article very interesting. It hits all the topics he mentions.

Anyway here is what this guy said