Normal battery sag for battery on ebike and where to hookup a battery meter?

[unclejemima]

I've got 2x 18v Rigid power tools batteries wired in a series running on a 350w 36v brushed controller on a 250w 24v brushed motor. So the motor spins faster than its designed...but its fine for the little I use it and seems to work good.

Regardless, I've got a battery gauge/voltage readout hooked up to one of the controller leads that outputs voltage. I'm not certain the exact function of the output as the controller writing is in Chinese, but it's small gauge, like 18 or so...so it's not a motor lead, I assume its for accessories or something...anyhow, when I hookup a meter to it, it reads about the same as the battery voltage...perhaps a bit lower. I think it shows about 41v, where the actual battery output shows about 42v.

All works fine, but when I apply full throttle with no load (IE, the bike is on the stand)...the voltage drops just a bit. But when I'm on the bike, the voltage drops significantly. Say I'm around 41-42v charged, when I load up the bike at full throttle, it drops to like 38v on brand new, fresh, fully charged batteries.

Is that normal? Or is it perhaps because my battery gauge is not hooked up directly to the battery leads, rather an output from the controller?

Let me know!

 

[E-HP]

I've got 2x 18v Rigid power tools batteries wired in a series running on a 350w 36v brushed controller on a 250w 24v brushed motor. So the motor spins faster than its designed...but its fine for the little I use it and seems to work good.

Regardless, I've got a battery gauge/voltage readout hooked up to one of the controller leads that outputs voltage. I'm not certain the exact function of the output as the controller writing is in Chinese, but it's small gauge, like 18 or so...so it's not a motor lead, I assume its for accessories or something...anyhow, when I hookup a meter to it, it reads about the same as the battery voltage...perhaps a bit lower. I think it shows about 41v, where the actual battery output shows about 42v.

All works fine, but when I apply full throttle with no load (IE, the bike is on the stand)...the voltage drops just a bit. But when I'm on the bike, the voltage drops significantly. Say I'm around 41-42v charged, when I load up the bike at full throttle, it drops to like 38v on brand new, fresh, fully charged batteries.

Is that normal? Or is it perhaps because my battery gauge is not hooked up directly to the battery leads, rather an output from the controller?

Let me know!

You didn’t specify what capacity the packs are (2Ah, 4Ah, etc.), or which generation they are. Rigid used Sony cells in a lot of their packs, and cells like the vtc4 or vtc6 have pretty high discharge rates (30A) so depending on whether they have one vs two parallel groups which will affect sag, the sag you mentioned is probably normal, or pretty good.

 

[Nix Liou]

I've got 2x 18v Rigid power tools batteries wired in a series running on a 350w 36v brushed controller on a 250w 24v brushed motor. So the motor spins faster than its designed...but its fine for the little I use it and seems to work good.

Regardless, I've got a battery gauge/voltage readout hooked up to one of the controller leads that outputs voltage. I'm not certain the exact function of the output as the controller writing is in Chinese, but it's small gauge, like 18 or so...so it's not a motor lead, I assume its for accessories or something...anyhow, when I hookup a meter to it, it reads about the same as the battery voltage...perhaps a bit lower. I think it shows about 41v, where the actual battery output shows about 42v.

All works fine, but when I apply full throttle with no load (IE, the bike is on the stand)...the voltage drops just a bit. But when I'm on the bike, the voltage drops significantly. Say I'm around 41-42v charged, when I load up the bike at full throttle, it drops to like 38v on brand new, fresh, fully charged batteries.

Is that normal? Or is it perhaps because my battery gauge is not hooked up directly to the battery leads, rather an output from the controller?

Let me know!

That's pretty bad sag, but I would not have high expectations for the powertool pack setup anyway.

As E-HP said, the capacity affects it. If the packs are 5s1p 4Ah with 21700 cells, then you effectively have a 10s1p pack, and you will be pulling over 2C discharge. With new and good quality cells, this would be light work. Cheaper cells, aged cells, or imbalanced cells will perform worse with more sag and deliver less capacity.

Many power tool batteries don't do a very good job keeping balance, and put the cells under a lot of stress, resulting in low cycle life as well, so age is more of a concern than usual.

 

[Risky Rob]

Isn't the motor expected to work harder under more load (your weight)?

 

[sternwake]

No power tool brand has their 5S batteries come off the charger at 21 volts. I usually see 20.6 or 20.7 on my Ridgid 2.0 and 4.0 battery, but my Makita 4.0 AH can be lower at 20.2 and 20.3 and my Dewalt is usually 20.7.

Pretty sure my Ridgid 2.0 uses Lishen cells and my 4.0ah uses EVE 20P.
I was going to swap the Lishens for Ampace JP30, but put it off. Very impressed with the design of the enclosure, and am reluctant to disassemble it.

I sometimes have to put a 4.0 Ridgid battery in an adapter into my lawn mower which uses 2 batteries in series. The ridgid comes off the lawnmower 0.5 to 0.6v higher, and the Ridgid battery in adapter in my leafblower has a more responsive trigger than the Makita 4.0ah battery.
Have ordered a 10 21700 cell Makita kit for the leaf blower. BAK45D and 0.2mm copper are going into it. If the Fat BAK's fit.

 

[unclejemima]

Ok. I'm using a set of 4.0ah NON-max branded Rigid.

I have a set of 6.0ah MAX branded Rigids I'll give a try and see. I thought the exaggerated sag might be where I'm hooking up my battery meter to...what is off one of the leads from the ebike controller. Because it was giving out around 40v...I figured it would be mimicking battery voltage...but i suppose it could be loosing some voltage along the way through the controller.

Is is best to hook the voltage meter directly to the battery leads?

 

[neptronix]

That voltage drop is due to the load. this seems relatively normal for a 4ah pack ( that's really small ), so a decent amount of voltage loss is expected. there should be less on the 6ah.

You're missing amps and watts info. This would be really helpful to understand things more.
https://hobbyking.com/en_us/turnigy...asm5yOgR4CP1cMtP2Mo1DyKijuv_SnEbrlD1M4d5_KUy2

 

[docw009]

A motor doesn't pull much current spinning a motor in the air. I put a current meter on my 20A controller with a 500W AKM128 motor. It fluctuates around 2/3 amp with wheel spinning in the air It peaked at 1.6 amp during the initial startup, However, I know the controller can surge close to its full 20A on a hill.

I used to run inexpensive hoverboard (36V4Ah) packs on a similar bike and get around 12-14 miles range. A four volt sag on full throttle sounds about right. I would often run two in parallel, and cut that sag in half, After three years, those packs were so weak that a single pack fresh off the charger would sag 10-12 volts and shut off the bike, Time to retire them, but they served me well., I also had a 36V2.5Ah Ryobi that I thought was fun to use. Maybe 8-10 miles.

If you want to get technical, batteries are modeled with an internal resistance from the cells and also from the internal wiring. Figure on about a quarter ohm in total for a small battery. Using Ohm's Law. If you pull 10 amps, that's 2.5V, and 20 amps is 5 volts,

You can hook up the battery meter anywhere. It draws a tiny current so it won't load the circuit. That allows it to read an accurate voltage.