Daly 20A Commonport BMS cutting power on mobility scooter - why?

[Boxman39]

I'm using a Daly 20A commonport BMS for my 8s 24v LiFePo4 battery (charging it with my Rhenogy + 1 solar panel). Charging seems to go fine, and it handles my 230v inverter up to 500W as per testing.

However, when I connect it to my mobility scooter - its intended use - and try to drive, it will cut out within a second. I have to disconnect the battery and reconnect, and it comes back alive.

Did some measurements to see if I'm not spiking the inrush current (even though the spec-sheet says 60A sparkle current), and my findings are:

- Voltage doesn't drop more than 0.2v before the BMS cuts out. From 26.3 to 26.1. I see no transients
- Amperage drawn is 0.7A when the BMS cuts out
- If i feather the throttle so it stays below 0.5A, it sloooowly starts to move but as soon as it picks up speed and exceeds 0.7A, it cuts power.
- If I connect the battery such that I bypass the BMS (directly on the battery -+ clamps), the scooter works absolutely fine for the entire day
- When it cuts, the battery voltage drops from 26.3 to 0 and then 0.5v, until I disconnect the battery upon which it jumps back to 26

Also, when connected to an inverter, the BMS runs >10A comfortably to my power drill. Haven't managed to cut it out, so it seems to be something the scooter is doing to trip the BMS up.

Which things can make a Daly low-power BMS cut out? My understanding is either low voltage on one of the cells (not applicable) or overcurrent (>60A seems much, since it's not inrush current either).

Any ideas?

 

[sunshine_eggo]

Impossible that you're only pulling 0.7A. 24V * 0.7A = 16.8W - about 0.02 hp.

I would never trust a BMS surge rating until I knew the surge was no more than double digit milliseconds.

All the evidence points to you exceeding the BMS current rating.

 

[Boxman39]

The same measurement method says there's 0.7A drawn from the battery in the scooter before it cuts out and 10A+ drawn from the battery when connected to the inverter. The method is accurate. I'm feathering the throttle, it's on the brink of moving and then once it goes a tiny bit it cuts.

It's a tiny scooter.

Same method says the scooter draws 0.7A too when I connect it bypassing the terminals and use the same throttle-feather method. However, bypassing the BMS I can just floor it and draw around 10A max.

The method is accurate, I don't believe it is overcurrent. I'm asking if theres any other means that could trip a cheap Daly BMS up.

 

[Substrate]

- If I connect the battery such that I bypass the BMS (directly on the battery -+ clamps), the scooter works absolutely fine for the entire day

There's your answer. Stop using a bms on top of a battery in a motive-power application.

Instead, for this non-critical scooter application, use a balance charger. Here's a typical 10A unit, that has internal power supply.

https://www.amazon.com/Charger-Battery-Balance-Discharger-Adapter/dp/B07R18YNZQ/ref=sr_1_7?

The ignorant call this a "lipo charger", not knowing it also does nimh, lead acid, LiFePO4, NMC, you name it. You learn how to use it. Use the LFP or LiFePO4 settings!

Like you would with a battery-based bms, you run the sense / balance leads to the proper battery terminals.

You'll be a pro motive-power user lickety split!

NOTE: unless your scooter already incorporates an LVD disconnect, a Victron LVD or similar would be a highly recommended addition.

 

[Boxman39]

My only real reason for using a BMS was to have an individual cell based low voltage disconnect and overvoltage charge protection, balancing comes with that but was less critical since they're all matched cells.

While I appreciate your suggestion and while the combination of charger + LVD disconnect would certainly do the job, it would also set me back another 100 bucks for what was supposed to be a cheap as possible replacement for my lead-acid batteries, which in the long run would turn out cheaper than replacing them often. Currently this entire 24v 18Ag battery pack + BMS is clocking in at 110 bucks, so you can understand my reluctance.

The BMS that I'm using and that should be appropriate only cost me 27 bucks. Does anyone know of any other reasons why a BMS cuts power? What kind of protections does a Daly BMS have? I've got one similar to this, albeit probably an older model: https://dalyelec.en.made-in-china.c...-Battery-Pack-PCM-for-Electric-Golf-Cart.html

I see that it has a short-circuit protection built in too. Since it's not overcurrent nor low voltage, could it be that the scooter somehow trips the short-circuit protection? How does short-circuit protection work, what does it do to detect? How is it different from overcurrent protection? Then possibly I can build a workaround.

 

[Substrate]

My only real reason for using a BMS was to have an individual cell based low voltage disconnect and it would also set me back another 100 bucks for what was supposed to be a cheap as possible replacement for my lead-acid batteries, which in the long run would turn out cheaper than replacing them often.

Ah, bummer - I wish I had known this objective sooner. Because in a motive-power application with surge currents, merely replacing like for like doesn't save you any money because the LFP gets hammered harder. Albeit at first it's a bit more peppy and a small increase in range. But your cycle-life is shorter than expected with like-for-like.

That's why for motive-power replacements, you always go larger in capacity. The larger the better (or try to fill the battery tray up).

Well, I'll let others chime in. Knowing that you are only trying to charge up an 18ah battery, then this balance-charger cost goes down. A non-counterfeit SkyRC iMax B6AC V2 is about $60. Grab some jxt-xh 4S leads (5 wires in all) and you're done.

But I see what you're saying. Maybe on your next build you'll consider the design considerations above.

 

[Ampster]

The BMS that I'm using and that should be appropriate only cost me 27 bucks

It could be that the difference between $27 BMS and a $120 BMS might be the answer. My experience with motor loads is what led me to that conclusion. I wish I could offer more concrete advice.
I am running a chipper shredder on 28 volts and using a BMS rated for 200 Amps.

 

[Substrate]

Or, parallel another 18ah battery reducing the inrush current load to the existing bms's in half?

So hard to give a definitive answer at this distance.

 

[uksa007]

You need to work out your inrush current

How many motor and how big?
AC:

Motor current calculator

Calculate the full load current of AC induction motors with this Free Online Motor Current Calculator. Includes Formulas.

www.jcalc.net

DC:

EE07a: DC MOTOR STARTING CURRENT, BACK EMF, ARMATURE CURRENT AT SPECIFIC SPEED CALCULATOR

Visit the post for more.

bookofengineering.com

Size your BMS x2 bigger.
Always run a BMS!

 

[Boxman39]

In the interest of a proper internet archive, I did end up resolving it. Turns out the input caps (2x 1000uF) of the PWM box of the scooter were shot - one completely broken and the other holding 4.7nF. The box thus drew high frequency transients that messed with the BMS. Replaced the caps, and the thing works flawlessly with BMS now.

The PWM does the soft-start so DC motor inrush current wasn't applicable and also really didn't align with the observations of an extremely soft start still cutting the thing out with 0.7A avg current on the multimeter.. Just an unfortunate defect in the box' input.

Thanks for your suggestions, do appreciate it!