Noob needs help with Headway 24V20ah pack

[theGoPedal]

Hi, first timer here.

I have a 24V cordless electric lawnmower and I wanted to upgrade it's battery from lead to LiFePO4 for increased range. I ordered a 24V20ah pack from 'Michelle at Headway', who quoted me a very good price for this pack size. I'm happy to say the transaction went smoothly and my battery arrived approx 2 weeks after I ordered it.

When I got the battery, it's voltage was 28.0V and everything looked good. However, when I hooked it up to the lawnmower it failed to spin the motor. Here are the details: When I turn the mower on, the blade motor turns about 1/4 of a turn and then stops. There is a smaller, secondary motor for the wheels (it's a self-propelled model) and this motor spins fine, no issue there. So, my guess is the BMS that came with the battery is limiting the current which is keeping the blade motor from spinning.

When I ordered the battery, I was asked what the working current of the lawnmower was. I didn't have a meter that could measure this, so I guessed it to be 20A. There is a 40A safety fuse on the mower, so I guessed the working current would be half of that. Maybe I guessed too low.

My question is, can I see what the current limit is from looking at the BMS board? And can I adjust anything on the BMS to increase the current?

 

[bigisland]

Hi Gopedal, First off I am just typing here cause I'm sittin at the key board here and blabin.
So I know nothing about the true facts of electrons, but I'm not stupid so heres my guess.
The battery is fine, highest probability says so.
So either your mower aint workin, but you problly know it is. Or
You got it wired up wrong, high probability as well.
Most logical answer though battery can't provide enough current to power mower.
So hopefully you will find out soon when some one here can tell you how to trouble shoot it, how much grass you cuttin where are you at.

 

[sangesf]

That's a tough one, because at 20ah, the Headways should be able to handle At least 60amps, and I highly doubt that the lawn mower blades would pull more than that.. IF the headway person set the BMS at somewhere between 20-40a and the initial startup of the blades pulls more than that, then the BMS will trip...
I would suggest, for only about 30sec, wire the SLAs in parallel with the Headways and see if the mower blades start.. If they don't, then there's something wrong with the mower. If it does start, then pull the SLAs off and see if the mower blades stay on, if they do, then it's definitely thr BMS tripping on the initial high current draw, if they don't continue spinning, then your mower blades are pulling way too many amps then they should and there's a problem there, UNLESS just the SLA's power it fine, then again, either the BMS is tripping because of way too high power demands for the normal operation of the mower (blades) or there is an intermittent short somewhere between the batteries and motor.
(P.S. SLAs can handle HUGE amp draws for a limited amount of time, even more then most lithium batteries (just not as efficient as them))

 

[neptronix]

Here's an idea. Get the motor spinning in an area where there is no resistance, and then move it onto the grass.

No, it is not possible to adjust the current limit in the BMS. Unless you are an electrical engineer and real good with a soldering iron
I really think you should have got lipo or a123 for this application, hate to say that but.. yep.

I would also say you need some kind of amp monitoring device, that would certainly help right now. Hobbyking sells a turnigy watt meter for pretty cheap, and there are other things such as a watts-up that will do the job too.

Good diagnostic tool for things like this.

 

[neptronix]

wire the SLAs in parallel with the Headways and see if the mower blades start..

Um, ya know the operating voltages for the SLA and lifepo4 are different enough that one battery is going to try to dump as much current into the other as soon as you connect them in serial due to the voltage differential right?

The SLAs should be at 13v each so 26v, the headways fully charged would be 28v

http://www.solarnavigator.net/battery_charging.htm

 

[sangesf]

Yeah I know that, that's why I said for 30sec just to crank the motor..
Shouldn't hurt anything for 30 sec

 

[theGoPedal]

Thanks, everyone, for the responses.

No, it is not possible to adjust the current limit in the BMS. Unless you are an electrical engineer and real good with a soldering iron

I had a feeling the BMS wouldn't be adjustable. Do you know if there are any markings on the BMS that would indicate what the current is?

I really think you should have got lipo or a123 for this application, hate to say that but.. yep.

That's interesting, I thought for sure the Headway's would be able to handle this application. Why do you think this, are the Headway's C rates not high enough?

I think what I'll try next is to disconnect the BMS from the battery and see if I can get the motor spinning without it. Then I'll know for sure if it's the BMS that's causing the issue. Stay tuned...

 

[sangesf]

You don't have to disconnect the BMS, just run wires from the input of the BMS

 

[theGoPedal]

You don't have to disconnect the BMS, just run wires from the input of the BMS

Yep, that's right. I just have to bypass it, thanks.

 

[neptronix]

Yeah I know that, that's why I said for 30sec just to crank the motor..
Shouldn't hurt anything for 30 sec

Actually the second you plug the two together is when this transfer of charge / discharge starts and can be rapid.
Just like how they tell ya to parallel lipo only when the voltages are practically the same.... there's a reason for that

Gopedal: some manufacturers mark their BMS, others don't. Usually they don't.
The headway C rate shouldn't have any probs.. it's rated for 100 amps output in that pack. I think the BMS is the bottleneck.

 

[sangesf]

When just connected together there will only be a .5a differential and during the initial start of the motor, there won't be much of a transfer either... They aren't in series, it's parallel so no I'll effects in the very short term (15 seconds or so). It's not like Lipo, I'm talking lifepo4 and sla.. No detrimental effects, I promise.

 

[dnmun]

i think the thermal relay on your mower is opening up which is why the other motor continues to run. or else there is a bad connection in the wires that opens up when it gets hot.

did you open up the mower yet?

 

[neptronix]

When just connected together there will only be a .5a differential and during the initial start of the motor, there won't be much of a transfer either... They aren't in series, it's parallel so no I'll effects in the very short term (15 seconds or so). It's not like Lipo, I'm talking lifepo4 and sla.. No detrimental effects, I promise.

Parallel is where the transfer occurs.
Series is where no transfer occurs. Then the problem is the set as a whole getting disbalanced.
I think you got something mixed up here.

And it's a voltage differential that makes energy flow rapidly from one place to the other.
A 12v lead acid battery sits at about 12-13 volts, tops.

Thus the headway pack at 28v would shoot as much juice into the lead acids as it could to try to make the lead acids 14 volts.

The lead acids have a charge rate of about 0.3C and the headways would try to give them a 5C-50C charge.
Good way to make a lead acid battery do this:

http://en.wikipedia.org/wiki/Automotive_battery#Exploding_batteries

But i think that's not what theGoPedal is trying to do.

Please don't recommended someone parallel batteries of various voltages without knowing the effects first hand yourself.

 

[sangesf]

Actually I have done it, a MILLION times (well at least 15 times).
I use my lifepo4's as a help to jump start cars, trucks, vans, even a BOBCAT!
And I'm even talking at LEAST a 100a draw for a couple of sec and using 5s(16v) lifepo4 to 12v SLA..
(yes I know that's with a low battery, but the effects are the same because there really isn't that much of a voltage difference between nominal and low voltage on an SLA)


It won't hurt anything for 15sec while trying to start up the mower.


Try this... Take a 12v lifepo4 pack and a 12v SLA and put a meter between them and check amp draw "unloaded", you will see a VERY low (less than an ampere) of "draw", THEN take a 12v 100w bulb, put a load on that setup and see what happens, you might be surprised at the results..

 

[TylerDurden]

It won't hurt anything for 15sec while trying to start up the mower.

Should be fine.

Those 40A breakers vary in quality. They do take time to open too, being thermal devices.

I agree the BMS is the likely culprit... the startup current is prolly quite high (near stall).

 

[theGoPedal]

Good news to report: Yes, the BMS was the culprit. I connected the battery directly to the blade motor and it started right up. Quite powerfully, too, I'm happy to say. Seems like there's more 'oomph' in this battery compared to the old SLA, but that might just be my imagination. So I'm off to search for a 8s LiFePO4 BMS that can handle the start-up draw. Note to other first-timers: learn about your application needs and make sure the BMS cut-off current isn't set too low!

Thanks again to everyone who responded. I've been lurking on this board for a little while and I'm happy to have something to contribute.

 

[dnmun]

the current limiting built into the BMS is there to protect the battery from being damaged. i don't think we ever learned what was wrong with your battery. also if you offer FREE advice to other newbies, please consider that your advice could also create damage to their equipment too if they follow it.

 

[theGoPedal]

Yeah, I don't think there's anything actually wrong with the battery. I'm pretty sure made a mistake when I ordered it. I think that when I was asked what the 'working current' was, they meant the cut-off current. Since I replied with an answer of 20A, that's what the BMS cut-off is set to. The motor probably needs 25-30A to get started.

And I heartily agree about not taking advice from newbies without getting more opinions. I was only trying to say this: Be more diligent and do your homework before jumping in with both feet, as I now wish I had.

 

[TylerDurden]

the current limiting built into the BMS is there to protect the battery from being damaged. i don't think we ever learned what was wrong with your battery. also if you offer FREE advice to other newbies, please consider that your advice could also create damage to their equipment too if they follow it.

Caveat emptor, caveat consuasor.

Seems like a typical bms to me... the motor has a high startup current and the bms may hit LVC for a moment.

 

[999zip999]

It's hard enough trying to type in english as far as talking across the ocean ? Hay signalab has 60 bms. I don't know about 24v. I think you found the problem. Great. The mow I like the mow I do.

 

[auraslip]

SLA can be charged to 2.45 per cell (14.7v for 6 in series) and it's perfectly fine to parallel a 12v lead acid battery with a 4s lifepo4 group. It's been done many many times.

That being said, 20ah of headways make a fine choice and any one saying otherwise doesn't know what they're talking about. I mean, I don't know what your mower is rated at, but pulling 60a continuous would probably over heat the motor, so there is no need for higher power here. In fact, 8s lifepo4 is perfect for this application as 7s lipo is a bit cumbersome to deal with!


I'm thinking of doing the same thing with a mower! How much did the pack cost? What mower is it going in? Can you do some pictures or videos cutting grass?!

I wonder if using the bms for charging, hvc, and balancing only would work for you. Then just use a cell-log for LVC.
That's how I would do it! Chances are you won't be able to use the whole 20ah anyways....

 

[sangesf]

SLA can be charged to 2.45 per cell (14.7v for 6 in series) and it's perfectly fine to parallel a 12v lead acid battery with a 4s lifepo4 group. It's been done many many times.

Thank you for agreeing with me..
Some people just want to be "more knowledgeable" then every one else..

 

[theGoPedal]

I got some time to work on my project and have made some progress. I thought auraslip's suggestion of bypassing the BMS during discharge was a good one, so I soldered a second negative lead from the battery to use while discharging the battery. I will look into getting a cell-log to monitor discharge, but the mower came with a built-in voltage gauge, so at least I have something to monitor voltage while mowing. The mower now starts without issue and I'm ready to mow.

However, now there's another issue. I wanted to break in the battery a bit before I tackled the lawn (the grass is so high from all the rain), so I let the mower run for a half-hour while sitting on the driveway just to get a few ah out of the battery. When finished, I took the voltage of the battery, it was 25.4V. I then connected the battery to the charger, but the second LED stayed green. It didn't turn red to indicate charging, as I expected. I disconnected the battery and took the voltage of the charger, it was 28.7V. I put the battery back on the charger and left it on for 4 hours, but the light always stayed green and the battery is still at 25.4V. I'm quite sure the charger isn't charging the battery.

I opened up the charger case and took some pictures. I didn't see any thing obviously wrong, no burn marks and all the connections seemed solid. I've written Michelle at Headway about this, but I haven't gotten a response yet.



View attachment 1

 

[auraslip]

Your a smart man to break in a new battery. Might do a few light cycles to 50% dod before you do some serious yard work.



The charging issue might now be on how the bms, battery, and charger is hooked up. Yo can hook the charger up to the battery through the discharge cables, and if that doesn't work your charger is for sure bad. Other than that, the BMS probably needs to be hooked up correctly for it to properly charge.

 

[theGoPedal]

I tried connecting the battery to the charger with both the discharge cable and the BMS cable. No change, the light stays green.