motor cuts off

[ecgoeken]

I just finished converting my bike to an e-bike using a 36V/500W 26" rear wheel golden motor kit.  I was able to get some free sla garage door batteries.  im using 6 total.  they are 12v 4.5Ah.  I have two sets of 3 in a series and wired the two series in parallel to give me a 36V 9Ah battery.  soon after I got it running I started to encounter some problems.  I would twist the throttle and before it got fully accelorated the motor would cut off.  I would simply twist it again and it would get going again.  this problem, however, gradually starting happening on a more frequant basis.  now it does this every time I begin to accelorate.  the power meter still reads a full charge.  since I am new to this, I am not quite sure as to what is causing this loss of power.  has this been a problem for anybody else?  what can I do to fix this problem and get my ebike ready for commuting?

[Mel in HI]

I would say low voltage cut off from accelerating too fast.  Anyone else have a suggestion?

Free garage door batteries...  Old and weak I suspect?  I am not an expert, but I suspect that the large draw is causing a voltage drop on the batteries which is causing the controller to cut out to protect the batteries.  Lack of draw allows the voltage to come back up and then work again when you twist again.

I would try not accelerating as hard. 

[ecgoeken]

the batteries are brand new off the shelf.  (benefits of knowing somebody in the industry)  if the low voltage cut off is the problem, does anybody know how I would go about fixing this?  is there a way I could get rid of the low voltage cut off?  or would this be hazerdous in some way?  otherwise the acceloration is fine and the power meter reads full.  its just after a short acceloration period the motor cuts off.

[Mel in HI]

The batteries are brand new off the shelf and you get them for free?  Wish we could all be so lucky, but I really wonder about these 'new' batteries and how long they were sitting. 

But anyways, I still say to try not accelerating so hard and see how that works. 

[OneEye]

I concur with Mel, this sounds like a low voltage cutoff under load, which most likely means your batteries are sagging excessively when drawing the amps.  Hook up a voltmeter to the batteries and watch the readings as you apply power.  This should give you an idea if they are actually dropping below the programmed low voltage cutoff, or if your controller is erroneously cutting out at a higher than expected voltage. 

If one of your six batteries is defective it could very easily create this sort of problem.  Even "new" batteries are occasionally duds.

[ecgoeken]

thanks for the insight.  I will go ahead and try this.  but if this turns out to be the case, I would like to try and fix it so that I can accelorate as fast as I want.  if it means I have to get different batteries that can take the current draw, is there a rating on batteries for the amount of current draw they can take?  i am only familier with V and Ah.  again, thanks for the help.

[Mel in HI]

Actually OneEye has the better suggestion, to look at the batteries individually to see if one is bad, pulling the voltage down. 

The larger the battery pack, the better you will be able to accelerate since each battery would take less of the percentage load, but it will be heavier too.  Alternatively you could go with something other than SLA, read up on this board, there is lots of talk about different battery chemistries.  But, if the SLA batteries are FREE, I would stay with them and just find out which battery is pulling you down.  Even if you had to buy that one battery to replace the dud, you would be talking WAY less money. 

Remember that accelerating hard will shorten the distance you will be able to go, since you will be using the energy less efficiently.

[ecgoeken]

thanks a ton.  I will put a volt meter to each cell in my pack and try and find the dud.  also, do you know where to find the low voltage cut off on this type of controller?  oh and one more thing, do I have to apply the volt meter when there is a load on the batteries?  when I lifted the tire off the ground and hit the throttle it work fine.

[Mel in HI]

One more thing... You asked "is there a way I could get rid of the low voltage cut off?"  It can be adjusted with some soldering to modify the controller, but I really wouldn't.  It's there to protect the batteries from being discharged too far, and if you do discharge your batteries too far, you will be replacing them really fast as it can cause a short in the cell from the lead plates deteriorating.  Or at the very least it will cause problems in charging them.  

[ecgoeken]

does ayone know if I need to test the cells in my battery pack while they are under load.  if this is the case, how would I go about doing that?  sorry if this question seems elementary but I am pretty new to this stuff.  also, if anyone knows what the voltage I need to be above or where to find this info, I am oblivious to this as well.

[ahend]

To make it easy, connect a volt meter into the battery monitoring leads coming out of the controller, the ones your LED "fuel" gauge plug into. This can be done quickly with a set of small alligator clips (make sure they do not short together or elsewhere!). Turn on the volt meter and ride the bike around under various throttle conditions. If you are dipping to 31 volts or below, the controller should inhibit power to the phase windings - the low voltage cutoff. If you want to monitor each battery, connect your meter up across each one in turn and ride around with the volt meter. (hopefully you will not turn up on You Tube slamming into a telephone pole with kung fu grip on your multimeter! - be careful) Check each battery this way and then compare the readings to weed out any weaker batteries.


I've read that voltage sag is acute in lead acids. You might try another three (free?) SLA's to connect in series and then parallel in to make a 13.5AH pack. That would likely eliminate the early cutout during hard acceleration.
Andy

[ecgoeken]

thanks a lot for the advise Andy.  I will go ahead and hook up a volt meter to my batteries tonight and take it for a ride and see if I cant isolate the problem.  cant wait to start using the bike to commute to work!

[ecgoeken]

i checked my batteries last night and it appears that they are definatly tripping the low voltage cut-off.  I was thinking, instead of adding more heavy batteries, would it be possible to add a large capaciter to the battery assembly to take care of the dip in voltage caused by a heavy initial load?  does anyone have any thoughts on this or have tried it on there own packs?

[Electrobent]

the problem is finding one of adequate voltage.  Maxell (Maxwell?) makes 48V ultracapacitors out of 2.7V cells.  I don't know they do this.  I thought putting caps in series lowered the capacitance.  Any  way they are big and heavy  and expensive (though two cost the same as one $3200).  I think they are for buses at this time.  But I, too, am interested in finding a suitable physically small ultracapacitor  as I think it would be better suited to high current than batteries.

[OneEye]

A capacitor could only help if this was a brief instantaneous rush of current that was tripping something.  That was the case for some folks using battery setups with an electronic BMS that would shut down the pack on an overcurrent condition regardless of how brief.  Capacitors could then be used to smooth out an instantaneous high current reading.  In this case it seems the current demand is too high for the batteries for more than just an instant.  Today's capacitors just don't have the energy density needed to overcome that barrier.  More battery capacity or a lighter touch on the throttle are what is called for.  One more set should get you up above 12Ah, which may be able to handle the current without quite as much voltage sag.