Li battery cutting out

Hi,

I have a LiFePO battery from BMS battery that is about a year old and has had 50-100 charges. Recently the range has dramatically reduced but I am not sure why.

The battery has a BMS - all cells are balanced equally when charged and when discharged. The bike works fine when the battery is fully charged. The issue is that the battery cuts out when going uphill after about 5 miles or so. The battery used to be able to do much more than this before (not sure of the range, but easily managed 20 miles). It seems likely that the problem is with the electrics or electronics. It would be useful to know how to determine if there is an issue with the BMS, the charger or the motor controller.

I am at a bit of a loss and wondering whether I should start replacing parts.

I don't believe the cells are dud because the battery gets to over 42v (36v battery) and all cells are balanced.

 

I would be grateful for any useful suggestions.

 

Regards,

Justin

Which battery is it? Can you give us a link? What motor and controller do you have?

To figure out what's happening a voltmeter would be useful. You might want to get one of these so that you can see what's happening with voltage in real-time:

Waterproof Digital Voltmeter DC 15V To 120V Red Led Voltage Digital Panel Meter | eBay

The battery is:

36V 12Ah 38140 LiFePO4 Battery 12 Cells EBike Battery Pack 30-60A

36V 12Ah 38140 LiFePO4 Battery 12 Cells EBike Battery Pack - BMSBATTERY

 

Motor and controller:

Bafang BPM 36V500W

Bafang BPM 36V500W Rear Driving Bike Conversion Kit - BMSBATTERY

 

I've just ordered one of those meters ...

I think you'll have to wait for the meter to see what's happening unless you can figure out a way to connect a multi-meter while you're riding.

 

What's the voltage hot from the charger and after it's cut out? We did have one of those chargers that cut out before the battery was half charged, and it finally stopped charging even though it was still giving 43v and able to drive the controller and motor directly without a battery connected.

I did wonder if the battery was getting a full charge. It looks like the battery charges through the BMS - do you know if this is so?

If so, the issue could be with either the BMS or the charger.

 

Having the voltmeter on the bike will help me understand if the battery is working as expected.

How can I determine if the charger is working as expected?

You mentioned that your battery was balanced before and after the charge. How did you determine that? Normally, you have to take off the shrink-wrap and fibreglass shrouds to get at the BMS so that you can measure each cell voltage. Is that what you did?

 

If you have or can borrow a multimeter, you set it to 200v DC and measure the voltage on the charger connector, but that's not the whole story. You also need to check the battery on its output wires. The two should be nearly the same after charging.

 

Most BMSs control both the charge and the discharge. The charge does indeed go through the BMS, and so does the discharge. There are mosfets that work like switches for both, which are controlled by components that monitor each cell. If any cell reaches a minimum or maximum voltage, the mosfets switch off, which blocks the current. You could have a faulty cell, which could prevent full charging or cut off discharge too soon if it goes down faster than the others, but you said that the battery was balanced before and after. If so, that would rule out a faulty cell, because they can't all be faulty.

I did unwrap the battery to check the cells. The BMS has a couple of connectors with wires from each cell - easy to check the cell voltages - they were all identical, both when fully charged and when the battery had been used and was cutting out on hills.

The voltage on the charger connector is the same as the battery voltage 43.4 after charging.

Does this voltage suggest that the battery is likely to have been fully charged?

43.4v is fully charged, so that rules out the charger. Do you have LEDs on the throttle? If so, what was happening with them during the ride? Next time it cuts out, measure the battery voltage in its discharged state.

The battery was charged to 43.4v, then the bike was ridden for about 6 miles. The LEDs stay on green for most of the time, the green dropping off slight on steep hills.

Bike works fine for the first few miles, then the battery begins to cut out on steep hills - fine on the flat and slight inclines. THe LEDs were on green apart from the hills where the green light went out, but the amber and red were still illuminated. The battery cutout happens when the amber light is illuminated. After 6 miles the battery voltage was 39.5v at the battery output and 39.6v at the charger lead.

I am afraid that i don't know if this battery voltage is what you may expect.

You have all the symptoms of a 48v controller with a 36v battery. The later KU123 controllers have a connector that you have to connect for 36v and disconnect for 48v. If you have this one, has the connector become disconnected?

This sounds interesting - I was never aware that the same controller could be used for both 36v and 48v.

As far as I can determine there is no connection for this - I have checked through all the connections over and over and all seem to be in order. I wonder if the issue may well be a faulty controller.

The bike was left for 8 hours or so with the switch left on - I cannot remember if the cutting out happened immediately after this. Do you know if this could cause a problem with the controller.

In any case, I think that it would be useful to try another controller.

Is there someway I can get one of these promptly or would I have to order it from china?

It should be possible to fix your controller, assuming that it's the problem. Below is a picture of the latest one's LVC. Could you open yours and see if it's similar, or maybe start with a photo of the outside to identify which version it is. A detailed photo of the inside would also help, for which you need to use an image host like photobucket.

 

I don't believe that the cause of your problem is directly linked to leaving it switched on. I have a spare new latest version KU123 if we can't fix yours or if we need to swap for testing.

 

To open the controller, you need to undo the four screws at the wiring end and the three screws in the side, then everything slides out.

http://i1116.photobucket.com/albums/k572/saneagle1/20121210_195022.jpg

Wow, that's fantastically helpful.

I'll check the controller out - not that I am clear what I am looking for. I'll endeavour to photograph it.

Please could you clarify, when the controller sees the voltage drop below a set level it will cut out aswell.

This means that there are 2 low voltage cutouts, one in the battery and one in the controller?

Wow, that's fantastically helpful.

I'll check the controller out - not that I am clear what I am looking for. I'll endeavour to photograph it.

Please could you clarify, when the controller sees the voltage drop below a set level it will cut out aswell.

This means that there are 2 low voltage cutouts, one in the battery and one in the controller?

 

Correct. A 48v controller cuts out at about 37.5v, which is what seems to be happening with yours. When everything was new, how far were you able to go and how hilly were your journeys?

I live in cornwall and it's very hilly.

I have a 6 mile journey to work and I used to be able to make 4 journeys (24 miles) but the battery tailed off on the last journey.

The battery doesn't even last one journey now before the cutting out happens.

Photo and image hosting, free photo galleries, photo editing

 

I'll send a photo the guts a bit later ...

OK - i've uploaded images ...

Sorry, that doesn't seem very clear in those posts.

There are a number of images of the outside and guts of the controller in photobucket.

Follow the link below:

 

Photo and image hosting, free photo galleries, photo editing

The photos are too small. I need to be able to read the numbers on the resistors. Haven't you got a 5 mega pixel camera? Maybe whatever has sent them to photobucket cut them down - check your settings, I know Saneagle used to have the same problem until he found the setting.

 

Your previous range sounds about right, so it doesn't make sense that your controller changed from a 36v one to 48v, although I used to use about 6 to 8aH on my 30 mile journey, which wasn't too hilly.

 

When Saneagle had the same problem, he was getting 10aH from his 20aH battery, so maybe you were originally getting something like 8aH from yours because it holds onto its voltage better. With a bit of deterioration, you could lose 1/2 a volt, which would make a huge difference to the cut-off point, so it's still possible that you've got a 48v controller.

Please check the link again:

Photo and image hosting, free photo galleries, photo editing

 

I've uploaded higher resolution photos - not sure if I can even read the numbers from the circuit board!

From the photos, as far as I can see because they're still a bit blurred, your controller is identical to mine in that corner. To save writing it all again, there's an explanation here in post#5.

http://www.pedelecs.co.uk/forum/electric-bicycles/13365-problem-ku123-controller.html

Looking at your photos, the two pads at ZX are not joined, which makes it 48v. For 36v they need to be joined, which means soldering a blob between them or soldering a short piece of wire across them.

You might want to check that there's nothing joining them on the other side.

The latest KU123s have two red wires soldered to those pads, which have connectors outside the controller so that you can join them for 36v or unjoin them for 48v. I removed them from mine and just put a blob of solder on, but the soldering wasn't as as good as it looked, which is why I had the problem. It worked at the second attempt.

 

Assuming that this will fix your problem, report back on your new range. I think it'll be going up to about 40 miles.

OK, ZX pads were already connected with a blob of solder. I replaced this with a tiny piece of wire and resoldered. Then I rode with a charged battery for about 2 miles. The battery completely cut out going up a steep hill and never cut back in. Before it would cut out and cut back in a few seconds later. The LEDs were all illuminated, but the motor could not be engaged. I checked all the connections and charged the battery up again, but the bike is now unresponsive. It seems sensible for me to try a new controller.

OK, ZX pads were already connected with a blob of solder. I replaced this with a tiny piece of wire and resoldered. Then I rode with a charged battery for about 2 miles. The battery completely cut out going up a steep hill and never cut back in. Before it would cut out and cut back in a few seconds later. The LEDs were all illuminated, but the motor could not be engaged. I checked all the connections and charged the battery up again, but the bike is now unresponsive. It seems sensible for me to try a new controller.

 

I don't understand. You photos show them very clearly not joined, or were they joined on the back. Could you post a photo of what you did?

Assuming that the pads were already joined, putting a piece of wire across then can't change anything, so the new cut-out could be completely unrelated, because you disconnected everything. Disconnect the brakes and PAS and try it again. Also have a very close look at the thin red ignition wire. Is it still properly energised with 36v?

The pads were joined from the back with a blob of solder. All i did was replace to remove this solder, and replace it with a small piece of wire which extended from one pad to the other. The copper wire helps to retain the solder. This was all quite neat and I would not expect that this caused an issue. The bike worked fine for a couple of miles.

It seems that the previous issue was with the controller. I would like to acquire another controller to get the bike working. Could you help with this? If so, please you could let me know how to proceed.

I'd be very surprised if there's anything wrong with the controller. You should do some simple tests before you write it off.

 

Check whether you have 5v on the throttle or PAS red wire. Did yoy try it with brakes and PAS disconnected?

 

My new KU123 is £35 including postage if you want it.