LiFePO4 battery cutout

[frank9755]

We experienced the first cut-out from our Ping LiFePO4 yesterday. It happened near the top of a long, slow hill (half a mile, perhaps 1 in 12). My wife was riding with the throttle fully open and the power faltered, then cut out. The battery was about half discharged, and we'd just had a lunch stop for about an hour, so the cells could have re-balanced, which may or may not have been a factor.

The battery regained power after a few seconds, and was fine thereafter.

I've done a few searches on here and elsewhere and it sounds like LiFePO4 cuts out much less than other chemistries. However my new controller is rated 18A (vs 15 for the original Tongxin) so the current drain may be quite high for a 10Ah battery.

I understand there are two things that could be happening. Either;
1. the sustained load leads to voltage drop below the controllers low voltage cut-off - as with the Ezee batteries of a year or two ago. This is what I first thought was happening but now think it's unlikely
2. the load leads to the BMS cutting out to protect an individual cell which displayed a drop in voltage. I think this is most likely.

I'm keen to work out what is going on, as I'm likely to buy a second battery for this bike soon, to increase the range. So, is there any way that these cut-outs can be prevented? Would backing off the throttle on hills (perhaps only having it 80% open) do the trick? Would it be less likely to happen with a higher capacity battery? Or is the set-up likely to keep doing this because of its power requirement or otherwise?

Thanks

Frank

 

[flecc]

Either or both backing off the throttle or a higher capacity battery will help Frank. High current consumption peaks are inevitable with motors of useful power and I don't see that 18 amp controller as in any way excessive.

Although you've drawn a distinction between the overall voltage drop of the old eZee battery and an individual cell drop, the cause almost certainly remains the same, the poor high current delivery of lithium cells in general.

In this respect technology has gone backwards from previous perfection, that being the NiCad battery which was capable of total near instantaneous discharge. The NiMh that followed greatly reduced that ability and now with lithium we have even worse current delivery volumes, with only larger capacity alleviating it.

Sadly there's no realistic alternative in sight at present, but at least your battery type has long life potential.
.

 

[Tiberius]

Hi Frank,

You say it is a 10 Ah battery but not what voltage. Ping recommends keeping the discharge to 2C or below - 20 A in your case, but I don't think the internal BMS enforces that. If there is an overcurrent protection then it is set much higher. So that does suggest it is a low voltage cutout tripping. What state of charge were you at?

Some BMS have an output line to operate the brakes or pull the throttle low when the battery is stressed. All the ones I have seen are on/off controls, whereas what you really want is a proportional analogue one - so that it would lower the throttle to the point where the battery was just able to cope.

That could possibly be done with a special throttle control circuit instead of a BMS modification. Failing that, you just need more Ah. Bigger batteries, multiple batteries.

Nick

 

[frank9755]

Thanks Flecc, Nick,

My battery is 36v; I think it was about half discharged, perhaps slightly more.

So - do back off the throttle on long hills, and if I get a new battery, definately go for a 15Ah or higher!

 

[Danny-K]

Thanks Flecc, Nick,

My battery is 36v; I think it was about half discharged, perhaps slightly more.

So - do back off the throttle on long hills, and if I get a new battery, definately go for a 15Ah or higher!

Frank, I was puzzled as to why you chose only a 10 amp battery for your motor in the first place, as to my mind the only reason for choosing Ping's LiFePO4 batteries was that you could obtain virtually double the capacity for much the same money as a branded, (and nicely fitting), standard amp battery - in my case, that's 10 amps.

For instance my Powacycle 26 volt battery comes in the 10 amp size only, (contacted the supplier - nothing with a larger capacity to offer me). But I am tempted with one of those Ping Liposuction 24 things, (visions of touring in the summer ie., running the standard battery and then the Ping to take over when the standard battery exhausts its charge), as monitoring eBay I've seen Ping's 24V 12 amp batteries sell for just £20 difference to the closing price of a 24v 20 amp battery - but not always, you have to cat-and mouse it a bit, and hope for a quiet day on the listing you have your eye on.

As I get a terrific range on the standard 10 amp battery anyway, my imagination goes into overdrive just thinking about hooking up a secondary, (20 amp) battery, as well as retaining the 10 amp battery, ( will always keep the branded 10 amp for the lighter weight on shopping trips etc., and the way it fits neatly, unobtrusively, into the housing on the bike) both costing approximately the same amount.

In my case, the larger capacity would be for extended travelling range, in your case to prevent the safety cut-out from activating under heavy load, so yes, if there's not much difference in price and you don't mind the extra weight, (and larger size), the larger capacity Ping LiFePO4 you're mulling over sounds preferable in your case.

 

[john]

Frank,

What you might be able to do is to modify your controller to reduce the current limit. My controller had a limit of 30A but I changed this to 20A to put less strain on the battery. Failing this, it is fairly easy to limit the throttle and you could make this switchable.

As flecc says, 18A is not over excessive though (I use 10Ah 36V Yesa LiFePO4). Just wondering how may cycles you think the battery has done?

 

[Tiberius]

Hi Danny,

Don't mess around with ebay. Just email Ping direct and ask for what you want. He's doing what many people do and using the ebay auction as a shop window.

He can even make your chosen battery in different shapes.

Nick

 

[frank9755]

I was puzzled as to why you chose only a 10 amp battery for your motor in the first place...

I originally bought it for a different purpose (2nd battery for my Wisper, taking the range up to 60 miles), when I'd given up all hope of getting the Tongxin back on the road. At that time the larger Ping batteries were significantly more expensive than the 10Ah ones, and I didn't see us wanting to go on 80-mile rides!

...to my mind the only reason for choosing Ping's LiFePO4 batteries was that you could obtain virtually double the capacity for much the same money as a branded

Double the weight too - which is a significant disadvantage especially if you are carrying it in a pannier or on a makeshift mount. Best to get a battery that is the size you actually need rather than the biggest possible if it may not work out as convenient to use!

 

[frank9755]

Frank,

What you might be able to do is to modify your controller to reduce the current limit. My controller had a limit of 30A but I changed this to 20A to put less strain on the battery. Failing this, it is fairly easy to limit the throttle and you could make this switchable.

As flecc says, 18A is not over excessive though (I use 10Ah 36V Yesa LiFePO4). Just wondering how may cycles you think the battery has done?

Is that your Tongxin, John? What controller are you using on it that gives 30A?

The battery has had only light use so,in truth,I was a little disappointed to have the cut-out. I bought it in March but it's only had, I think, 8 charge cycles. On the other hand, my first Tongxin set-up was hard on batteries as it did cause my Wisper 13Ah one to cut out, which it has only done on the steepest hills on the Wisper bike.

I was thinking of current limiting but the two methods that came to mind didn't appeal:
1. a Cycle Analyst - would be nice but costs almost as much as a new battery!
2. putting a lower fuse, say 15Ah, in the circuit - which would probably work but would mean stopping to change a fuse every time.
Is there another simple way to do it, without opening up the controller?

To limit the throttle I was thinking of just putting a little scale next to it so that the rider (my wife) could see how far open it was, so she could have it fully open on the flat but ease off a bit on hills.

 

[john]

Is that your Tongxin, John? What controller are you using on it that gives 30A?

No, I gave up on Tongxin because I got fed up with repairing the motors as they fell apart. I'm now using a Crystalyte 409 which I only really bought as a cheap backup but is a bit quicker than the Tongxin.

The battery has had only light use so,in truth,I was a little disappointed to have the cut-out. I bought it in March but it's only had, I think, 8 charge cycles. On the other hand, my first Tongxin set-up was hard on batteries as it did cause my Wisper 13Ah one to cut out, which it has only done on the steepest hills on the Wisper bike.

That does sound quite disappointing. It would be worth monitoring the battery voltage as you are using it to see how much it drops. It should certainly be holding up well at 50% DOD. Once you collect some data I would be inclined to contact Ping and see what he says.

I was thinking of current limiting but the two methods that came to mind didn't appeal:
1. a Cycle Analyst - would be nice but costs almost as much as a new battery!
2. putting a lower fuse, say 15Ah, in the circuit - which would probably work but would mean stopping to change a fuse every time.
Is there another simple way to do it, without opening up the controller?

I modified my Crystalyte controller; just a matter of adding a resistor, once you know where and what.

To limit the throttle I was thinking of just putting a little scale next to it so that the rider (my wife) could see how far open it was, so she could have it fully open on the flat but ease off a bit on hills.

I have a resistor in series with mine which I can switch in and out.


One other thought, could it be that it was not as fully charged as you thought? Certainly worth doing another test.

 

[frank9755]

It would be worth monitoring the battery voltage as you are using it to see how much it drops. It should certainly be holding up well at 50% DOD. Once you collect some data I would be inclined to contact Ping and see what he says.

How could I monitor this? Would I need a cycle analyst, or is there an easier way?

One other thought, could it be that it was not as fully charged as you thought? Certainly worth doing another test.

Possible, but it took < 3 hours to re-charge in the evening. Will try to give it another proper test next weekend.

No, I gave up on Tongxin because I got fed up with repairing the motors as they fell apart

I hope some of the guys clamouring for Tongxin kits read this!

 

[Tiberius]

I was thinking of current limiting but the two methods that came to mind didn't appeal:
1. a Cycle Analyst - would be nice but costs almost as much as a new battery!

Frank,

Good though the CA is, it doesn't do this the way you would need.
The CA can be programmed for current and speed limits, but it implements them not as limits but as targets in a PID servo loop, and unfortunately it doesn't work too well. It has only a one way pull on the throttle and the end results are dependent on matching the loop parameters to the system. In practice, you will experience overshoots, slow settling, and the wrong setting.

Nick

 

[john]

How could I monitor this? Would I need a cycle analyst, or is there an easier way?

I just wired a voltmeter/multimeter to the battery and kept an eye on it as I went along.

 

[frank9755]

The battery is fine! We went out for another ride today and my wife got 20 miles out of it before it cut out on a fairly steep hill. She'd got up three significant hills already - about 850 feet of ascent - and had focused on keeping the speed up on the hills (ideally at 8mph or higher, but never below 5mph). That did the trick. I think it was cutting out because of lack of torque from the overgeared motor, rather than a battery problem.

I think 20 miles range at mostly 15-20mph is ok for a 10Ah battery. I'm sure it would get 25+ on a flatter route.

Thanks everyone for helpful comments.

Frank