Controller current (Amps) and LiFePO4 batteries

[itdoesntmatter]

Hi all,

I decided to take on a project of adding two Kugoo M4 48v/500w motors to a Razor E300 but I'm a little confused about the Amp controller/battery correlation, so before I buy another controller, I hoped someone could guide me.

I have one cheap but seemingly good quality Chinese controller which is rated at 31amps. The plan was to buy an identical one and run the two together (one for each hub motor). They would be powered by Liitokala 32700 LiFePO4 batteries, rated at 35a continuous and 55a max. And they will be in a 16s2p pack, which will double the current....(he says with misplaced confidence)....

On the surface, this seems acceptable but:
1. I'm not sure I trust the battery Amp rating of 35a/55a.
2. I saw a test where the battery reached 62 degrees C/143F when continuously drained at 20amp. 60 degrees C is the operating max for LifePO4.

I am 100kg, the batteries will add 5kg and to get home I have a medium 2mile incline. I typically go at full speed (load) but if the scooter is actually quick (currently overvolting 24v motor), I'm guessing I won't need to go flat out....

So, should I be looking at getting a different controller under 25amps?
Or, do you think the fact there will be two motors and I wont be at full load due to the increased speed, I could get away with the 31amp controllers?

Feel free to shake your head and expose my ignorance! I am not precious about my ego.

 

[amberwolf]

I decided to take on a project of adding two Kugoo M4 48v/500w motors to a Razor E300 but I'm a little confused about the Amp controller/battery correlation, so before I buy another controller, I hoped someone could guide me.

The battery must be able to supply more than the current required by all devices in the system under worst case simultaneous load.

So if you have two 31A controllers, you need a battery that can supply *at least* 62A continuous.

If a battery has two ratings, the lower one is probably continuous (but may have some time limit, or other environmental conditions limits, like temperature), and the other is a peak, usually for a few seconds at most (say, 5 or 10).

If those batteries get that hot at only 20A, I would call them 5A or maybe 10A batteries, max. A battery shouldn't heat up at it's usable continuous rating. (they all heat up some, because they all have resistance...but anything with that much heat has very high resistance and will sag in voltage quite a lot, so you won't get nearly the watts out of them to the wheel that you could otherwise, because a significant amount of power is being wasted as heat. I can't detect the temperature change in my EIG packs during usage at 80A peaks for a few seconds, ~20A continuous; even though there is nowadays significant voltage drop because of their age, but they are large with a fair bit of thermal mass. Still, they certainly don't get hot, even when they start out hot from sitting in the summer desert sun at the store, etc, or start out near freezing in the coldest winter).


Note that going a certain speed creates a load all by itself.

So does going up a hill. (which works exactly like accelerating from a stop)

Doing the two together adds both loads.

To find out what load you'll actually be at, you can use calculators or simulators like those at ebikes.ca or kruezotter.de, for your specific riding conditions (actual slope, actual wind, weight, speed, etc. It doesn't really matter what motor/etc you're running for how much power it will take to do the job; it'll vary slightly depending on efficiency, but it's the conditions/etc that determine the power required.

 

[itdoesntmatter]

@amberwolf, thanks for the reply. After some research, taking much of what you have said as confirmation of what I thought I knew and adding vital knowledge I didnt give enough consideration to, I'll be looking for a lower Amp controller or finding a pair of cheap 36v motors which fit to get me going.

I have used the calculators before and when doing it again, they appear to theoretically support my suggestion (could be me doing it incorrectly!) but while LifePO4 has far better temperature tolerance, lower resistance and more sustained voltage hold than li-ion (but lower energy density), continuous at near max is foolish at best, stupid at worst, so worth a miss; even if the stated (clearly not real, which tests appear to suggest 15a) current from the batteries would be 70amp.
And there's not enough room to run four of them in series (should be parallel, as Amber corrected below) to 'be safe' and push to 140amp (real world 60amp) limit.

Interestingly, the 48v/500w low current limiting controllers are few and far between, unless you want to pay through the nose and some of those which were standardised a few years ago, such as the Lenzod 500w V3, arent abundant.

Again, thank you for your advice. It's good to be sent back to the drawing board.....on the plus side, my Maletrics Spot Welder arrived and I've been practicing on old laptop batteries and upgrading my power tools.

 

[amberwolf]

I have used the calculators before and when doing it again, they appear to theoretically support my suggestion (could be me doing it incorrectly!)

You'd have to link to the specific ones used and state / show the configurations you used, for us to see if that's the case.

but while LifePO4 has far better temperature tolerance, lower resistance and more sustained voltage hold than li-ion (but lower energy density),

Resistance is specific to the cells used, and pack construction method. There's plenty of high resistance LFP cells, and plenty of low resistance in ohter chemistries. Plenty of ways to make low resistance interconnects, and also to make them high resistance.

If by "sustained voltage hold" you mean less voltage sag, that's a direct consequence of internal resistance, so no matter what the chemistry is, the lower the resistance the less sag there is, and the less wasted power inside the cells.


You just can't trust what sellers in general say the specs of their packs are; you have to get them to tell you the specific manufacturer and model of cell used, and show you the actual interconnect methods and materials, etc. Most wont' do this (or they will lie). The ones that will tell you, you can verify if they are telling the truth by matching up the specs they provide with the theoretical specs from the specific cells/etc used (which you'll have to work out what those would be in the specific pack configuration they state).

Same for sellers of just individual cells--if you can't get trustworthy complete reliable specs on them confirmable thru other independent sources, you can't really know what they are.

There are also many places selling literal recycled garbage as if it was new, and claiming it can do the same thing it could when it was new.

The only general thing to avoid these with is "if it looks too good to be true, it probably is", and then doing your own checks as much as that is possible before buying, and trusting your feelings that if it doesn't seem right, avoid it.

continuous at near max is foolish at best, stupid at worst, so worth a miss; even if the stated (clearly not real, which tests appear to suggest 15a) current from the batteries would be 70amp.
And there's not enough room to run four of them in series to 'be safe' and push to 140amp (real world 60amp) limit.

Sustained current at any "maximum" that is a peak limit is "bad" regardless of cell type. If it's a continuous maximum rating, then you can run at that until the cells are drained. But you need to verify the *cell* max, not just the *pack* max advertised...


Running batteries in series does not increase their current (A) capability or Ah capacity, only their voltage (and their Wh capacity).

If you need higher A current or Ah from a battery made up of other smaller ones, you must put them in parallel.

If you need both, you'll have to put a set of them in series for the voltage, then parallel as many as needed of those sets to get the current required.

Interestingly, the 48v/500w low current limiting controllers are few and far between, unless you want to pay through the nose and some of those which were standardised a few years ago, such as the Lenzod 500w V3, arent abundant.

If you get controllers that can be programmed via setup software / apps, then you can usually also change the current limit while you're in there. Most of the FOC type controllers (VESC, Phaserunner, etc) can do this.

Some aren't programmable via setup software, but do have a way to reduce the max current within their displays, like KT, and I think Lishui. (boht of which also have opensource OSF firwmare that can replace the factory software to give different options. (threads about these around the forums).

If they're not programmable, you can still lower the current limit in a crude way by removing one or more of the shunts in it, if it has more than one; this is something you'd have to know about a particular controller in advance to choose it for this, as it's pretty typical for them to have two, three, or four. That means you can cut the current by half, thirds, or quarters. If there's only one, it requires replacing the shunt with one that has higher resistance, proportional to the change you want to make.

Or you can shave material off of the shunts, testing current measurement as you go, but this is not very precise without testing/shaving/testing/shaving until it reaches the point you want, and time consuming.

Any physical modification also risks damage to the controller from ESD or other handling issues.

 

[itdoesntmatter]

Thanks again, @amberwolf.

Was a mistype regarding current, I meant parallel not series. I have corrected my previous post, with thanks.
I am also fortunate enough to have the vital data on the batteries....because people are testing them to prove they are/arent correct datasheets!

By sustained current, I mean that LifePO4 has a more stable drain than Li-ion, and then it drops off a cliff; which from what I understand means greater voltage stability at full load.

Excellent point about the modding, which is why I'm staying away. I looked into current limiting this controller, as it has surprisingly good wiring and decent mosfets but for all that hassle, I'd be better saving up and getting a better ESC or VESC! On that, I did not realise you could control the current as you've described, I will look into that. That's incredibly helpful advice and a potential money (as time is money) saver, plus lets me upgrade in the future.

 

[amberwolf]

Was a mistype regarding current, I meant parallel not series. I have corrected my previous post, with thanks.
I am also fortunate enough to have the vital data on the batteries....because people are testing them to prove they are/arent correct datasheets!

Which people, and which specific batteries? Do you happen to have a link, or at least names that could be looked up? (shared info helps others that may read this thread later, or are just doing websearches, etc).

FWIW, every different cell out there will be different from other cells even of the same chemistry, so just saying they're LiFePO4 doesn't say anything about it's capabilities; you have to know which specific cells were used, in what configuration, under what conditions, for any testing or other info to have any meaning.

By sustained current, I mean that LifePO4 has a more stable drain than Li-ion, and then it drops off a cliff; which from what I understand means greater voltage stability at full load.

Not sure what you mean by "more stable drain".

if you mean that it's discharge curve is flatter, stays closer to the nominal voltage for more of the curve..sure, but unless you have some specific reason for requiring that, it's not relevant to most usages, and doesn't have anything to do with sustained current draw.

Excellent point about the modding, which is why I'm staying away. I looked into current limiting this controller, as it has surprisingly good wiring and decent mosfets but for all that hassle, I'd be better saving up and getting a better ESC or VESC! On that, I did not realise you could control the current as you've described, I will look into that. That's incredibly helpful advice and a potential money (as time is money) saver, plus lets me upgrade in the future.

There are threads about various types of controller mods around here; most shunt mods are people trying to increase current by adding solder / etc to them, but this just means the controller then has no idea what the real current is, and can't protect itself against overcurrents.

Shunt mods that *decrease* current still mean the controller doesn't know what the real current is...but it will always be *less* than the max the controller could handle, so isn't likely to damage anything.