Current query

[drsolly]

I have a 250 watt geared hub motor, running at 24 volts. I ran it from 8s Lipo, that's 33.6 volts. I took it out for a test run with a ammeter/wattmeter monitoring, and the amps maximum was 45 amps. Nothing too arduous, just along a local road and down and up a bit of a hill.

The controller is an infineon 12fet, programmable, and I've set the parameters to 20 amps maximum. So my question is this, because I'm still trying to understand this stuff.

How come I was pulling 45 amps?

 

[Ykick]

How do you know the meter is accurate?

Did you ever use the same meter with old 24V battery setup? If so, what did it indicate under the old setup?

The meter may be averaging quickly enough to see "spike" current but if it sustains at 45A while you have it programmed for 20A perhaps try 10A max and see what it does?

RC Lipo doesn't sag like many batteries so power tends to exhibit higher peaks than saggy old SLA or laptop cell battery packs.

If sustained, 1500W will likely destroy that motor.

 

[drsolly]

The meter is one of these:

http://www.ebay.co.uk/itm/171123750032?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I know that the voltage is accurate, and when I use a battery, it reports 4.5 AH drain on a 5AH battery, which makes me think that the AH measurement is correct, and that, I think, means that the amps measurement must be right, otherwise it couldn't show an accurate amp-hour figure.

I'll try to think of a way to test the current measurement. Maybe it's showing a very short-term peak, like when I start up going up a hill.

 

[gwhy!1]

The meter is one of these:

http://www.ebay.co.uk/itm/171123750032?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I know that the voltage is accurate, and when I use a battery, it reports 4.5 AH drain on a 5AH battery, which makes me think that the AH measurement is correct, and that, I think, means that the amps measurement must be right, otherwise it couldn't show an accurate amp-hour figure.

I'll try to think of a way to test the current measurement. Maybe it's showing a very short-term peak, like when I start up going up a hill.

I expect the overcurrent delay setting is still enabled in the controller thats why you are seeing the higher current

 

[drsolly]

Yes, it's set to one second. That sounds like a good explanation.

Using the software from the vendor, I don't see how to set it to zero.

What I can do, is temporarily run an ammeter on the handlebars, using 4 sq mm wire to take the current from the batteries up to the front and back again. I'll try that tomorrow.

 

[drsolly]

OK. I mounted a Turnigy 7-in-1 multimeter on the handlebar, and ran four lengths of 4mm sq wire from the battery to the meter and back down to the controller. It looks awful, but it's only for testing.

I got a peak current draw of 37 amps on the Turnigy; my old wattmeter was reporting 39, so they're agreeing, to within a mouse's whisker. But that peak draw wasn't repeated, it was just when I started up. So I think gwhy! is right, it's the overcurrent, which I have a delay of one second on. At full speed, slightly uphill, it's pulling 20 to 25 amps from an 8s lipo pack, which is close enough to the limit I set with the Infineon to make me happy. I also installed a speedometer, and the speed made me happy too - this is a 250 watt motor, or so it claims.

So I'm happy now that I haven't made some mistake, things are performing as per spec.

 

[dnmun]

if it measured 4.5Ah on a 5Ah pouch it doesn't sound that accurate. but the turnigy wattmeter i have is 10% high too.

 

[drsolly]

I don't run the batteries down to 2.7v, I switch them out at 3.2v. So I wouldn't expect to get the full 5ah

Also, thinking about how a meter that measures AH works, I'd be fairly happy if it were accurate to 10%, provided it was consistent about it. What I want to know, is if I'm only getting 3ah from my 5ah battery; then I know it's time to retire it.

I'll try to remember to take a picture of the horrible multimeter mounting, so you can all have a good laugh.

 

[Ykick]

I don't run the batteries down to 2.7v, I switch them out at 3.2v. So I wouldn't expect to get the full 5ah

Also, thinking about how a meter that measures AH works, I'd be fairly happy if it were accurate to 10%, provided it was consistent about it. What I want to know, is if I'm only getting 3ah from my 5ah battery; then I know it's time to retire it.

I'll try to remember to take a picture of the horrible multimeter mounting, so you can all have a good laugh.

4 years experience with this chemistry it doesn't work that way. One cell usually dies early while exhibiting severe voltage sag which then translates to capacity loss.

Unless you live monitor and/or test IR frequently enough to plot a "curve of death" - waiting for the pack to lose noticeable capacity will most likely operate a cell in dangerous zone for RC Lipo.

The nose is another useful tool to detect ruptured pouches with no external physical damage. I've got 2-3 cells that have ruptured from the inside-out due to some form of internal corrosion.

The thing about meters is that it's good to have more than one to compare when things seem off.

 

[drsolly]

Unless you live monitor and/or test IR frequently enough to plot a "curve of death" - waiting for the pack to lose noticeable capacity will most likely operate a cell in dangerous zone for RC Lipo.

The nose is another useful tool to detect ruptured pouches with no external physical damage. I've got 2-3 cells that have ruptured from the inside-out due to some form of internal corrosion.

The thing about meters is that it's good to have more than one to compare when things seem off.

Each time I discharge a battery, I check the AH capacity that I got out of it. Is that close enough tolive monitoring, do you think?
Also, I have an voltmeter mounted on the handlebar, so I would see excessive voltage sag quite quickly. And also, if the voltage sags under 3.2v on any cell, the beeper will start beeping.

 

[gwhy!1]

I dont use a ah meter on any of my bikes I just use a voltmeter to monitor the complete pack voltage, I stop riding when the voltage get down to around 42v ( its a 44.4v lipo setup ) and this equates to around 15-20% capacity left, then I check this capacity when recharging, and I cant see a problem with that... a volt meter is much more use than a ah meter , and the only way to improve that is to use cell level monitoring, which can cause more trouble than its worth. So as long as you keep a eye on the voltage as well as the ah's you will be fine.

 

[drsolly]

I am using cell level monitoring. When I plug a battery into the bike, I also plug a cell monitor in. If any cell falls under 3.2, it starts beeping at me.