Is my watt meter lying to me?

[ClintBX]

Hi ESFMs,

I just range tested a second pack I'm adding to my frame. It's a 48v 14Ah. In the weeks leading to this pack's arrival, I calculated that it should get me at least 20 kms of range. And tonight, I managed to squeeze about 24 kms until the LVC made my throttle redundant. So, I'm actually quite satisfied with that result.

However, I also set up my wattmeter to ensure that I had most of the rated capacity, as I've done on previous packs. What I found weird was that it was under rating my output of Ah. I stopped at the 5km Mark to check the reading and it read 1.726Ah. But I know from testing previous packs that I should have been in the 3Ah ballpark.

By the end of my range test, my wattmeter stated only an 8.6Ah capacity. I expected a slightly lower read out than the rated 14Ah but that's a massive 5.5Ah short.

This seems especially odd as the cells are the same that I have in my bigger 35Ah pack. Sanyo GA 3500mAh.

How accurate are wattmeters for measuring Ah? And is it normal for them to be off by such a margin? I've only used this wattmeter a couple of times and had it since around May or June. Maybe a little later. Come to think of it, this might have been my first proper use of it.

Clinton

 

[E-HP]

Does the meter use an external or internal shunt?

 

[john61ct]

There are dozens of types, some better than others.

Which is yours?

 

[miro13car]

Clint
what wattsmeter do you have?
Cycle Analist for sure does not lie , is accurate for sure in calculating Ah and Wattshours.
I second question above - does your device uses external or internal shunt?
in general I would not trust cheap eBay, Aliexpress , etc. wattsmeter

 

[ClintBX]

Yeah, it's one of the cheap ones. One of those ones with the blue casing. Have no idea about how shunts work, so can't tell you of it's internal or external. Tell me more about shunts please.

I never expected to have pinpoint accuracy but I didn't think they could be so wildly off. Maybe this one is part of a bad batch. My previous ones where a lot closer to the truth.

 

[john61ct]

Please link to the listing through which you purchased.

An inline type has the shunt (current sensor) inside.

Others the shunt is placed at the point in the wiring where you want to measure current/power, and connected by thin wire to the remote display.

 

[fechter]

It would be nice to have a second meter to compare readings.

Another thing is due to voltage sag under load, most packs will hit cutoff well before their rated capacity. If you were running much lower current, you would get closer to the rated capacity.

 

[docw009]

I've found that the cheap ebay wattmeters are consistent for AH, as I once strung the three that I own in series and they all read the same.

However, they would vary by 1/2 volt on 42 volts. And the WH values would be inconsistent when riding, so I would only record the AH used in my "battery economy" logs.

 

[john61ct]

Ah is the better measure anyway.

I only use Wh when conversion between different voltages is required

 

[obcd]

Depending upon the value of the shunt, it will generate a slight voltage drop when some current passes trough it.
Maybe that's why they don't all measure the same voltage if you place them in serie?
When there is no current flowing, they should all measure the same voltage.

 

[docw009]

The voltage difference is connected separately to the same voltage source. There's no trimmers inside for calibration.

 

[dogman dan]

One possibility not mentioned here so far, is that you could be wearing out the shunt in your watt meter. I have cooked off a few for my stand alone CA's, and had to replace them.

So it could be out of calibration, compared to 6 months ago or whatever if your shunt in the meter is starting to fail, and heat up.

 

[ClintBX]

Now I know I got a bad batch. I went to use it on my main 35Ah pack to see if maybe the battery pack is the variable. The damn display wasn't activating. I just had a line of rectangular blocks all the way along the top half of the display remaining indefinitely. Did I fry it in my range test?

I'll be doing an autopsy of it later.

 

[dogman dan]

Those are pretty affordable, but at that price, they might not last all that long. I fried mine forgetting that it had a 60v limit, and got a CA so it could handle a 72v battery.