Any ideas why Capacity Tester is reading low?

[manly]

My capacity tester is reading less amps than my Overkill Solar BMS says is being taken.

I took some pictures as an example. This is a 12v, 280ah battery. The BMS showing 7.7 amps being drawn while the capacity tester shows only 7.03 amps being drawn.

Doesn't seem there shouldn't be that much drop through the wires, and all the nuts are tight. Scratching my head here... any ideas? It did the same thing on the first battery I put together (cells 1-4). Capacity tested at only 243 amp-hours.

 

[timselectric]

Could be voltage drop across the wire and connections. Which inversely affects amperage.

 

[mmame]

Could be voltage drop across the wire and connections. Which inversely affects amperage.

Voltage drop would explain a difference in voltage readings (and Wh readings) - not in Amps. My guess: The amp offset is due to read errors caused by the BMS. The BMS has a much wider Amps reading range whereas the shown capacity tester's max current is a fraction of the BMS's one. Thus, large reading differences will occur - especially on lower currents.

 

[MisterSandals]

Scratching my head here... any ideas?

I'd check with a quality 3rd tester, like a clamp meter.

Dire Straights: "2 men say they're Jesus, one of them must be wrong."

 

[timselectric]

Voltage drop would explain a difference in voltage readings (and Wh readings) - not in Amps.

Actually, watts is the constant. Power being used is Power being used. Volts and amp change.
If voltage goes down, amperage goes up. And vise versa.

 

[mikefitz]

Volt drops don't effect amps flowing in a circuit .
Using the BMS value for current gives a better result for capacity

Mike

 

[timselectric]

Volt drops don't effect amps flowing in a circuit .

It absolutely does.

 

[mikefitz]

I should have made the point with more words.

If you have two current measuring devices in series in a circuit then any resistive volt drops within that circuit will not have any effect that will cause the readings to differ.

Mike

 

[Bud Martin]

The same current flows through the shunt of both Amp meters (BMS shunt and the tester shunt which is basically in series with each other), the difference reading may be due to poor calibration of the meters.
Did you very the reading with the clamp on meter?
Did you perform Zeroing out procedure for the Amp meter of the BMS and the capacity tester?

 

[mmame]

The JBD/Overkill BMS also support amp reading calibration (as seen on the Daly which imho use similar BMS IC), don't they? If yes, just perform a calibration. You might need a third Jesus/Ampmeter to proof the readings...

 

[moon]

Hi,
I have the same tester. The stated amperes ont the Display are not correct by mine. The display shows nothing although already over one ampere is taken. Check it with an amp meter. Probably yours is the same.

 

[manly]

Thank you everyone for the replies. I'm out of town at a trade show but once I return back I'll check with a clamp on amp-meter as suggested. Hopefully it's as @moon states and my cheapo tester is the one off.

Fwiw, the discrepancy is between Volts AND Amps:

Chinese Tester: 12.9 Volts * 7.03 Amps = 90.7 watts

VERSUS

BMS: 13.1 Volts 7.7 Amps = 100.1 watts

 

[cods4]

I have the same discharge tester and found that it matched my Victron smartshunt and clamp meter pretty closely. That's just one datapoint though, Not saying yours is going to be the same.
This was discharging a 48v 800ah bank over about 10 days.
At the end of it the aliexpress tester showed 40604Wh and the smartshunt showed 40200Wh

 

[timselectric]

I have the same discharge tester and found that it matched my Victron smartshunt and clamp meter pretty closely. That's just one datapoint though, Not saying yours is going to be the same.
This was discharging a 48v 800ah bank over about 10 days.
At the end of it the aliexpress tester showed 40604Wh and the smartshunt showed 40200Wh

I would call that pretty darn close.

 

[Bud Martin]

Thank you everyone for the replies. I'm out of town at a trade show but once I return back I'll check with a clamp on amp-meter as suggested. Hopefully it's as @moon states and my cheapo tester is the one off.

Fwiw, the discrepancy is between Volts AND Amps:

Chinese Tester: 12.9 Volts * 7.03 Amps = 90.7 watts

VERSUS

BMS: 13.1 Volts 7.7 Amps = 100.1 watts

What Voltage does the Capacity tester show if you set the current to 0? this will tell you how much the Voltmeter of the tester is off from the BMS Voltmeter.

 

[brb58]

The current should be identical. I can totally see the voltage being different because of voltage drop in the cabling

 

[mmame]

The current should be identical. I can totally see the voltage being different because of voltage drop in the cabling

Exactly: To get proper W(h) or Voltage readings on capacity tester on higher currents, a four wire version is required.

And current readings could be off due to poor calibration and/or measurement errors/tolerances: With an effective current of 10A, a BMS with a measurement range of 200A and 1% error could show anything between 8...12A while still being in stated 1% tolerance...

 

[A.Justice]

I love my JBD BMS, but it's not very accurate when it comes to being an ammeter. I would say that mine reads +/- 10% depending on the day. I also have a similar cell tester, and it matches my calibrated Fluke within a few millivolts and milliamps, except under a high(er) load, where it succumbs to reading a bit of the voltage drop. My renogy shunt is dead on at readings OVER 1 amp, but it's pretty much guessing on loads that are under 1a. My $17 Amazon shunt is by far the most accurate of any of them.

My personal "tolerance" for cheap ammeters is about 10%. If I can get a "pretty good enough idea" of how much juice the cell has left, I'm content.

Not really "advice", but I just wanted to let you know that misreadings and off calibration are totally normal for cheap devices.

 

[Bud Martin]

The current should be identical. I can totally see the voltage being different because of voltage drop in the cabling

I am looking to see the VOLTAGE delta between the Voltmeter of the tester and the BMS Voltmeter.

 

[brb58]

I am looking to see the VOLTAGE delta between the Voltmeter of the tester and the BMS Voltmeter.

yeah...I would be measuring that with a meter at both locations since I really trust neither the BMS or the capacity tester. 4 feet round trip of 10 AWG wire @ 7A would be less than 0.05V.