Why does my battery have idle draw?

[HylanderUS]

Okay beginner here, I might be misunderstanding something so bear with me:
I built the milk crate system from one of Will's videos, with a 50Ah 12V Ampere Time battery. All works well, except the whole thing has some parasitic drain, so the battery empties itself over the course of a few weeks. I measured and found there's constant draw of 0.3A. Disconnected everything to figure out what's drawing, and then just for baseline I measured amp draw between the battery poles themselves....and it's 0.3A. Wth? Is this normal? Am I measuring something wrong? What is even drawing power here, is it the internal BMS of the battery? At 0.3A the 50Ah battery will drain itself in ~150h (right?), so that's not really a viable thing to have as "backup storage" if it becomes unusable within a week. Is this normal?

 

[FilterGuy]

and then just for baseline I measured amp draw between the battery poles themselves....and it's 0.3A.

What do you mean by 'between the battery poles'?

A BMS will have a parasitic draw, but 300mA does seem excessive.

 

[HylanderUS]

What do you mean by 'between the battery poles'?

A BMS will have a parasitic draw, but 300mA does seem excessive.

I set my voltmeter to "amp" and hold the black stick to the black battery pole and the red one to the red pole. Without anything connected to the battery, it's just sitting there on my workbench by itself

 

[robby]

I set my voltmeter to "amp" and hold the black stick to the black battery pole and the red one to the red pole. Without anything connected to the battery, it's just sitting there on my workbench by itself

Your putting the meter in Amp mode and putting the probes across the Red and Black terminal on the battery? That would cause a short and blow the fuse in the meter. I suspect that you did not move the red probe wire on the meter out of the Voltage input side and what you are seeing is just some zeroing out error in the meter. Do not put the red wire into the Amp side of the meter or it will kill the meter or blow it's fuse. The Amp mode is an inline mode on the meter, meaning you put one probe on the battery positive and the other lead onto the loads positive side. The problem is that the load cannot exceed what the meter can handle and in most cases that is 10A.

It's going to be hard to test for parasitic current draw on a sealed battery with a BMS inside. As @FilterGuy pointed out the BMS is drawing some power but it is not going to be as high as 300ma.

 

[HylanderUS]

Ohh, thanks for the info, I've been doing this all wrong... ?
I ordered a meter w/ a hall sensor to get actual amp readings, and in the meantime I took all fuses out and now it's not discharging anymore (i.e. the voltage reading doesn't go down after a day).
So I think it was just the idle draw of the MPPT controller that was always "plugged in"... i couldn't find any specs on that (rich solar 20a), but it would be reasonable for that to draw a few W at idle. And my wrong way of "reading" produced always the same results, so I thought the battery was drawing w/ nothing attached...doh.
Anyway, there's no reason for the mppt (or anything really) to be on when the crate is "in storage mode", so I'll just take the fuses out when I put it away. Or I guess I could wire in a "master switch" directly on the battery lead?

 

[chrisski]

So I think it was just the idle draw of the MPPT controller that was always "plugged in"... i couldn't find any specs on that (rich solar 20a), but it would be reasonable for that to draw a few W at idle. And my wrong way of "reading" produced always the same results, so I thought the battery was drawing w/ nothing attached...doh.

If its a few weeks its probably a lower draw than .3 amps.

SCCs and Shunts can draw amps. A cheap shunt is supposed to be 50 mV and a better one 5 mV. However I can’t do the math to turn that into time, other than 10 times as quick for a cheaper shunt.

My SCC might have an idle draw of 40 ma. THat would take weeks to deplete a 100 ah battery.

THis is my non-UL approved switch for my crate build:

Amazon.com

Sonce panels need to be disconnected before the batteries, I just need to remember to disconnect the panels first and then shut it off. I’ve forgotten a few times, but the SCC is fine.

I’ve got the crate out right now cooking up some food In a crock pot.