This is in regards to the following unit:
DROK DC 0-120V 0-100A Digital Multimeter
https://www.amazon.com/gp/product/B01M5CWR2P/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1

I’m using this in an RV application to monitor my battery.

When I’m charging the battery, the aH indicator counts down. This is expected and I understand why this is – it’s like showing me my “tank is being filled up.” What I don’t understand is why the aH goes into the negative. And if I stop the charging when its in the negative (and start using the battery again), its starts count the aH from the negative (not from zero). So if I start at -2.0 aH, by the time I am at 0 aH, I actually used 2 aH. So my battery level has been used by 2 aH, but the display will only show 0 aH used.

Can you explain why this goes into negative aH?