With included hardware, the battery attaches to the fold-out stand on the mcHF's housing and does not interfere with that functionality. In addition to the 12V output the battery also has two 5V USB plugs for charging a phone or other device.
The battery arrived poorly packed without sufficient packing material to keep it from shifting around in the box. It got worse from there.
Prior to charging the battery I opened it to have a look at the battery and supporting circuitry. As with the shipping package the battery was unsecured in the housing and would slide from side to side as the case was tilted.
|Battery slides to the left...|
|...and to the right|
The wires leading to the USB jacks changed gauge halfway there via a poor solder connection wrapped with electrical tape.
|Battery, charger and hardware for attaching to mcHF|
Then I set up the battery analyzer to discharge the battery at 400mA - the receive current draw of the mcHF. This resulted in a measured capacity of 3.4 AH.
At a lower discharge rate, and if discharged to a lower voltage than I would prefer, the battery probably will give 5 AH of capacity so the advertising is correct although the battery's label is overly optimistic.
If you have a mcHF and are tempted to add a modular battery I'd recommend avoiding these eBay batteries like the plague.
A better solution would be to buy an empty case (if possible) and add a battery and protection circuits from a reputable manufacturer.
.UPDATE (26 June 2019):
A comparison of a lower discharge rate (100mA) shows little difference in battery capacity from the 400mA discharge rate.