Battery Tender and RFI

I have a battery tender paired up with a 12Ah SLA battery. I found that when the unit is plugged in and charging the battery, it generates a lot of RFI up through VHF. When powered up the unit creates full scale interference on my 2 meter station. I like to use this battery on my QRP/experimental radios as it lets me go outside away from outlets. Keeping the battery topped off is important. Unplugging it anytime I want some peace and quiet on the RF spectrum is a bit inconvenient.

Pondering on possible solutions for this I came up with stuffing the whole setup into a metal box and adding additional filtering on the AC input. This keeps it quiet when everything is sealed and it is just maintaining the battery. I had an ammo can that I have just used as a storage box in the past. I modified this with a few holes to mount an EMI filter and allow the power cord to come through the case. I did not have a grommet for the power cord so I applied a liberal amount of hot glue for support and to prevent chafing.

New battery tender setup. Nice and quiet. -AC9LF

The negative side of the battery is bonded to the metal ammo can along with the ground connection on the 3 wire 120VAC plug. The lid is also bonded with wire, star washers and bolts to the main case. This is to make sure there is a good electrical connection. I did not want to rely on the hinge for a connection. I no longer see noise emissions from it on my s-meter in the 2 meter rig. 

The somewhat haphazard insides. -AC9LF

If I use this as a power supply with the charger/tender plugged in it will still generate a lot of hash in the radio. This may be due in part to the lid not being sealed when using the DC connection. I did not route this wire through the case. I was hoping that I might be able to use this as an "automatic backup" power supply for my 2 meter base station. I may be able to remove the conducted noise with more filtering at the DC side of the battery tender.

For now I can keep the battery topped off without the interference normally caused by this thing. I can still unplug it to use the battery. As a bonus it features a nice handle and carrying case for the battery!