Dual battery BDS- isolation needed?

[Allen Rowand]

Thinking about adding a second battery connector so that I can hot swap batteries without bringing the bag down. I was considering using diodes to isolate the batteries, but I don't want the voltage drop that would bring. Do I even need isolation? I don't want to run both batteries at the same time, just connect a second then disconnect the first.

[Derek H]

I’m not an electrical engineer but I think in general you’d be fine IF you’re using two of the same type battery (voltage and capacity) and you wire them in parallel. I have a dual NP1 holder box that does exactly this and it works great. 

 

Now if you want to combine different power sources like say an AC adapter and a battery you’re going to want a circuit with diodes to isolate each power input. Look for a Schottky type diode for the lowest voltage drop (under 1/2 volt). Probably want a fuse in this kind of arrangement somewhere as well. You can get a dual Schottky diode in a small form factor IC style package that has three metal tabs. Two input one output. 

[Allen Rowand]

Thanks, I figured parallel would be fine. The batteries are the same voltage and capacity.

 

Back when I worked on Cadac mixing consoles they had diode bridges to combine the main and backup power supplies. I'd always tweak the output voltage up to correct for the voltage loss.

[Wyatt Tuzo]

Just a note that if you DO simply wire in parallel, the batteries will equalize one another, and you will technically be drawing from both at the same time. The voltage will remain the same across each battery. This will still allow you to hot-swap (assuming you aren't close enough to total depletion that the increased current draw to the second battery isn't enough to tank it when the first is pulled).

Again, this should all work fine, but it's not like you'll be drawing from one batt until its time for the backup to take over.

[Allen Rowand]

I'm trying to avoid carrying both batteries in the bag, so rather than use them in parallel to extend the mAh capacity I'd swap. Run on one battery until it's low, then connect a fresh battery and remove the first. When both are connected the fresh battery may begin to back-charge the old one, but only for a few seconds.

 

I think my bag should be good for a day on one battery, but I want to have a plan in place before I prove myself wrong!

[Derek H]

Yeah I don’t think the drop is a huge issue. The diode circuit will draw from whichever source has a higher voltage. So you’ll want a 15V power supply or better if you’re using lithium ion batteries.