BDS dual NP cup hot swap issue

[JesseF]

 

So here’s the situation. I’m running a dual NP cup remote audio BDS system. I use the dual NP cup so I can hotswap batteries and not power down my bag. 

 

It’s worked great so far, but the other day it went a little haywire. I was powering two ADX5D and an 888, not a huge load. At about 13.7v reading on my 888 I went to hot swap batteries. I unplugged one NP and the voltage was reading 13.5v and I got a notice from the recorder that voltage was low. When I put a new NP battery into the cup it went straight from reading full to empty. I thought I might have just not charged that one so I tried a second NP and the same thing happened. 

 

I took the two dead NPs and put them on charge and immediately they popped back up to full. I took both low voltage NPs out of the cups, put both of the ones I just put on charge in the BDS, without hot swapping with a used battery still in a cup, and they worked fine for the rest of the day. 

 

So, is this an issue of there being to wide a spread between voltages of the NPs for a hotswap to work? Like the BDS tries to average the voltage and just ends up sapping the new battery in one second? I’ve done many hotswaps at higher voltages no problem.  

[Izen Ears]

Do both batteries drain equally or does one drain before the other?  There must be some magic in the BDS that limits the output voltage, otherwise you'd have 32 volts coming outta there, right?

 

I use two batteries but I never even thought of getting a dual cup. I just use two BDS boxes.

 

That behavior almost sounds like old batteries that can't hold a charge anymore, but if they lasted all morning that doesn't make any sense.

 

Dan Izen

[JesseF]

I'm pretty sure they are wired in parallel, so the voltage is not additive.  Batteries wired in series would double the voltage which is obviously bad.

 

The batteries drain at the same time.

 

They are fairly new batteries, from a year or two ago.  So they should be good.

[The Documentary Sound Guy]

My guess is that you accidentally triggered some protection circuitry in the BDS when you plugged the fresh battery in, and that circuitry reset itself.  Once the BDS was unplugged and given a chance to rest with no batteries attached, the protection circuit reset itself and went back to normal operation.

I think it's not unreasonable to think that having to big a spread in voltage between the two batteries (i.e. a fresh one and a dead one) could have caused this, but there's no telling for sure at this point.  Maybe be a bit more conservative in your low battery threshold if you want to play it safe?  Or you could try reproducing it by deliberately running one of the batteries way down and plugging in a fresh one next to it.  13V is not an unreasonable threshold to swap Lithium Ion batteries, but maybe your particular setup isn't stable that low.
 

[Derek H]

I agree that it sounds like some battery protection circuit kicked in. Either in the BDS (poly fuse) or something maybe more sophisticated in the battery itself.  Maybe the level of charge had nothing to do with it and there was a small arc or something that kicked a fuse. 
 

I’ve had this happen when hot-plugging a device with a barrel connector (bad idea) and there’s a tiny arc and the whole bag shuts down. Give it a ten second rest and it powers back up. Poly fuse. 

 

 

Still thinking about this… maybe what happened when you hot swapped was the NP fresh off the charger at 16V and put it in parallel with the 13.5V NP too much current rushed from the charged NP to the low NP (batteries in parallel will balance each other’s voltage this way) and it tripped the low battery’s charge current protection. 
 

The basic remote audio BDS doesn’t have anything fancy built in for managing two batteries as far as I know. 
 

A better way to run two batteries together might be to have a circuit that doesn’t allow the two batteries to interact. Like a Schotkey diode. The PSC triple bds for example has some circuit on it that does this. It has two separate input connectors too. This also lets you have an AC adapter in one input and battery backup in the other. Sorry not trying to sales pitch PSC stuff but just an example of other options to handle this scenario.

[Tim Norris]

I have been doing this for 30 years or so... running dual NP-1 type batteries and hot swapping using many power distro devices from the modern AudioRoot, BDS in several different iterations and even my first couple of hand crafted Peter Engh rigs using various battery pairs from old Sony NiCad to IDX NP-9X.

 

I have never had problems unless I allow the batteries to get too low to power the bag. I generally swap both batteries at no lower than 14.1v. Because the batteries drain equally, I figure they should be swapped together. I have enough battery pairs to run for several days without charging. I do appreciate the Scorpio mixer with its on-board batteries. The SL6 was nice, too. Yes, I always run the on board batteries with the dual NP-1 cups.

 

I hot swap only when I must...

 

Cheers,

Tim