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Seamless Dual Battery Switching


Dave Williams

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I've had batteries on the brain lately.

 

Not just from a production audio point of view, but also for off-grid and house-scale solar applications. I'll be purchasing a spot welder and building various types of battery packs some time soon, and have also been thinking about setting up a decent cart power distro system for a big upcoming job, but the idea of seamless battery / power switching was giving me a bit of trouble. A simple diode arrangement is supposed to work well, but I wanted more manual control than that, while also keeping it as simple as possible. The other constraint I put on myself was to try and come up with a solution that could be scaled from small in-bag systems up to large high current applications (potentially with relays / contactors instead of switches).

 

This is what I eventually came up with and I think it solves those issues elegantly (but I'm not an electronics expert by any means).

 

image.png

As you can see, the switching circuit consists of 3 diodes and two SPDT switches (or relays / contactors). The idea is to have two modes (transfer & run). In 'transfer mode' it simply acts the same way as a regular diode power share circuit, where the current flows from the battery with the highest voltage, and cannot flow from one battery to the other. Once the preferred battery is selected, switching back to 'run mode' will then bypass the diodes and connect the selected battery directly to the load. So with this system you could have two batteries connected but preferentially drain one all the way down before switching to the full one (or leave it in transfer mode if you preferred). Previous iterations had a problem where there would be a break in power when using a break-before-make switch to change modes, but the diode across the mode switch should allow current flow for the brief interval of time that there is a break between contacts; this diode then gets fully bypassed once the contact is made. As far as I can tell this gives perfectly seamless switching (changes in output voltage notwithstanding) so long as you only select between the two batteries when in 'transfer mode'. If you attempt a switch between A & B while in 'run mode' the break-before-make switch should cause a very brief blackout.

 

My thoughts are that this could easily be done with manual switches, or perhaps a microcontroller and relays would be a good upgrade because then some voltage sensing could be used to automatically switch batteries if one gets to a pre-set voltage level or similar.

 

I've read that it's often fine to use Schottky diodes to minimise voltage drop in power sharing circuits, but I wanted to fully bypass them so that there would be the possibility of having some sort of charger option further down the chain (perhaps a charger / inverter?), and this way whichever battery is selected could also be charged by that. I need to research more about which types of charge controllers can also power a load at the same time without upsetting their charging algorithm. I figure if a solar charge controller can do it, then there must be some AC powered chargers that won't mind either.

 

It also occurs to me that you could use this circuit as a UPS type of setup if left in 'transfer mode' and one of the inputs was from an external AC to DC power supply, so long as that voltage was higher than the battery, and if you're happy for the diodes to stay in circuit, the battery would take over the instant the external dc was interrupted.

 

Another thought is that if uninterruptible power was important to you in a mobile bag context, while also saving weight, it might be possible to have a single 21700 cell connected to a boost converter to be a temporary power supply only while swapping the other battery over (I think a single 21700 could power a standard bag for over half an hour maybe?)

 

Does anyone have any thoughts or feedback?

 

Cheers!

 

Dave.

 

 

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Most of your questions here are over my head but I wanted to share something I noticed on a DIY project I did. 
 

I assumed that two dc sources feeding one load but isolated with diodes would perfectly draw from just the higher voltage source but in my case I found that the load pulled power from both sources but it was not in equal amounts. More power was pulled from the higher voltage source than the lower voltage source. I’m not sure if that’s to be expected but that’s what I ended up with in my case. 

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I thought this issue had been solved in the world of RV vehicle electrical?  Years ago I had my Econoline work van modified by a truck-repair place to have two batteries that the alternator kept topped up, but drawing from the 2nd battery, there for sound equipment power via an invertor, did not affect the truck or "starter" battery.   So maybe there is a mass produced solution for this from RV equipment companies?  I apologize if I've misunderstood your project.

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You could also cover the drop during switching with a capacitor.  The capacitor only needs enough capacity to keep the voltage up under load for the few milliseconds that a relay would take to switch sources.  If you only wanted one or the other battery and never both then you could potentially drop the diodes.

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I‘m not sure why you want both batteries to drain sequentially rather than simultaneously? The time it takes until both are finally depleted would be the same, so maybe it doesn’t matter much, plus your way allows for more time to change batteries, but with my way you‘ll have less loss in the stand-by battery. Maybe not really an issue. 
 

Not quite what you’re looking for, I know, but Remote Audio make a dual NP-1 cup that allows for seamless battery changes. To scale things up a bit when needed I bought a dummy NP-1 with an XLR4 input which I can use for any power supply. 


Funnily enough, just today I ordered a DC-AC converter which also accepts AC as an input and will switch over from battery to AC once the battery is near empty. I use it with my small solar installation….

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2 hours ago, Wandering Ear said:

You could also cover the drop during switching with a capacitor.  The capacitor only needs enough capacity to keep the voltage up under load for the few milliseconds that a relay would take to switch sources.  If you only wanted one or the other battery and never both then you could potentially drop the diodes.

 

Yes I was originally considering a capacitor to cover any break-before-make situations, and it might still be a good idea to include one anyway to reduce operator error on the A/B select switch in 'run mode'.

 

1 hour ago, Constantin said:

I‘m not sure why you want both batteries to drain sequentially rather than simultaneously? The time it takes until both are finally depleted would be the same, so maybe it doesn’t matter much, plus your way allows for more time to change batteries, but with my way you‘ll have less loss in the stand-by battery. Maybe not really an issue. 
 

Not quite what you’re looking for, I know, but Remote Audio make a dual NP-1 cup that allows for seamless battery changes. To scale things up a bit when needed I bought a dummy NP-1 with an XLR4 input which I can use for any power supply. 


Funnily enough, just today I ordered a DC-AC converter which also accepts AC as an input and will switch over from battery to AC once the battery is near empty. I use it with my small solar installation….

 

I simply just wanted options. Yes definitely the two batteries in parallel would last twice as long (or build a bigger battery in the first place), but I also wanted the flexibility to choose otherwise and also to bypass the diodes for integrated charger options.

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I should clarify that this circuit wasn't necessarily intended specifically for bag/cart or even audio applications. I wanted it to be applicable to them, but also cover other scenarios as well.

For example this is a really cool DC input ATX power supply - https://hdplex.com/hdplex-800w-dc-atx-with-12v-63vdc-input.html that could be used to build a server or other always-on computer setup, or some other sort of application where continuous operation was important.

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3 hours ago, The Documentary Sound Guy said:

Wow ... 800W is a hell of a power draw for a battery source!  Not that a computer is likely to draw that continuously, but still... that's 10A on each input wire.  I'd be nervous about drawing that much power from a single battery.

 

Haha yeah only 67A at 12V... But that's what I love about batteries, you can keep adding more! Given suitable bus bars that is 😁. Plus 24 or 48V means less current for the same power out. I have a little fantasy of adding some sort of motor to my cart for going up hills / stairs. It could even incorporate some sort of regenerative braking for going back down again haha.

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