# Can two batteries be connected both in series and parallel using an isolated 1:1 DC/DC converter?

I have two 7.4V batteries. I want to use the series 14.8V voltage to drive a load but I also want to use them paralleled to drive a lower-voltage load.

So would it be possible to connect an isolated 1:1 DC/DC converter to one of the batteries and connects its Vout+ and Vout- to the + and - of the other battery?

There's the additional requirement that the grounds of both loads have to be shared.

• Couldn't you also connect them in series or parallel and use that voltage to drive a converter? Commented Aug 27, 2020 at 16:09
• @user253751 Yes, step-down the batteries in series or step-up the batteries in parallel, but my question is specifically about using an isolated 1:1 converter.
– xnor
Commented Aug 27, 2020 at 16:14

So would it be possible to connect an isolated 1:1 DC/DC converter to one of the batteries and connects its Vout+ and Vout- to the + and - of the other battery?

Trying to do so with an off-the-shelf converter would just lead to trouble and frustration -- without a specially-built circuit either the top battery would just go along for the ride, or the bottom one would.

To really do this you'd either need to carefully match battery currents, or battery voltages. Neither of these are things that an off the shelf 1:1 converter does -- those things usually assume that isolation is your goal, not exact voltage duplication.

If you took the equal-current approach, you'd need to monitor the current coming out of the bottom battery to the 7.4V load, and match that current with the contribution from the top battery. It would be, if not entirely straightforward, fairly direct to do this -- but you'd need to monitor currents, which isn't trivial (or is trivial with the use of expensive chips).

To do the matched voltage approach you'd need to be able to measure the voltage across just the top battery, compare it to the voltage across just the bottom battery, and regulate any difference to zero. (This is probably the easier approach, BTW -- but still not trivial).

Either way, you'd basically end up building your own converter from scratch -- at which point you may as well build a non-isolated buck converter, because you certainly don't need isolation.

So would it be possible to connect an isolated 1:1 DC/DC converter to one of the batteries and connects its Vout+ and Vout- to the + and - of the other battery?

Step in the buck regulator - feed it from your 14.8 volt (series battery) and it should very efficiently step down to provide you with a lower voltage supply. It'll be more efficient than an isolating DC-to-DC converter because it won't use a lossy transformer.

In addition, trying to parallel the output of a DC-to-DC converter with an existing battery is going to require careful design to ensure load sharing. Don't bother, just use a buck regulator as previously mentioned.

• Dangit -- I should have mentioned this possibility in my answer. Unless there's issues of keeping the batteries balanced (7.4V sounds a lot like two LiPo cells in series) this is probably simpler & better. And if it's battery balance you're worried about, you should balance all four cells, don't just balance them two by two. Commented Aug 27, 2020 at 21:57