# Switching high voltage battery from series configuration to parallel configuration for charging

I have a question regarding switching batteries from series to parallel to charge them.

I have 36 modules of 1Sx11P Li-ion batteries connected in series. During normal operation they drive a car with a high efficiency motor, and are recharged via solar panels and two max power point trackers.

However, when the car is not running off of solar power or is parked in the garage I would like a way to charge the vehicle with a battery charger or power supply with a cable that could plug into the car. I have been reading some descriptions about people doing similar things for beefy RC airplane batteries by connecting them in parallel and charging many more than the battery charger could originally charge. Here is an image from this website:http://www.rchelicopterfun.com/parallel-lipo-charging.html

Or other configurations like on this picture

The advantages of parallel charging would be very big for me as it would regulate a lot of the cells voltages without the need for external balancing circuitry, though I do have that available when the car is in serial hookup.

The batteries aren't all that accessible in the car either, so plugging in two different harnesses isn't really option for this idea.

What I want to know is, is their a way I can implement a circuit that can flip back and forth between series and parallel for the batteries that could handle the high voltage and large currents I would be dealing with? Do you have any recommendations on how many batteries I should look into having in parallel while charging?

I would appreciate some insight as this is a little out of my typical realm of electronics.

• Why not charge the batteries while they are connected in series? Seems like a much easier approach. Commented Jun 19, 2015 at 22:44

## 1 Answer

This is the universal conundrum of parallel vs. serial connection of cells/batteries, for both discharge, & re-charge. There are pros & cons for each scenario - efficiency/losses (P=I^2.R), safety (Lithium is an unforgiving mistress), convenience / accessibility, longevity, equalisation.

RC hobbyists trade convenience with a willingness to disconnect packs from their gear, and then connect them to a charger, which gives them flexibility to arrange cells in whatever series or parallel arrangement makes best sense for each discharge & recharge phase of their play-flow (work-flow - geddit? ;).

As you say, your application doesn't facilitate this physical flexibility - the cells aren't easily accessible, nor is changing harnesses. You could engineer a complicated switching scheme with either relays or mosfets, to allow you to reconfigure the array for discharge & recharge, but that's a lot of work, a lot of cost, and extra copper/mosfet losses.

If it were me, I'd suck it up & go for a Li-Po recharge solution that supports the number of series strings with cell/pack-balancing capability (which of course is critical in series configurations). That may mean you need to take your power source & use a switching boost voltage regulator to get the voltage you need (i.e. more than the number of series cells * 4.2V) to achieve this, and then a charger to match.

If you really want to stick with hobbyist-level chargers like the one in your photo that can handle a handful of cells in series, then one approach I can think of is to power each of those rechargers via isolated DC-to-DC converters (in turn powered from your solar/MPPT supply, or bulk mains-powered PSU) - that lets each charger handle a handful of cells in series, and provides the isolation needed between each charger as the string voltage escalates along the string. Here's some examples of the kind of DCDC modules I mean. One challenge of this approach is how to control each of those rechargers electronically (instead of humanely) whilst maintaining the isolation needed between them, but that's a separate question :).

• btw, about that 'other' configuration picture you attached: either there's a module on each of those capacitor-isolated recharge outputs to properly handle Li-Ion charging that just isn't shown (i.e. at least a 2-stage constant-current followed by constant-voltage recharge controller), or whoever drew it has no idea how to recharge Li-Ion. Commented Jun 20, 2015 at 1:18
• Yep, it does seem like nobody has the good solution for how to switch between Series and Parallel. I've been waffling on what to do, but I think you've help make up my mind to do the Series recharge of the batteries. Now I just have to make sure that the balancing is happening so we don't get any Lithium fires! :D Commented Jun 20, 2015 at 14:24