Battery pack switching Series to Parallel

[Dollarday]

Hi Everyone,

Basically, I want to switch a battery pack from a series configuration to a parallel configuration (during charging) and back again (during discharging) using MOSFET switches.

Why is this nifty?
a) Hobby shops sell something similar, but you have to manually connect and disconnect the batteries yourself: http://www.progressiverc.com/paralle...er-t-plug.html
b) You can charge any battery pack from the same source if they are connected to this circuit
c) Your cells are connected in parallel while charging, which causes your battery pack to balance itself without a balancing circuit (during charging anyway)

I Have attached a picture of analogue switches required and a picture of the circuit I built using MOSFETS in the simulation program Crocclips. (Which seems to work in simulation)

I know I can probably use relays, but MOSFETS are so much more elegant...

Would the design work with the N-Channel mosfets I use? Otherwise, any design suggestions are welcome.

 

[(*steve*)]

You need to realise a couple of things.

Firstly, a power mosfet has an integral body diode in parallel with the channel. This means that it does not operate like the perfect theoretical fet you have drawn. The cure for this can be to place two mosfets in series so that their body diodes are back to back. This means you need to turn both mosfets on to achieve a low resistance path, but when stwitched off, they really are! Placing a diode in series with the mosfet is not a good solution as this will cause an undesirable voltage drop.

The second consideration is that you need to provide gate voltages suitable to turn the mosfets on. this may depend on the circuit topology, and the voltages may well be outside the bounds of your battery voltage.

The third consideration is switching speed. You need to be careful to turn one set of mosfets off before turning the next set on. If you're not careful you could short your batteries for a brief period. In the worst case, this could result in a mosfet failing (typically short circuit) leading to a far more significant short and subsequent risk of fire.