> > Nevermind my schematics. They won't work. I'm having a LOT of trouble w= ith > > the MOSFET switches. All I want is a relay type thing that will consume= next > > to no power, but I can't generate the appropriate gate-source voltage l= ike a > > real SSR does with a photocell. How would I go about doing this? What you were suggesting was fine, just takes some playing to get all the switching and conduction paths right. > Next crazy idea: capacitor charge pump for driving the MOSFET gate so I c= an > put N channel MOSFETs anywhere I want, even on the high side. MOSFETS are a potentially very good solution if you are going to do this. (In the example you started with I'd probably stick with the series string and voltage clamps per cell, but here I'll stick to the current series/parallel switching question_. Care needs to be taken re MOSFET reverse "body" diode from Drain to Source. This is an "intrinsic" diode which occurs as a result of the construction method and must be taken account of. It may not be relevant here depending on final circuit but I mention it 'just in case'. For a static switch - one way for charge, other way for discharge, you need minimal gate current. The excessively enthused could probably charge a capacitor from th series battery string and use it to power the switch in the parallel battery state ! :-) - BUT it is unlikely to be an extremely good idea. To get a "high side" supply to allow all N channel FETs to be used you need an extremely modest power level. This could use inductive flyback with dedicated SMPS IC or a 555 or whatever. (My oft mentioned super cheap MC34063, cheap small inductor one cap and a few R's would do the job (plus a smoothing cap or few). As would any of a thousand other SMPS ICs (few as cheaply) A capacitor pump would also be OK but need to be N stage if driving FETs for 3 batteries in series from a single battery-voltage supply. With Schottky diodes you probably need 4 multiplier stages if powered from a single cell voltage. You can probably arrange a system which bootstraps itself off the increasing available battery voltage as the arrangement starts to switch from parallel to series. But a basic inductor based SMPS seems more attractive. A feature of MOSFETs which is often not appreciated (or allowed for) but which may prove useful here, is that they are true 2 quadrant switches. That is, they will conduct D-S with either polarity while Vgs is always positive. So they are both a bidirectional current path when on AND can be used "upside down" as long as Vgs is positive for an N channel device. The downside of "can work upside down" is "always will work upside down". That and the reverse body diode and yo may have to worry your circuit for a while before it lies down and behaves. Bipolar transistors o not have this nice bidirection when on feature, and will/may voltage block in reverse voltage up to a point, but behave interestingly after that point. So - NOT hard but needs care in design to make sure that current doesn't go via unintended paths. MOSFETS may be uses to make a true bipolar on/off switch by connecting two in series, sources joined, gates joined, switch contacts =3D the two drains, You can then have some fun getting switching voltages to the now floating source/gate connections but that is also doable. Quick BOTE switch suggests 6 FETS needed. Batteries T M L (Top middle low) Bt Bm Bl F1 ... F6 N Channel FETs. F1 Bt- Bm+ F2 Bm- Bt+ F3 V+ Bt- F4 V+ Bb- F5 Bt- Gnd F6 Bm- Gnd Bt+ - Vout Bb- Gnd Switching left as an exercise for the student. Back diode checking left ... (look OK at a glance) R --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .