24v supply from multiple 12v cells

hi, I need a 24V supply capable of providing about 20 amps to field charge large LiPO batteries. The charge process at 10A would take about 30 mins. I have available a large number of ex-UPS 12V batteries, each 12V, 26Ah. I would think that 6 cells configured in 3 pairs would give me sufficient 24V capacity to charge 3-4 times in a session. I would then like to be able to quickly switch the batteries over to a 12V "second-battery" charger relay that I have installed in my 4wd, so that when I am driving, I can re-charge the cells. I have a 4amp current limiter, as these sealed cells do not like a faster charge rate. Durability of the batteries is not really a problem, as I have about 20 of them avaialable.. so if some fail, no problem. my question is, is there an easy way to re-configure the batteries for 12V charging, and quickly re-configure as a 24V supply. I have seen the series - parallel circuit using two changeover relays.. but will I need to use 2 x relays for each pair of cells... ??I am not a complete idiot....some parts are still missing !!

https://www.the12volt.com/relays/page5.asp#spvr

Voice Coils in diagram = batteries.   Amplifier output = battery charger

I like your signature line.

I was thinking of replying (my girlfriend is going to kill me!), but some things are worth dying for... That signature is a gem! If only I had have thought of it - I might not hate i am an idiot for having my perfect name! I can see myself using that signature line a lot! I was going to add the usual diatribe about matched batteries in series (eg, not 2 in parallel with 1 in series, or big in series with small etc). The paralleling of unmatched batteries (but all 12V) when charging should be fine (unless a faulty battery pull down the charger and discharges the others). And maybe some basic calcs for the UPS battery drains... But now I have to confront my executioner. (No, she's great, but us men have to exaggerate!)

On second thought, do not use this diagram. This may put 24 volts worth of battery across your 12 volt battery charger. Mods please delete my previous post.

hi oldspark, you can have the signature line ... I stole it from another forum in any case ( I belong to about 10 RC helicopter forums !!) so.. nobody seems to have a practical opinion on my battery issue ? I know I can undo the leads and reconnect all 6 cells in parallel to charge them, and then re-connect them in 3 x 2 series to give me my 24V LiPo charger power supply. but this is fraught with danger having so many leads floating about.. I can see a lot of sparking and wire melting happening in the process.. I was thinking maybe something like all twelve wires go to a home-made flat terminal block, each wire separate. A connecting plate could be applied across the 12 terminals which is configured for the 3p x 2s wiring. The connecting plate has the 24V output lugs. Another different connecting plate could be applied that has the 6p wiring, with a 12V input lugs for charging ?? something like this rough pic I did in ms-paint ?? The blue plates A & B could clamp on to the temrinal plate to change the wiring config ? just not sure exactly how the 3p & 2S cpnfig should look ??      I am not a complete idiot....some parts are still missing !!

ok... this would be the 3P& 2S wiring plate that should work.. anyone see any issues.. I am not a complete idiot....some parts are still missing !!

PS - I missed chanen1953's reply. That too I'll leave till later... PPS - and links are now underlined too! I think I'm in love! Interim reply: There must be a way, though it's a matter of simplicity. 2 batteries is easy, but extending to 6... Hang on, it should reduce to 2 batteries - ie, 2 lots of 3 batteries each with their +ve & -ve tied together - and then just interconnect them as if 2 batteries. [ How obvious! Why didn't I think of that before? ] I'll find that circuit later... I did find 2 links that seem interesting for misc reasons - namely

Parallel Battery Switch (which shows the battery's internal resistance - that's handy when explaining battery behavior), and Connecting Batteries in Series and Parallel at the Same Time. The latter's left fig (below) shows the normal LONG-TERM connection method, but the RHS version is what I was thinking of above, and that is okay for short term. (If desired, can explain why the RHS is bad.) (with thanks to overlandresource.com) I'll also check i am an idiot's linked pic later... I think it is okay, I'd just ensure that parallel for 12V is a de-energised relay in case the relay lets go during charging. ...But just in case i am an idiot's requested deletion is still required, Admin here are great! Notice how I have stopped underlining weblinks? I finally realised Admin have made changes so the stand out better (I once suggested that as a "trivial" improvement) - ie, larger font. I noticed the same too for keywords. IMO it's those "trivial" GUI/HFE things that make a site so good (even if it takes IDIOTS like me a while to see things are done...). Those trivial codings aren't always so trivial, though more so usually another one of those thousand "few minute jobs" WHEN one finally gets a round to it (the famous round tuit). Ah yes, great Admin, great contributors with great names and signatures... what else could a person want?

See the UPDATE below.... (This reply was deleted and replaced 31Aug11 to send notifications etc after the update.) Well, after being put in place by some Noob! LOL! No - my newest hero tuvee found that circuit I was thinking of, namely from 12 to 24v automatic swap: UPDATE - WARNING! - if the bottom relay arm connects down to +12V whilst the top arm is still connected up to +12V, that means the Aux battery is shorted! That could be made "safe" with a fuse between the Aux battery's +12V & the relay, but see the above mentioned 12 to 24v automatic swap thread for further developments & solutions. But I'll check its suitability later [see UPDATE above], though otherwise it should be fine, just replace each battery with your 3 paralleled batteries. Isolation between the paralleled batteries can be added later - if you want that, ie, if housed in a permanent mobile carriage and you want to avoid manual isolation of paralleled batteries. It's just SPST switches or relays between each battery (or a 4- or 6-pole <whatever> relay/switch etc). Now to complete yesterday's oil change (car), and muse over these Rookies! (Then I might review my search technique for the12volt!)

thnaks oldspark..all very good.. but now getting much too complicated.. especially when I want to revert to 6 x 12V wired in parallel for charging.. your solution now has a lot of components, and relays.. which add up to a significant cost..    I think I like my own manual wiring plate idea.. as the switchover between battery configurations does not need to be automated.. just a simple "unbolt a plate" and "bolt on a different one" when charging required. simple mechanics that cannot go wrong .. provided ti is made well. Thanks all fir the suggestions and thoughts .. excellent ! I am not a complete idiot....some parts are still missing !!

It's only one DPDT relay, though probably 2 SPDT relays could or would be used (noting what I say in the other thread - ie, at least add a fuse in the upper +ve line to the relay). The default (de-energised relay) state is all in parallel for charging. When energised you have your series 12V connection (ie 2 lots of 3 parallel batteries in series with each other). My comments about extra relays to isolate all batteries relates to NOT leaving batteries in parallel for long periods, but IMO that is probably better done by manually disconnecting each interlink (between +ve terminals). [ In principle, leaving 6 batteries in parallel means you have to replace 36 in the same period as you'd normally expect to replace one. That's based on 6x the probability of a battery failing and that a failed battery will fail the other paralleled batteries - hence replace ALL batteries 6x as often. ]