9 or 18V from DPDT switch?

I'm having a hard time seeing this. The idea is to make the series connect giving either 9 or 18V from 2 9V batteries. Is it even as simple as that?

Thanks,
Caine

Series/parallel? or do you just mean series or a single battery?

Either way, yes.

Keith

it needs to be either 18V or 9V depending on the switch position. So, that would be series and a single battery.

Here's a better option. It shares the load when in 9V mode, for longer-lasting staying power and (theoretically) equal load on both batteries whether in 9 or 18V mode.

Keith

Thanks Keith,

Thats not even close to the stuff I was drawing!

Cheers,
Caine

I may be reading it wrong, but shouldn't the bottom switch contacts be reversed?

Yup, the bottom contacts are backwards. The general idea is still sound, though.

Peace,
Al.

You're quite right!

As drawn there'd be fireworks! (well, it was the 4th yesterday!) the throw of the lower half of the switch should indeed be reversed.

-Hastily drawn, with half a mind on how to make a GIF and upload and make a link and what's for lunch and "ooh, a Bird!" and "what's that dear?"...

:roll:

:grin:

Keef

Thanks, PRR

Caine

If you were to use Keith's approach make sure you buy and use the same vintage batteries. I have seen paralleled batteries work fine in many apps, but if they are mismatched badly in terms of degree of discharge or age, bad things can occur.

Even series batteries are problematic sometimes. The new Lithium primary (i.e., non-rechargeable) AAA cells were a natural for a portable speaker system I did. But they don't know how to make them all that consistently yet, and when subjected to a tiny standby current for a series string of four, I found widely different cell voltages after a while and greatly reduced lifetime compared to expectations.

I always blindly thought batteries in series or parallel would "equalize" for lack of a better word. I guess that is a false assumption.

Caine

Unfortunately, yes. Battery chemistry is very complicated, for both rechargeable and primary cells.

If the energy density isn't too large at least you are fairly safe.

Lithium rechargeables are so dangerous that the vendors won't even sell them directly. They come packaged with a bunch of electronics to guard against abuse. If you are a big company and have taken the training and have the liability insurance you can get raw cells.

I spent months on a system that was going to use lithium rechargeables only to be re-directed, when the top honcho flew too close to the sun, to use less elegant but much cheaper user-replaceable primary batteries. I was actually relieved; I learned a lot, and although it was a lot of wasted work I didn't have to have nightmares about fires and injuries.

Cool. I thought about drawing PRR's version alongside the first one that I drew (badly! ).

I long ago gave up mixing and matching batteries of questionable provenance, it's more trouble than it's worth. Assuming same battery type and similar battery condition, the method I tried to draw was intended to try and maintain approximately equal aging on both batteries.

So, bcarso: any thoughts on the long chains of NiMH batteries that they use in Hybrid cars? -When the cars start to age, I bet that you run the risk of ending up replacing a battery every few days, and getting pretty darned fed up with it?

Keith

Yeah---I don't know what they are doing, but there is probably some pretty fancy management of the cell voltages going on. Initial matching and consistency have got to be critical.

An old friend of the family back in the UK just bought a Toyota Prius, and (being of a design/engineering persuasion) one of the things he asked about -although very generally- was cell replacement and management. He was told by the sales droid that there was a lengthy chain of Ni-MH cells and some fairly involved monitoring of each cell individually. When a cell goes bad the car will probably still run, and it will apparently tell you that one of the cells needs to be replaced. You presumably go to the dealership, where some diagnostic plug-in leads them to the dodgy cell which they duly swap and bill you for.

The reason your comments on cell consistency have touched me is that in series/parallel cell arrangements, the random varying condition of differently-aged cells "shuffles the load" around unpredictably, hence changing out a few cells one at a time in a hybrid vehicle means that you probably have a slowly accelerating failure rate, until you get into the thick part of the curve, where you're replacing cells frequently. At that point, perhaps each cell that you switch out results in a tougher load for the older cells, triggering/perpetuating an unpleasant cycle.

In Florida, with the heat for so much of the year, (coupled with the humidity) I'd expect that running on batteries around town is one thing, but what do they do for Air Conditioning? -I think my friend's Prius (in the UK) doesn't have Air Conditining, so the question probably didn't occur to him. None the less, The load and the relentless heat probably make battery condition and different age an interesting challenge...

Actually, the more I think about it, the more I want to sell shoes for a living!

Keith

[quote author="SSLtech"],
Actually, the more I think about it, the more I want to sell shoes for a living!

Keith[/quote]

Hmmm. Feet tend to stink, especially in warm and humid climes. Wouldn't be my choice.

Real estate? No, everybody is doing that now and the bubble has to burst sometime soon anyway.

Plastics? No, that was so mid-20th century.

real estate's bubble has JUST burst, thanks to SCOTUS

The answer lies in.... boost gauges!

No, really.