More smaller batteries vs. Less bigger batteries

[order99]

Well, if you don't mind doing all the tedious connections yourself, I don't see why not-just remember to use several parallel strings of series, otherwise you'll end up with a very high Voltage pack of 2.7 Amps.

And Bennelson-if you're going to Turbocharge a PWM controller with a Contactor(and risk the sudden jolts damaging it) why not just use a Contactor Controller with Amp Limiters on the Batteries and freewheeling diodes on the circuits to prevent Contactor wear? The old Analogue Controllers probably won't care about the sudden additional power, and the diodes should reduce Contactor wear, with the Amp Limiters just helping your EV FE a bit...

http://www.diyelectriccar.com/forums...ion-27537.html

There's an interesting idea being discussed here, of Volt-controlling the battery output with a software-driven Auto-switcher...it logs in one battery at a time as needed, and in order of the strongest battery currently in the string. If this method is perfected, it would make a cheap and effective alternative to PWM, I think.

Mind you, I don't see how the Switcher or the Contactor methods would work with NiMH or Lithium single packs-you'd probably have to keep them separated into discrete packs and separate chargers or something...

[Daox]

Originally Posted by The Atomic Ass

That's actually incorrect. If you have 2 batteries in parallel, and you're pulling 100A at 12V, then you're pulling 50A from each battery. But if you have them in series, and you're pulling 100A at 24V, you're pulling 100A at 12V from each battery. In series your amperage draw per battery is always equal to your total pack draw, with the exception of cell imbalance.

Alright, that does make sense. Thanks for correcting me and clearing that up.

If the amperage was split between the two batteries you'd end up with the same amount of power with the 12V and 24V system which really doesn't make sense when you calculate power output in watts.

example:


parallel setup of 2 batteries: 100A @ 12V = 1200W

series setup of 2 batteries: 100A @ 24V = 2400W

Each battery in the series setup must support half that wattage. But each battery is only 12V. Therefore, you must pull 100A @ 12V to get 1200W which is half of the 2400W.

I hope that makes sense to others...

[spoil9]

Originally Posted by Wilden

Just wondering, since the consensus here seems to be that more batteries is better, what do people think of using AA batteries.

You can get a pack of 4 2.7amp 1.5v rechargables for about £5 here in the uk, sometimes cheaper. If you got a load of these, and set them up to output 72v and maybe 100amps would that be as effective as using barger batteries? And more importantly, would it be cheaper?

Although I'm still in school for EE, just doing a little math here.
to get the 72V from rechargable AA (which only put out about 1.4v) comes out to needing 51.4 batteries wired in series. So we'll say 52 batteries since you buy them in packs of 4.
Then to get the 100A from 2.7A, you would need 37 strings of batteries.
37 strings of 52 batteries would equal 1,924 batteries, (or about 481 packs of 4).
481 packs at £5 would be £2,405 (US$3496-ish).

I'm thinking you would need to create your own holders for the cells unless you can solder a metal strip to the tips and then bolt a wire to the end of that.

Other than being tedious like order99 said, how does that compare to other options cost/ weight wise?

Off subject, I thank everyone for their input, this has been a very interesting read for me.

[spoil9]

Originally Posted by Daox

example:


parallel setup of 2 batteries: 100A @ 12V = 1200W

series setup of 2 batteries: 100A @ 24V = 2400W

Daox,
My understanding was that batteries in series gets their voltages added, and batteries in parallel get their current added. So wouldn't that be 200A @ 12V = 2400W for the parallel setup also? (Assuming that is 100A @ 12V for each battery)

[rmay635703]

Originally Posted by Wilden

Just wondering, since the consensus here seems to be that more batteries is better, what do people think of using AA batteries.

You can get a pack of 4 2.7amp 1.5v rechargables for about £5 here in the uk, sometimes cheaper. If you got a load of these, and set them up to output 72v and maybe 100amps would that be as effective as using barger batteries? And more importantly, would it be cheaper?

I doubt it, just to throw in an opposing view here but generally 2 things are true of lead acid batteries (if thats what you plan on using)

1. The lower the voltage the more reliable the battery, this is ALWAYS true
2. The higher the capacity the more reliable, this is almost always true.

Smaller capacity lead acid batteries tend to be much more fragile and last fewer cycles as compared to larger capacity batteries.

Now from a controller and motor standpoint the higher the voltage the less current, cabling etc you need but HV systems are not always more reliable, better or more cost effective, what you save on cabling could easily be offset by higher controller costs and higher battery failure rates (if you use lead).

Basically put in the lowest voltage and highest capacity that you can live with while still achieving the range and weight you need, there are always going to be trade offs one way or the other. Go really high voltage and your controller will be expensive and the batteries will be tiny and potentially unreliable.
Go too low of voltage and your cabling and weight might be heavy and your speed too low.

Good Luck

[Wilden]

Originally Posted by spoil9

Although I'm still in school for EE, just doing a little math here.
to get the 72V from rechargable AA (which only put out about 1.4v) comes out to needing 51.4 batteries wired in series. So we'll say 52 batteries since you buy them in packs of 4.
Then to get the 100A from 2.7A, you would need 37 strings of batteries.
37 strings of 52 batteries would equal 1,924 batteries, (or about 481 packs of 4).
481 packs at £5 would be £2,405 (US$3496-ish).

Ah, interesting, I thought you could set it up so you had a string of 52.4 in series which then leads to the 37 in parallel, rather than making a giant square of batteries as you are suggesting. My knowledge is somewhat lacking on the subject, but this would mean that you would only need 90odd batteries, rather than nearly 2000.

Here's a quick paint job of that I mean, sorry about the quality, my brain doesnt work at these times (7am here in the UK). In this case you're looking at spending only about 23 packs*£5 which is £115.

[The Atomic Ass]

Originally Posted by Wilden

Ah, interesting, I thought you could set it up so you had a string of 52.4 in series which then leads to the 37 in parallel, rather than making a giant square of batteries as you are suggesting. My knowledge is somewhat lacking on the subject, but this would mean that you would only need 90odd batteries, rather than nearly 2000.

Here's a quick paint job of that I mean, sorry about the quality, my brain doesnt work at these times (7am here in the UK). In this case you're looking at spending only about 23 packs*£5 which is £115.

Not quite sure where you're going with this, but it doesn't look fruitful. I'll admit my schematic reading is somewhat poor so I haven't a clue what your diagram represents. (I see a bunch of diodes?). Remember that even if you artificially raise the voltage, to draw 1 amp at 72v requires drawing 48 amps at 1.5v

To get to a goal of 72 volts, you would actually need 48 batteries, wired in series. This would give you a pack with a spec of 72v, at 2.7 Ah. (I'm assuming you meant the batteries you can get are 2.7 Ah). This certainly won't get you far, as it's only ~194 Wh. Perhaps it would get a small motorcycle up to 30mph before running out of juice.

Any proper electric car is going to require at least 5 KWh, so if you need ~5 packs to reach 1 KWh, you'll need 25 packs to reach 5 KWh. That's 1,200 batteries, for just 5 KWh. And that still won't take you very far.

Using small batteries is certainly going to allow more flexibility for pack size and placement, but there's no free lunch. You're going to use A LOT MORE smaller batteries than larger ones.

[Wilden]

Dont worry about not being able to read it. Just my bad drawing. The symbols are meant to represent batteries, and my agument was that, if you can have 48 AA batteries in series to produce 72v at 2.7amps, and you can have 37 batteries in parallel to produce 100amps at 1.5v, surely, if you join those two circuits together you could produce 73.5v at 102.7amps. Or am I talking a load of rubbish?

[spoil9]

Originally Posted by Wilden

Dont worry about not being able to read it. Just my bad drawing. The symbols are meant to represent batteries, and my agument was that, if you can have 48 AA batteries in series to produce 72v at 2.7amps, and you can have 37 batteries in parallel to produce 100amps at 1.5v, surely, if you join those two circuits together you could produce 73.5v at 102.7amps. Or am I talking a load of rubbish?

We are basically talking the same thing, just using different numbers and words to describe it.

I got my numbers by dividing 72V by (1.4V per battery) to equal 51.4 batteries, then just rounded up. This I assumed would be wired in series to equal about 72V @ 2.7A.

Then each 'string' as I called them (I may be using the wrong term here) would be wired in parallel which would add the current to get your 100A.
To find this I took 100A and divided (2.7A per string of batteries wired in series) to equal 37.03 which I rounded to 37. (Cause rounding up to 38 would add another series of 52 batteries)

The end result is still 72V (ish) at 100A (ish) not taking into account wiring resistance, heat build up, improper matched cells, etc.

[micwich]

well you can get hold of ultra max aa batterys which are 800mah per battery that cost £1 per pack of 2 it doesnt say what voltage they are tho.