More smaller batteries vs. Less bigger batteries

[bennelson]

Yeah, I believe that when doing a series/parallel combination, most people do a single string (series) of PAIRS of batteries (parallel).

I can't be too sure about that though. I have seen maybe 10 cars converted to EV's and I think ALL of them were series battery arrangements.

But yeah, rule of thumb is higher system voltage is better. I can't wait to see Tom G.'s 300V Dodge Neon finished....

[Daox]

Its always in your best interest to operate with all your batteries in series. The general rules, from what I heard are something like this. If you have a 100Ah battery, don't pull more than 80A (not sure if its 80% or lower) from it to maintain good battery life. If you have 12 of these 100Ah batteries, you can run 144V or 72V or 36V or whatever parallel setup you want. However, you'll want to limit current draw to a max of 960 amps (80% of 1200). 960A at 144V is a heck of a lot more power than 960A at 36V, and you don't sacrifice capacity or anything else to get more power. However, your components are just a lot more expensive.

[MazdaMatt]

Daox - keep in mind that if you are using parallel combos, the current FROM EACH battery is divided by the number of batteries in parallel.

So where 12 batteries in series can handle X current, 12 batteries in 4 3-battery strings, could handle 3X current.

They will also have a combined series resistance that is lower. So most definately, parallel batteries are going to provide you with the capacity to push more amps.

[Daox]

The current is always divided amongst how many batteries you have. If you have two batteries and are using 100A, you are pulling 50A from each battery. It doesn't matter if they are in series or parallel. You have two batteries that are splitting the amperage draw. In the series setup you will be getting twice the power and half the battery life. In the parallel setup you will get half the power, but twice the battery life.

Resistance is something to think about, but I think its fairly small when you figure in the bigger picture.

[Clev]

Originally Posted by MazdaMatt

Ben,

You say higher voltage makes for the same work at lower current. This is true, P=VA... but isn't the force generated by an electric motor ONLY proportional to current? If i recall correctly from electromagnetics class (way back), F=cAT (c is a constant, Amps, Turns).... (I'm really sketchy on the electromagnetics details).

Help

That's where the controller comes in. This is not technically quite correct, but by controlling the duty cycle, the controller essentially outputs high current at very low voltage at low RPMs, and outputs lower current at higher voltage at high RPMs, while drawing the same current/voltage at the battery.

[The Atomic Ass]

Originally Posted by Daox

The current is always divided amongst how many batteries you have. If you have two batteries and are using 100A, you are pulling 50A from each battery. It doesn't matter if they are in series or parallel. You have two batteries that are splitting the amperage draw. In the series setup you will be getting twice the power and half the battery life. In the parallel setup you will get half the power, but twice the battery life.

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.

Also, power is a meaningless term without context. The same number of batteries, wired any way you can think, should still output the same WATTS. Wired in series you get full VOLTAGE, wired in ONLY parallel you get full AMPERAGE. But you'll still get the same number of watts from the batteries.

Now, here's an idea no-one has seemed to hit on, at least in the grocery-getter field. How about having a pack normally wired for series, but center-tapped. Flick a switch on the dashboard, and several contactors switch over from series to 2 parallel packs. Half the voltage, double the amperage, and great for acceleration. As a matter of fact, the Killa-Cycle does just that, only on the motor side. The motors are run in series for one part, and parallel for the other part of the run, and I've forgotten which is which.

[bennelson]

Citicars, which are going on 40 years old, used a series/parallel configuration of batteries.



They used a set of contactors for basic speed control. Kind of loud (lots of clicking from those contactors) but they worked well.

About the only upside of that system was that they actually could pull pretty high amperage, and you could get some decent acceleration out of them.

Still better just to use a good PWM controller though.

[The Atomic Ass]

I was thinking of a combination of the two. Have the parallel/series switch before the controller, and feed the controller full voltage for normal driving, and half-voltage when you need to GO!

[bennelson]

That somehow assumes that your amperage is somehow being limited by the batteries, NOT the controller.

Unless you are drag-racing or doing something else equally unusual, being able to pull more amperage from the batteries isn't going to help any, and having lower system voltage will drop your speed.

One "hybrid" system I have thought about before is to have contactors switch between the controller to the motor, and an extra-long series string of batteries straight to the motor.

Imagine you have a 72V system - 6 x12V batteries. You have a 72V controller, and upgrading to a bigger one costs more than you want to spend.

You could have one or extra batteries and contactors to disconnect the controller, and connect the other batteries in series with your main string. You now have a 82 or 86V OVERDRIVE for max speed!

In fact, many EV potentiometers have a microswitch on the minumum potentiometer end. You could just as easy put one on the high end to control the contactors.

Press down the gas pedal - start going - press further - go faster - press all the way down - CLICK! - FULL MAX VOLTAGE/MAX SPEED!

[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?