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Author Topic: 2 Q's about Batteries  (Read 2431 times)

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Offline SeagullOneTopic starter

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2 Q's about Batteries
« on: August 31, 2007, 12:12:40 PM »
Okay, before I continue with my robotics project, I have just a few questions concerning batteries and electronics so I know for sure what I'm doing in my circuitry.

Here's what I want to know about the battery's relationship to the electronics: The electricity doesn't actually start in the battery, but it moves throughout the entire circuit. The battery just applies the force for the electricity to move. So my big question is, is it a push or a pull? Does the battery push the electricity through the components at a certain rate and that determines the force? Or do the numerous components suck the life out of the battery, and that determines the force? I want to know to determine how long the batteries in my robot will last. I'm using a 12V 20 amp hour battery in my robot. I know that the battery is able to last for twenty hours, if its constantly providing only one amp of current throughou the cycle. So for instance, if I used two 2amp motors, in my robot, not counting any other components, the battery would last for about 5 hours, right? But if I used two 1 amp motors in my project, the battery would last about 10 hours? Do I have it right?

Another thing, I know that if you wire batteries in series it increases amperage, right? (Or was is parallel. Have to read up on it again.) So if I wired 2 of the 12V 20amp hour batteries to increase current, does that mean the batteries will push twice as much power through the circuit, or does that mean I'll double the life span of the power supply cycle?
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Offline dunk

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Re: 2 Q's about Batteries
« Reply #1 on: August 31, 2007, 01:40:14 PM »
hi SeagullOne,
the easiest way i can explain it is to think of it like plumbing. (this is doesn't really bear much similarity from what is actually happening but it's a good model to work out the numbers.)
you battery is the water supply.
the voltage is like the water pressure.
the motors are like pipes which allow a certain amount of water through. the amount of water going through is like the current.
the pipes/motors will only take a certain amount of water pressure (voltage) before they burst.
the pipes/motors will allow a certain amount of water/current through them at their rated pressure/voltage.
if you supply less pressure/voltage the water/current passing through them will drop.
if you supply more pressure/voltage the water/current passing through them will increase but at the risk of damage.

now, if you connect 2 supplies/batteries in series you get twice the pressure/voltage.
if you connect 2 supplies/batteries in parallel you get the same pressure/voltage but they contain twice as much so you can draw water/current for twice as long.
if you connect 2 supplies/batteries in parallel you must make sure they are at exactly the same pressure/voltage otherwise water/current will flow from one supply/battery to the other. (make sure both your batteries are at the same state of charge.)

if you connect 2 pipes/motors in parallel to a supply/battery each will still get the same pressure/voltage so each will continue to draw the same water/current.
now my analogy starts to break down a bit here...
if you connect 2 pipes/motors in series to a supply/battery the pressure/voltage will be split between the 2. each pipe/motor will only get half of the pressure/voltage so will allow less water/current through.

so, applying this to your situation,
Quote
I'm using a 12V 20 amp hour battery in my robot. I know that the battery is able to last for twenty hours, if its constantly providing only one amp of current throughou the cycle. So for instance, if I used two 2amp motors, in my robot, not counting any other components, the battery would last for about 5 hours, right? But if I used two 1 amp motors in my project, the battery would last about 10 hours? Do I have it right?
yup. right on.

Quote
Another thing, I know that if you wire batteries in series it increases amperage, right? (Or was is parallel. Have to read up on it again.) So if I wired 2 of the 12V 20amp hour batteries to increase current, does that mean the batteries will push twice as much power through the circuit, or does that mean I'll double the life span of the power supply cycle?
if you wire them in parallel you would get the same voltage out of them and double the life expectancy.
if you wire them in series you would get the double the  voltage.
if you are connecting multiple battery packs you have to be carefull to make sure they are the same specification, same level of charge etc.

imagine connecting a fully charged battery in series with an empty battery.
the empty battery's voltage has dropped off so it is not supplying the circuit with much current any more but the fully charged battery is still forcing current through it.
this situation is very bad for the battery.

if you connect a fully charged battery in parallel with an empty battery the fully charged battery will try to recharge the empty one.
this is not too bad for rechargeable batteries (as long as the difference in voltages is not too high so it doesn't happen too quickly) but would be disastrous for non rechargeable batteries....

i hope this makes things clearer. if not, please ignore all of it. it worked for me when i was trying to figure all this out.
it also bears no real resemblance to how the system actually works so don't expect to be able to pass any physics exams with it....

dunk.

Offline HDL_CinC_Dragon

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Re: 2 Q's about Batteries
« Reply #2 on: August 31, 2007, 04:32:12 PM »
if you connect 2 supplies/batteries in parallel you must make sure they are at exactly the same pressure/voltage otherwise water/current will flow from one supply/battery to the other. (make sure both your batteries are at the same state of charge.)

would diodes fix this problem, Dunk?
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Offline dunk

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Re: 2 Q's about Batteries
« Reply #3 on: August 31, 2007, 04:40:45 PM »
Quote
would diodes fix this problem, Dunk?
yup. but it's an inefficient way to do things as you get a voltage drop across the diode.
better to never connect uneven batteries.
better still to just use one battery the correct size to start with.

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Offline SeagullOneTopic starter

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Re: 2 Q's about Batteries
« Reply #4 on: August 31, 2007, 05:14:34 PM »
All right! Thanks for the help guys. Boy, I've been waiting to solve that problem for quite some time now.

Just one more question. I still haven't bought a battery charger for my 12V 20amp hour batters. I've been looking on planetbattery.com and they look like they have some good deals. I just want to make sure I get the right charger for my 12V lead acid batteries so I don't undercharge the batteries every single time or blow up the house or something like that ( :. I just want to know I get this straight:

Planet battery has serveral rechargers for seal lead acid battery. Some for 6V and some for 12V. I know I want a 12V one, but I'm a little confused on what the amp-hour amounts are for each charger: for instance, the difference between a 2-10 amp hour recharger and a 20 to 60 amp hour recharger. Does the amp hour lables on the recharge mean thats the most it can recharge up to (in which case I'd have to get a recharger with at lest 20 amp hour labling), or is that the recharge rate? In which case I could settle for a recharger with at least 10 amp hour rates and wait for the batteries to recharge over two hours per cell?

Thanks guys. This is really clearing some things up for me!
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Offline Admin

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Re: 2 Q's about Batteries
« Reply #5 on: September 29, 2007, 02:10:30 PM »
Quote
Planet battery has serveral rechargers for seal lead acid battery. Some for 6V and some for 12V. I know I want a 12V one, but I'm a little confused on what the amp-hour amounts are for each charger: for instance, the difference between a 2-10 amp hour recharger and a 20 to 60 amp hour recharger. Does the amp hour lables on the recharge mean thats the most it can recharge up to (in which case I'd have to get a recharger with at lest 20 amp hour labling), or is that the recharge rate? In which case I could settle for a recharger with at least 10 amp hour rates and wait for the batteries to recharge over two hours per cell?

The charger datasheet should specify . . . no other way you can know. If its a smart charger, it wont charge any faster than your battery can handle.

Also consider this when looking for batteries/chargers:
http://www.societyofrobots.com/robot_parts_list_battery.shtml

 


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