Starting Point

If i bought batteries off ebay, how many would i need for 4 drill motors and what would i be looking for? If i did go this route I would be interested in your charger but what kinda price are we talking?

ok 2 of these would do fine

http://cgi.ebay.co.uk/7-2v-NiMH-3700mAh ... 3357365ba0

this shop/seller has been very helpful to me and loads of other roboteers

the charger new would be £75 but mine is used but in good condition and comes with box and instructions and all the leads, (btw this is the type you can just plug into a wall socket)

i would be looking for £50 however i have said to someone else that they are welcome to buy it but i will contact them to see what they say and repost later

let me know if interested

alex

I would need 4 or 8 for 4 motors? and yeah if hes not interested let me know

2 would be fine, 7.2 V batteries n series make 14.4 V , an ideal voltage for a 12 V drill motor.

im running 12 volt drill motors on 14.4 volts on 100cm blue wheels and its a really nice nippy speed that you get. so i recommend it also:)

So the two batteries will power all 4 motors for a reasonable time then?

yeh easy

I advise doing the mathmatics for the problem rather than assuming they will be fine.

The basic way to think about it, batteries capacity is given in Amps per hour (Ah). In other words if a battery is sold as 2Ah then it will provide 2 amps of current for a solid hour (at least in theory it will). Now we can use this to work out if the battery will last in battle. A battle is around 3 minutes long. That is 1/20th of an hour so the 2Ah battery will be able to supply 40amps for that battle (again in theory, it does depend on the batteries internal chemistry if it can make this figure). Good batteries should always be able to tell you their constant discharge rate and their short term discharge rate. The second being used when a large amount of current is required at a motors start up. With regards to a drill motor, I'd say a good average for the time in battle will be of the order of around 10 to 15amps average over the entire battle. Lets say because you are using 4 drill motors each motor needs 10Amps average to push featherweights around for a full 3 minutes. So your drive will require 40amps continuously over the entire battle. You can then use this figure to see if your battery will last the full time.

There are always assumptions in this calculation (there always are in any engineering question) but the key is to reduce their impact on the real world solution. I therefore always like to over engineer robots, throw in more capacity than is required and beef things up. Of course this comes with weight problems but its all part of the bigger problem of coming up with a reliable machine.

Cool so by that logic 3.7 provided by those batteries should be enough (i assumer 3700mAh =3.7Ah)

it does i let you work out Gary's math

you will see with robnoteers a slight split one side being the math side really getting things perfect so they 'shouldn't' go dodgy others will guess and hope for the best. saying this i myself differ with approaches depending on what robot i'm building

have fun,
alex