Battery discharge rates
Turbo Mini Forums

Right the situation thus far I have an airsoft M134 mini gun that is supposed to run off of a 12v 4.2amp hourslead acid battery. Which is weighty when you are also holding onto 20 kgs of a.e.g. I want to swap the lead acid battery for a LiPo setup of 11.1v 6300-8400 milli amp hours. Now what I want to do is build a LiPo setup that will give me the same discharge rate as the lead acid battery thus keeping the barrel assembly rotation speed the same.

Is there a way of working out the discharge rates of the two sets so that I can build a LiPo pack to achieve the desired result?

Own the day

The volts will control how fast it spins the discharge rate will dictate how long it spins for
Double the a/h double the time it lasts.. roughly speaking

i would consider a low voltage cut off for the lipo they go bang when you discharge them too much

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As Brett says the speed of the motor is controlled via the voltage. Thus moving from 12v to 11.1 would yield 7.5% or so less speed.

However, the lipo battery can supply 6.4-8.4 amps for an hour where as the old one can supply 4.2 amps for an hour. This is an increase of between 50% and 100% for battery life.

Edited by Sam on 4th Sep, 2012.

Ahh, if only batteries worked as their numbers suggested. But if you work on the below then it'll be a good start.

I've previously spent weeks having batteries on seperate discharge rates to establish what they actually do.

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Ill agree with wil nicad's tend to do what they say on the tin but not for long nimh dont like to be discharged fast but will still be going strong 2000charges later lipo + liion's are just arses unless you spend big money i havnt done anything with them for 6/7 yeas so maybe tech has gotten better for lipos

I went from 14.4 ish to 11.1 lipo in my rc cars
The rpm difference especially with brushless motors is quite liniar so 10% lessV = about 10% less rpm like sam says

Edited by Brett on 4th Sep, 2012.

Yes i moved to the darkside

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Cheers for the input.

Own the day

As stated above, motor speed is a function of battery voltage and motor speed constant (rpm/volt)

A lipo/liion battery will comfortably handle a much higher discharge rate for a much extended number of cycles compared to that of either lead acid, nimh or nicad batteries if charged using the right charge curve for that battery chemistry.

The major advantage of lithium battery technology is that of the relatively flat discharge curve allowing a constant voltage output for up to 95% of the charge capacity which will help you maintain your required barrel speed.

Also remember when constructing your battery pack that you can attach batteries in parallel to gain an equivalent pack capacity and then attach these packs in series to gain the required voltage if you can't find 2400mAh packs in the right size.

also note that the fully charged voltage of a 3 cell lipo is about 12.6v.

Lipo's can be quite pricey though, you can save cash by going for a lower c rating. im assuming your application wont be drawing huge current?

12.6v, that explains why the barrel speed seemed to be up a fraction, this could also point to a rather lazy lead acid battery pack that has made me notice the increased barrel speed from the LiPo packs. The batteries I have opted for can supply 63amps constant and 126amps for the first 5 seconds. They are 30c 11.1 2100mAh was planning to run 4 in parallel. the control box is fused with a 60amp fuse. I have no idea what safety factor you apply to decide what size fuse you decide to fit to handle component current draw. does anyone know the rule of thumb for this?

Edited by Mr Joshua on 8th Sep, 2012.

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