# Increasing Battery life Question

#### spagettilegs

Dec 4, 2018
1
I have a little machine that fires tennis balls for my dog and while I can plug it into the mains electricity, it's also portable. I can insert 6x D type standard batteries which last for a half hour or so..firing every few seconds.

However I want to bring it onto a tennis court to fire balls for me and what I would like to do is rig up something externally I can wire up to it where it could last for hours.

I have a 12v portable power source with a cigarette lighter type output. I was hoping I could use this in some way?

Last edited:

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Maybe 4 2V lead acid cells. Do you know anyone who's decommissioning a submarine? (You probably don't want cells that large)

#### BobK

Jan 5, 2010
7,682
Alkaline D cells have a capacity of about 12Ah, so, if is lasting only 1/2 an hour, it pulls an outrageous amount of current. So, maybe the submarine batteries Steve suggests are just about right.

You say you have a 12V portable source. How much current can it provide? And what capacity?

A small portable generator might be your only hope.

Bob

#### CircuitMaster

Dec 17, 2016
48
"24A" for half an hour, which is very unlikely. It might be time to replace the batteries with rechargable ones and buy a car cigarette lighter charger for "9VDC" batteries. I will post the links for them later.

Isn't 12A/h too much? I see them with 600mA/h ?

#### BobK

Jan 5, 2010
7,682
This datasheet shows 12000 mAh at 250 mA. It is down to about 10000 at 500mA, which is the highest current the characterize it at. If the drain in 1/2 hour it is probably drawing several amps, but not close to 24, which I doubt that it could ever supply. Using it an application this far away from what the datashhet shows is battery abuse.

http://data.energizer.com/pdfs/e95.pdf

Bob

#### BobK

Jan 5, 2010
7,682
Not true. There are lithium ion batteries rated for 30C or full discharge in 2 minutes. These are used to power drones mostly.

Bob

#### WHONOES

May 20, 2017
1,215
Not true. There are lithium ion batteries rated for 30C or full discharge in 2 minutes. These are used to power drones mostly.

Bob
You are spot on with your observation.

#### dave9

Mar 5, 2017
1,181
Check the current rating of the lighter output on your portable power pack, it may not be suitable, and/or if your lighter adapter plug is not high quality, it may not be suitable for the current needed.

Yes you could use a portable 12V source but you'll want a switching DC/DC buck converter to drop it down to 9V. Others have suggested doing a current calculation and you might want to more carefully observe the runtime to get a closer estimate, helpful in determining the current capacity (plus some margin) needed for the converter. Since it is going to be an intermittent load this will ease the current requirement some.

#### Audioguru

Sep 24, 2016
3,653
The contacts on a battery holder for size D alkaline cells probably overheats with only 2A.
of course a "super heavy duty" Chinese carbon-zinc size D cell probably has a low capacity of only 1Ah.

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,792
I think I would revisit the design of the little machine that fires tennis balls for your dog. Why is it powered with D-size dry cells, and how much current does it draw throughout the firing cycle? Is this a commercially available contraption, or something you cobbled together from, say, used washing machine parts? What is the mechanism that fires each tennis ball? Can you post some pictures or design drawings?

Before wasting a lot of time and money on re-purposed lead-acid submarine batteries, see if there is something that can be done to reduce the power requirements. I mean, how complicated can it be to mechanically throw a tennis ball? Baseballs are a completely different problem because of their size, hardness, and terminal velocity plus whatever spin is imparted to create a realistic pitch for the batter. If you have adapted a baseball pitching machine to shoot tennis balls, I would expect a really big cart to carry the battery that powers it. So, please provide us with more information.

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,792
Looks like this was yet another drive-by poster looking for a quick answer. Not finding it, they moved on. They left one (and only one) post on December 4, 2018 and joined EP the same day. No further activity.

IMO, a moderator should probably close this thread to further replies, at least until the OP shows up again and asks a moderator to re-open it.

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,792
The contacts on a battery holder for size D alkaline cells probably overheats with only 2A.
of course a "super heavy duty" Chinese carbon-zinc size D cell probably has a low capacity of only 1Ah.
Ha! I thought everyone knew that Chinese carbon-zinc D-size cells consisted (at most) of a single AAA cell packed tightly into sand to bring the weight in line with other commercial D-size cells. You be real rucky to get a hunred milleramps for ten minits from one of them puppies.

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