Robot Batteries: What to use?
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Robot Batteries: What to use? Daniel Rudy 10-01-2008
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Posted by Daniel Rudy on October 1, 2008, 7:19 am



Hello Group,

Over the past few days, I've been looking at the various battery
technologies for robotics use. The specifications that I am working
with is a 24v rail with moderate to heavy pulsed current draw. I've
written up a table below of my perceptions of the different types. I'm
leaning toward either NiCd or NiMH, but the cost of NiMH is prohibitive
with the higher capacity cells. I would love to use a size F cell with
a capacity of 12-15 amp-hours, but at about USD$25.00 per cell on
average, buying 20 of them gets to be real expensive. So my question is
what are you using?




Lead Acid:

Nominal cell voltage 2 volts. Comes in 6 volt or 12 volt packages.
Requires 2-12 volt or 4-6 volt batteries to make the 24 volt requirement.

Pros:
Charge/discharge characteristics are well known.
Cost is cheap.
Can supply high current.
Deep Cycle versions.
Maintenance free and Sealed versions.

Cons:
Large and heavy.
Toxic (Lead, Antimony, Sulfuric Acid).
Liquid Electrolyte (Except gel-cells).
Construction is application specific (Car batteries cannot be
deep-cycled, deep cycle cannot support high current pulses).



Nickel-Cadmium:

One of the oldest rechargeable battery technologies.
Nominal cell voltage 1.2 volts.
Requires 20 cells to make the 24 volt requirement.

Pros:
Charge/discharge characteristics are well known.
Cost is cheap.
Simple charging circuit.

Cons:
Requires periodic deep discharge.
Toxic materials (Cadmium)
Do not handle high current loads very well.
Low energy density.
Can develop memory effect.
Requires overnight charging.



Nickel-Metal Hydride:

Nominal cell voltage 1.2 volts.
Requires 20 cells to make the 24 volt requirement.

Pros:
Charge/discharge characteristics are well known (Similar to NiCd).
Higher capacity (F size cells can be 15Ah)
Can support high discharge rates.
Can use NiCd charging circuit.
Non-toxic.
Can be fast charged.

Cons:
Higher capacity cells are expensive.
Can develop memory effect.
Cannot be deep discharged.
Can overheat on high draws.
High temperature will significantly shorten service life.



Lithium-Ion (Standard):

Nominal cell voltage 3.6 volts.
Does not evenly divide into 24 volts. Closest is 6 cells (Lower than
spec) or 7 cells (Higher than spec).

Pros:
Highest energy density of any technology available.
Can support high current loads.

Cons (** SAFETY ALERT **):
Requires special charger.
Requires over-charge/over-discharge protection circuit.
Requires cell balancer for optimum performance.
Will not tolerate electrical or physical abuse.
Will not tolerate high temperatures.
Will vent, flame out, or explode when abused.
Drawing too much current will cause the cell to vent, flame out, or explode.

Cons:
Expensive.



Lithium-Ion (A123 Systems):

Nominal cell voltage 3.3 volts.
Does not evenly divide into 24 volts. Closest is 7 cells (Lower than
spec) or 8 cells (Higher than spec).

Pros:
High energy density.
Can support high current loads.
Does not have the safety issues of standard Li-Ion batteries. They do
not blow up.

Cons:
Expensive.
Not available to general public.
Requires special charger.
Requires over-charge/over-discharge protection circuit.
Requires cell balancer for optimum performance.
(?) Will not tolerate electrical or physical abuse.
(?) Will not tolerate high temperatures.



--
Daniel Rudy

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Posted by Robin on October 1, 2008, 8:02 am


Have you looked at the Cyclon lead acids? Not a lot of amp
hours but half a dozen of the D size will start a car.



Posted by BobH on October 1, 2008, 8:18 pm


Daniel Rudy wrote:
> Hello Group,
>

> Nickel-Cadmium:
>
> One of the oldest rechargeable battery technologies.
> Nominal cell voltage 1.2 volts.
> Requires 20 cells to make the 24 volt requirement.
>
> Pros:
> Charge/discharge characteristics are well known.
> Cost is cheap.
> Simple charging circuit.
>
> Cons:
> Requires periodic deep discharge.
> Toxic materials (Cadmium)
> Do not handle high current loads very well.
> Low energy density.
> Can develop memory effect.
> Requires overnight charging.

I disagree on the high current loads, NiCads handle high current loads
just fine. They also can be fast charged.

BobH

Posted by Daniel Rudy on October 2, 2008, 9:36 am


At about the time of 10/1/2008 5:18 PM, BobH stated the following:
> Daniel Rudy wrote:
>> Hello Group,
>
>> Nickel-Cadmium:
>> One of the oldest rechargeable battery technologies.
>> Nominal cell voltage 1.2 volts.
>> Requires 20 cells to make the 24 volt requirement.
>> Pros:
>> Charge/discharge characteristics are well known.
>> Cost is cheap.
>> Simple charging circuit.
>> Cons:
>> Requires periodic deep discharge.
>> Toxic materials (Cadmium)
>> Do not handle high current loads very well.
>> Low energy density.
>> Can develop memory effect.
>> Requires overnight charging.
>
> I disagree on the high current loads, NiCads handle high current loads
> just fine. They also can be fast charged.
>
> BobH

I see what you mean.

http://www.powerstream.com/RW.htm

It seems these were designed for that type of use. Sintered cadmium
electrode?


--
Daniel Rudy

Email address has been base64 encoded to reduce spam
Decode email address using b64decode or uudecode -m

Posted by John Nagle on October 10, 2008, 2:34 am


Daniel Rudy wrote:
> Hello Group,
>
> Over the past few days, I've been looking at the various battery
> technologies for robotics use. The specifications that I am working
> with is a 24v rail with moderate to heavy pulsed current draw. I've
> written up a table below of my perceptions of the different types. I'm
> leaning toward either NiCd or NiMH, but the cost of NiMH is prohibitive
> with the higher capacity cells. I would love to use a size F cell with
> a capacity of 12-15 amp-hours, but at about USD$25.00 per cell on
> average, buying 20 of them gets to be real expensive. So my question is
> what are you using?

What do you mean by "heavy pulsed current draw"? You need to
put numbers on all these things.

Incidentally, if you don't have numbers, I recommend getting a
clamp-around DC ammeter (yes, these exist, and cost about $100)
and finding out. I've found that to be a very useful instrument
in robotics work.

                John Nagle

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