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'[OT]: Battery AH question'
2001\05\31@100609 by John Pearson

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I want to run a 850 watt AC motor for 10 seconds off 12 volt batteries.
How many 12 volt batteries and what AH would they need to be. I calculate 6.5 batteries (80 volts)?, but the AH part has me puzzeled.

Thanks for any help

John

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2001\05\31@101436 by Thomas C. Sefranek

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John Pearson wrote:

> I want to run a 850 watt AC motor for 10 seconds off 12 volt batteries.

What VOLTAGE AC?

> How many 12 volt batteries and what AH would they need to be. I calculate 6.5 batteries (80 volts)?, but the AH part has me puzzeled.
>

You should be considering an INVERTER.
(I hope you don't believe you can power an AC moter from 80 volts DC....)
Double the load power as the battery supply, 50% efficency.
1,700 watts at 12 volts is 142 amps. (Like starting your car!)

142 amps for 10/3600 hour is .4 amp hour.
BUT, you will NOT be able to demand 142 amps from a tiny battery.

>
> Thanks for any help
>
> John
>
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2001\05\31@104607 by Alan Beeber

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I'll help with the AHr question, but I'll let others
establish why you need to re-think this problem.

AHr is simply Amps (current) divided by time (one hour).
This describes the capacity of the battery. For instance, a
10 AHr battery should be able to 10 amps of current to a
load for 1 hour. You should be able to get larger currents
for shorter periods of time.

However....

As the current from the battery increases, the battery
voltage goes down, potentially down to zero. This makes it
difficult to run DC-AC inverters, etc., from a small
battery, and can cause damage. A larger battery can hold the
voltage up for a longer period of time.


John Pearson wrote:
{Quote hidden}

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2001\05\31@110649 by David VanHorn

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>
>As the current from the battery increases, the battery
>voltage goes down, potentially down to zero. This makes it
>difficult to run DC-AC inverters, etc., from a small
>battery, and can cause damage. A larger battery can hold the
>voltage up for a longer period of time.

Hence the "cold cranking amps" rating on car batteries.

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2001\05\31@113745 by John Pearson

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Can't I run an AC motor with DC? I was hoping to avoid inverters.


-----Original Message-----
From: Alan Beeber <KILLspamabeeberKILLspamspamMEDIAONE.NET>
To: RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU <spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU>
Date: Thursday, May 31, 2001 7:46 AM
Subject: Re: [OT]: Battery AH question


{Quote hidden}

6.5 batteries (80 volts)?, but the AH part has me puzzeled.
{Quote hidden}

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2001\05\31@142000 by Steve Smith

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AC battries ?

Assumeing that a battery is fully charged  (Ah uasally quoted at the 10 hour
rate) 10Ah = 1 A for 10 hours this does not mean that you can get 10A for 1
hour out of them at this level of discharge to a voltage of 1.7vpc (for lead
batts) typically derate by 50% at this sort of discharge current ie 10A a
10Ah battery may last 30 minutes. try looking at the Yuasa web sites and
check out the battery curves for sub 1 minute the current and duration is
available in a table with a period in the seconds order it may be possable to
discharge to 1.6vpc if the load can tolerate it.

The easy way is to calculate the locked roter current on the motor and then a
small UPS so that it wont blow when starting the motor (typically a factor of
about 6) 850 * 6 = 5100w (a 5kva ups) with a 10 minute battery is probably
the most sensable soulition. this way the battery is big enough and also
supports an ac motor !

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2001\05\31@143036 by Nigel Goodwin

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In message <001001c0e9e7$7f019060$752e5440@xero>, John Pearson
<RemoveMExeroEraseMEspamEraseMECMC.NET> writes
>Can't I run an AC motor with DC? I was hoping to avoid inverters.

NO!!.

If it's an AC/DC motor (like most electric drill motors), you can run it
from either - but if it just says AC, there's no way it will work off
DC.
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2001\05\31@143848 by Marcelo Yamamoto

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From the following site: http://www.usbattery.com/faq.htm

Q:What size battery do I need for my application?
A: Determine how many amperes your application needs from the battery and
for how long. Multiply the two to obtain Ampere Hours required. Increase
this by 20% for a safety cushion, and from our capacity charts match a
battery which will deliver this many AH for the required time, and voltage.
Connecting batteries in parallel adds AH, and connecting in series adds the
voltage. In either case the energy (WH) storage capability is increased by
the amount of energy each additional battery provides.

Marcelo Y.

{Quote hidden}

calculate 6.5 batteries (80 volts)?, but the AH part has me puzzeled.
{Quote hidden}

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2001\05\31@172526 by D. Schouten

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> Assumeing that a battery is fully charged  (Ah uasally quoted at the
10 hour
> rate) 10Ah = 1 A for 10 hours this does not mean that you can get
10A for 1
> hour out of them at this level of discharge to a voltage of 1.7vpc
(for lead
> batts) typically derate by 50% at this sort of discharge current ie
10A a
> 10Ah battery may last 30 minutes.

This is called the 'Peukert' efficiency. Peukert says that battery
size shrinks with increasing discharge current.

Daniel...

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'[OT]: Battery AH question'
2001\06\01@162924 by Peter L. Peres
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He probably wants to run a AC motor (i.e. 110Vac) off of an inverter and
has the luxury to be able to choose the batteries for it.

In this case a simple PIC-driven H-bridge can feed the motor from a set of
series coupled batteries. Since it is square wave drive it will have to be
120V (10 batteries), the current will be around 10A (less than, but not
accounting for starting current which could be 100A if no special measures
are taken to limit it). The efficiency will likely be >90%. The Ah rating
would be 10(A)/360 ~= 30mAh. In reality one would choose the smallest
battery that is rated at 10A continuous and 100A peak probably. 10 medium
size (5 to 7Ah) SLA batteries in series would cut it probably.

Then there is endurance. How many cycles does it need to support, what
about maintenance, how does one charge 10 batteries in series...

Peter

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2001\06\01@173336 by Peter L. Peres

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You can run a AC motor on DC if it is a 'universal' motor (with wound
stator and rotor and brushes). Check carefully to avoid letting out the
smoke out of it. A H bridge is a simple way to obtain AC (square wave)
from DC. It is not very good for motors but it will work.

Peter

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