### Author Topic: IC That Will Indicate When I Have Low Battery  (Read 3451 times)

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#### billhowl

• Supreme Robot
• Posts: 376
##### Re: IC That Will Indicate When I Have Low Battery
« Reply #30 on: November 20, 2011, 07:39:38 AM »
For SLA each cell had max voltage of 2.45V, hence 24V batteries can go up to 29.4V, so calculate your resistor values base on 30V.

#### Sylvestre

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• Posts: 221
##### Re: IC That Will Indicate When I Have Low Battery
« Reply #31 on: November 20, 2011, 03:36:29 PM »
Ok here are my new calculations

Resistor 1: 16500 Ohms
Resistor 2: 3300 Ohms

3300/19800=.16666666667

.166666666666667*30= 5 volts

30/19800= 1.5 ma wasted

I'm not sure how to choose a power rating for the resistors.  I have two motors that run at 400 watts each.  Does this mean I have to get a +800 watt rated resistor?
« Last Edit: November 20, 2011, 03:40:15 PM by Sylvestre »

#### billhowl

• Supreme Robot
• Posts: 376
##### Re: IC That Will Indicate When I Have Low Battery
« Reply #32 on: November 20, 2011, 05:21:08 PM »
Calculate for Power (P)
P = I 2 x R
P = 0.00152 x 19800
P = 0.04455W

So power rating for the resistors you can get 1/8W (0.125W) or 1/4W(0.25W)

#### Sylvestre

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• Posts: 221
##### Re: IC That Will Indicate When I Have Low Battery
« Reply #33 on: November 20, 2011, 08:11:59 PM »

#### billhowl

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• Posts: 376
##### Re: IC That Will Indicate When I Have Low Battery
« Reply #34 on: November 20, 2011, 09:00:29 PM »
The first one 16,5K was okay, but the 3.3K need not to have 3W, this will do

http://www.mouser.com/ProductDetail/Xicon/271-33K-RC/?qs=sGAEpiMZZMu61qfTUdNhG%2fJtDnsRrrptaPf%2fpx2qOkY%3d

#### Sylvestre

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##### Re: IC That Will Indicate When I Have Low Battery
« Reply #35 on: November 20, 2011, 09:04:57 PM »
Wow thank you!  I must have overlooked that on accident.

Thanks a lot!

• Supreme Robot
• Posts: 11,696
##### Re: IC That Will Indicate When I Have Low Battery
« Reply #36 on: November 25, 2011, 11:10:58 AM »
If I am hooking up my two sealed lead acid batteries in series to produce 24v, will I get a little over 24 volts?  Should I recalculate my resistor values for 24 or 25 volts?
Best to measure with a multi-meter the total fully charged voltage. But it'd definitely be more than 24V for sure.

#### Sylvestre

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##### Re: IC That Will Indicate When I Have Low Battery
« Reply #37 on: November 27, 2011, 07:58:33 PM »
I would hook it up to a multimeter, but my multimeter is only rated for 10 amps while battery (two batteries in series) is 24 volt at 50 amp.

• Supreme Robot
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##### Re: IC That Will Indicate When I Have Low Battery
« Reply #38 on: November 27, 2011, 08:17:17 PM »
I would hook it up to a multimeter, but my multimeter is only rated for 10 amps while battery (two batteries in series) is 24 volt at 50 amp.
When you measure the battery, there should be zero amps. You're only measuring voltage.

Measure each battery individually, then add the two voltages to get the total.

#### Sylvestre

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• Posts: 221