# [EVDL] Estimate of Battery Heat output when charging?

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I'm working on the math, trying to figure how much heat input will be needed to keep the batteries warm this winter. They are in boxes insulated to R5.

Dimensions:
Rear: 12 Batteries US 8VGCHC
40.5x27x14 (outside Dimensions)
Total Surface Area: 28.31 sq.ft.
Temperature Differential 60 deg. (0 outside, 60 in the box, hopefully)
Calculated Watts needed to keep warm: 339

How much of this heat could come from the charging of the batteries? I am hoping to charge from a 120V outlet, and don't think I have 500 Watts to spare (including front box)

Andy
'88 Mazda 323
Solectria/ElectroAuto Manual Transmission Kit
18 US Battery 8VGCHC

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1 - 4 of 4 Posts

#### EVDL List

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72,624 Posts
Discussion Starter · ·
Hello Andy,

Here is a Btur formula we use for calculating the amount of insulation is
needed.

Btur's = SF x u factor x TD

SF is the exterior surface between the ambient air and the inside
heated area.

u factor is equal to 1/R where R is the R-factor or resistance of the
insulation.

TD is the Temperature difference you want to maintain between the
heated area and outside ambient air.

Btur's is the amount of heat units you lose per hour. 1000 watts is
equal to about 3412 btus or 3.412 btus per watt.

Lets say you do not have a insulated battery box, so the R-factor is 1 and
the u factor is 1/R or 1/1 = 1 u factor

The formula now becomes:

Btur's = 28.31 SF x 1 u x 60 TD

Btur's = 1698.6

Watts = 1698.6/3.412 = 497.8 watts

Therefore it would take about 500 watts of continuous heating per hour to
maintain 60 degrees F. at 0 degrees.

A uninsulated metal container only adds a trace gain, because steel has
about 0.5 R factor per foot thickness.

Lets see what happens when we add 0.5 inch of foam board to the outside of a
battery box which has a R-factor of 2.5 R or (1/2.5) = 0.4 u factor:

Btur's = 28.31 sf x 0.4 u x 60 = 679.44 Btur's

Watts = 679.44 / 3.412 = 199.13 watts

Therefore with 1/2 of foam it will take about 200 watts per hour at 0
degrees F.

Now if you double the thickness of the insulation, the watts will drop by
one half:

No insulation ........... 500 watts per hour
1/2 inch foam ........... 200 watts
1 inch foam ............. 100 watts
2 inch foam ............. 50 watts

Dead air space of 3/4 inch adds about 3 R factor.

A vacuum space using two chamber box where the inside box does not touch the
outside box and apply a vacuum, the R factor goes in to the 1000's.

Example of my insulated battery box, that does not require any additional
heat to maintain 65 degrees at 35 below zero.

My battery box is about 47.5 SF of outside surface area which includes the
bottom, sides and top.

The insulation on the bottom is 2 inches of foam with two layers of marine
nylon carpet, 1/4 inch solid fiberglass which has a R factor of 15 R's
losses 105.6 btuh's

The insulation on the cover is 1/4 inch solid fiberglass, 1/2 inches of foam
and nylon carpet which has about 6 R's losses 265.6 btuh,s.

The insulation on the sides is 1/4 inch solid fiberglass, 2 inches of foam
board, nylon carpet, 4 inches of dead air space, nylon carpet and another
layer of 2 inch foam board for a R-factor of about 40 R factor losses 2.5
btuh,s.

Different R-factors require three calculations for each area. So at
-35 F. the total btuh,s is about 375 btuh which is about 110 watts per hour.

I no longer park the EV outside more than 2 hours in any temperature. The
inside of the battery box temperature is always between 70 and 80 degrees
even when the outside ambient air is at 100 degrees.

When I charge the batteries, it takes in the garage air which maintains a
constant 70 degrees temperature through a exhaust fan circulation that is
exhaust either through a hose going to a garage door port and/or ceiling
fans.

Roland

----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: Wednesday, September 26, 2007 9:43 PM
Subject: [EVDL] Estimate of Battery Heat output when charging?

> I'm working on the math, trying to figure how much heat input will be
> needed to keep the batteries warm this winter. They are in boxes
> insulated to R5.
>
> Dimensions:
> Rear: 12 Batteries US 8VGCHC
> 40.5x27x14 (outside Dimensions)
> Total Surface Area: 28.31 sq.ft.
> Temperature Differential 60 deg. (0 outside, 60 in
> the box, hopefully)
> Calculated Watts needed to keep warm: 339
>
> How much of this heat could come from the charging of the batteries? I
> am hoping to charge from a 120V outlet, and don't think I have 500 Watts
> to spare (including front box)
>
> Andy
> '88 Mazda 323
> Solectria/ElectroAuto Manual Transmission Kit
> 18 US Battery 8VGCHC
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev

#### EVDL List

· Registered
Joined
·
72,624 Posts
Discussion Starter · ·
Does anyone have any experience with radiant barriers? either in the cab's of the EV's or in the battery boxes? It might reduce the amount of insulation needed, and in the battery boxes leave more room for the batteries.

Thanks

Kevin Conley
In the Chicago cold weather district.
[email protected]

----- Original Message ----
From: Roland Wiench <[email protected]>
To: Electric Vehicle Discussion List <[email protected]>
Sent: Thursday, September 27, 2007 12:18:54 AM
Subject: Re: [EVDL] Estimate of Battery Heat output when charging?

Hello Andy,

Here is a Btur formula we use for calculating the amount of insulation is
needed.

Btur's = SF x u factor x TD

SF is the exterior surface between the ambient air and the inside
heated area.

u factor is equal to 1/R where R is the R-factor or resistance of the
insulation.

TD is the Temperature difference you want to maintain between the
heated area and outside ambient air.

Btur's is the amount of heat units you lose per hour. 1000 watts is
equal to about 3412 btus or 3.412 btus per watt.

Lets say you do not have a insulated battery box, so the R-factor is 1 and
the u factor is 1/R or 1/1 = 1 u factor

The formula now becomes:

Btur's = 28.31 SF x 1 u x 60 TD

Btur's = 1698.6

Watts = 1698.6/3.412 = 497.8 watts

Therefore it would take about 500 watts of continuous heating per hour to
maintain 60 degrees F. at 0 degrees.

A uninsulated metal container only adds a trace gain, because steel has
about 0.5 R factor per foot thickness.

Lets see what happens when we add 0.5 inch of foam board to the outside of a
battery box which has a R-factor of 2.5 R or (1/2.5) = 0.4 u factor:

Btur's = 28.31 sf x 0.4 u x 60 = 679.44 Btur's

Watts = 679.44 / 3.412 = 199.13 watts

Therefore with 1/2 of foam it will take about 200 watts per hour at 0
degrees F.

Now if you double the thickness of the insulation, the watts will drop by
one half:

No insulation ........... 500 watts per hour
1/2 inch foam ........... 200 watts
1 inch foam ............. 100 watts
2 inch foam ............. 50 watts

Dead air space of 3/4 inch adds about 3 R factor.

A vacuum space using two chamber box where the inside box does not touch the
outside box and apply a vacuum, the R factor goes in to the 1000's.

Example of my insulated battery box, that does not require any additional
heat to maintain 65 degrees at 35 below zero.

My battery box is about 47.5 SF of outside surface area which includes the
bottom, sides and top.

The insulation on the bottom is 2 inches of foam with two layers of marine
nylon carpet, 1/4 inch solid fiberglass which has a R factor of 15 R's
losses 105.6 btuh's

The insulation on the cover is 1/4 inch solid fiberglass, 1/2 inches of foam
and nylon carpet which has about 6 R's losses 265.6 btuh,s.

The insulation on the sides is 1/4 inch solid fiberglass, 2 inches of foam
board, nylon carpet, 4 inches of dead air space, nylon carpet and another
layer of 2 inch foam board for a R-factor of about 40 R factor losses 2.5
btuh,s.

Different R-factors require three calculations for each area. So at
-35 F. the total btuh,s is about 375 btuh which is about 110 watts per hour.

I no longer park the EV outside more than 2 hours in any temperature. The
inside of the battery box temperature is always between 70 and 80 degrees
even when the outside ambient air is at 100 degrees.

When I charge the batteries, it takes in the garage air which maintains a
constant 70 degrees temperature through a exhaust fan circulation that is
exhaust either through a hose going to a garage door port and/or ceiling
fans.

Roland

----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: Wednesday, September 26, 2007 9:43 PM
Subject: [EVDL] Estimate of Battery Heat output when charging?

> I'm working on the math, trying to figure how much heat input will be
> needed to keep the batteries warm this winter. They are in boxes
> insulated to R5.
>
> Dimensions:
> Rear: 12 Batteries US 8VGCHC
> 40.5x27x14 (outside Dimensions)
> Total Surface Area: 28.31 sq.ft.
> Temperature Differential 60 deg. (0 outside, 60 in
> the box, hopefully)
> Calculated Watts needed to keep warm: 339
>
> How much of this heat could come from the charging of the batteries? I
> am hoping to charge from a 120V outlet, and don't think I have 500 Watts
> to spare (including front box)
>
> Andy
> '88 Mazda 323
> Solectria/ElectroAuto Manual Transmission Kit
> 18 US Battery 8VGCHC
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev
_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev

#### EVDL List

· Registered
Joined
·
72,624 Posts
Discussion Starter · ·
Hello Kevin,

The radiant heat barrier is a bubble wrap type of plastic with dead air
spaces that is sandwich between two aluminum sheets that is to block 97% of

I look at the roll I have, that I use to insulated behind my door panels,
front and rear fire walls and ceiling. I completely gutted out the interial
of the EV and install all new panels and a custom made hinge down dash unit.

This stuff I use is made by www.reflectixinc.com. In a very cold climate, it
is best to not install it against the door and ceiling sheet metal. Install
it just behind the inside door panels and attach it to the headliner, so you
have a air space between this insulation and the sheet metal, or you may get
condensation.

This stuff has only a 3.5 R factor, so I back it up with a 1/4 inch vinyl
top foam pad that has a plastic film on it which prevent glue from absorbing
into the foam.

On the file walls I sandwich the radiant heat barrier between two layers of
1/4 inch vinyl top foam and glue that to the firewalls with 3M 88 spray
glue. Do not use the other 3M spray glues because it will melt the plastic.

The R-Factor is about 2 R'w for each layer of 1/4 inch foam and with with
the heat barrier, this makes about 7.5 R factor.

I then overlay this fire wall insulation with a carpet foam back and then
ran the carpet all the way up to the bottom of the windshield.

For the floor in the passenger compartment and in the bed of the pick up
box, which has a lot of deep ribbing and contours. I pick up some of that 2
inch thick soft blue foam which is more dense than the soft yellow or white
foam from a upholstery shop and carved it out with a electric bread knife to
fit the floor which then made the surfaces flat. Cover with a carpet and
set the insulated battery box on it with fiberglass spacers, so as not to
compress the foam too much.

Try the below formula for the 3.5 R-factor heat barrier to find out how much
wattage it will take to maintain the temperature you want.

Roland

----- Original Message -----
From: "kevin conley" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Thursday, September 27, 2007 2:38 PM
Subject: Re: [EVDL] Estimate of Battery Heat output when charging?

> Does anyone have any experience with radiant barriers? either in the cab's
> of the EV's or in the battery boxes? It might reduce the amount of
> insulation needed, and in the battery boxes leave more room for the
> batteries.
>
> Thanks
>
> Kevin Conley
> In the Chicago cold weather district.
> [email protected]
>
>
>
> ----- Original Message ----
> From: Roland Wiench <[email protected]>
> To: Electric Vehicle Discussion List <[email protected]>
> Sent: Thursday, September 27, 2007 12:18:54 AM
> Subject: Re: [EVDL] Estimate of Battery Heat output when charging?
>
>
> Hello Andy,
>
> Here is a Btur formula we use for calculating the amount of insulation is
> needed.
>
>
> Btur's = SF x u factor x TD
>
> SF is the exterior surface between the ambient air and the inside
> heated area.
>
> u factor is equal to 1/R where R is the R-factor or resistance of the
> insulation.
>
> TD is the Temperature difference you want to maintain between the
> heated area and outside ambient air.
>
> Btur's is the amount of heat units you lose per hour. 1000 watts is
> equal to about 3412 btus or 3.412 btus per watt.
>
>
> Lets say you do not have a insulated battery box, so the R-factor is 1 and
> the u factor is 1/R or 1/1 = 1 u factor
>
> The formula now becomes:
>
> Btur's = 28.31 SF x 1 u x 60 TD
>
> Btur's = 1698.6
>
> Watts = 1698.6/3.412 = 497.8 watts
>
> Therefore it would take about 500 watts of continuous heating per hour to
> maintain 60 degrees F. at 0 degrees.
>
> A uninsulated metal container only adds a trace gain, because steel has
> about 0.5 R factor per foot thickness.
>
> Lets see what happens when we add 0.5 inch of foam board to the outside of
> a
> battery box which has a R-factor of 2.5 R or (1/2.5) = 0.4 u factor:
>
>
> Btur's = 28.31 sf x 0.4 u x 60 = 679.44 Btur's
>
> Watts = 679.44 / 3.412 = 199.13 watts
>
> Therefore with 1/2 of foam it will take about 200 watts per hour at 0
> degrees F.
>
> Now if you double the thickness of the insulation, the watts will drop by
> one half:
>
> No insulation ........... 500 watts per hour
> 1/2 inch foam ........... 200 watts
> 1 inch foam ............. 100 watts
> 2 inch foam ............. 50 watts
>
> Dead air space of 3/4 inch adds about 3 R factor.
>
> A vacuum space using two chamber box where the inside box does not touch
> the
> outside box and apply a vacuum, the R factor goes in to the 1000's.
>
> Example of my insulated battery box, that does not require any additional
> heat to maintain 65 degrees at 35 below zero.
>
> My battery box is about 47.5 SF of outside surface area which includes the
> bottom, sides and top.
>
> The insulation on the bottom is 2 inches of foam with two layers of marine
> nylon carpet, 1/4 inch solid fiberglass which has a R factor of 15 R's
> losses 105.6 btuh's
>
> The insulation on the cover is 1/4 inch solid fiberglass, 1/2 inches of
> foam
> and nylon carpet which has about 6 R's losses 265.6 btuh,s.
>
> The insulation on the sides is 1/4 inch solid fiberglass, 2 inches of foam
> board, nylon carpet, 4 inches of dead air space, nylon carpet and another
> layer of 2 inch foam board for a R-factor of about 40 R factor losses 2.5
> btuh,s.
>
> Different R-factors require three calculations for each area. So at
> -35 F. the total btuh,s is about 375 btuh which is about 110 watts per
> hour.
>
> I no longer park the EV outside more than 2 hours in any temperature. The
> inside of the battery box temperature is always between 70 and 80 degrees
> even when the outside ambient air is at 100 degrees.
>
> When I charge the batteries, it takes in the garage air which maintains a
> constant 70 degrees temperature through a exhaust fan circulation that is
> exhaust either through a hose going to a garage door port and/or ceiling
> fans.
>
> Roland
>
>
>
>
> ----- Original Message -----
> From: <[email protected]>
> To: <[email protected]>
> Sent: Wednesday, September 26, 2007 9:43 PM
> Subject: [EVDL] Estimate of Battery Heat output when charging?
>
>
> > I'm working on the math, trying to figure how much heat input will be
> > needed to keep the batteries warm this winter. They are in boxes
> > insulated to R5.
> >
> > Dimensions:
> > Rear: 12 Batteries US 8VGCHC
> > 40.5x27x14 (outside Dimensions)
> > Total Surface Area: 28.31 sq.ft.
> > Temperature Differential 60 deg. (0 outside, 60
> > in
> > the box, hopefully)
> > Calculated Watts needed to keep warm: 339
> >
> > How much of this heat could come from the charging of the batteries? I
> > am hoping to charge from a 120V outlet, and don't think I have 500 Watts
> > to spare (including front box)
> >
> > Andy
> > '88 Mazda 323
> > Solectria/ElectroAuto Manual Transmission Kit
> > 18 US Battery 8VGCHC
> >
> > _______________________________________________
> > For subscription options, see
> > http://lists.sjsu.edu/mailman/listinfo/ev
> >
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev

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