From: John De Armond
Subject: Re: Electric Blanket on Inverter??? - the test
Date: Thu, 01 Feb 2001 17:24:00 -0500
"Glen R. Fotre" wrote:
> Has anyone had any experience (pro or con) running their electric blanket on
> in inverter? Mine is 350 watts so it would seem that a 400 watt inverter
> should do the job. Maybe "do the job" to the coach battery too??
OK, so I got curious about this and decided to test it. My MH is
parked right outside my bedroom so I decided to run an extension
cord out to the RV, power my electric blanket from the rig's
inverter and measure what kind of power it used. Interesting
The blanket is the Sunbeam dual control King size. I pulled the
room down to 65 degrees before starting, which is what I keep it at
at night. I put a wattmeter in line with the blanket to measure the
power draw and used my E-meter to measure battery parameters.
Upon power up, the blanket drew 100 watts with both controls set to
"6". The E-meter read the following:
Volts - 13.45
Amps - 9.6
Ah - 0
After two hours, the watt meter read 50 watts (probably only one
thermostat on at that moment) and the E-meter provided the following
V - 12.60
A - 8.0
Ah - -9.0
Hrs - 22.5
The last two readings are the number of amp-hours used and the
estimated hours remaining at the current discharge rate. The
E-meter learns the battery bank's characteristics so this hour
estimate is quite accurate.
I did not hook a duty cycle meter to the blanket so I have to
estimate the duty cycle. I'm estimating that the heating elements
were on approximately 20% of the time.
What is interesting is that this blanket uses just a little more
power than wifey's 12 volt blanket even though it's much larger.
I'm sure that if the thermostats were turned all the way to "10"
that it would use more power but still not out of line. One can
certainly run the thing all night without draining the battery. It
is also apparent that the blanket will run on a small, say 150 watt,
inverter. I have no idea why they mark the blanket as consuming 300
A note about the E-meter. I finally got around to installing it
properly a couple of weeks ago, complete with proper grounding and
voltage sensing. I'd had it taped to the dash for months. It works
MUCH better now. This instrument is something that I could not
imagine doing without now that I have it. It fairly quickly learned
the characteristics of my battery bank. When it says the battery is
approaching full discharge, it is. The "hours remaining" is also
quite accurate. The averaging period for the "hours remaining" can
be set anywhere from instant to "average since last charge". I have
it set at 4 minutes which gives me a good running average while
remaining responsive. Unfortunately the meter has also told me that
about half my battery capacity is gone. I knew I took a hit from
the hard discharge I suffered a few weeks ago at the hands of a
vandal but I didn't think it was that bad.
It is also very interesting to watch the E-meter while my cordless
battery charger is working. Recall that it is currently controlled
by a Cruising Equipment/Heart Interface InCharge 3 stage alternator
controller. Probably because of the capacity hit from the hard
discharge, the bulk charge ends at about 50% capacity instead of the
typ 80% claimed by CE. An hour's worth of absorption (constant
voltage) charging brings it up to 85-90%. It takes several hours'
worth of trickle charging to put in the final 10-15%.
My E-meter has all the options which includes a serial port for data
logging, a battery thermister for temperature compensation and a
contact closure for low battery alarm. The latter feature is very
valuable. I currently have it hooked up with a latching battery
relay to disconnect the batteries from the house when the voltage
hits 10.6. I intent to eventually have it hooked up to start my
generator automatically. I have figured out how to make my Onan AJ
fast charge directly, the only changes being a different control
Anyone who make significant use of their batteries, particularly if
they rely on marginal charging sources such as solar cells, really
ought to have an E-meter. It takes all the guesswork out of battery
management. West Marine sells the meter labeled as the Heart
Interface Link-10 at a significant discount from list.
From: John De Armond
Subject: Re: Battery state of charge
Date: Fri, 30 Nov 2001 02:52:44 -0500
You can't tell state-of-charge from the terminal voltage. The only
reliable method is to use an amp-hour meter that measures the
discharge, does Peukert compensation and computes amphours. The
Cruising Equipment (now Xantrex) E-meter is probably the best of the
inexpensive amp-hour meters.
Unfortunately Xantrex has run the price up on the instrument but
even at the listed price, it's a good deal. It is amazing how wrong
your gut is about battery condition, once you see the actual
numbers. I have the version with the optional RS-232 output and the
low voltage contact closure output. I use that output with some
homemade logic to crank my genny when the batteries get low. Nice.
Rob Gendreau wrote:
> I'm trying to figure out how much power I have left in my battery. I've
> got a Lifeline 4d AGM battery as an auxiliary. I have measured the
> voltage with a multimeter. I've seen some charts that list how much
> power I should have left at different voltages, but these vary quite a
> bit. So...how can I tell how much I've got left? I assume temp affects
> it, and I assume I need to measure it with power off and when it's not
From: John De Armond
Subject: Re: Battery state of charge
Date: Fri, 30 Nov 2001 15:15:10 -0500
> I think that the specific gravity, compensated for temperature,
> is the only way to determine an accurate state of charge for
> the battery. The amp-hour meter probably does not do the temperature
> compensation, nor account for the efficiency of high charge and
> discharge rates. It just records the current sent "to" the battery
> and the power taken "from" the battery, without regard to temperature,
> initial state of charge, and battery efficiency........ Better than
> terminal voltage, tho. Terminal voltage is pretty useless except as
> a VERY rough guide......
You probably ought to know what you're talking about before
posting. The E-meter manual is available online for downloading so
there is really little excuse other than laziness for such an
The E-meter solves the Peukert exponent for discharge rate
compensation. It does temperature compensation via either a
thermister or a keyed value (if one doesn't want to bother with the
thermister.) It has a provision for programming the battery's
tempco (available from reputable battery mfrs) and comes with a very
reasonable default It learns the charge efficiency and has a manual
trim parameter (that I've never had to use.) Plus it has a
programmable time-at-rate-of-charge to define fully charged. The
meter keeps track of the number of charge and discharge cycles, the
max discharge, the charge efficiency and several other statistics.
All are available either via the display or the RS232 port.
Specific gravity does NOT reflect state of charge unless a) the
battery has been through a vigorous equalizing cycle (whereupon the
battery is charged by definition) or b) the battery has sat for a
period of time. Especially with the densely packed and tightly
bound plates of ruggedized batteries such as golf car batteries,
there is very little electrolyte convection. The acid liberated
during charging and consumed during discharge stays within the plate
matrix until gassing stirs the electrolyte or enough time passes
that it diffuses out.
From: John De Armond
Subject: Re: Xantrex Link 10 temp sensor?
Date: Tue, 15 Jul 2003 23:47:41 -0400
Standard 10kohm thermistor. I got mine by cutting the sensor and cord off of
one of those el-cheapo digital thermometers with the outdoor sensor. That way
the thermistor is already encapsulated and sealed to the end of a thin wire.
I drilled a small hole down from the top of the neg terminal (doesn't matter
which, really) but not all the way through, dropped in the sensor and applied
a little epoxy to seal it in. The terminal temperature down below the case
line is more representative of the plate temperature than just sticking it to
the case. Sticking it to the case would probably do but I tend to go a bit
On Tue, 15 Jul 2003 22:21:31 GMT, "chateauV10" <firstname.lastname@example.org> wrote:
>Does anyone know what type of sensor Xantrex uses for the Link 10 battery
>monitor temp sensor? I can imagine it'seither a diode or a thermistor. I ask
>because I'd like to install one to measure & display battery temperature in
>my on-board set-up. Xanterx say to go to West Marine for the thing, and I
>can imagine it will cost $50 before I'm out of there, all for a $0.50
>component and a dollar's worth of wire....
From: John De Armond
Subject: Re: Electrical (a bit long)
Date: Thu, 31 Jul 2003 02:56:35 -0400
On 30 Jul 2003 17:59:24 -0700, email@example.com (Jim) wrote:
> I glanced over at the meter mentioned in N.J.'s post; at the time I
>didn't have time to read all the specs. Was interested enough to go
>back and see exactly how they were arriving at amp hours remaining..
>it seems that the meter requires the user to input the amp hour
>capacity of their battery bank, and the meter does a simple 'plus or
>minus' of amp hours based against that number.
And most importantly, it also does the peukert compensation for loss of
capacity vs discharge rate.
> (I've been considering getting one of the old 486 computers out of my
>shed and wiring up a IO board to measure battery capacity of small
>(12v 7.2ah) SLA batteries.. calculate a 20 hour discharge amp rate,
>monitor voltage until it drops to a specified value and determine how
>long it took to get there.)
I experiment with and build custom PEVs (personal electric vehicles). As I
work with a number of batteries, I have the need for just such a device. I
constructed one of much simpler design that works quite well. It consists of
a bank of switchable lamps for the load, lamps acting as a constant current
load over a range of voltage; a resettable minute meter (like an hour meter
but calibrated in minutes) a relay and an undevoltage relay. This is all
wired so that when the relay is energized and latched, the load is applied to
the battery through one set of contacts and power is applied to the minute
meter through the other. When the battery voltage reaches the undervoltage
trip, the relay latching current is interrupted and the relay opens. it will
not reclose as the battery voltage recovers. From the discharge rate and the
elapsed time I compute AH.
I can set the discharge rate from 1 to 50 amps in roughly one amp increments.
By discharging the battery over a range of rates I can compute the Peukert
exponent, something the E-meter needs to accurately compute remaining AH.
The E-meter is very accurate even in high rate environments. It's not unusual
for a PEV battery to be discharged at up to 7 or 8C. Once an accurate Peukert
exponent is arrived at, the E-meter will faithfully track regardless of the
> What would be neat is for Neon John's mentioned meter to actually
>determine battery bank capacity. It would require some additional
>components (and programming), but as batteries age their capacity
>decreases, and being able to measure that would be handy.
it's easier just to enter the value. The meter does measure the actual
capacity for use in several stored parameters including the battery charge and
discharge efficiency. These, along with max depth of discharge, number of
cycles and a few other parameters can be recalled to the display and in the
case of my super deluxe whiz-bang unit, output via the serial port to a
>WRT. voltage; as long as it's understood that voltage readings will
>differ depending on current draw (meaning that at a 20 hour current
>draw a 100% discharged battery will read 10.5 volts, and without a
>load the 100% discharged battery will read a different voltage) it's
>my belief that voltage is an accurate way to determine state of
>charge.. especially when measuring state of charge on batteries that
>have 'rested' for a day or so without any discharge current.
yes, voltage can give a rough indication of charge under the right conditions
and provided the reading is temperature compensated. Unfortunately the right
conditions never happen when we RVers need to know. That is, almost by
definition our batteries have not sat idle for hours when we have to decide
whether to crank the genny and charge or try to make it through another night.
That's what I love about the E-meter. I can look up and see, "well yes, I've
used 100 ah and since I have a 300 ah battery I can go another night, run the
MAXXAIR on high and maybe run the computer." Or "I'm a little short but I can
get by with just X hours of genny time until tomorrow."
For the under $200 and few hours' install time involved, I just can't see
enduring the stress of guessing and not knowing.
One other comment on the "don't go below 50%" advice. This is, of course, a
personal decision based on each individual's values. If one is the sort who
begs pennies on the curb to buy batteries, then this rule is perhaps
important. For me with my 3 110ah Sam's deep cycle batteries that cost less
than $70 ea, getting every minute of service life out of 'em isn't important.
Convenience is much more important. If I have to replace 'em a year earlier
than someone else, so be it. When necessary or convenience dictates I don't
give a second thought to going to 80% discharged or below. So far I've gotten
about 3 years' life out of each set using my rig almost every weekend. $70 a
year for batteries is something I can afford.
From: John De Armond
Subject: Re: Battery Monitoring Options
Date: Tue, 01 Jun 2004 12:34:19 -0400
On Tue, 01 Jun 2004 06:52:25 GMT, John <John@nospam.net> wrote:
>I've learned a few things with my dry camping experience this weekend.
>One thing I've learned is that I have a lot to learn!
>My Prowler 25ft 2003 comes with a panel that, on the push of a button,
>shows whether my black, grey, fresh, or battery levels are emptry, 1/3,
>2/3 or full.
>This is not going to be detailed enough for my needs. What are my
>options for better, including more detailed, monitoring of my battery
>charge status and health?
About the only affordable battery "gas gauge" is the Xantrex Link-10 (formerly
I've used one of these in my RV for probably 5 years and could not imagine dry
camping without it.
I recently bought another one for an EV project. The best price I could find
was from here:
Search for "link-10". I highly advise also buying the temperature sender. If
you don't, the meter uses a fixed (normally 20 deg C) temperature for its
temperature compensation calculation.
If you don't want to spend the $50 and are handy with a soldering iron, here's
a little tip. The sender is a standard 10kohm thermistor like that on just
about all the cheap indoor/outdoor digital thermometers on the market. that's
the route I go. I just buy an el-cheapo thermometer, lop off the sensor and
toss the display. I drill a little hole in the top of a battery post, insert
the sender and fix it in place with a little RTV.