Power is defined as the first derivative of energy w.r.t time...i.e
how much energy is dissipated, generated or transmitted per unit
time. The rate of the energy provided by the supply to your charging
battery depends on the rate of the charging current. The total energy
being supplied minus certain losses is going to be the total energy
storage of the battery divided by 2 (if it is half charged/dicharged
when recharging commences). The total energy can be calculated by
multiplying the terminal voltage by the amp hours value not forgetting
to convert hours into seconds to obtain the energy in joules. Power is
in joules/sec....or watts
You still can't derive power without stating a charge time and even then, you
don't normally charge lead-acid types at constant current, so the power input
will decrease as the battery gets fully charged.
You can estimate the ENERGY required though.
A watt describes only an instantaneous condition of power dissipation.
You (or your battery) don't consume watts of energy, but joules of it
(watt-seconds, or watt-hours, or even calories).
You can measure approximate charging loss, in watts, by measuring the
surface area of the battery, measuring the temperature of the surface
in degC, then calculate
P = (Tm-Ta) x As / 1000 (=/- 10%)
P= power in watts
Tm= measured surface temperature (degreesC) - averaged over the entire
surface would be more accurate.
Ta= ambient temperature (degreesC)
As= surface area cm^2
Measuring Watt-seconds requires the integration of this value over a
continuously monitored period, as the losses will vary with rate of
charging and state of charge over the life of the cell, for any
Some battery chemistries are more exothermic on charge than others,
and all may exhibit abnormalities outside of regular recommended
It's easier to just take a real world measurement. These work
surprisingly well for how inexpensive they are
There's a European 240V version of it out there as well. Plug the UPS
you wish to measure into it with no load and look at the wattage drawn,
it will be higher during charging than it will once the battery is fully
I suppose it depends on what information he's really looking for.
A simple voltage and current test with a multimeter at the battery
itself will tell you how much energy is being poured into the battery at
I don't think the OP mentioned anything about a UPS. Just wanted to
point out that a thermometer, ruler and timepiece can all be useful in
electronics, as can pencil and paper.
Unless the wattmeter can be stuck in the actual charging circuit, your
measurement is only going to be vaguely related to what you're trying
I'd be interested to know how these useful little meters react to low
He did, in more than one post he said this is a battery in a UPS.
Those watt meters only work with AC, but as I said, it depends on what
he really wants to measure. If his goal is to know at what rate energy
is being put into the battery, then a multimeter to measure DC amps
while charging is the appropriate tool for the job. If what he wants to
know is how much energy it draws from the wall to charge the battery,
taking into account energy lost as wasted heat in the charger and
battery, then the watt meter is an easier way to get all the data and
measures true power, taking power factor into account, just as the
kilowatt meters used for billing do.
In a nutshell, he hasn't given enough information regarding what data he
wishes to measure.
There's another way, likely easier. You can monitor watt-hours coming in, and
those coming out, and eventually you'll get a ratio that tell you your
average losses, and the longer you monitor, the better the accuracy.
As both methods depend on some device's determination of state or completion
of charge, that's the point where most loss occurs, because once you reach
upper limit, input energy must either be stopped or diverted. I guess with a
mains device that is switched off bar a tiny monitor PSU those losses don't
matter much but it is interesting when you use a solar panel array, you
become quickly aware of how much useful power is shunted to oblivion, no
matter how sophisticated the charger, once the storage is full on a sunny
day. Some chargers make it quite clear what you're missing...
As a crude guess, I'd say my 240 AH of 12V storage gives out about 90% of
what it received, not counting shunting loss when fully charged. If I want a
better reckoning I'll have to use a better meter than what I'm using now. I
don't think cheap regulators are very accurate, and they don't come with much
documentation either so it's hard to see exactly what they're doing.
An *ideal* 12V, 140AH battery will deliver 140 amps for 1 hour, or 1A
for 140 hours, or 10A for 14 hours, at 12V, before it is completely
The supplied energy would be ...
E = V x I x t
= 12 x 140 x 1 /1000 kWh
= 1.68 kWh
If you could fully charge such a battery in 1 hour, then you would
require an average power input of 1.68kW. In 10 hours, the power would
- Franc Zabkar
Please remove one 'i' from my address when replying by email.
Thanks. So if everything is ideal and your calculation is correct then the
UPS will draw power at an average rate of 168W or 168 Joules/sec right? What
do you mean by 'In 10 hours, the power would be 168W' ? I just want a figure
in Watts to try to compare it with other electric appliances e.g 100W bulb
etc. Also I want to know if my 2 KV generator would manage to charge it
alongside running 3 ceiling fans (of about 150W each)
I was economical with my language. What I meant was, if a completely
discharged battery were to be fully charged in 10 hours, then the
required power input would be 168 watts. Of course I'm talking about a
*perfect* battery and a perfect charging system. The figures are
really only a theoretical minimum.
You really need to know what sort of charging current your battery
could tolerate. Furthermore, a 12V lead-acid battery would normally be
charged at a constant voltage of between 13.8V and 14.1V, so the
current draw would be dependent on its internal resistance. A
partially discharged battery would need to be current limited so as
not to overheat it.
- Franc Zabkar
Please remove one 'i' from my address when replying by email.
Your generator will definitely handle it, it's just a question of how fast
you can go with the available power. If you want a quick guide go back and
(re)read my posts about a solar power charger into 210 AH (said 240 AH
earlier, that was an error).
Battery charging is a complex study despite the steady and well known
qualities of lead acid batteries. You've been given good guidance for
calculating, based on ideal conditions, but by the time you add in
corrections for conditions only you can assess because you're there and we
aren't (like temperature, charge/discharge rates, age of batteries, etc) your
best bet is to get detailed anecdotal evidence based on long term
observations. That's why I tried to give you some.
If you really want to measure it, you need two cheap multimeters, one reading
the voltage across the battery, the other reading millivolts across a small
length of heavy wire whose resistance you know as exactly as possible. This
second gives you the current, and it's better than switching the meter to
ammeter mode and placing it directly in circuit because it's safer, and you
get to use that meter on other stuff easily if you want. Take readings
frequently. And observe the polarity of the reading on that second meter,
that one tells you whenther the flow is going in, or coming out. If you end
up wanting a greater guide, consider some cheap analog to digital system so
you can log the readings automatically. For a UPS though, you'll likely only
need to do frequent readings during each charge or discharge run.
If you want a fast guide, and to avoid a lot of work monitoring them over
time, you really need to read accounts of average yield in real situations.
Look for web sites and forums about DIY solar power for such accounts, look
closely at any that seem similar to yours, in capacity, charge times, etc.
And if all you want is a guide to what is drawn out of a mains connection,
get a Kill-A-Watt meter like James Sweet pointed out. Those do all your
monitoring and averaging for you because they're designed to watch long term
use of intermittent loads, like fridge compressors, and unsteady ones like
TV's and hi-fi's.
But in the end if you really want to know the ratio of input to output yield,
you're going to have to look more closely at the batteries themselves.
Starting with existing observations in other systems is the easiest and
fastest way to good a clear idea.
Also, get the maker's data sheet for whatever batteries you have. If the
Powersafe data I have is anything to go by, you'll likely get all the info
you need, except for the charger efficiency.
(Powersafe batteries are great, banks punt them out after five years of their
nominal 10 year lifetime because banks need very high security for data
stores, they're top quality gear, with a likely 7 years remaining life or
better by the time they end up on eBay for prices that make serious
installations possible at low cost.)
It sounds like this is exactly the tool for the job. It's an
impressively powerful "instrument" given how inexpensive it is. While it
does record kilowatt hours, it also will give instantaneous readings of
volts, amps, volt-amps, power factor and watts. The only thing it
doesn't do as well at is seeing large spikes in draw, like the startup
of a motor, there is no analog display and the digital one does not
update quickly enough for this.
In regards to UPS on a generator, be aware that some UPS's do not like
the output of some generators, and will refuse to switch out of battery
mode. Cheap generators tend to have really dirty nasty output.
Don't I know it.. killed my SCI Pro One synth in 1985. A performance that
would have been way before its time too, if it hadn't. Was going to do a solo
industrial sort of thing in the Demolition Ballroom, a squat a load of us set
up for punk gigs, at a time when acid house and punk, the main 'parents' of
the industrial/techno scene, watched each other from a mistrustful distance
like a two strangers who couldn't believe they could get on... The delicate
controller IC in the synth got fried by a generator someone had hired and I
couldn't get a note out of it when setting up. Likely one of the most
disappointing moments of my life. Things were chaotic enough at the time that
finding repairs, or money for them, were beyond reach.
I guess another caveat to observe re UPS's and any kind of off-line source is
that they might not offer much control over what charge rates can be had,
though I assume some kind of trickle charge is in order. I read on some blog
that a USP can eat as much as 30 WATTS just sitting idle. That sucks, if
true. I don't even beleive it, once a battery is charged a UPS has to eat
less than that, surely?
I was in B&Q the other day and was looking at some cheap generators and
they had warnings on them saying not suitable for electronic equipment.
However, it seems a lot of generators have a 12V DC output. Why not slap
this across the 12V battery in your UPS and charge directly?
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