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Old 31-08-2013, 02:18   #1
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Efficiency of Battery Chargers

Various battery chargers I've owned over the years are never rated for their maximum AC current draw. My AC ammeter is located downstream of my battery charger, so I do not have immediate access to information about how much power it is drawing.

I guess I could get out the clamp meter, but does anyone know offhand what the efficiency of these things is? So a 24v * 70 amp charger, for example, at full chat is outputting around 2000 watts -- so how many amps of AC does it need? If it's 90% efficient it would presumably be 2222 watts which at 230v is a bit less than 10 amps. If it's 80% efficient, then 2.5kW and 11 amps.

Anyone understand this at all? Inquiring minds want to know.
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Old 31-08-2013, 05:17   #2
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IOTA lists the max AC input amps of each charger, on the web site. If you do the math for a few, it may give you a %.
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Old 31-08-2013, 06:21   #3
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Re: Efficiency of Battery Chargers

Iota is unusually efficient.

Your marine battery charger suffers from two problems: electrical inefficiency where some of the power is converted to heat - about 20%, and a poor power factor, sometimes as low as .75 which increases amps but not heat.

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Old 31-08-2013, 06:27   #4
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Re: Efficiency of Battery Chargers

I have had two chargers on Hawk. The current one (sterling) is 89% efficient. The prior one was 67% (prosign). Those are actual "in Place" measurements, not marketing material.
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Old 31-08-2013, 06:37   #5
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Re: Efficiency of Battery Chargers

Actually, the Iota is pretty inefficient since it does not have a power factor correction in it.

Our Victron's measured efficiency is 86%.

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Old 31-08-2013, 07:11   #6
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Great stuff; thanks everyone.
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Old 31-08-2013, 07:49   #7
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Re: Efficiency of Battery Chargers

Probably the reason some vendors do not give a max current specification is because there are different types of current meters in use that will give significantly different answers depending on how the meter is designed. The so-called true RMS current meters will only give the right reading under certain conditions. An analog panel meter may give a much different answer. Current input is also determined by input voltage (lower voltage causes higher current). Thus vendors may choose to avoid this confusing state of affairs by simply not giving the current specification.

Dockhead, if you are concerned about the current draw then that will sometimes be unrelated to the efficiency. Current draw can be considerably affected by power factor. A sometimes misunderstood fact is that low power factor does not usually contribute to low efficiency. There are many snake oil salesmen selling power factor correction devices as a means to vastly improve efficiency of equipment.

Efficiency is simply the ratio of power out and power in. Power is the average of the product of current multiplied by voltage in real time. Unless there is effective power factor correction this is not the same as taking a voltage measurement and multiplying it by a current measurement.

It can be difficult to measure the power input to a non power factor corrected battery charger because it requires equipment that most boaters do not have on board. A clamp meter and a volt meter will not give accurate results. A real time watt meter is about the only way to measure power into one these devices.

Sorry for the diatribe on efficiency and power factor but it is an oft misunderstood subject.
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Old 31-08-2013, 09:31   #8
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Quote:
Originally Posted by transmitterdan View Post
Probably the reason some vendors do not give a max current specification is because there are different types of current meters in use that will give significantly different answers depending on how the meter is designed. The so-called true RMS current meters will only give the right reading under certain conditions. An analog panel meter may give a much different answer. Current input is also determined by input voltage (lower voltage causes higher current). Thus vendors may choose to avoid this confusing state of affairs by simply not giving the current specification.

Dockhead, if you are concerned about the current draw then that will sometimes be unrelated to the efficiency. Current draw can be considerably affected by power factor. A sometimes misunderstood fact is that low power factor does not usually contribute to low efficiency. There are many snake oil salesmen selling power factor correction devices as a means to vastly improve efficiency of equipment.

Efficiency is simply the ratio of power out and power in. Power is the average of the product of current multiplied by voltage in real time. Unless there is effective power factor correction this is not the same as taking a voltage measurement and multiplying it by a current measurement.

It can be difficult to measure the power input to a non power factor corrected battery charger because it requires equipment that most boaters do not have on board. A clamp meter and a volt meter will not give accurate results. A real time watt meter is about the only way to measure power into one these devices.

Sorry for the diatribe on efficiency and power factor but it is an oft misunderstood subject.
Not a diatribe at all; I've learned a lot and am very grateful that you took the trouble to explain it. Among other things, I've learned that I know even less about this subject than I thought.

One question I have - do we actually care so much about watts? Shore power and gender capacity are limited by current. Will my True MRS clamp meter give me the same amps as that d**ned breaker on a shore power connection sees?
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Old 31-08-2013, 09:57   #9
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Quote:
Originally Posted by Dockhead View Post

One question I have - do we actually care so much about watts? Shore power and gender capacity are limited by current. Will my True MRS clamp meter give me the same amps as that d**ned breaker on a shore power connection sees?
In general, yes the RMS meter will give the current as perceived by a circuit breaker. But the meter should be way more accurate than the breaker. A breaker is also a time sensitive device so it will trip at many different current levels depending on how long the current is present. If you stay below 90% of the breaker rating it should not trip unless it is damaged.
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Old 31-08-2013, 21:23   #10
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Re: Efficiency of Battery Chargers

Quote:
Originally Posted by Dockhead View Post
One question I have - do we actually care so much about watts? Shore power and gender capacity are limited by current. Will my True MRS clamp meter give me the same amps as that d**ned breaker on a shore power connection sees?
A circuit breaker is a safety device. Its job is to disconnect AC power if the draw from the load exceeds an arbitrary point, so that the unacceptably high current doesn't cause overheating or other failure in the circuit.

There are tolerances and safety factors built into the whole system, so as long as the breaker will reliably trip above a current that's reasonably close to its rated value, it's done its job.

If you have a problem with frequent tripping of a breaker, you need to determine if the current is really getting that high, or if the breaker is tripping at a value lower than it's rated capacity.

Re the actual efficiency of a battery charger, I'd hazard to guess that the inefficiency of the process of charging a lead-acid battery would swamp the electrical conversion efficiency of most chargers.
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