Questions about replacing battery charger

[skipmac]

Originally Posted by btrayfors

One hundred watts input power rarely equals 100 watts output power when dealing with power conversions. Here, we're dealing with converting 120VAC to 12-14 VDC. There are always losses involved. The Iotas are pretty good, however, and generally run about 80% efficiency.

An Iota DLS-75 has a maximum AC current draw of 17 amps, and a maximum inrush current of 50 amps (single cycle).

This sounds odd to me. If I'm doing the math right 17 amp draw of the Iota 75 (which by the way is what I have, with the IQ4 option) at 120VAC means the Iota is drawing a little more than 2,000 Watts. At 80% efficiency and assuming a power factor of 1 then a little over 1600 Watts is output as DC which at 14.4VDC is over 110 amps? Either I'm doing the math wrong or the Iota is seriously underrated.

I tried to run the Iota75 with a 1200 Watt portable gennie and it would not fly. If the batteries were depleted at all the gennie would seriously bog down. Didn't wait to see if I could produce smoke, just went ahead and switched it off.

[djmarchand]

Skipmac:

In the world of AC, watts= volts x amps x power factor. The IOTA chargers are not power factor corrected and have a pf of maybe 0.7. Also on the DC side it takes about 14 v to charge at 75 amps. So given that watts AC x conversion efficiency= watts DC then- 120 x 17 x 0.7 x conversion efficiency = 14*75. Solving for conversion efficiency gives about 75%. Maybe efficiency is higher and pf is lower, but the bottom line is that it takes a LOT of AC amps to make DC amps and not a simple ratio of 120/12.

That is why generators can trip when hooked to battery chargers. The high AC current trips the breaker.

FWIW the manufacturers data for input AC current is almost always conservative. I had a Freedom inverter/charger which is rated at 100 amps DC charging. The AC input spec was 21 amps. I never saw more than about 18 amps even with pretty well discharged batteries.

[stevensuf]

you need to realise the power factor is phasing, ie the current will lag the voltage by some degree, probably about 40% in the case above so as the voltage swings for zero to 120 max, the current will be lagging behind and out of phase, more likely the max current draw will be when the voltage is around 80v, as i have recommended before, try and get a charger with a power factor as close to 1 as possible, this means the current and voltage will be almost in synch , meaning far less amp draw as the peak power is taken at peak voltage, nb the charger is not drawing 17amps at 120v more like 17 amps peak at a point where the output voltage is about 70-80 volts, poor power factors are just cheapness plain and simple, its something we pay for when we play our electricity bills, the companies just measure amps and assume at peak voltage, you may pay for a kw hour running a motor and yet receive only 1/3 a kw hour in reality, running your on generator you wont loose out that way, but with a poor power factor you are going to need a larger generator than you would have with a low power factor unit.

[stevensuf]

nb power factors can be corrected using the correct value of capacitance on the supply line to slow down the voltage to put it in or near phase with the current

[DotDun]

Originally Posted by btrayfors

The only way to push up the amperage beyond the battery's safe charging rate is to raise the charging voltage higher than about 14.4VDC for a 12V battery. Smart chargers and regulators prevent this from happening.

A charger offers the battery a target voltage (dictated by what 'stage' the charger is in) and delivers current to it's maximum capability or to match internal resistance of the battery. This isn't necessarily the optimum charging rate for the long term health of the battery.

Trojan recommends a charging rate of 10-13% of rated AH for the T-105, the battery will accept much more. An example of (4) T-105's for a 12v @ 450AH bank, per Trojan, a 110A charger is too big, a 50A charger is closer to manufacturer recommendations.

[goboatingnow]

The only way to push up the amperage beyond the battery's safe charging rate is to raise the charging voltage higher than about 14.4VDC for a 12V battery. Smart chargers and regulators prevent this from happening.

factually wrong

Dave

[btrayfors]

Originally Posted by DotDun

A charger offers the battery a target voltage (dictated by what 'stage' the charger is in) and delivers current to it's maximum capability or to match internal resistance of the battery. This isn't necessarily the optimum charging rate for the long term health of the battery.

Trojan recommends a charging rate of 10-13% of rated AH for the T-105, the battery will accept much more. An example of (4) T-105's for a 12v @ 450AH bank, per Trojan, a 110A charger is too big, a 50A charger is closer to manufacturer recommendations.

Dotdun,

Yeah, but.....

What cruiser is going to charge his bank of 4 T-105s for more than 4.5 hours to replenish the 50% capacity used?

You gotta be reasonable here, folks. Trojan only recently reduced their recommended charging rate. However, in conversations with Trojan engineers you get a different story, including that typical charging voltages with modern chargers are set way too low for effective charging and for the health of the battery.

My own experience with T-105s over the past 20 years or more -- and I have a total of 10 of them now, incuding 8 on the boat and two at the home ham shack -- suggests that 20% of capacity is none too fast a charging rate for them.

Once again, a flooded battery is going to take whatever amperage it's going to take for a given voltage, and no more. If that amperage doesn't result in excessive heating, that should be fine for the health of the battery.

Bill

[goboatingnow]

Once again, a flooded battery is going to take whatever amperage it's going to take for a given voltage, and no more. If that amperage doesn't result in excessive heating, that should be fine for the health of the battery.

Lets put this dangerous myth to bed

A discharged battery presents a very low internal resistance. It will in essence accept any amount of current a charger is capable off, even if that current is detremental to the battery. If you connect a big charger to a small bank, you will damage and possibly explode Lead acid batteries, its why there a typical C/5 to C/10 limit on most.

Dave

[forsailbyowner]

Ive been living aboard and cruising for 5 years without a shore battery charger. IMHO youd be much better off spending the same money for solar that you are planning on spending on a genset and charger. Ive had the same 6 golfcart batteries the whole time and theyre holding strong. My setup uses a diversion regulator hooked up to a 12v element that heats water when the batts reach full charge. Solar prices have dropped dramatically and having a robust system isnt terribly expensive. Im a huge energy consumer running my fridge on high and my 400 watt sewing machine, lights, music and fans on while I work sewing onboard. Another thin that the charger wont do is keep you powered up when under sail, the solar i have keeps my autopilot radar chartplotter etc going for days on end. I did get kind of carried away with almost 600 watts of panels, 4 out of 10 0f the panels i hook up only when underway.

[Maine Sail]

Originally Posted by goboatingnow

Lets put this dangerous myth to bed

A discharged battery presents a very low internal resistance. It will in essence accept any amount of current a charger is capable off, even if that current is detremental to the battery. If you connect a big charger to a small bank, you will damage and possibly explode Lead acid batteries, its why there a typical C/5 to C/10 limit on most.

Dave

Balmar has a different take regarding alternators.. I have also charged many customers batteries on my bench from dead with a large smart charger and the wets all take some high in-rush charge but then settle down at around 25% of C or less. These have been mostly deep cycle batteries not thin plate starting batteries.

Quote From Balmar:
"Forget the rumor that an oversized alternator will destroy your batteries ... the truth is that the acceptance rate of your batteries will dictate how much amperage the alternator will provide."

[DotDun]

Originally Posted by btrayfors

Trojan only recently reduced their recommended charging rate. However, in conversations with Trojan engineers you get a different story, including that typical charging voltages with modern chargers are set way too low for effective charging and for the health of the battery.

My own experience with T-105s over the past 20 years or more -- and I have a total of 10 of them now, incuding 8 on the boat and two at the home ham shack -- suggests that 20% of capacity is none too fast a charging rate for them.

At least you've given me some comfort that my 90A alternator won't completely ruin my (4) T-105's. I'll stick with my 50A shorepower charger, when at the dock, time charging really doesn't matter.

[goboatingnow]

Originally Posted by Maine Sail

Balmar has a different take regarding alternators.. I have also charged many customers batteries on my bench from dead with a large smart charger and the wets all take some high in-rush charge but then settle down at around 25% of C or less. These have been mostly deep cycle batteries not thin plate starting batteries.

Quote From Balmar:
"Forget the rumor that an oversized alternator will destroy your batteries ... the truth is that the acceptance rate of your batteries will dictate how much amperage the alternator will provide."

Id have to say that a strange statement , Lead acid batteries have acceptance rates at certain states of state that are determental to the battery. The balmar statement is true, but it isnt the best for the batteries.

[skipmac]

Originally Posted by djmarchand

In the world of AC, watts= volts x amps x power factor. The IOTA chargers are not power factor corrected and have a pf of maybe 0.7.

Yes, it has been a while since college but still remember the formula and noted the pf question in my calcuations. Since I don't have the specs for the Iota at hand I made an assumption (always dangerous) that the power factor of the Iota would be similar to the power factor of the Victron mentioned by Bill. Where do you get the pf rating of 0.7 for the Iota?

Originally Posted by djmarchand

Also on the DC side it takes about 14 v to charge at 75 amps. So given that watts AC x conversion efficiency= watts DC then- 120 x 17 x 0.7 x conversion efficiency = 14*75. Solving for conversion efficiency gives about 75%. Maybe efficiency is higher and pf is lower, but the bottom line is that it takes a LOT of AC amps to make DC amps and not a simple ratio of 120/12.

Yes again. If you notice I used a DC voltage of 14.4V in my calculation and efficiency of 80% again based on Bill's estimates. Am very aware of efficiency losses and pf corrections required to convert the AC power to DC power. Thought that was taken into account in my original post.

[ADMPRTR]

I was speaking with a sales person regarding the correct size of charger and they mentioned something that I found interesting.

Let's assume one has 300Ah in the house bank. Using the 20-25% sizing recommendation on the charger would dictate a 60-75AMP charger. However, as one would not discharge the bank below 50%, the actual capacity to charge is only 150Ah, which would require a charger of 30-40 AMPs which should charge a fully diminished bank in roughly 5 hours.

Which is the correct target? 40 or 75AMP?

Thanks,

Andrew

[steamgoat]

Good thread ADMPRTR, if I may join in. After lots of reading I am still not 100% sure of this. 4 T105's in paralell/series, normally on Solar power.
I have a Honda 2000. I would like to buy a charger for this. Will the Honda power a Iota DLS 75 without problem or should I be safe and get the Iota 55.
Thanks, Bruce.