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Old 31-03-2014, 04:52   #1
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Battery Charger Power Draw Questions

I have a Victron Multiplus charger/inverter, rated at 70 amps * 24v as a charger.

Since it manages the AC power on board, it is wired in upstream of my AC ammeter. I cannot therefore see the power draw of the charger itself.

I guess it will be simple enough to move the toroidal inductive sensor to the right place, but in the meantime I'm curious about the power consumption of battery chargers, which is nowhere mentioned in the manuals.

70 amps * say 26 volts is just under 2kW (1960 watts to be exact). So if the total losses are say 20%, the maximum AC power draw is going to be about 2.4kW -- does that sound right?


Here's the next question. Now I understand that the bulk charging phase is constant current -- so the charger is supposed to be putting out its maximum current (70 amps?) until the voltage rises to the absorption voltage level. So as long as the "bulk" pilot light is glowing, the charger is putting out 70 amps? And the current starts to taper off only after the charger goes into absorption mode? That doesn't seem right to me.


Anyone have any insights?
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Old 31-03-2014, 04:59   #2
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Re: Battery Charger Power Draw Questions

Quote:
Originally Posted by Dockhead View Post
Here's the next question. Now I understand that the bulk charging phase is constant current -- so the charger is supposed to be putting out its maximum current (70 amps?) until the voltage rises to the absorption voltage level. So as long as the "bulk" pilot light is glowing, the charger is putting out 70 amps? And the current starts to taper off only after the charger goes into absorption mode? That doesn't seem right to me.
It'll put out 70 amps as long as the batteries accept that much. As the battery SOC rises, the current draw goes down. Chargers work to a voltage set point, the charger really has no clue where the current is going. As an example, 50% of the charger output could be going to house loads and only 35 amps into the batteries.
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Old 31-03-2014, 07:46   #3
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Re: Battery Charger Power Draw Questions

Install the Blue Power Panel on your Multiplus and all of your questions will be answered by it. And it is the best way to configure and control the MP.

Mark
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Old 01-04-2014, 11:17   #4
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Re: Battery Charger Power Draw Questions

Quote:
Originally Posted by Dockhead View Post
.....Here's the next question. Now I understand that the bulk charging phase is constant current -- so the charger is supposed to be putting out its maximum current (70 amps?) until the voltage rises to the absorption voltage level. So as long as the "bulk" pilot light is glowing, the charger is putting out 70 amps? And the current starts to taper off only after the charger goes into absorption mode? That doesn't seem right to me....
Quote:
Originally Posted by Dockhead View Post
...Here's the next question. Now I understand that the bulk charging phase is constant current -- so the charger is supposed to be putting out its maximum current (70 amps?) until the voltage rises to the absorption voltage level. So as long as the "bulk" pilot light is glowing, the charger is putting out 70 amps? And the current starts to taper off only after the charger goes into absorption mode? That doesn't seem right to me....
During the bulk stage the charger is acting as a "constant current" charger and the voltage slowly builds up to the absorption voltage that has been set. When it reaches this voltage the charger changes to a "constant voltage" charger and the battery, not the charger, decides how much current it will accept.

On my Victron the time to get to absorption voltage t - depends on how charged the batteries are, so after a time of 5 x t at the absorption voltage charger drops to the Float voltage and the current going into the batteries will drop quite a lot.

On many cheap chargers it will drop to float after 1-3 hours at absorption - whether the batteries are charged or not!!!

See this simplified diagram of what's happening:
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Old 01-04-2014, 13:32   #5
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Re: Battery Charger Power Draw Questions

Install an AC ammeter in the supply line, or use a clamp-on ammeter on the ungrounded supply wire and read the amperage draw directly.

Bill
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Old 01-04-2014, 13:52   #6
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Re: Battery Charger Power Draw Questions

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Originally Posted by sailinglegend View Post
During the bulk stage the charger is acting as a "constant current" charger and the voltage slowly builds up to the absorption voltage that has been set. When it reaches this voltage the charger changes to a "constant voltage" charger and the battery, not the charger, decides how much current it will accept.
That's what I had thought, but what DotDun says above contradicts.
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Old 01-04-2014, 13:53   #7
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Re: Battery Charger Power Draw Questions

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Originally Posted by btrayfors View Post
Install an AC ammeter in the supply line, or use a clamp-on ammeter on the ungrounded supply wire and read the amperage draw directly.

Bill
I guess that's the only way to get to the heart of the matter. I do have a clamp meter!
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Old 01-04-2014, 17:20   #8
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Re: Battery Charger Power Draw Questions

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I guess that's the only way to get to the heart of the matter.
No, there is also that Blue Power Panel someone mentioned earlier…

Mark
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Old 01-04-2014, 17:48   #9
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Re: Battery Charger Power Draw Questions

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Originally Posted by colemj View Post
No, there is also that Blue Power Panel someone mentioned earlier…

Mark
That does look pretty cool. I have been jealous of space on my instrument panel, but might have another look at that. Thanks.
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Old 01-04-2014, 17:55   #10
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Re: Battery Charger Power Draw Questions

If you know the DC amperage of your charger, just convert it to AC amps at the appropriate voltage difference (110 ac to 12 dc, or 220 ac to 24 dv) just do the math, forget the blinin' conversion to watts, it's all linear anyway.

Most every single quality battery charger and for that matter regulator manual (Welcome to Balmar), as well as most boat electrical books, explain three stage/phase battery charging and have for the past 15 or more years.

Good luck, the information is out there. The ac power draw on a 50A dc charger here in the USA is approximately 1/10th of the dc amps rating (1/12 - 120 v vs, 12 v - plus inefficiencies).
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Old 01-04-2014, 17:59   #11
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Re: Battery Charger Power Draw Questions

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Originally Posted by Stu Jackson View Post
If you know the DC amperage of your charger, just convert it to AC amps at the appropriate voltage difference (110 ac to 12 dc, or 220 ac to 24 dv) just do the math, forget the blinin' conversion to watts, it's all linear anyway.

Most every single quality battery charger and for that matter regulator manual (Welcome to Balmar), as well as most boat electrical books, explain three stage/phase battery charging and have for the past 15 or more years.

Good luck, the information is out there. The ac power draw on a 50A dc charger here in the USA is approximately 1/10th of the dc amps rating (1/12 - 120 v vs, 12 v - plus inefficiencies).

That's the point he doesn't know the amperage. Keep up.

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Old 01-04-2014, 18:04   #12
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Re: Battery Charger Power Draw Questions

Quote:
Originally Posted by Stu Jackson View Post
If you know the DC amperage of your charger, just convert it to AC amps at the appropriate voltage difference (110 ac to 12 dc, or 220 ac to 24 dv) just do the math, forget the blinin' conversion to watts, it's all linear anyway.

Most every single quality battery charger and for that matter regulator manual (Welcome to Balmar), as well as most boat electrical books, explain three stage/phase battery charging and have for the past 15 or more years.

Good luck, the information is out there. The ac power draw on a 50A dc charger here in the USA is approximately 1/10th of the dc amps rating (1/12 - 120 v vs, 12 v - plus inefficiencies).
Well, that's pretty obvious, and in fact that calculation is already in the original post. But that doesn't answer the question. What is the efficiency of the conversion? You will need more watts of power going in, than what comes out (70 amps * 26 volts). How many more?

And the other question was whether our battery chargers put out their full rated current (in my case, 70 amps) during the entire bulk stage. There have been contradictory opinions expressed here.
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Old 01-04-2014, 18:08   #13
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Re: Battery Charger Power Draw Questions

I have a 75 amp Iota charger, 110v in, 12v out. It's rated at 18.2A current draw. Since yours is a 220v in, 24v out, I'd think it would be very close to 18A at full power.
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Old 02-04-2014, 00:13   #14
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Re: Battery Charger Power Draw Questions

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That's what I had thought, but what DotDun says above contradicts.
I think what he says agrees exactly with what I said. My simplified diagram doesn't take into account the current going into boat loads.
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Old 03-04-2014, 06:18   #15
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Re: Battery Charger Power Draw Questions

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That's what I had thought, but what DotDun says above contradicts.
It's simple ohm's law. For a charger to be 'constant current' it would need to raise voltage level as the internal resistance of the battery goes up.

One can only assume that 'constant current' implies if your batteries accept 70 amps at say 25.5v, the charger will raise the voltage up to the maximum (26 as an example) to keep 70 amps flowing. Once the maximum voltage is reached, assuming it won't go any higher, current flow will depend on the internal resistance of the batteries, the charger can't do anything to change that.
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