How much ac power does a dc charger draw? (Math)

[Nan_wpg]

All you need to know for gen sizing is the input wattage. The manufacturer will tell you what that is.

Ok so the 20a charger has input v of,120. 120 x 20 = I need a 2400 watt generator? Seems high

 

[Checkthisout]

Ok so the 20a charger has input v of,120. 120 x 20 = I need a 2400 watt generator? Seems high

Do you have a link to the charger you're considering?

Edit:

Oh look. They don't tell you the a/c input for that charger. Must not UL listed.

 

[Bud Martin]

Ok so the 20a charger has input v of,120. 120 x 20 = I need a 2400 watt generator? Seems high

20A charger: 20A is the battery DC charging current, not AC input current.

 

[Warpspeed]

I d-nt think anything. In fact I know nothing about lifepo4.

I thought the,current,stayed the same up until they are fully charged. Voltage is also,high all the,way to the top and then drops to,float.

current only,drops down for la battery?? Or am I wrong?

Current stays the same because current is controlled by the charger.
Once the voltage reaches the set charging voltage, the current starts to fall.
The higher the initial full charging current, the longer the ramping down takes.
Its certainly not fully charged once it reaches the set charging voltage.

There are several different lithium iron chemistries, they differ but the general characteristics are quite similar.

The curve below shows what happens if you charge at 1C.
That is charging at 100 amps for a 100 amp hour battery.
Once it reaches full charging voltage, it has only reached 60% charge state.
It takes several times as long to reach full amp hour capacity.

Suggest you visit "battery university" its an excellent resource, the best on the internet for battery facts and figures.
https://batteryuniversity.com/article/bu-216-summary-table-of-lithium-based-batteries

 

[Nan_wpg]

Do you have a link to the charger you're considering?

Edit:

Oh look. They don't tell you the a/c input for that charger. Must not UL listed.

But they do list the ac I nput??? Input voltage: 1 Phase 120VAC 50HZ/60HZ ± 2HZ

 

[Nan_wpg]

Current stays the same because current is controlled by the charger.
Once the voltage reaches the set charging voltage, the current starts to fall.
The higher the initial full charging current, the longer the ramping down takes.
Its certainly not fully charged once it reaches the set charging voltage.

There are several different lithium iron chemistries, they differ but the general characteristics are quite similar.

The curve below shows what happens if you charge at 1C.
That is charging at 100 amps for a 100 amp hour battery.
Once it reaches full charging voltage, it has only reached 60% charge state.
It takes several times as long to reach full amp hour capacity.

Suggest you visit "battery university" its an excellent resource, the best on the internet for battery facts and figures.
https://batteryuniversity.com/article/bu-216-summary-table-of-lithium-based-batteries

Thanks for the link. I’ll do some reading. I’m new to lifepo4 batteries.

 

[Checkthisout]

But they do list the ac I nput??? Input voltage: 1 Phase 120VAC 50HZ/60HZ ± 2HZ

They don't list input wattage or amps which is what you need to figure out exactly how much power it will draw.

Otherwise, use your estimate based on output plus like 20%.

 

[Nan_wpg]

20A charger: 20A is the battery DC charging current, not AC input current.

Yes, I know the 20a is charging current. I multiplied current by voltage (120) to get wattage as I thought dc and ac was all the same and only power factor and efficiency mattered Ac input current should be 2400 watts?/ no?

 

[Nan_wpg]

They don't list input wattage or amps which is what you need to figure out exactly how much power it will draw.

Otherwise, use your estimate based on output plus like 20%.

So the charger is junk? Stay away?
who makes a decent 48 v charger? Do I look for up listed? Something that lists ac input current?

 

[Bud Martin]

Yes, I know the 20a is charging current. I multiplied current by voltage (120) to get wattage as I thought dc and ac was all the same and only power factor and efficiency mattered Ac input current should be 2400 watts?/ no?

The 20A is the DC charging current to the battery, you cannot mutiply AC input Voltage to the DC charging current to get the AC power input.
For example (assume 100% effi. to simplify the calculation)
Charger is rated at 12VDC 20A so the power is 12V x 20A = 240W
So the AC current will be 240W/120Vac = 2A of Ac current, so 120Vac x 2A = 240W.
If effi. is 90% then it will be 240W/0.9 = 267W pulling from the AC outlet.

 

[Nan_wpg]

The 20A is the DC charging current to the battery, you cannot mutiply AC input Voltage to the DC charging current to get the AC power input.
For example (assume 100% effi. to simplify the calculation)
Charger is rated at 12VDC 20A so the power is 12V x 20A = 240W
So the AC current will be 240W/120Vac = 2A of Ac current, so 120Vacx 2A = 240W.
If effi. is 90% then it will be 240W/0.9 = 267W pulling from the AC outlet.

so my linked charger requires 1066 watts from genny?

(48 x 20 = 960 watts. 960/120 = 8 a. 8 x 120 = 960 watts /.9 = 1066 watts?

this is the math needed for dc chargers?

 

[Bud Martin]

so my linked charger requires 1066 watts from genny?

(48 x 20 = 960 watts. 960/120 = 8 a. 8 x 120 = 960 watts /.9 = 1066 watts?

this is the math needed for dc chargers?

Yes, based on 48V nominal, as the the battery Voltage goes up toward the set point the Wattage will go up with it, also the Power factor comes in as explained.

 

[Checkthisout]

So the charger is junk? Stay away?
who makes a decent 48 v charger? Do I look for up listed? Something that lists ac input current?

Not junk. It's just lame of them not to have a rating tag for how much juice the thing will draw.

 

[Nan_wpg]

Not junk. It's just lame of them not to have a rating tag for how much juice the thing will draw.

I can always put the charger on my killa watt (meter) and charge it “on grid”. Then I’ll know the true power it needs, and buy my genny accordingly.

 

[Checkthisout]

I can always put the charger on my killa watt (meter) and charge it “on grid”. Then I’ll know the true power it needs, and buy my genny accordingly.

Well why didn't you say that in the first place?

 

[Nan_wpg]

Well why didn't you say that in the first place?

I don’t have the charger.

just learning and wanted to know the math.

I don’t just want to know what to do, I like to know why I’m doing it.

 

[Checkthisout]

I don’t have the charger.

just learning and wanted to know the math.

I don’t just want to know what to do, I like to know why I’m doing it.

I know. I was just giving you a rib'n. If you're committed to the battery and voltage, yes of course buy the charger and batteries and just plug it in and see what it draws, then buy generator.

This is the smartest way.

 

[Nan_wpg]

I know. I was just giving you a rib'n. If you're committed to the battery and voltage, yes of course buy the charger and batteries and just plug it in and see what it draws, then buy generator.

This is the smartest way.

I figured it was ribbin, lol. Glad we both understand that.

our current honda 2000 can accommodate a 48v charger. Not 2. I’d have to upgrade to a 3200w Honda. (3200i is a nice looking inverter).

I appreciate all the help!