# converter charger limited by generator

JsMom
Registered Users Posts:

**6**✭✭
Please forgive my ignorance if I get this wrong as I am trying to learn how all this works.

I "think" I may have battery charging using a generator and converter charger figured out.

Am I correct in that no matter what your converter charger amp rating is, it will still be limited by the amp output of your specific generator?

For example, if I have a 1700 (running watts, not surge) generator, and I plug a 55 amp converter charger into the 120 volt outlet on said generator, the most my converter charger will pull from my generator is 14.16 amps?

Also, is that 14.16 amps per hour that is going into my battery bank?

Also, is that 14.16 amps per hour that is going into my battery bank?

#### Howdy, Stranger!

It looks like you're new here. If you want to get involved, click one of these buttons!

## Quick Links

#### Categories

- 25.7K All Categories
- 199 Forum & Website
- 107 Solar Forum News and Announcements
- 1.2K Solar News, Reviews, & Product Announcements
- 146 Solar Information links & sources, event announcements
- 850 Solar Product Reviews & Opinions
- 246 Solar Skeptics, Hype, & Scams Corner
- 20.4K Solar Electric Power, Wind Power & Balance of System
- 3.3K General Solar Power Topics
- 6.2K Solar Beginners Corner
- 908 PV Installers Forum - NEC, Wiring, Installation
- 1.9K Advanced Solar Electric Technical Forum
- 4.8K Off Grid Solar & Battery Systems
- 344 Caravan, Recreational Vehicle, and Marine Power Systems
- 977 Grid Tie and Grid Interactive Systems
- 593 Solar Water Pumping
- 769 Wind Power Generation
- 610 Energy Use & Conservation
- 438 Discussion Forums/Café
- 206 In the Weeds--Member's Choice
- 68 Construction
- 82 New Battery Technologies
- 82 Old Battery Tech Discussions
- 3.8K Solar News - Automatic Feed
- 3.8K Solar Energy News RSS Feed

## Comments

79✭✭Your 55 amp converter is 55 amps at 12 volts. You feed your converter 120 volts and it steps it down to 12 volts. The converter will draw a maximum of 5.5 amps at 120 volts.

Look at it this way:

Your converter output is 12 volts 55 amps. You can calculate watts by multiplying volts * amps so the converter can output 660 watts.

If we have 660 watts coming out we have to have 660 watts going in. Right? So 660 watts/120volts = 5.5amps.

In the real world there are some losses in the conversion and something called "power factor" that affect the efficiency of the conversion.

Bottom line: Your 1700 watt generator should run your 55 amp converter.

6✭✭(lets assume efficiency loss is calculated in already)...then it would take me 7 1/2 hours to replace those 40 amps into my battery bank?

What I'm trying to figure out is what I need in a generator and a converter charger, in order to replace those 40 amps in the shortest amount of time possible.

30,511adminAmp*Hours and Watt*Hours are an "amount: --- Like driving 120 Miles (=60 mph * 2 hours)

And Amps is a "partial unit"... It describes the rate of current flow, but does not contain the "pressure" (voltage).

Watts are a complete unit... They contain both the flow and and the pressure:

- Power = Watts = Volts * Amps

So, you have to look a bit of math when converting between 12 VDC and 120 VAC -- Without any losses or other fudgfactors:- 50 amps * 12 volts = 600 Watts (p=v*i)
- I = P/V = 600 Watts / 120 volts = 5 amps

So, for the battery charger--Which is just an "energy converter" (takes high voltage / low current from the utility power and efficiently converts it to low voltage / high current needed to charge the battery bank).Now--You will see people talk about Amps and Amp*Hours all day long with cars, boats, and RVs when working with their power systems (battery, generator, 12 volt loads, etc.). And that works very well because, generally, everything is running at one voltage--12 volts typically, sometimes 24 volts for boats and larger trucks/RVs).

However, when we work with Solar Power systems, AC inverters, and such... Talking in Amps is no longer "good enough". The easest method is to use Watts (and Watt*Hours). When we talk about Watts, we are including both Amps and Voltage (p=i*v). So, when we say we have 600 Watts of "something" (loads, charger, etc.)--We can easily convert that number to different Amperage values based on the usage/need. For example, some common voltages, and their current values at 600 Watts:

- P=V*I; I=P/V
- I = 600 Watts / 12 volts = 50 amps (at 12 volts)
- I = 600 Watts / 24 volts = 25 amps
- I = 600 Watts / 48 volts = 12.5 Amps
- I = 600 Watts / 120 volts = 5 Amps
- I = 600 Watts / 240 volts = 2.5 Amps

So--From a "systems" point of view, all the above 600 Watt power systems are 600 Watts of Power--But their Amperage (current flow) is dramatically different depending on their "working voltage".If I was working on my house AC power, I would be using 120 VAC everywhere for my standard outlets and lighting--And just talking in Amps is fine. But if I was paying my utility bill, they care about Watt*hours -- Or really kilioWatt*Hours (100 Watts * 10 hours = 1,000 WH = 1.0 kWH). And because my utility feed is 120/240 VAC split phase--The utility needs to convert everything into kWH because they charge (mostly) for the energy I use, not the 10 amps at 120 VAC (1,200 Watts) or 5 amps at 240 VAC (still 1,200 Watts).

Now, to your battery bank numbers... You can work with either Watts and Watt*hours, or Amps and Amp*hours--As long as you are consistent in your units and values.

For example. Say you have a 12 volt @ 100 AH battery. And you use 50% of its capacity... You could do this in Amps:

- 100 AH * 0.50 = 50 Amp*Hours used
- 50 AH / 55 Amp charger = 0.91 hours to charge

Or you can do this in Watts:- 100 AH * 12 volts = 1,200 Watt*hours of storage
- 1,200 WH * 0.50 used = 600 WH used
- 12 volts * 55 Amp charger = 660 Watts charging
- 600 WH used / 660 Watts charging = 0.91 hours to charge

You cannot directly use the 5 Amps of 120 VAC current and divide that into the 50 AH @ 12 volts of battery to get 10 hours of charging time (using a battery charger/converter).Now the dirty little secrets... Your battery bank will probably take 55 Amps of charging (from 50% to 80% State of charge) until it is ~80% full (this is the bulk charging cycle, maximum current from the charge controller), then the battery will be at the charging voltage set point (say 14.75 volts) and it will need to be held there for ~4-6 hours (absorb charge) to get 80% to 100% full (for lead acid battery chemistry).

So an estimate for genset run time, 100 AH @ 12 volt battery bank, 55 Amp @ 12 volt charger, and 50% battery capacity would be:

- 100 AH * (0.80 - 0.50 state of charge) = 30 AH "bulk"
- 30 AH bulk / 55 Amps = 0.55 Hours
- Absorb cycle for deeply discharged FLA battery = ~6 hours
- 0.55 hour bulk + 6.0 hour bulk = 6.55 hours of genset runtime

Now--The above is just made up numbers... The maximum rate of charge typically used in solar/off grid FLA batteries is around 10% to 20% rate of charge... For a 100 AH battery bank, that would be a 10-20 Amp charger. A 55 Amp charger is really "too large" for this small of battery bank (in my above made up example). But the numbers are "close enough" for a first pass estimate.As you get deeper into the battery / genset charging issues... You find that there are more and more little "fudge factors" that make things even worse... For example, I used 12 volts as the charging voltage... In reality, the battery bank is charging around 13.6 to 14.75 volts, which changes the energy amount:

- 55 amps * 13.6 volts = 748 Watts to battery
- 55 amps * 14.75 volts = 811 Watts to battery

The typical "more accurate" estimate of how much your 1,700 Watt genset can charge for a 12 volt battery bank is closer to:- 1,700 Watt (really VA) rated genset * 0.80 continuous load (run residential genset at 80% of rated load for hours at a time) * 0.85 AC battery charger efficiency * 0.67 power factor (typically for "cheap" battery chargers) * 1/14.75 volts actual battery charging voltage = 52.5 Amps "supported" 12 volt output "cheap" battery charger

As confusing as the math can seem for 12 VDC power systems... The math for true 120 VAC power analysis is 100x more complex (if you get into the real engineering details). Using "fudge factors" like Power Factor roll a whole bunch of other AC related issues into the math.The above equation is an estimate--And if you have a 55 Amp charger on your 1,700 Watt genset--It should be fine. Most of these equations/predictions are within about 10% accuracy (+/- 5.25 amps) at best...

Anyway--Details matter. Telling us about your system and needs (battery bank voltage / AH rating / battery type), battery charger type, your energy needs (RV, camping), weekend, off grid living, etc.... We can give better answers (or at least less confusing answers) that address your needs and system--Generic answers get really convoluted very quickly (we don't know if you are talking about a VW Bug, or a Mac tractor/trailer)...

-Bill

8,762✭✭✭✭✭But a battery charger is just ON for hours and can overheat the alternator and engine.

|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||

|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar

gen: http://tinyurl.com/LMR-Lister ,

6✭✭My set up is going to be in the trunk of a toyota camry, living in it long term while I travel the entire country sightseeing. I'll have 1-225 AH FLA deep cycle battery to start, running a cpap every night that consumes appr. 33 amps, and soon after I get on the road, adding another 225AH FLA deep cycle battery ( running the 2 in series/parallel) to run the cpap everynight, and a refrigerator part time and a part time fan maybe...I guesstimate AH usage to be between 40 to 50 AH's a day.

Since I will be on the move a lot, I was going to use a small generator when I have the opportunity to be in a forest away from shore power to charge my batteries using an AC to DC converter charger, ( I don't need and inverter) and eventually buy a 200 watt solar panel to be able to use my generator less often.

6✭✭30,511adminFirst, need to confirm your CPAP machine power consumption... 33 Amps @ 12 volts (which is a rate). Or 33 Amp*Hours @ 12 volts. My guess is "Amp*Hours" (which makes sense).

That would, for example, be ~3.3 amp load for 10 hours (overnight) which is 33 AH (3.3a * 10 hours = 33 AH @ 12 volts).

So, you have a 12 volt @ 225 AH battery (pretty good size battery to haul around in a car)... Using 33 AH overnight:

- 33 AH / 225 AH = 0.098 = 9.8 % of battery bank capacity

That is not a lot of power, if you charged every day, then the absorb would be around 2 hours or a bit less.If you ran the genset every 2 days, then ~20% use of battery storage--Then something like a ~2-4 Hour absorb charge (plus 1-2 hours of bulk charging).

Unless you have some reason for a second battery string, I am not sure I would get one. That is a lot of weight and space.

You should also have a hydrometer (if flooded cell battery--Rinse the hydrometer after every use with distilled water to prevent the float from getting sticky in storage).

https://www.solar-electric.com/search/?q=hydrometer

And a DC Current Clamp DMM lets you check debug your DC power system (is it getting charging current, what is the battery voltage during charging/resting, etc.). This is a mid-price DC current clamp DMM (about $110). You can find DC current Clamp meters down below $50 if you look around.

https://www.amazon.com/gp/product/B019CY4FB4

There are other Battery monitor systems that can help you manage your battery bank... Prices are not cheap, and with a voltmeter (or DC clamp/dmm), you can keep a pretty good eye on your system.

One of the other reasons I would suggest a smaller battery bank, you can use a smaller charger and genset... A 10% rate of charge (25 amps) would should run fine on a Honda eu1000i inverter-generator. Much less fuel usage, much quieter than the large gensets, and much smaller (and should be more fuel efficient). Quiet enough that you could run the genset and power the lights/computer/etc. while charging your battery bank in the evening for a few hours before you hit the hay:

https://mayberrys.powerdealer.honda.com/products/generators/eu1000i

I will suggest you look at your choice of battery chargers... Here are a couple that are good quality--nice to very nice (just starting points for your search--Link is to the host of our forum, but you do not have to purchase from them):

https://www.solar-electric.com/cotek-cx1225-12-volt-25-amp-advance-battery-charger.html (cost less, but lots of features)

https://www.solar-electric.com/xantrex-804-1220-02-truecharge2-20-amp-battery-charger.html (very nice)

If you are tight for space/weight--A lithium Ion LiFePO4 type battery may be good for you. They charge quicker (basically no 2-4 hour absorb cycle) and are very light weight... Not cheap, and some other issues too (such as cannot charge below freezing, cannot power load if somewhat below freezing). Here is an example--You may want 100 AH or 50 AH (depending on your other loads):

https://www.solar-electric.com/residential/batteries-battery-storage/deep-cycle-batteries.html?manufacturer=547

Li Ion batteries are not a simple choice... Read about them and get more advice (here/other places). And you can find other Li Ion (LiFePO4 recommended as a start)--Many are direct shipment from China...

-Bill

8,762✭✭✭✭✭|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||

|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar

gen: http://tinyurl.com/LMR-Lister ,

6✭✭you are dead on with the amps and ah's for my cpap.

I'm going to get an amp clamp, multimeter and hydrometer too.

I want another battery for the compressor fridge I plan on getting, but after some more consideration and it's minimal amperage draw, I may opt for a 100ah battery, and not run the batteries in series/parallel.

Lithium Ion is too far out of my price range....as for the weight of the FLA batteries, I'm hoping to be able to get a hitch installed and buy a small 4x6 trailer to carry the 2 batteries, the generator, my hand tools and a few other small things, my car can safely pull 12 - 1400 lbs, so I figure after a few months, that will be the way to go, and I can enclose the inside of the trailer with a lid for when I start using solar too.

I've put about 2 years of thought an planning into this, and I'm almost ready to hit the road....I just needed to get the charging issue explained to me better, and you did that, so thank you

6✭✭I was wanting to do that too, I'm just leery about harming my alternator or car battery......if I should need to get the H*ll out of dodge in the middle of the night, i want to make sure my car will start right up without a problem