Watts to Ampere Hours

Clue 1: how much energy is there in a 42VDC, 20.8Ah battery?

Clue 2: how long would need to be allowed to charge it?

Clue 3: what is the first divided by the second?

Rhetorical question 1: what on earth gets taught in CGSE Physics classes these days?

1. 873.6 Watts

2. (10% to play safe over) 10 hours or longer?

3. 2.01923077 should be perhaps Ohms, for what it serves the purpose in his question?

What can somewhat forget after CGSE classes? I even do not know what CGSE means.

Sorry

1. 873.6 Watts for 1 hour

1a. 87.36 Watts for 10 hours

1b. 8.736 Watts for 100 hours

theoretical.

PIE

Power (watts) = Volts x Amperes

How about an efficiency factor in there also when using an inverter? Maybe yes? Maybe no? Maybe maybe?

Good Luck, Old Salt

good call !

Definitely a factor to consider if designing and building a charging system from scratch.

If purchasing an already assembled unit with a predetermined efficiency rating the formulae would be accurate to identify the batteries requirements then you would simply match the numbers to the charger/inverter OEM specifications. Yes?

Why do you want to know? What will you do with this fundamental knowledge? Why do you not trust your own ability to look things up? Do you know anything about rechargeable battery chemistry? What will you do with a formula that uses symbols you do not understand? Where's Waldo? Why do people expect they can use an answer without comprehension? How many inane questions can there be in one reply?

If you are interested in it, I have found and used a good site for basic to intermediate level battery information;

http://batteryuniversity.com/

It is good for what it is but will never make anyone the guru of batteries and battery chemistry. I often use it as a refresher when things can get more confusing than what the memory can keep straight. Won't put you asleep but won't win an academy award.

Try it, it may be of interest.

Good Luck, Old Salt

You Sir are a gentlemaqn and a scholar ! many thanks, lee

This appears to be a small motor cycle battery or similar, it can run at best a very tiny inverter....tiny, tiny, only good enough to run say a mobile phone charger or similar....and not for very long either....

A 100 watt inverter will take almost 10 amps continuously......such a battery will soon collapse.....discharging for any length of time below 12.6 volts will eventually damage the battery.....

Is that your intention?

I have always planned such things usually more with the final load in mind, not the other way round....

No information about final voltage, frequency / output power needed either....

Lots of inverter styles are either efficient but with a "square" sine wave (not suitable for sensitive electronics), or have a good well formed sine wave but are not efficient...seldom do you see one that achieves both...unless you believe the advertising hype....

Is this a homework question?

This is for a small robot used by the Military. I am trying to size the inverter for the charger for the 42.5 VDC @ 20.8 Ampere Hours battery

Something is very strange here. An inverter changes DC to some form of AC. A charger does the opposite, with some form of control to avoid overcharging.

Are you charging the robot battery from some other (larger, different voltage) battery?

I suspect a language translation problem here.

out in the field AC is not available, therefore usually use the Humvee battery which is DC, then use a DC to DC converter to the actual charger for the robot battery to be charged....I am trying to size the DC to Dc converter.....

thanks

OK, now that makes more sense.

The next thing to know is how rapidly you want to charge the battery. The faster you try to charge it, the more power will be required. Unless you have a charger designed for the specific battery chemistry of the robot battery, you are in danger of overcharging and destroying the battery.

If you already have the charger on hand, then it should specify required input power, voltage, waveshape (square wave should be OK for most). Just specify an inverter with a rating 10-50% higher than the input power required by the charger, and you should be good.

A typical car battery (12V 40 AH) has about 1.8 million Joules of energy. I have no idea how much your Humvee battery contains but your fully charged robot battery contains about 3.1 million Joules of energy. Before including the energy lost from cabling, step up conversion, battery heating and a plethora of other losses, you cannot fully charge your robot battery with just your Humvee battery. You might be able to use the alternator of your running Humvee to charge these batteries. However, battery chemistry of this robot battery will limit how quickly and efficiently this battery can be safely recharged. Then there's a little uncertainty in the power output of your Humvee alternator.

If charging this robot battery with a Humvee was my engineering task I'd just add a second alternator to the engine with an appropriate output voltage, power and voltage regulator for charging this higher voltage battery.

great Idea !

Lee

While looking into the second alternator for installation, look at a single alternator to take both the truck and the robot load. It might be not be feasible to locate the extra space for the alternator, belting or wiring

Alternators up to and exceeding 300amps at 12 volts are available readily. I'm sure there are also higher voltage alternators since many trucks and big equipment, especially off-road construction, have 24 and 48 volt systems. Check with a large truck parts outfit or contact high output alternator suppliers/manufacturers.

Try: http://www.elreg.com/high-output-alternators.html

http://www.delcoremy.com/Home.aspx

Good Luck, Old Salt