Inverters

[mike killay]

Wanting to charge my battery from the campervan battery, I have looked at inverters.
However my charger says input= 240 volts at 1.8 amps i.e. =432 watts
The output though is 42 volts at 2 amps= 84 watts.
Does this mean that the efficiency of the charger is only 19.4%
OR, have I got this wrong? Do I really need a 500 watt inverter?

 

[danielrlee]

Have you looked at RC chargers, since they run straight from a 12v supply?

 

[BAH48]

My Bosch charger :-
230v 1.5A .... 36v 4A

With power losses, a 500 watt inverter would be about right. Some might argue that it's on the small side, better to have some reserve capacity.

 

[trex]

your charger outputs at 42V, 1.8A * 42V = 75.6W
Assuming that up to 20% of the input energy is wasted in heat, the inverter needs to supply at most 95W.
A 250W inverter costs £10 on ebay that plugs into the cigarette lighter socket would do.

 

[BAH48]

It's the input amperage into the charger that is important, a 250 watt inverter will be overloaded. I use a 1000 watt inverter to charge my batteries, bigger than it has to be - obviously, but it works reliably.
An inverter for £10 - don't waste your money, buy a quality item that will last.

 

[mike killay]

But 1,000watts at 12 volts is 83 amps.
What sort of 12 volt battery do you have?

 

[SRS]

I have a 500w and can run two bosch chargers together happily.

You will find that if you measured the physical current taken on the 240v side, it will be less than the data on the label. In fact if you do that whilst charging a flat battery you'll have you answer.W= I x V

 

[BAH48]

I don't use all 1000 watts - why would I, I'm only charging 1 battery. The point is with electronic components it's better not to stress them too much if you want them to last. A 500 watt inverter will not have to work so hard. But go ahead, buy a cheap 250 watt inverter - see how long it lasts. My advice is always build in some redundancy.

 

[oigoi]

I don't use all 1000 watts - why would I, I'm only charging 1 battery. The point is with electronic components it's better not to stress them too much if you want them to last. A 500 watt inverter will not have to work so hard. But go ahead, buy a cheap 250 watt inverter - see how long it lasts. My advice is always build in some redundancy.

I agree it is better to have a bit of spare capacity in the inverter. Not just to reduce the stress it is under, but also to allow for the sudden momentary inrush of current that occurs when you switch the charger on.

 

[BAH48]

Common sense really. It's not rocket science.

 

[mike killay]

Everybody seems to be missing my point,
Are our chargers that inefficient that they lose 80% of the electricity between input and output, or have I got it wrong?
BAH48 says that his Bosch charger requires 230 volts at 1.5 amps, i.e.345 watts.
But SRS tells us that he runs two such chargers on a 500watt inverter.
Something does not add up.
I am quite happy to buy a 1000 watt inverter if it is truly necessary, but would then worry about the c rating of the LB.

 

[oigoi]

Mike your charger won't be 100 percent efficient but it won't be as inefficient as the input amperage would suggest. The 1.8a might be a surge figure when initially switched on. In my experiences charging from an inverter and lead acid batteries there is approximately 30 percent loss through the inverter and charger i.e around 130w being drawn from the lead acid battery is putting about 90w into the ebike battery. Best way to find out the efficiency of yours is measure it

 

[BAH48]

The rating on the charger is a maximum and as the battery charges up the current will reduce. The inverter will also supply an overload current, but not continuously. This is why SRS can charge 2 batteries, but as an electrician I could not condone the practice. If a customer asked me to supply an inverter I would have to make sure the nominal output of the inverter was equal to or more than input of the equipment. Then if a problem arose I could not be deemed negligent.
The closest inverter for your charger is 500 watts. That is what I would supply if you were a customer.
My 1000 watt inverter is far to big, but I had it in my garage so I might as well use it.

 

[mike killay]

Thanks, got it now
Mike

 

[SRS]

For interest, I physically measured the current consumption whilst charging when I arrived at work this morning.

Bosch Gen1 Classic charger and 400Wh battery.

The battery was down to 1 led bar and the mains measured at 242v.
Current drawn at mains voltage was 1.1 amps.

266W, I guess that this rate may vary a little with mains voltage and possibly temperature but does explain why I can runs two chargers off a 500W (nominal) inverter.
The instruction books states that my inverter will shut down at 600W.

A sensible inverter as stated in an earlier post would be 500w for a single charger, this giving the invertor an easy time of it. 350W ought to be the absolute minimum.

 

[trex]

the Bosch charger can do fast charging. Mike has a 42V 1.8A charger, it can't use more than 95W.

 

[SRS]

the Bosch charger can do fast charging. Mike has a 42V 1.8A charger, it can't use more than 95W.

Forget the output rating and efficiency.
Its output may be rated at 95W but it is the input current that matters.

The classic charger output is rated at 144W, a lot less than the requried invertor rating.

If you wish to play it safe, take the input (label) rating as worst case scenario, add 100-150 watt to the rating and you'll have a capable setup.

 

[trex]

that's what I saw on a watt meter that the woosh charger (mains plug) was plugged into.
when the battery was very flat, the reading was about 90-93W, dropping down to about 80W in the main, then down to about 7W near finishing. 2-3W when the LED went green.

 

[BAH48]

Why would the charger be rated at 1.8A if all it takes is 0.4A ?
Assuming that your figures are accurate, then either the charger is faulty or the rating is incorrect. You still need to match output to rated input as a general rule.
.

 

[trex]

these Lithium-ion chargers are switched mode power supply. The input current is smaller because the input voltage is higher.
http://en.wikipedia.org/wiki/Switched-mode_power_supply