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I'm fairly interested in electronics but not very educated in it.

I was wondering how I would go about safely converting your average li ion battery pack to a usuable voltage (240v ac) for things like bulbs and such.

I've so far got multiple packs I charge them using 5v usb chips of various kinds and run them all in series. I have a 5v solar panel which in the summer can trickle them and ive created a charging bridge from 12v dc to the 5 v dc and thus the batterys themselves and a small solar panel trickle charge (unfortunatly right now the battery is not deep cycle I know thats bad)

if you were to wire them in paralel would the voltage step up like with conventional batterys?

I was hoping I could run a 5v transformer in reverse ? but have yet to figure out the maths on how many amps I would have. I know I would need 2 diodes to make a rectifying bridge and then hoping to put it threw a 5v to 240v transformer

this is all just a spiral off my hydro / sterling engine 5v generator to battery bank for off grid basic gadgetry.

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    \$\begingroup\$ A rectivying bridge is for AC -> DC not for DC -> AC \$\endgroup\$ – PlasmaHH Mar 15 '15 at 21:02
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    \$\begingroup\$ Consider preservation of power and when improvising an inverter like this, you should probably be very happy with 50% efficiency. Assume your battery bank can deliver 1A at 5V, then consumed power will be 5V × 1A = 5W. At an efficiency of 50% for the converter, this would be 5W × 50% = 2.5W output power. At 240V this would mean 2.5W ÷ 240V = 10mA. If the capacity of the battery is 2000mAh, that would mean you drain the battery in 2000mAh ÷ 1000mA = 2h, but if you check the datasheet of the battery you'll probably come to the conclusion that the realisticly usable capacity is considerably lower \$\endgroup\$ – jippie Mar 15 '15 at 21:09
  • \$\begingroup\$ FYI, to increase voltage, you wire batteries in series, not parallel. \$\endgroup\$ – DoxyLover Mar 15 '15 at 21:34
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    \$\begingroup\$ If you want to light things from Li battery packs you're much better off looking at LED lighting than standard light bulbs. \$\endgroup\$ – pjc50 Mar 15 '15 at 23:47
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The device, or circuit, you are looking for is called an inverter. You are right that it has a transformer connected backwards, but it also has a lot more besides. A block diagram of a typical inverter system might look something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

The H-Bridge is used by the controller system to generate an alternating current (AC) waveform. The transformer then steps the waveform up to the target voltage.

It's not a nice clean sine wave like you'd get from the mains, but instead what is called a "Modified Sine" wave (green line in this graph):

                                            enter image description here

Due to limitations in the way the H-bridge works there has to be a gap (or dead zone) between driving in one direction and driving in the other. This gives the waveform its characteristic three-level appearance.

Inverters are not a very efficient system, since a proportion of the power is wasted as heat and even noise (they usually hum). It is far better to find a low voltage solution to your problem. Most appliances nowadays (excluding normal white goods) actually run on low voltages, and contain a power supply to reduce the voltage down to the typical 12V/5V/3.3V that you find everywhere now. A much better solution is to find products that allow you to bypass that power supply and provide the needed voltages (or a subset of them - typically 12V is used) directly to the device.

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You are probably best to consider wiring your batteries in series to get 12 volts. Then buy an 12 volt inverter (very common) that will connect to your battery pack and convert it to 240 volts AC. The inverter will be built for lead acid batteries which run a little different voltage than a 12 volt lithium battery does, but you can improvise.

From there you can investigate what the inverter is doing and consider making your own that works with 5 volt (or 4.2 volt) lithium battery packs.

Also, when a lithium ion battery is full, stop the charging. You mention trickle charging with the solar panel which usual means continuing to charge a battery slowly and continuing to charge it indefinitely, even when it is full. A good way to blow up a lithium ion battery.

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