# How do we charge a small 12V battery from a large 12V battery bank

In a youth organization that I am part of we are building a traveling stand alone system, which will be put up on various fairs and music festivals. On one of the festivals there is a wish that we can charge 12V car batteries for people, that they can in turn use for playing music in their camps.

We will have a 1050Ah 12V battery bank. It is actually much bigger than what we wanted, but we got it extremely cheap from a bankrupt company.

We will have smth like 1500W solar panels.

How should we go about charging other peoples batteries?

I guess I should look for a 12V to 12V charge controller? But does that even exist?

I know that there are 12V to 12V solar chargers, but I am not sure how they work from just one battery to another. I guess a 12V battery to 12V battery should be able to lift the voltage.

To charge a lead acid battery you need more than 12V. Any charger would be a combination of a DC-DC boost converter and some circuitry to control the charge current.

Probably the easiest off-the-shelf solution which you could piece together in a hardware store would be a DC to AC inverter coupled with any standard lead acid charger. It wouldn't be as efficient as a DC-DC converter charger, but would be a lot less specialised. There can be issues with cheap modified square wave inverters and some devices - I'm not sure if that would be an issue here.

There is devices which will do what you want. This company seem to have a whole range of them. (I've never had any dealings with them, they were just the first result when I googled "DC DC lead acid charger".

Finally, you could build something yourself. DC DC boost converters are fairly ubiquitous and a lead acid battery charging IC like the uc3906 is available in a DIP form factor and has a fairly low part count requirement. It would require some design work and knowledge of electronics though, so if you're looking for something as simple and rugged as possible this isn't the ideal approach.

• An inverter would definitely be the easiest way to go. They are plentiful enough that you can run down to Walmart and pick one up. And you get the flexibility that you can charge/power anything that takes normal household AC, which may come in handy for charging cellphones, flashlight batteries, radios, etc. – Grant Apr 24 '14 at 16:16

I do 12V battery to 120V inverter ops for outdoor festivals all the time. You really ought to use a pure sine-wave inverter to power up chargers, especially for power tools, because modified sine-wave inverters (most all the cheap ones) produce a waveform that heats up the transformers in the chargers so that power is wasted and they may even burn out.

There are inexpensive pure sine-wave inverters available these days, so you won't have to pay an arm and a leg for a medium-power one that's suitable for this application. But if you're planning to use and inverter to power any mechanical appliances (those that have motors), you'll need to buy an inverter with a continuous power rating about 10 times the rating of your appliance because they draw huge surge currents when the motor first starts up.

The easiest solution might be in a few steps, if you have any money left.

1) Buy a charge controller for the 1500 watt solar panels so that they can re-charge the large lead acid battery that you have. I hope that the solar panel voltage matches that battery voltage and then you can just connect it to the charge controller. You can get a reliable charge controller off of e-bay for $10-20. While you are charging peoples lead acid batteries, the solar panels are keeping your batteries nicely charged. It is always a good idea to keep your lead acid batteries as fully charged as you can. 2) Buy the biggest wattage 12 volt AC inverter that you can afford (wihtin reason that is not too big for your battery bank). You attach the inverter to the huge 12 volt battery bank that you have and then you can plug in any devices to the inverter and charge your devices including ipods, lapops and other batteries. 3) Buy a 12 volt lead acid battery charger that you would also plug into the inverter and use it to charge the various smaller lead acid batteries that you want to charge. You can find these at any automotive store or in the automotive department of target or Walmart. 4) Have fun. If I were you, I would not make my life to complicated , and I would juts buy any charging module that includes a Dc To DC buck converter from Ebay or Aliexpress. Like the one from the link below. This one is less than$5, including shipping. I have use many of these kind of modules with great results.

• You may want to edit your answer and use the link button – Voltage Spike Mar 9 '16 at 23:32