How do I charge a small battery with a large battery?

Aro2220

If I had a big car battery and wanted to charge a smaller battery with it (say a laptop), how would I do that without breaking the laptop?

First of all, a battery has a certain voltage and DC current which is the same thing a laptop battery uses, but you'd have to ditch the AC adapter because that has a transformer on it for converting AC to DC.

I know that I=V/R but how can I charge one battery with the other battery (assuming they are rechargeable in the first place). If I connect the leads directly and one is a much bigger battery won't that damage the smaller battery? Is there a way to reduce BOTH the incomming voltage and current so that it isn't harmful to the battery?

How would you do this? A capacitor?

rcgldr

You could use an inverter to convert the car battery DC to AC. Assuming your laptop battery is a LiPo battery, those can catch fire or burst open if improperly charged. Radio control models use special chargers for LiPo and other types of rechargable batteries to avoid any issues.

Aro2220

I don't think it would be very efficient to do it that way. First of all, if we assume the power input is going to be renewable, likely solar, then efficient is key. Plus, the computers are going to be very low voltage, probably not even a laptop but an ultra power pc for some home functions running linux or something.

If you convert from DC to AC then from AC back to DC I think that's inefficient, no? I suppose it depends on the quality of those parts.

You would need an inverter then a transformer. It would be better if I could use a capacitor or something to control the voltage/current. I'm just not sure how.

rcgldr

My concern was safety not efficiency. Many of those radio control model chargers use a 12 volt DC source, so that would work if you know the parameters of the laptop battery. Those chargers are fairly complex, in order to safely charge LiPo batteries.

mrspeedybob

You may be able to get an adapter designed to charge your laptop from a 12 volt power outlet in a car. If you can then that would likely be your best option. If not then the inverter will be your best option. What you want to do is not the job of a simple capacitor or resistor, you are talking about a fairly complex piece of equipment. It will be cheaper to buy an extra solar panel to compensate for the inefficiency then it will be to buy all the hardware needed to build something yourself (and a half a dozen laptop batteries that you will screw up trying) ;-)

Aro2220

I don't understand what good it would do buying another solar panel. Without some kind of battery to level out the variable power input (solar energy is not a constant) you're either not going to have enough voltage/current or you'll have too much. Besides, most laptop chargers use weird voltage/currents that are not easy to get to with typical cells (for example, how many 2v cells would it take to get exactly 8.3V?)

I'm trying to figure out what is required to transform DC energy from one voltage/current to another. I'm not going to learn anything if I go to bestbuy and buy a car adapter for my laptop and plug that in.

If the voltage was alright would the current be fine? Would the laptop just pull as much current as it needs as long as the voltage isn't so high it blows everything up? Or do you need to really control the current too?

I'm confused.

Anyway, the reason I was wondering about this is because I stumbled across ultra-low energy pc designs using really tiny ITX motherboards and atom-like processors and stuff. One desktop used 4 watts of energy and I saw another one using 2. I was figuring that with an ultra low energy PC, Linux and some programming ability you should be able to build a green smarthome. Or really you could do a whole bunch of neat things where having a portable computer that has infinite energy would be useful.

I just want to understand how things work. I really think it would be handy to be able to wire up solar panels to a battery system and have nice clean power that works with sensitive electronics.

rcgldr

Current is an issue. The charger needs to be "smart" enough to limit the amount of current in a discharged battery, which will have lower voltage than a fully charged battery, so that would be an issue with a near constant voltage source.

Is there an option with the lap top to use a dc voltage source for power (this would mean that there is some type of logic inside the lap top to contol battery charging)?

meBigGuy

Generally you build/buy a power supply that converts the car battery to the proper voltage for the device you want to operate. If you create the right voltage, and are capable of supplying the required current, then the system will work. This assumes you are powering a device which contains a battery charger to manage the charge currents and profiles. (check out wikipedia's article on charging lithium ion batteries)

Power supplies take many forms, and battery chargers also take many forms. Depending on what you want to power, and how efficient you need to be there will be different answers. But one thing is for certain: Actually charging batteries without damage is never simple (unless you are an engineer with experience). Building power converters (DC to DC or whatever) is generally complex also.

Centauri1st

I'm just a student, but a really simple method would be build a circuit where

1) Limit current to about 10% of the smaller batteries amp-hour current with a current limiting circuit. You can google current limiting circuits and figure out which is best for you as they can go from lossy to fancy and efficient. you could just slap a resistor in there and rate it with ohms law, or you could use an LM317. Or fancier switching circuits like the lm2937.

2) Connect that to a voltage limiting circuit to limit voltage to around 1.5X battery voltage. so a 6 volt battery would be voltage limited to 9 volts if your charging off a 12 volt battery. The voltage needs to be less than the "gasification voltage" of your smaller battery. Again, if you just use a voltage regulator like an lm317 its burns off the extra voltage as heat so your conversion efficiency will suck.

Now that your current and voltage are limited the smaller battery cannot be overcharged.

NOTE: this is a 10% trickle charge, so it will take 10 hours to recharge the small battery. any higher and you risk damaging the smaller battery. If you want to charge faster or a 90% efficient circuit, you will have to invest in a battery management IC like a MAX 700 series or LM2576.

I hope that helps get you on the right track. Study hard!

Cheers