Battery System Design

Bad, bad, bad. A "car" battery is not suitable for use as a stationary battery. It's made for quick bursts of high current needed to power a starter motor and not the comparatively low continuous drain of a laptop. Even "deep discharge" batteries don't do well in stationary service.

What you want is some manner of "solar" or gel battery. For examples: Battery size depends on long you plan on running the laptop off the battery. (Hint: Target specs and other numbers are always useful). Fill in the blanks: and don't forget that lead-acid batteries don't age gracefully, and lose capacity when cold.

Ok, you want cheap. You're about to make another mistake.

Here's why you DON'T want a "car" charger: All of these blew up in various different ways while trying to float charge a pair of big 6v flooded lead-acid batteries on a mountain top radio site. I was only able to repair 2 of them because the others had epoxy slopped onto the PCB or had IC markings ground off. They were finally replaced by a proper Statpower/Xantrex charger: No dead batteries or blown chargers for about 6 years so far. (Note: The fuse is inside the box and difficult to replace. It will blow if you reverse the 12v leads. Have some spares handy).

However, none of this really matters because you're about to make yet another mistake. The LiIon battery in the laptop is going to die rather quickly. If your karma is lacking, it could short and catch fire. LiIon batteries do NOT like to sit at full charge and in a hot running environment, such as the insides of a laptop. Modern laptops even have a feature in the battery control app to limit charging to about 50% of full charge, to extend battery life. This is also why you don't find many UPS power backup systems running on LiIon batteries.

More: <

So, how does one do this? First, kindly disclose the maker and model number of your laptop. I don't like working in the dark.

Most laptops require a minimum of about 18VDC to charge, but will run the laptop on 13.6VDC nominal. I found one that runs it quite nicely at 11.0VDC (possibly a Dell D620). Test yours with an adjustable power supply. However, most laptops are NOT designed to operate in this manner. I managed to kill the charging circuit in my Dell something laptop during such a test. No warranty expressed or implied for following my advice. However, it works, you should be able to run the laptop directly from the 12v battery pile, without the LiIon battery and without the 19VDC charger.

Otherwise, go unto your favorite vendor and buy a computah made to run off of a wide range battery source, typically for automotive computahs. ATX/ITX based mini boxes are common. Most run off a PicoPSU power supply such as: Note that you want the version with the wide range input (blue connector), not the one labeled just 12v input (yellow connector).

Good luck.

You want an UPS. Buy an UPS: it will work way better and probably be cheaper than your "thing".

Bye Jack

Car batteries dont last well like this. Burglar alarm or leisure batteries would be better suited

NT

What pippo said. What you're describing a laptop plugged into a UPS, so go get a UPS.

If you want cheap, get a used UPS. If you want a long hang time, get a UPS that allows for external batteries and use lots of _appropriate_ batteries -- i.e., deep discharge batteries. They use that sort of UPS's in data centers, so if you're lucky you can get something surplus that lacks the bells and whistles that a data center manager may want, but that will work just dandy for you.

Agreed and I have the hole in my bank account to prove it. The problem is identifying a quality battery. I simply use the weight of the battery and calculate the battery energy capacity in watt-hrs/kg or Joules/kg. I have a chart (somewhere) of a bunch of UPS batteries that I made for a customer to convince him to buy decent batteries. Basically, heavy batteries have more lead and therefore can tolerate more normal abuse.

I'm a big fan of Trojan Batteries, but be prepared for sticker shock.

Overkill: Retired telco batteries. My guess(tm) is about 40 years old.

Hints:

1. Don't try to charge to 100.000% of capacity. You'll go over.
2. Make sure the charger has temperature compensation that tracks the battery, not the charger or room temperature.
3. Emergency disconnects on the 12V line are a good idea. I use overpriced Perko switches (available at marine supply stores).

Ummm... That inverter thing isn't going to charge the battery. The Statpower/Xantrex contrivance I recommended is a battery charger. What you suggest will work, but then the OP will have 3 boxes to deal with:

1. Battery charger.
2. 12vdc to 117vac inverter.
3. 117vac laptop power supply. My not so humble opinion is to find a computah power supply that is designed for the unspecified purpose that will run on battery power.

Also, I've a bit too many problems with the cheap 12VDC to 117VAC cigarette lighter inverters. They work, but not for long. On my list of repairs are usually (in order starting with most common).

1. Cooling fan craps out.
2. Cigarette lighter connector becomes intermittent or melts.
3. Internal overvoltage protection diode melts the 12v wires instead of blowing the fuse.
4. Overheated bulging and leaking electrolytics. Most of these problems could be easily solved, but not at the commodity price point.

Ummm... If this is the Intellipower that you're recommending, I would think Intellipower is more expensive. Also, I got a deal on the older model 10A Xantrex chargers when they switched to newer TrueCharge versions. Zero failures, except when abused.

One thing I like to consider is frequency. Is this something called into action once a year when the grid power goes off? Or maybe something that runs the batteries flat every day...like a solar installation might.

Then, there's "The laptop is to run all the time until battery depleted." What then? Does anything have to recover? How does a laptop behave if you bring the voltage up from zero to full over several hours?

Another thing to weigh is the cost of failure. If it's life support, you wanna make sure it never fails. If it's a minor inconvenience that may happen once a year, you can live with considerably less robustness...aka cost.

Also assumes you really need a 90W laptop and a 20W one (or a smartphone) won't do.

So far, your only stated parameter is "inexpensive". The devil is in the details. It's easy to put a lot of effort into solving 99% of the problem only to discover a fatal flaw in the plan.