Sizing a battery charger

Think this through before you start spending money...I didn't. I found a brand new 5KW generator at a garage sale for $100. I couldn't pass it up. Then I found a transfer switch for $50. Might as well get that too. Then I found some big wire at a garage sale for $20. oooh, need that too. Then the $15 each plugs to hook it together.

Then, I started thinking about how to do it. The electrical stuff was trivial. But the permit to install it cost more than the generator. And my service entrance doesn't meet current code, so there's risk that the inspector might require a major $$$$ change there if I modify it.

My major concern was to run the furnace air handler so that the pipes wouldn't freeze in winter. So, I'd just put a plug on the furnace electrical wire. Well...the electrical code forbids that. General consensus, verified by talking to the local inspector, suggests that the "rule" is generally ignored. But what is the fire insurance underwriter gonna say when my furnace juice doesn't meet code and the place burned down? His job is to DENY claims.

NONE OF THE PROBLEMS HAD ANYTHING TO DO WITH ELECTRONS. Bottom line...anybody wanna get this generator out of my garage???

A sensible approach is to put a pair of isolation diodes and an additional deep-cycle battery in your car or truck. Stick an inverter adjacent to the battery and you're done. You have a general purpose portable power source that can supply a LOT of power for a short time. And it's always ready. And it comes with a two-ton charging machine.

Another option is to buy a used UPS. The batteries are always dead, but the rest often works. Make sure it's a 12V system, not all are. Hook it up to a deep cycle RV battery and plug it in. They're not designed to run for a long time, so you'll probably need a fan on the heat sink.

What follows is a rant. You know where your "next" button is...

The hardest part of any project is deciding what you want to accomplish. It's easy to lump multiple disparate objectives together and poorly specify the desired result.

The power going out for a few days after a hurricane is a camping trip. If you're on life support, that's an emergency. Having a solar powered led infrastructure in your house is a hobby.

The definition of a hobby is spending WAY too much time and money on something that gives you pleasure, but the result is inconsequential. Lithium batteries and solar is a fine hobby. Just expect to spend way too much time and $$$$ on it. ;-)

If I were building a lithium standby system out of 18650 lithium cells, I'd start by figuring out how many peak amps I'd want and put that many cells in parallel. So, for 100W of 120VAC out of an inverter, I'd have at least 10 cells in parallel. If you want longer run time, put more cells in parallel. Then I'd figure out the voltage output I wanted. For a load that could tolerate 9V-12.6V, I'd series three. for 12-16.8V, series 4. Notice that these numbers exceed the requirements on one end or the other for most inverters. So, I'd go buy 40 brand new cells from the same batch from a reliable vendor and weld them up in sets of 10 with high-current plugs.

Remember that random 5 year old Chinese cells reclaimed from worn out laptop battery packs and sold on EBAY are not what you want. You want recently manufactured cells from a reliable source. Those are EXPENSIVE!! Fry's gets about $13 each for 'em. There's a reason a Tesla car costs a hundred grand.

I'd charge them to 3.8V, yes exactly the same voltage. The optimum voltage may be different. Point is you want

40% of so state of charge. You want the state of charge to be EQUAL. Then I'd stick them in the fridge at just above freezing. One school of thought holds that at near freezing the actual state of charge has less effect on cell life...YMMV.

When the hurricane watch goes up, take 'em out, warm 'em up and charge each pack to 4.2V...Not about 4.2V... LESS than or equal to

4.2000V. Notice that I didn't say plug them together and charge to 16.8V. I said make sure the state of charge is equal and you don't exceed 4.2V on any pack. Plug them together and run your system. Have a mechanism that disconnects the load when any cell pack gets below 3V or so. Don't rely on watching a meter. Fall asleep and you can snuff your batteries.

THESE NUMBERS WORK FOR SOME, BUT NOT ALL battery chemistry variants. ASK THE VENDOR FOR NUMBERS and use those.

There's not much solar energy available in the wake of a hurricane, so you don't need the solar part. IFF the cells are balanced and you don't go overboard, you can get away with a few charge cycles without monitoring every pack individually, but I'll deny saying that. There's typically a pressure disconnect in each cell. If you over-stress the system and one cell disconnects, that puts even more stress on the others and the dominoes start to fall. You have to balance the probability of some fault against the consequences of that fault and how much liability insurance you have.

Then I'd look at the plan and decide that it's clear that I should go buy a 12V lead-acid battery instead.

Garden lights make damn fine garden lights...leave them where they belong.

18650s make horrible laptop batteries, but they're the best we can afford. Leave them in laptops.

Are we having fun yet?

y

If you want to sell the generator, you might let people know where you live.

If I were you , I would keep the furnace hard wired so it is per code. But could you not have a terminal strip some where between the breaker and the furnace. Then in the event of a power failure, you can disconnect the furnace from the breaker and connect it to the generator. Not as quick as using a plug, but should be a five minute job to unwire it from the house and connect it sans transfer switch to the generator. Could be done without the terminal strip, but you do want it so it is super easy to change.

Dan

I was surprised too. Here is a shot of the battery next to a NICAD Energizer "AA":

Exact same dimensions, way different voltages!

Correction: NiMH - not NiCAD

They only burst into flames when one drives a silver spike through its heart.

I have one such application. I maintain a few mountain top radio sites and weather stations. A few don't have commercial power and run on solar and wind. I really don't relish the thought of dragging a charged battery up the mountain.

That depends on the fuel storage facilities. After Hurricane Katrina, most operations were powered by emergency generators. That worked just fine for about 7-10 days, when everyone ran out of gasoline. The systems running on solar plus batteries kept running (if the panels hadn't been trashed by the hurricane). However, those running no battery solar grid tied systems had a big problem because some (not all) require commercial power to operate.

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You're right, they're all over EBAY.

I consider myself to be a knowledgeable, observant and careful person. But it wouldn't be the first time I plugged the wrong thing into a compatible socket and blowed something up. I can imagine Miranda SixPack in a hurry to get that personal appliance back online. "You've got second degree burns on your what???"

Let me repeat that the typical voltages I mentioned apply to older lithium ion technology and do not apply to these.

Quick search suggests that LiFePO4 is even trickier to charge properly than lithium ion.

I have a pile of 18650s from old laptops and a battery tab welder. I have a computer controlled charge/discharge controller and have done a bunch of experiments to characterize their behavior.

I still haven't found any application where the benefit of using them outweighs the hassle of using them.

Still think you're betting on the wrong horse. Lead-Acid is safer/easier/cheaper for what you're trying to do.

Many years ago, we used to get 3V "Lithium" batteries in D cell packages.

They worked better than most batteries at low temperatures and had good shelf life.

I still have one in the freezer. It says: eternalcell LITHIUM SDX

Power Conversion, Inc

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Yeah, I think a gel-cell is the way to go.

It's called 14500 when it's a lithium cell, it's possibly 1% shorter than a nominal AA

if there's a special term for the 3-cell NiCd variant that's about the same size as an AA I don't know it.

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in what do you use those cells?

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G. Morgan wrote in news: snipped-for-privacy@Osama-is-dead.net:

He could get one of theose auto "jump-start" portapacks,that have the gel cell battery,emergency light,air pump,and power socket,with jumper cables,all in a nice neat package.

and maybe get a good,desulfating charger. Also a solar panel trickle charger.they don't cost much.

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Jim Yanik
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=20

hot.

The idea is decent, the thinking it all through needed some help. Any high capacity battery has some interesting and potentially dangerous attributes. Not the least of which how it responds to fault currents. = If there is no protection against excessive fault currents you have a recipe for fire or worse.

Different battery chemistries have different characteristics. Most batteries dislike to hate complete discharge, but NiCd thrives on it. = Few battery chemistries like float service, but most lead-acid likes it. Variations in chemistry and cell construction can have surprising = effects, know your cells well before building this.

?-)

Thanks for all the input guys. I will have to do more research before I build this thing now, you've convinced me of a fire hazard.

I may stay away from lithium's and just use a 7-10 Ah gel-cell. I'll find out how many little panels I can stack to keep a trickle with these cheap solar panels. It may turn out to be cheaper just to buy some bigger 5W panels instead of messing with dozens of small ones.

It was a headlamp intended for use when climbing/hiking in cold weather. Just one cell. They were expensive, but the long shelf life was worth it.

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I'm a hobbyist. I've been cobbling together useless crap for half a century. I was mostly using stuff I had on hand. If you're gonna buy stuff, you have more options. Keep your objective in mind. If cost has any bearing on the project, there is no cost effective solar solution. And solar is not portable. You have to find a location to mount it where it gets sun year round. If it fell off the window shelf and your battery discharged, you're SOL. Ditto if you mount it on the roof and the squirrels ate the wire. Once you screw it to the wall, you lose any portability advantage. And you'll get so little power in the aftermath of a hurricane, that you won't miss the connection.

If you're after emergency power, do you really want a system that has a battery and some wire and a fuse wrapped in duct tape hooked to some socket wrapped in duct tape into which you plug some adapter that gets a light working? It's all about the packaging. I'd want something with a handle that I could pick up with one hand, flashlight in the other.

I don't recommend this one, but it was easy to click the link:

I'd get one without the jumper cables but add a 120VAC inverter built-in. Without the inverter, they can be had for not much more than the cost of a new battery. And for light loads, you can get cheap inverters that plug into a cigarette lighter socket; still meeting the one-hand rule.

And you can charge 'em with something like this:

Really doesn't make any difference whether it's one watt or five watts. Neither will be of any real use in an emergency, and either can keep the battery topped off.

Cobbling together a bunch of solar cells removed from other stuff is ill-advised unless you already have the other stuff. The problem is not the circuit. The problem is getting them soldered together without breaking 'em. And making sure the flux doesn't corrode. And sealing them against weather using something that doesn't block the sun today or turn yellow and block the sun next year. And keeping the water out so they don't corrode and break the connection. And finding a way to limit the voltage and current. You want to be able to have some charge in the winter without cooking the batteries in summer.

A wall wart solves all those problems. And many more not mentioned.

Designing an electrical system like this is trivial. The devil is in the details: short circuit protection, connectors, charger, mechanical details of the package...the one-hand rule.

FWIW, if you believe the graphs published on the battery sites, you can get 4-ish amp hours out of a 7AH battery at a one-hour rate. Your car battery can supply this amount of energy without breaking a sweat...and a LOT more in short bursts. The system weighs a ton, but it's on wheels ;-)

Looking back on all my hair-brained projects over the years, I wish I had simplified my objectives, bought instead of built stuff and spent more quality time with the (ex)wife.