Sizing a battery charger

The form of your question suggests that you're interested in more than just car batteries. Each battery technology and sub-technology has specific requirements. Best source of info is the vendor for the EXACT battery you're charging. Even car batteries have sub-technology differences.

Remember that most battery chargers are used infrequently and "not even close" is close enough to get the desired result...your car started.

Google will give you temperature curves and voltage/current charge profiles. Typically, more than one current and voltage is used to determine the charge profile.

And you might make tradeoffs...like for solar when you have a use it or loose it charge source.

Maintenance during prolonged storage is a subject of much disagreement. I subscribe to the theory that some amount of higher current cycling is preferred over continuous trickle.

There's lots of snake-oil out there for batteries. A solar energy group might be a place to get guidance.

Lots depends on exactly what you're trying to accomplish and how much you're willing to spend to get that extra bit of performance... depending on your definition of "performance".

There is no one answer - it depends significantly on the battery chemistry and slightly on ambient temperature as well if you want to be precise. Lead acid is fairly robust. Keeping a car battery alive just requires replacing the losses due to self discharge on average and not letting it ever get completely flat. Try:

For a quick intro at the better than an unregulated current source charging profile.

Regards, Martin Brown

You are very perceptive. I am thinking about keeping 3.7V rechargeable lithium's charged. I disassembled some solar landscaping lights and isolated the charging circuit (a 3v rechargeable lithium was in there).

Open circuit on the solar panel is about 4.5V, once it goes through the regulator I get about 4V.

I'd like to buy some more panels and put them in series to get 12V, thus keeping a 12v high-current source available if the lights go out (I live in hurricane alley). I can simply plug the 12V into an inverter for

110V AC, or leave posts sticking out where I can pick off 3V DC for a 22/2 cable that I can run through the house, adding 3V LED lamps where needed.

I do have a gasoline generator, but solar for LED lighting is a no-brainier in that situation.

G. Morgan schrieb:

Hello,

running a 3 V DC cable through the house is not a good idea, you will loose too much voltage over the long cable.

Bye

Thanks Martin,

The batteries will be lithium. I'd like to put enough of the 3.7V ones in a series/parallel circuit to get 10-15AH or so. It will be able to provide 3V DC and 12V DC in case I have to use an inverter.

Will high temps (110°+) make this much harder to do? I would like to keep the batteries in the garage to keep the wire run as short as possible to the panels.

I'll compensate for voltage drop in the copper.

I'll compensate for voltage drop in the copper.

Thanks Martin,

The batteries will be lithium. I'd like to put enough of the 3.7V ones in a series/parallel circuit to get 10-15AH or so. It will be able to provide 3V DC and 12V DC in case I have to use an inverter.

Will high temps (110°+) make this much harder to do? I would like to keep the batteries in the garage to keep the wire run as short as possible to the panels.

There are power fail LED lamps that have a battery storage in them and turn on when the power in the plug they are in halts.

Buy some flameless candles.

That is what the higher voltage is for.

Depends whether you want to burn your garage down or not. Lithium battery recharging is about the most volatile battery chemistry that you can imagine. It is not for nothing that portable PC batteries have anti self imolation disable circuits in them to protect from adverse conditions. Even then some units have notoriously caught fire.

Lithium cells are most unforgiving and potentially very nasty if you don't do *everything* *EXACTLY* right. Unless you are expert in safe charging techniques do not even think of reusing lithium cells.

Lead acid is relatively easy provided that you don't accidentally short out the terminals and follow the charging rules more or less.

Low voltage fuses (or cutouts) in any high capacity battery circuit are essential to avoid fires. A modest high capacity battery will source a hundred amps or more for plenty long enough to make copper wire red hot.

Regards, Martin Brown

Depends on a lot of variables, but I'd be surprised if you could get more than 30mA out of a yard light solar cell. And the batteries are going to be very low capacity. I buy my solar yard lights from the dollar store. They've been thru three levels of profit and transshipped several times from China and they still sell for a buck. Can't be very good solar or lithium cells in there.

available if the lights go out (I live

Executive Summary: Give it up!!! That really is a no-brainer.

Lithium batteries are a POOR choice for what you want to do. High cost, low capacity. Solar is a POOR choice for what you want to do. You don't get much sunlight in the period around a hurricane.

Depends on chemistry, and you probably can't find out what the exact chemistry is used in the lights...but for the types of cells used in older laptop computers, you MUST keep the battery voltage below 4.2V and above 3ishV. In series, they must be balanced. It's a battery management nightmare for no real benefit. Battery management failure can cost you your house in a ball of fire.

So, go buy a deep cycle lead-acid battery and stick it in the garage. Put it in a box with a vent to the outside so your furnace doesn't ignite the hydrogen it gives off under overcharge.

Decide on a charging regime...I suggest a 1-2A trickle charger with good voltage control and hit it for an hour every once and a while. Numbers to be determined by the exact battery and charger. Run your inverter or leds off the 12V. Stick about 3 leds in series and use a resistor to get the current you want at 10-14.2V range of battery voltage.

Use the generator to charge the battery for prolonged outages.

For general lighting, use LED flashlights/lanterns. $20 at Costco will get you a lantern that runs off 8 D-cells and claims 100 hours run time...and it's almost guaranteed to light up when you push the switch after a few years sitting there in readiness.

Keep it simple.

I was gonna suggest a more robust permanent system if you're serious about solar, but the hurricane is gonna blow the panels off the roof anyway...or put a tree thru 'em.

I lived on the Gulf Coast for 18 years. I know a little about trying to keep outside stuff from being destroyed by a hurricane...you don't!!

That's what I'm playing with. $2 ones I saw at Big Lots that are 1.5V, plus some bigger 3V landscaping lights I found somewhere and just held on to them. Hell, I didn't even know they made 3.2V lithiums in "AA" size until I took the two big ones apart.

The goal is to build a portable solar box with really cheap parts that are already in these things. I figure if I stack enough of them together I could maintain a pretty stout power supply.

With the (3V LED) lamps removed I can modify them a little to easily clamp to a bus, alligator clips maybe. Then for the bus I grab a box of

22/2 and run it where lights are needed for 3V.

I'll start small, just a box that lights up the LED's - but I would like to eventually build a battery bank of some sort (auto batteries?) to handle 12V high-current to power multiple inverters along the bus. Even if I use 18/4 the voltage drop will be negligible, the whole cable length would be less than 200' in my estimate.

That was an idea for a quick portable system that can be deployed in 15 minutes. I have some 3000 mA 18650's that can deliver a lot of power. I just don't know how to charge them using solar correctly.

Great idea. That is a second project. May even include a transfer switch to a circuit or two.

These 3000 ma 18650's could do some damage, I agree. I just liked the idea because they pack so much power into a small container. I thought if I can keep them charged with cheap solar cells I'd always have a high-capacity 12v source.

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

here's what I did during the 2004 aftermath of Hurricane Charlie; I had a portable "stick" fluorescent that ran on 8 AA cells,but also had a jack for a 12V wallwart. I had made a cable to connect my 12v 20AH scooter battery to the light,or to a pair of quiet 12v fans salvaged from old PC power supplies. The fans made it bearable to sleep at night. I also had a fluorescent "camping lantern" that ran on 4 D cells. after the 12V gel cell died from old age,I made up a 8 D-cell battery holder that can connect to either the light or fan assembly. alkalines have a pretty good shelf life.

If you don't cycle the SLA batteries,they sulfate and die. "use or lose".

--
Jim Yanik
jyanik
at
localnet
dot com

Lithium or Lithium-Ion? If lithium, they're not rechargeable. If Li-Ion, there are various chemistries that require slightly different magic formulas for safe charging.

However, I think you had better reconsider your objective. Li-Ion batteries are very different from other rechargeable batteries. Li-Ion hates being fully charged and hates being overheated. A hot Li-Ion battery, maintained at 100% charge, is going to have a rather short lifetime:

I've found that many laptops now charge to somewhat less than 100% of charge. Laptops are the worst case, with a nice hot environment and

100% charge to insure that the replacement battery industry will be a growth market. The short lifetime at 100% charge is also the reason you don't see any standby power supplies (SPS/UPS) with Li-Ion batteries.

That won't do much for what I'll guess(tm) is your backup power system. I pile of 18650 3A-hr cells might supply 10-15AH, but for a limited amount of time. Let's play with the numbers:

To get 12V at 15A-hr, you'll need more than the obvious 20 cells. That would work if you wanted to discharge it exactly once, because it would run the cells all the way to total discharge. You can do that if you want, but if you plan to preserve your investment in batteries, using only about 20% of the battery capacity would be better. That means you need: 5 * 4 * 5 = 100 cells where 5 = number of 18650 cells to get to 15A-hr 4 = number of 18650 cells to get 12VDC 5 = only 20% discharge allowed. You might consider buying a spot welder to assemble the 100 batteries.

I'll assume 110 Fahrenheit. No, it will make it impossible to do. High temperatures and Li-Ion batteries do not mix. Read on what the hybrid automobile makers are putting into their battery packs. There's quite a bit involved in the cooling. You won't have as bad a heating problem, but you'll still need to remove the heat somehow.

What panels? Electric breaker panels? Solar panels? Using a solar panel to "top off" a standby battery is a waste of money. If you size the panel big enough to recharge the battery in 24 hrs, you'll have an expensive panel on the roof doing nothing most of the time. If you make it small enough to be economical, it will take days to recover from using your standby system. If you want emergency power, use an AC line powered charger to charge you backup system for short term outages. For anything more than a few hours, get a gasoline, propane, or diesel generator.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

And are inclined to protest by self immolation if provoked.

I have yet to find an application where a solar panel can beat physically moving a larger lead acid battery to and from mains power for recharging.

The one where unexpectedly a freebie that I got at a trade show came in useful was one of those solar powered mobile phone rechargers. Our local mains got taken out by a major thunderstorm as did the East Coast mainline train system and this little gadget allowed me to have a working mobile phone despite the thing deciding to play dead soon after.

Nothing beats a generator if you are off grid for any length of time.

Regards, Martin Brown

I can't argue with your voltmeter, but I do have some observations... Rechargeable lithiums in AA size are a product liability disaster waiting to happen. Joe sixpack is gonna stick 'em in every AA socket he can find.

The two-cell yard lights from 5 years ago contained two AA NiCd batteries. The current batch of $1 yard lights has a single AAA NiCd with a QX5252 boost converter to run the LED. The spec sheet is extremely terse, but there seems to be no mechanism to control the charge current or voltage. At least, there's no spec for it. You get whatever the solar cell puts out.

Lithium batteries are a poor match in cost and functionality for a yard light.

Certainly possible, except for the cost/value/stout ratio. But my experiments suggest that it's unlikely.

It's certainly gonna vary with the design, but remember what they were designed to do. In the ones I've dissected, if there's light, there's zero output, none, not any. It's in charge mode. If there's no light, the up-converter runs and stuffs pulses of CURRENT into the LED load. I spent some effort trying to figure out how to make a

5V supply out of them for a solar powered microcontroller. I gave up.

You can buy 'em at any auto parts house or any department store or most anywhere. 12V lead acid battery with optional lights and optional

120VAC. Biggest bang for the buck.

This is a bad idea. You want 12VDC>>120VAC inverters as close to the battery as you can get 'em. Even

If you mean 18 gauge wire, 200 feet of it running 20Amps into an inverter is not neglibible...it's a FIRE.