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# Sizing a battery charger

G

#### G. Morgan

Jan 1, 1970
0
What is a good formula for the needed minimum current (and over voltage)
for charging batteries? Use a 12V car battery for example, how much
'trickle charge' is needed to keep them alive when not being used?

Thanks.

M

#### mike

Jan 1, 1970
0
What is a good formula for the needed minimum current (and over voltage)
for charging batteries? Use a 12V car battery for example, how much
'trickle charge' is needed to keep them alive when not being used?

Thanks.
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".

M

#### Martin Brown

Jan 1, 1970
0
What is a good formula for the needed minimum current (and over voltage)
for charging batteries? Use a 12V car battery for example, how much
'trickle charge' is needed to keep them alive when not being used?

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

G

#### G. Morgan

Jan 1, 1970
0
mike said:
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".

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.

U

#### Uwe Hercksen

Jan 1, 1970
0
G. Morgan said:
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.

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

G

#### G. Morgan

Jan 1, 1970
0
Martin said:
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.

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.

G

#### G. Morgan

Jan 1, 1970
0
Uwe said:
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

I'll compensate for voltage drop in the copper.

G

#### G. Morgan

Jan 1, 1970
0
Uwe said:
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

I'll compensate for voltage drop in the copper.

G

#### G. Morgan

Jan 1, 1970
0
Martin said:
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.

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.

M

#### MrTallyman

Jan 1, 1970
0
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

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.

M

#### MrTallyman

Jan 1, 1970
0
I'll compensate for voltage drop in the copper.

That is what the higher voltage is for.

M

#### Martin Brown

Jan 1, 1970
0
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.

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.

http://computer.howstuffworks.com/dell-battery-fire.htm

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

M

#### mike

Jan 1, 1970
0
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
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
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.
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!! G #### G. Morgan Jan 1, 1970 0 MrTallyman said: 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'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. <g>

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.

G

#### G. Morgan

Jan 1, 1970
0
mike said:
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.

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.

http://www.amazon.com/UltraFire-300...y-Charger/dp/B004LHFU62/ref=pd_bxgy_hi_text_y

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.

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

G

#### G. Morgan

Jan 1, 1970
0
Martin said:
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.

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.

M

#### Martin Brown

Jan 1, 1970
0
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.

And are inclined to protest by self immolation if provoked.
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

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.
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

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.
outages. For anything more than a few hours, get a gasoline, propane,
or diesel generator.

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

Regards,
Martin Brown

M

#### mike

Jan 1, 1970
0
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.<g> 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.
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.
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.
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.
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 I use 18/4 the voltage drop will be negligible, the whole cable
length would be less than 200' in my estimate.

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

S

#### Sjouke Burry

Jan 1, 1970
0
What is a good formula for the needed minimum current (and over voltage)
for charging batteries? Use a 12V car battery for example, how much
'trickle charge' is needed to keep them alive when not being used?

Thanks.

Trickle charge a battery only, when you want to destroy
the battery.
Periodic charge with a modern charger is better, there are modern
ones which will only restart charging when charge drops to 70 or 80
percent.
Else use a mains timer , to power the charger ~10 minutes a day.
That should keep your battery happy.

M

#### mike

Jan 1, 1970
0
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.

http://www.amazon.com/UltraFire-300...y-Charger/dp/B004LHFU62/ref=pd_bxgy_hi_text_y

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

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?

R
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