So, how DO I charge a 6V SLA battery?

On Mar 9, 8:17=A0pm, snipped-for-privacy@gmail.com wrote: > I bought a 6V, 6Ah battery from Home Depot to power my 3-year-old's > toy electric car (the kind that a kid can ride). =A0The previous battery > didn't hold a charge when I bought the car. >

What became of the charger that came with the car that showed you the original battery is now bad?

GG

Charger wasn't available. Got the car for $5 from a Thrift Store, intending to utilize the motor assembly only. Turns out the rest of the car was in pretty good condition, and the tiny 6V 4A-h battery worked for maybe a month, then no longer held a charge.

I suppose I could run to Wal-Mart and buy a $30 charger for their Power Rangers line, but that would not be nearly as satisfying as a DIY solution... plus, I'm not sure if their charger is tied to the 4Ah battery or not.

MD

Per the manufacturer's recommendations. But that's not what you wanted to hear, so read below.

Here is a way you can charge it: Build a float charger. Set the float voltage to 7 volts. Use a 9 or 12 volt DC wall wart (not your 6v wall wart) The schematic is on the first page of the datasheet

There are better ways to charge your battery, but this way won't cook it and is simple to build.

If you can't build, buy a charger recommended for that battery.

Ed

You can probably use your 6VDC wall wart. Unless it is a really hefty wart, it'll probably droop considerably when subjected to the battery. It may actually fail if the battery is discharged, since they often have internal fuses to prevent fires. So, use a 4.7 ohm 1W resistor in series with it just in case. If the resistor gets too hot to touch (which it won't) while charging, post again with specs for the wart.

Regards, Bob Monsen

My preferred solution to SLA charging is to use the TI/Unitrode UC3906. If you're not into building your own PCB then ready-to-assemble kits are available.

They do ship internationally if you can't find a souce for a similar kit nearer to you.

Bad idea, Michael the OP has stated that it produces 9V off load. The current drawn by the battery drops as it charges. If left plugged in past the full charge point the bettery will receive excessive voltage. Leaving the battery plugged in for a week will result in a dead battery.

Around there the DIY superstores sell very cheap flashlights with lead acid batterys and unregulated chargers. They are notorious for the batterys failing.

Bob

Yep, 9V off load; once loaded, voltage dropped to about 6.42V @

177mA. I monitored the voltage every half-hour or so until voltage rose to 6.77V 6 hrs later, then cut the juice (it was bedtime anyway). I was just trying to figure out the point where I should stop charging.

I'll study the other responses and see what I can do.

Thanks,

MD

datasheet

Thanks! Looks simpler than the L200.

Just wondering, where did you get the value of 7.00V? Should I apply a brief charge at 7.50V to get maximum capacity?

"At 2.37V, most lead-acid batteries start to gas, causing loss of electrolyte and possible temperature increases. The exceptions are small sealed lead acid batteries (SLA), which can be charged to 2.50V/ cell without adverse side effect."

MD

"Bob" wrote in message news: snipped-for-privacy@m44g2000hsc.googlegroups.com...

Do you know how much the current drops?

There is such a thing as overdesign. For example, it is possible to use a kit, a charger chip, or a discrete circuit like this one:

8 - 12 V input ------o-----o------------o-----o------. | | | | | | | | | .-. | | | | | | 1.38 Ohm 1W | | | | | | | >| |< | '-' | |-----o--| >| | .-. /| | |\\ |----o 4.7k | | | | | /| | | | | '----o | | '-' o------------)-----o---||-+ N-MOSFET | | | ||->

| | | ||-+ | | | | | | | 10k | | |/ \\| ___ | o---| |--o-|___|-o--------. | |>

I think they specify that you should take the battery off the charger after it is charged.

Regards, Bob Monsen

Here is the circuit in LTSpice format:

Version 4 SHEET 1 880 680 WIRE -112 16 -336 16 WIRE 48 16 -112 16 WIRE 208 16 48 16 WIRE 336 16 208 16 WIRE 336 32 336 16 WIRE -112 48 -112 16 WIRE 48 48 48 16 WIRE 208 64 208 16 WIRE -32 96 -48 96 WIRE -16 96 -32 96 WIRE -336 112 -336 16 WIRE 336 112 272 112 WIRE -32 160 -32 96 WIRE 48 160 48 144 WIRE 48 160 -32 160 WIRE 336 160 336 112 WIRE -112 176 -112 144 WIRE 208 176 208 160 WIRE 208 176 -112 176 WIRE 288 176 208 176 WIRE -112 224 -112 176 WIRE 48 224 48 160 WIRE -176 272 -224 272 WIRE 128 272 112 272 WIRE 144 272 128 272 WIRE 336 272 336 256 WIRE 336 272 224 272 WIRE -336 288 -336 192 WIRE 128 304 128 272 WIRE 336 304 336 272 WIRE -224 336 -224 272 WIRE -112 336 -112 320 WIRE -32 336 -112 336 WIRE 48 336 48 320 WIRE 48 336 -32 336 WIRE 336 336 336 304 WIRE -32 352 -32 336 WIRE -336 448 -336 288 WIRE -224 448 -224 416 WIRE -224 448 -336 448 WIRE -32 448 -32 432 WIRE -32 448 -224 448 WIRE 128 448 128 384 WIRE 128 448 -32 448 WIRE 336 448 336 400 WIRE 336 448 128 448 FLAG -336 288 0 FLAG 336 304 a SYMBOL voltage -336 96 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 9V SYMBOL pnp 272 160 R180 SYMATTR InstName Q2 SYMATTR Value 2N3906 SYMBOL res 320 16 R0 SYMATTR InstName R1 SYMATTR Value 1.38 SYMATTR SpiceLine pwr=1 SYMBOL pnp -48 144 R180 SYMATTR InstName Q3 SYMATTR Value 2N3906 SYMBOL pnp -16 144 M180 SYMATTR InstName Q4 SYMATTR Value 2N3906 SYMBOL npn -176 224 R0 SYMATTR InstName Q5 SYMATTR Value 2N3904 SYMBOL npn 112 224 M0 SYMATTR InstName Q6 SYMATTR Value 2N3904 SYMBOL res 240 256 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL res 144 400 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R3 SYMATTR Value 22k SYMBOL res -48 336 R0 SYMATTR InstName R4 SYMATTR Value 1k SYMBOL cap 320 336 R0 SYMATTR InstName C1 SYMATTR Value 1 SYMBOL voltage -224 320 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 5.1 SYMBOL pmos 288 256 M180 SYMATTR InstName M1 SYMATTR Value FDS4953 TEXT 408 280 Left 0 !.ic V(a) = 6 TEXT -370 458 Left 0 !.tran 10

Current dropped from 177 mA @6.42V to 140mA @ 6.63V about 90 minutes later. After that I just took voltage measurements only.

What will happen if I charge with my (underpowered?) wall-wart, just watching until voltage reaches 7.5V? I'm in no particular hurry, and can monitor the voltage every hour if necessary, at least for now. I'm guessing the reason for the 500mA is for a faster charge? Or is this necessary for the health of the SLA?

Thanks for the circuit,

MD

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You seem to know what you're doing, monitoring the voltage and current. SLA's have a charging protocol that goes in three stages: bulk, absorption and float. If the battery has discharged a substantial amount of its capacity and you put it on to charge it will pull a lot of current and draw down the voltage of the charger. It pulls down the voltage of the charger either because the charger has a designed- in current limit or because of the charger's output impedance -- for example, your wall wart can only put out a certain amount of current (it has an output impedance). All good and well, it means your transfomer is small enough not to boil the battery right off the bat! (Some manufacturers recommend initial charging current limit for small SLA's around .3 C, or 1.8 amps for your battery). Then as the battery takes on bulk charge, the voltage rises. With a properly regulated charger, this plateau will be somewhere around 2.4 to 2.5 volts. For an SLA, 2.5 is not too high. This is the "absorption" stage. The charger holds the voltage steady at the setpoint. Gradually, the current drawn by the battery begins to decline. When it reaches about .03 C (180 mA), the battery is considered charged and a "smart charger" will drop the voltage to float, about 2.2 or 2.3 volts per cell. Your wall wart won't behave is such a precise and predictable way, but it will work fine to charge your battery as long as you monitor it. Such an unregulated charger only poses a danger to your battery if you leave it on indefinitely, because the batttery will take on so much charge that it will draw negligible current, causing your charger to approach its open-circuit voltage, which is too high for the battery. Just disconnect before it happens.

datasheet

:-) From the same place you got the quote below! They give a range and I used 7.00, which is within the range. But read on for a little more detail. The smiley is because we've (posters on the newsgroups) have been searching for years for the answer I warn you (below) not to look for. :-)

Yes. But read the whole thing. "Correct settings of the voltage limits are critical and range from 2.30V to 2.45V. Setting the voltage limit is a compromise."

If you look above the part you quoted, you will see a table showing advantages/disadvantages. The 7 volt figure was chosen for maximum service life.

According to the site, you can apply a 7.5 volt charge for

2 hours, once every 6 months, provided the temperature is below 86F during the 2 hours, AIRC.

Now here is a warning: "they" (meaning many folks who write about batteries) _seem_ to play fast and loose with the numbers and the recommendations. You will find small differences in what is said, depending on where you read. There is no *exact* level or procedure that is 100% applicable. As the battery university says, any level (within the general parameters) is a compromize. The way the battery is used, how deeply and rapidly discharged, how rapidly charged and the charging procedure, and the ambient temperature all play a part in it. Obviously, the battery chemistry, construction and number of cells also play a part in it. So the warning is, don't expect to find a one size fits all, perfect answer to how to charge batteries.

You can spend a lot (relatively) and buy a 3 stage charger or you can build something real simple, or go somewhere in between.

Ed

I've got a lot of 6V SLAs. I charge them on the cheapest 6V/12V motorcycle battery charger that I could find at the local auto parts store. Cost was about $15 each & charge at a rate of about 1A/hr. I just hook 'em up overnight & they're ready in the morning. I've been doing this for many years & never had any problem.

JM

Yesterday I charged the battery for 6 hrs, and the voltage when charging ranged from 6.42V to 6.77V after 6 hrs.

Today I resumed charging, and discovered something weird: the voltage rose, then fell, during charging.

6.58V @ 7pm 6.86V @ 7:2pm 6.83V @ 7:28pm 6.82V @ 7:35pm 6.81V @ 7:40pm 6.82V @ 8:35pm 6.84V @ 9pm

In general, is it better or worse for the battery's health to split charging across several days?

Thanks,

Michael

Could be your local mains voltage increasing when people finish using electric cookers in the evening but the most likely explanation is a temperature effect. When the battery is drawing current it's internal temperature goes up a bit which shows up as a higher voltage.

It's normal for the terminal voltage of large lead acid batterys to go up a bit when drawing tens of amps from them off charge.

Provided the charging current is within the manufacturers rating it won't make a significant difference.

Bob

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I wouldn't think a few hundredths of a volt is symptomatic of anything. Another thing about those voltages in your chart -- they're pretty low, closer to float voltage than charging voltage. The voltage figures would have more meaning if you had also recorded the current draw. What was the state-of-charge of your battery when you recorded those voltages, and did you have a resistor in series?

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Current was just over 100 mA at end of charging (I stopped at 6.97V, bedtime again).

Battery history:

1) bought 6V, 6.5Ah battery from Home Depot for $21 2) charged to about 6.7 V 3) 3-year-old daughter drove the car from home to the park (3 blocks away), then back home 4) charged to 6.77V over 6 hrs 5) charged again to 6.97V over 4 hours (where I got the above readings).

No resistor; I figured I need all the power my little wal-wart can give without impeding anything. To measure current I'd manually disconnect, connect DMM in series, then re-connect.

Michael

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I'd say you've got it pretty much under control.

Put a voltmeter on it, and connect the wall wart directly to the battery || voltmeter. Sit and watch it. When it reaches ~7.2 volts, disconnect the wall wart, let the battery cool, check the voltage, and if it's 6.6V or more, you're good to go.

Good Luck! Rich