## Battery charger

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myp71
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### Battery charger

What is the best battery charger for a 12volt 4.5ah battery I thought that a automotive battery charger would work but it says on there not to use this type of charger now that i think about it this automotive charger is what did in my other 12v-8ah battery.
something like a 15 volt 500ma charger but how long do i charge it for<p>thanks
Myp71

Chris Foley
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### Re: Battery charger

Electronic Circuit Schematics -- Batteries<p>is a good intro to the business of recharging NiCad batteries. You probably have a 10-cell pack, which has a nominal voltage of 12.5VDC (1.25V per cell). First, you want to use a resistor or current sink to discharge the pack to 10V. Then, you charge it at 1/10 the Amp-hour rating (4.5 A-h would give you about 450mA) until you get the total amp-hours to about 1.4 times the rating. For the 500mA example, you would want to charge the 5A-h battery to 5*1.4=7A-h divided by .5A, so that would be 14 hours at a constant current of 500mA.<p>Assuming this is what you were heading toward in your earlier post, you want to get a 12VAC, 1A wall wart, use a bridge rectifier and 2200uF 25V cap to get an unregulated 16VDC, then use an LM317 constant current source (2.5 ohm 1 watt resistor -- just put 4 ea. 10 ohm 1/4W in parallel), and charge the battery pack for 14 hours. OK?<p>[ May 24, 2003: Message edited by: Chris Foley ]</p>

myp71
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### Re: Battery charger

Thanks Chris once again for all your help.<p>
Myp71

Rodney
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### Re: Battery charger

You do not say what type of battery. If it is a Gell Cell, requirements are quite different from NiCads. Go to rcbatteryclinic.com for some very good info on all types of batteries, their care and feeding etc.

Chris Foley
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### Re: Battery charger

Ooops -- Rodney's right -- my bad. Definitely check the battery type first.

myp71
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### Re: Battery charger

Ok thanks guys it is a sealed lead acid like a backup power for an alarm or also a u.p.s.<p>
thanks
Myp71

k7elp60
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### Re: Battery charger

Most manufacturers recommend charging at C/10 or in your case maximum of 450Ma.,at a constant voltage. If you set the voltage between 13.6 and 13.8 volts you can leave the charger on all the time and it will never over charge. The 317T will work fine as a constant voltage source. However to limit the current is another matter.
The easiest way is to use a wall wart that will produce less than 450Ma of current, that will be the current limiting. The follow it with a voltage regulator like the 317T. If the DC source has enought output voltage, about 16 volts you can use first a 317T as a current regulator,followed by another 317T as a voltage source.
Ned

Chris Smith
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### Re: Battery charger

k7elp60 ,........ he has a lead acid battery, your advice is good for Nicads.<p> Lead acid can take short bursts of mega amps to quick charge like in a car, or slow charges in the medium amperage, but most continous trickle charges cause sulfating of the cells. <p>Your better with large charges, short periods, and then stopping,..... for the longest life of your battery, like in a car. On and off top ups are ok for extended periods of non use. Use medium charges. [10 amps] <p>As long as you dont go over around 14.7 volts you wont be boiling away the battery, and you need at least 13,2 minimum, with what ever the battery wants to draw, within the limits of the terminal size or feed wire.

k7elp60
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### Re: Battery charger

Chris,
I am aware that his battery is a Lead-Acid. I'll bet it is a Gel-Cell. Which means the electrollyte is really in a jelly form, or is known as Dilute Sulfuric Acid. Float charging is an acceptable way to charge a gel-cell. POWER-SONIC recommends a charge voltage of between 2.25 and 2.30 Volts per cell. In this case 13.5 to 13.8 Volts.
I was incorrect about the charge current limit.
POWER-SONIC's Technical Handbook says the initial
charge current must be limited to C X .25, in this case 4.5AH X .25 = 1.125 amps. <p>If one is to charge a 12V lead-acid battery at the 14.7 volt range it is best to monitor the current and when it drops to .01C (45Ma in this case)the charger must be removed or battery damage will result.<p>Charging at the float level the charger never has to be removed.
Ned

gadgeteer
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### Re: Battery charger

NiThe general rule of thumb is to charge batteries at 1/10 their amp-hour-rate. Car batteries are usually in the 80-amp-hour range, so 6-10 amps is fine. Certain batteries DO take "quick-charge", but regardless of the battery type that will shorten the life. You can charge batteries either "constant-voltage" (current decreases as charge progresses), or "constant current" forcing a known current through a battery and monitoring the battery's voltage).<p>Lead-acid batteries HATE to be deep-discharged, and like to be trickle-charged; nickle-cadmium batteries LOVE to be deep-cycled (used until dead) and full-charged --- in fact, NI-CADS have memory, so if you only USE 10% several times, the battery will start going DEAD after only that 10% use. Hence the "deep-discharge". BUT --- if you discharge a ni-cad (or most other batteries) down to ZERO VOLTS, they likely will never charge again...<p>Lead acid exhibits a charge-slope, so you CAN feed it constant-current and watch the terminal voltage; but NI-CADS stay very flat over most of the charge-cycle, AND they stay flat over most of the discharge cycle too (you know this if you've watched a ni-cad flashlight SUDDENLY go dead). Nickle-Hydride has the benefits of ni-cad, but NO MEMORY. However, Ni-H has shorter shelf-life.<p>SHELF-LIFE (usable time after full charge):
Lead-Acid ......................... ~6+ months
Lead-Calcium (NaOH electrolyte).....1-3 years
Nickle-Cadmium .......................3 months
Nickle-Hydride .......................1 month
Lithium ..............................a year?<p>You can make a simple constant current charger, with a 120volt full-wave-bridge rectifier; place a NIGHTLIGHT BULB in series, and the bridge output connected to your battery. A 4-watt nightlight bulb will limit the current to 33 mA, a 7-watt to ~60mA. Use P = VA, or A = (bulb-wattage)/120volts (USA). But you MUST set a timer, 'cause there is no provision to STOP the charging, and you WILL overcharge the battery unless you're watching...<p>There are some house-hold rechargeable battery chargers that use ONE diode, and a 4-watt bulb; so the average current is about 17mA... (And the bulb gets half-power, so it almost never burns out...)

myp71
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### Re: Battery charger

thanks a lot guys. <p>Myp71
Ryan

Will
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### Re: Battery charger

I once had an invalid friend with a battery (Lead Acid) operated wheel-chair. Full load (Going up a hill to the pub) the drive motor would draw 22 amps. They could not get two runs up to the pub on one singlecharge. The chair was fitted with an integral crude battery charger which would completely re-charge the battery in 6 - 7 hours. The problem with this was that, once the battery was fully charged it would gas off seriously. As a consequence they could not put it on charge overnight but had to monitor it until it was gassing off and then switch it off.
I did some tests on the battery and discovered that, for most of the time when charged but not fully charged it's open circuit (Off-charge ) voltage wouild generally be 12 volts. When it was fully charged the OC voltage would rise to 12.6 volts. I also noticed (Abd I do not know why) that, when the charger was switched off it took several seconds for the OC voltage to decay to the values I have described.
I rigged up a cicuit using a 555 timer (With it's Vss held steady by a Zener diode and controlling a 30 Amp germanium transistor (Because it was available), via a 2N 2222 or something similar, and set the 555 timer so that it's sensing circuit detected when the battery volts dropped below 12.3 volts. it switched on for ten minutes then switched the gwemanium transistor off. If the voltage was below 12.3 the it immediately switched back on again - if not then it waited until it was. Consequently the battery would get fully charged as required but would then get charged for 10 minutes whenever required. It worked perfectly. With a new battery, once fully charged it would get a 10 minute charge once every 8 - 10 hours, as the battery condition deteriorated it might require a charge every 2 hours or so. Irrespective of battery condition the chair/battery could be left plugged permanently into the wall socket for ever without any chance of excessive gassing or damage.
The design was fairly simple and many of you could probably design it better. If anyone would like to have my dseign I'm certain I could resurrect it. Excuse the typos adn errors - I can hardly see in this light.
BB

gadgeteer
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### Re: Battery charger

Gee, Will, you'll ruin your eyes! (Didn't your mom ever tell you that???)<p>You can also use an SCR to auto-charge batteries. If you full-wave-bridge-rectify the output of a transformer, you can connect it to the battery through the SCR; and simply power the GATE through a resistor, and have a zener-diode switch turn on an NPN common-emitter to GROUND the gate (and stop switching it on). Because the rectified transformer power is not FILTERED, it actually falls to zero 120 times per second; so killing the gate-drive WILL switch off the SCR. If you use a DARLINGTON NPN, the base-current drain to your battery will be minimal. That's the way the store-bought auto-chargers work...

Bernius1
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### Re: Battery charger

I worked with truck batteries for years. Delco says 12.6v is full charge, and 12.0v is 'discharged'. That's because voltage & current follow exponential curves. Roughly, that means that 50% of current discharges between 12.6 & 12.3, while the last 5%-10% occurs from 12.0v to 6v. The BEST WAY to determine full charge on a lead acid battery is (1) check voltage (close to 12.6v) ,then load test as follows; press switch for 2-10 sec.,until rate of voltage drop slows notably, THEN release & WATCH recovery of the needle.
(a) Needle immed. returns over 12v,Batt good
(b) Needle slowly returns to 11.5v+,more chging.
(c) Needle drops to 8v, on release returns to 10.5v, slows, & then returns to 12v, one cell is bad. This is when the mech. says " it doesn't hold a charge". Will work,short duration.
(d)Needle doesn't retrn over 12v, replace.
Hope this helps.
Can't we end all posts with a comical quip?

Will
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### Re: Battery charger

Gadgeteer,
My eyes have already gone and my memory has too so I've forgotten what mama told me.
Your suggestion sounds good and practical - but have you ever tried it in practice ? As I said in my post, when I did tests on a battery the voltage decay once I switched off the charger was quite slow. So ! it seems to me that once your SCR switches on and the off after 16+ milli-secs - at the end of one half cycle then, if the volts hadn't decayed to battery open circuit voltage then (Because of the higher voltage reuired to overcome the battery internal resistance and charge it) wouldn't that prevent it from turning on at the next cycle ? Such being the case then the SCR might only turn on every tenth half cycle or so dependent upon actual rate of decay of the voltage from the caharging value to the open circuit value ? Perhaps I should try it
BB

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