SLA battery charger

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erkanyigiter
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SLA battery charger

Post by erkanyigiter » Tue Sep 21, 2004 4:17 am

i have a SLA which is 12V-1,3Ah and want to charge it with a constant current source. what are the optimum values for a safe charging. (For example 130mA up to 10V, and then 100mA up to 13V etc..) and what is the expected charging time at this conditions. thanks for your helps.<p>Erkan<p>[ September 21, 2004: Message edited by: yigiter ]<p>[ September 21, 2004: Message edited by: yigiter ]</p>
-Erkan

dyarker
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Re: SLA battery charger

Post by dyarker » Wed Sep 22, 2004 2:40 am

Depends on SLA battery type. General purpose, high rate, standby float.<p>For general purpose SLA, like you said, 130mA (1/10th Amphour rating). But when battery gets to like 14.5V switch to trickle charge. Check manufacturer's data sheet for the model battery you have.
Dale Y

camino75080
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Re: SLA battery charger

Post by camino75080 » Fri Sep 24, 2004 9:47 pm

What is an SLA battery? Anyway, in that R/C car hobby, we normaly charge our 2.4AH nicads at anywhere from .5A to about 5A. But, I dont know how it works in other applicatons. Just my two cents.

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jwax
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Re: SLA battery charger

Post by jwax » Sat Sep 25, 2004 5:13 am

camino- Sealed Lead Acid.
In RC recharging, you're doing a "fast charge", right? 5 amps into a 2.4 AH cell seems like a lot!
John

erkanyigiter
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Re: SLA battery charger

Post by erkanyigiter » Sun Sep 26, 2004 9:47 pm

as you said it is a "sealed lead acid" battery. i want to charge it with a microprocessor controlled constant current source in 2-3 hours and i don't want to damage it. in my opinion i must start with a fast charge and then i must reduce the charging current. but i don't know the nominal values.
-Erkan

camino75080
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Re: SLA battery charger

Post by camino75080 » Mon Sep 27, 2004 10:19 am

Sealed Lead Acid, DUH. What was I thinking? Anyway yeah its a lot of current, but it makes the batteries discharge fast so the motors have better low-end power and the slower charges cause a slower discharge for longer races.

dyarker
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Re: SLA battery charger

Post by dyarker » Mon Sep 27, 2004 10:07 pm

I have the spec sheet for a 12V 5AH SLA battery. It says recharge at no more than 2A, constant Voltage 14.5V to 14.9V. Extrapalate to 1.3AH:
1.3AHr / 5AH = 0.26
2A * 0.26 = 0.52A (520mA) maximum<p>0.5A * 3Hr = 1.5AH, about 3 hours to completely recharge 1.3AH battery at max sounds right.<p>Do not charge that fast unless the microprocessor monitors the battery temperature. At 35°C start reducing the current. I'm guessing that a sensor pressed against the side of the battery measuring 35°C means 40°C inside the battery. 40°C was the highest mentioned.<p>Without temperature monitoring, 130mA to 13V across the battery should be very safe. But, it will take 10Hr to 12Hr if battery is below 10V.<p>Trickle charge 13.6V to 13.8V at 25°C.<p>The spec didn't say, but I suspect a lower current if the battery is under 10V is a good idea. Also if battery temperature is is below like 10°C.<p>------------------------------------------
This is your project, do it your way. If I was building it, I would:<p>Set a voltage divider from battery to microprocessor ADC so that 15.94V in gives 5V to ADC. (16V would equal 256, so 15.94V would be 255 or max byte count.) A 12V battery should always be lower than that.<p>During charging, hold a thermistor or temperature IC against the battery with rubber bands or Velcro straps or some such. Twisted pair cable back to microprocessor.<p>Use a max current of 400mA (for faster recharge) adjusted down based on Voltage and temperature. Switch to trickle charge when battery reaches 14.6V.<p>If microprocessor has unused I/O pins, connect a DIP switch so other AmpHour size batteries could be charged. Maybe one switch to select 12V or 6V.<p>Have fun!
Dale Y

Bernius1
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Re: SLA battery charger

Post by Bernius1 » Wed Sep 29, 2004 3:05 am

Dale,
It seems to me that the reason for the thermistor is that at max current, and with the higher differential between the charger and low battery voltages, the total power into the battery is too high. I've tried cycling 12V into a dead 1.5V watch battery, and got a week's life out of it. But I was cycling until my thumb got hot ! Perhaps a PWM square-wave to provide a constant average current, so that as the voltage differential decreases, the current increases. Overall charge rate will be slower, but safer. What do you think ?
Can't we end all posts with a comical quip?

dyarker
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Re: SLA battery charger

Post by dyarker » Wed Sep 29, 2004 10:38 pm

no_vice,
The difference in voltage between the supply and battery is dropped inside the charger.<p>In a constant current charger dropped across the current regulator transistor. If the current is at/near max, the battery won't go above 14.9V because battery resistance is determined by plate size and chemistry.<p>In a constant voltage charger the difference is dropped across the voltage regulator to 14.xV. The battery resistance limits the current.<p>(The problem with either method alone, is how do you know when to stop charging?)<p>The simplest charging regulator is a resistor sized to pass near max trickle current at battery voltage of 13.6V. But it takes about forever (exageration). The resistor drops the voltage difference.<p>PWMing would be safer because the average current is smaller. Using a smaller continous current is the same thing. Trading time for safety, and I suggested that.<p>To reduce charge time, at/near max charge current is needed (either PWM average or continuous). The spec sheet stated the max current at 77°F(25°C) ambient. What if the temperature is higher, or the battery is enclosed?? It can't get rid of the normal heat of recharging fast enough, and burns out. Inside an enclosure on a hot day even half current could cause the battery to get too hot. Venting gas reduces AmpHour capacity. (SLAs have special valves to prevent explosion.) Warped plates at really high temps ends the battery's life.<p>((a chemistry expert can correct me if I'm wrong here, please) By "normal reharging heat", I mean heat due to battery resistance plus heat of exothermic chemical reactions in the battery. During discharge, the heat is due to resistance minus heat of endothermic reactions. Discharge partly self regulates; as load increases, voltage decreases. That's why max charge is lower than max discharge.)<p>For a reasonably short recharge time, I'd want a temperature sensor on the battery for any method.<p>In really cold conditions the chemical reactions slow down. Forcing a safe at 25°C current through the battery is energy coming in faster than the atoms can recombine to the charged state. I think its gas generation again. This isn't as bad as high temp, because the charging warms the battery so it isn't cold anymore (a self correcting problem). DO NOT recharge a frozen battery! (maybe can freeze at -20C°, I haven't checked that)<p>---------------------------------------------
Years ago I saw a schematic for a PWM charger for NiCd batteries. A large charge pulse followed by a short reverse pulse. The reverse pulse was supposed remove "memory effect". That charger had a temperature sensor tucked in between the NiCd cells.<p>For a fast charge, watch the temp. Lower currents are safer.<p>PWM will certainly work with SLAs, I don't see the need for the extra complication. (One reason might be to measure battery voltage between pulses when no current is flowing.)<p>-------------------------------------------
What I think is that there are as many ways to build a charger, as there are people who want to build a charger. You want pulses? Okay. You're willing to use a lower average current to avoid temperature monitoring? Okay too. It's all compromises. Go anywhere within specs you want.<p>Cheers,
Dale Y

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