Rapidly charging a lead-acid battery - Question - Page 1
Forum section: electronics / beginners

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: November 10, 2015, 10:36:21 pm
content: Hi againI want to design a charger that can quickly recharge a lead-acid battery (Optima D31A, not worried about lifetime), the charging information for the battery states that you can rapidly charge the battery using a maximum applied voltage of 15.6V at a non-limited current as long as the battery temperature stays below 54.7°C.So what I was thinking of doing is designing a SMPS with which I can easily change the output voltage to fall within 13.8->15V (specification for a normal charge) but I am not sure how to go about limiting the output current of the converter. I want to be able to limit the output current between 1->30A, but so far I have only come up with using a simple BJT on the output to achieve this.There must be a better way of limiting output current right?

author: Paul Price
author_rank: Super Contributor
timestamp: November 10, 2015, 11:45:10 pm
content: Limit current by adjusting the charge voltage while monitoring current with a low-resistance shunt. The battery will draw no current from the charger until the output voltage of the charger is greater than than battery voltage. After the charging voltage exceeds the battery voltage by just millivolts, the current increases exponentially. By using a PWM controlled step down converter to feed the battery, the charging current can be adjusted in tiny steps. In operation, the PWM would be slowly ramped-up while monitoring charging current and battery voltage to set and continually adj. the desired charge current.One easy way to do this is by using a cheap but fast MCU to monitor the battery temperature, the charging current and battery voltage and use these readings to control the PWM duty cycle of a buck converter and thus adjust both voltage and charging current. The MCU would supply the PWM signal.A high current N MOSFET would be used along with an high current inductor(approx. 100uH) and a high current schottky catch diode to form the PWM controller. Both the catch diode and MOSFET would require small heatsinks. A BJT could be used in a common-base circuit to control the gate of the MOSFET. A shunt resistor as small as .005 ohms could provide an output that could be amplified by a fast op-amp to feedback charging current to the A2D input of the MCU. The MCU could easily both control and report the results of the charging operation.

author: Psi
author_rank: Super Contributor
timestamp: November 11, 2015, 12:06:01 am
content: Be aware that fast charging a lead acid will cause it to outgas a lot of hydrogen. Exploding lead acid batteries does happen, and is not fun.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: November 11, 2015, 01:28:32 am
content: Quote from: Paul Price on November 10, 2015, 11:45:10 pmThanks for the in depth reply. Doing it this way would be perfectly fine for the rapid charge stage where from my understanding you would set the charging voltage to say 13.8V and don't worry about the charge current too much (as long as you keep battery temp below 54.7°C). But what about the float charge (should have added this info sorry) which says keep the float voltage at 13.2->13.8V and limit the maximum charging/floating current to 1A?Another thing I should have added is that my power source will be coming from a bunch of solar panels (300W in total). So doing some quick calculations showed that with a 90% efficient converter, given that I set my charging voltage to 13.8V my maximum charging current would be ~19.5A on a good day, and this is fine for the rapid charge stage. But from my understanding if I lower the voltage to say 13.5V for the float stage won't the peak charging current shoot up to 20A? Something that I can't do for the floating stage. Or am I just missing some basic understanding of charging batteries.EDIT: Link to datasheet for batteries,http://www.optimabatteries.com.au/wp-content/uploads/2014/07/Group-D31A-YellowTop.pdf

author: Paul Price
author_rank: Super Contributor
timestamp: November 11, 2015, 03:48:21 am
content: Please read again my reply. You control the voltage applied to the battery to control the charging current. Lowering the charging voltage/float voltage will reduce the charge current to any desired value and it is possible reduce the charge current to zero.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: November 11, 2015, 10:07:48 am
content: Quote from: Psi on November 11, 2015, 12:06:01 amHappened to me, make sure it doesn't happen to you! It's a bloody miracle that my friend wasn't blinded. He was leaning into an engine bay to look at stuff and turned around to pick up his cup of tea just at the moment I pulled the starter then BOOM. We found chunks of lead about 10 m away after we'd finished hosing him down (and the vigourously fizzing engine bay).

author: wiss
author_rank: Frequent Contributor
timestamp: November 11, 2015, 02:48:15 pm
content: I am building a PV-Lead-Acid charger as well now, I use an Arduino Nano, 65 kHz pwm on D6 to run a high-side N-FET and some Hall current-sensors to monitor current (ACS712), all picked up on ebay from china for a few dollars each.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: November 11, 2015, 09:52:22 pm
content: Quote from: Paul Price on November 11, 2015, 03:48:21 amAh yup my bad, I think I got it now.Quote from: DenzilPenberthy on November 11, 2015, 10:07:48 amYup will have to set-up some sort of temperature monitoring to be one the safe side.Quote from: wiss on November 11, 2015, 02:48:15 pmOh awesome, thanks for naming the part #'s, will have to have a better look at them

author: SL4P
author_rank: Super Contributor
timestamp: November 11, 2015, 10:15:52 pm
content: I'm not a big fan of Supercaps as Lead-Acid replacements (yet), but it sounds like your application could benefit - as youdon'tneed long discharge periods, anddoneed short charge cycles...

author: Seekonk
author_rank: Super Contributor
timestamp: November 11, 2015, 10:39:57 pm
content: I remember an article from GE R&D in the 80's doing fast charging by periodically (like once a second) doing a heavy discharge to collapse bubbles. Bubbles increase the resistance and lower maximum charging current.

author: Rick Law
author_rank: Super Contributor
timestamp: November 12, 2015, 04:47:03 am
content: Quote from: ANTALIFE on November 11, 2015, 09:52:22 pmMay I suggest the very simple to use LM35z.I use a Nano to monitor Volt, Amp and temperature. The temperature is monitored by the LM35z reporting back to the Nano. I do that monitoring during test of newly recovered laptop lithiums. The Nano will cut power to the charger (via 10Amp relay) once temperature reaches certain temperature.