Class D audio amp battery + dc help - Page 1
Forum section: electronics / beginners

author: malium
author_rank: Contributor
timestamp: September 08, 2013, 11:48:11 pm
content: Hello,Would need some help and pointers on how to approach to the power supply for the portable audio speakers I am trying to make...The speakers will be composed from the d-class amp which (TA2024) which needs 12V, around 2A power supply, which I have solved via the laptop like brick smps I found lying around. Now, I am having some difficulties in finding a proper way to solve the portable part... I need some kind of a battery backup, but as the person who I am making this for is, well, a kid, I am trying to make it as simple as possible. Another words, I need a solution which will be in the sealed box with the speakers and an amp, and will charge the battery when connected to the above mentioned power brick. Now, I want it to be "smart", and that when the adapter is plugged in, it plays and it charges, and to sense when battery is charged, stop the charging and play on. When adapter is unplugged, it uses the battery power.Now, my knowledge in this field is young and small, and I do not want to experiment a lot, so I would use some pointers. At first I opted for Li-on batteries, but all the drama on the internet with the explosions and so on, kind of scared me away. SLA is out of the question, as it is too heavy.What would Your proposition be for the battery type, what to use as a charger (batteries not removable from the enclosure), how to solve a dc adapter direct - battery charging and protection circuit, is there some finished circuits of this type to buy (like in laptops for example)?Many, many thanks for any help.

author: peter.mitchell
author_rank: Super Contributor
timestamp: September 09, 2013, 03:39:28 am
content: what is your desired runtime? and what kind of effort do you want to put into the charging circuit, also, what is an acceptable charging time?Lead-acid is out because it is crazy heavy, and you specified you don't want lithium, so that pretty much leaves you with NiMH, NiMH leaves you with long charge times or more complicated charge circuitry.

author: malium
author_rank: Contributor
timestamp: September 09, 2013, 06:22:22 am
content: Runtime, well, few hours would be fine. I would probably also add the solar panel to the circuit to help things out a bit. It is not that I do not want the lithium, I just want a "safe(r)" solution. Litium, but with all possible protections then. As I understand LiFePo4 should be safer-ish?Charging time not important, I am ready to buy something, or to build it, whatever it takes to finish it (pitbull sindrome).

author: peter.mitchell
author_rank: Super Contributor
timestamp: September 09, 2013, 01:53:15 pm
content: LiFePO4 is pretty safe, but imho still needs a reasonable protection and charge circuitry.Your best bet is probably NiMH, get some sub-c NiMH batteries, the sub-c ones have a bit more capacity than the AA variety and can provide their rated capacity at higher discharge rates, a typical size cell is about 3000mah.Put them in a series string, say 18 cells, and use a buck converter module to get your 12v, because NiMH cells don't like being charged in parallel(I assume you don't want to have to change from series to parallel when you charge/discharge)Charge them at 0.1c (1/10th their capacity per hour, 300mah battery, 300ma charge rate, fully charged in 10~ hours), pretty much all manufacturers batteries are able to be charged at 0.1c indefinitely without any issues (example).You can get cheap, reasonably high quality one in series strings for use in older RC models for examplethese.Simple enough and should get probably 3-4 hours runtime, remember, the amp/speakers will not use a constant "12V@2A", they will use the power based on how loud the music is, so even though you have say... 65Wh of battery, and say 50Wh useable after the buck converter (assuming 80% efficient), the amp won't be drawing 24W at 100% dutycycle, maybe 50%, so 12W on average, giving you about 4 hours of use.

author: malium
author_rank: Contributor
timestamp: September 10, 2013, 06:31:26 am
content: Thanks for Your help, Peter.Few things, if You would not mind:- why 18 cells? Is that better than having 10 of them (~12v)? Why?- Buck converter, got it. While at it, what with having 6v battery and stepping up? Possible?- What with the switching between power supplies? Or do You mean it would always be run from batteries? Could it be operative while charging?

author: malium
author_rank: Contributor
timestamp: September 11, 2013, 08:28:07 am
content: Quote from: malium on September 10, 2013, 06:31:26 amSorry I have to bump like this, I have some more questions, and no answersCould a begginer, but of the "want to learn" kind, and "I wont stop till it's finished" kind, use the BQ24721C IC in his diy circuit somehow? Is this the solution for my troubles (dc adapter inside, battery outside, automatic switching and charging)?How could one use this with just a basic soldering skill set, nothing advanced?Any comments or advices?Many, many thanks!