Battery selection. What battery do i need?

Thread Starter

Mattumber92

Joined Jun 26, 2020
9
I am making a portable sound system and I need a 24v 2a dc power supply. I'm thinking about using 18650s in a 4 cell 4 batteries per cell configuration but the cost is a bit much. Would 2 12v batteries be a better choice?
 

KeithWalker

Joined Jul 10, 2017
1,019
The best choice of batteries will depend on several things:
Lowest price of batteries and chargers will be with lead acid batteries.
Lightest will be lithium batteries.
If it is to be used often, and recharged after use, lead acid will give the longest life.
If it will sit on the shelf for long periods of time between uses, lithium batteries will be best.
Regards,
Keith
 

crutschow

Joined Mar 14, 2008
25,095
You need to decide how long you want it to run between charges, as that will determine the required ampere-hour capacity of the battery.
The system likely will not draw 2A continuously, so you need to measure the current with a multimeter under typical operating conditions (sound level, music type).
 

Irving

Joined Jan 30, 2016
763
Lithium batteries now give most bang for the buck, 3000-4000 charge cycles and maximum life time (>15y) but avoid the small cells as these are poor vfm. The safer LiFe chemistry is easier to manage/charge compared to the slightly higher capacity Li-Ion versions. Lead-acid will be 3x the volume and 1.5x the weight of Lithium and for peaky currents as you might find in a high power sound system will give a shorter run time than the same capacity Lithium.

As has been said, you need to say how much capacity and give a better definition of portability!
 

MrSoftware

Joined Oct 29, 2013
1,798
Also consider your application and decide if safety is a concern. Lithium batteries are among the least safe. So if it's a sound system that you take to the beach, maybe this isn't a big deal. But if it's going to be used indoors, especially if it's going to be used in a bedroom, think a little harder about the battery choice. Long story short, make a chart of your requirements. Technical requirements (capacity, size, ..), your budget, and your safety requirements if any. Then use that chart as a guide to help you choose.
 

Audioguru again

Joined Oct 21, 2019
1,562
24V at 2A in a stereo amplifier is 24V at 1 A per channel. If the amplifier is an old fashioned single-ended class-AB type then the output swing will be about 20Vp-p which is 7.07V RMS which produces 12.5W into a 4 ohms speaker and the amplifier heats with about 11W. Then the battery must supply 23.5W per channel which is about what you have when it plays a continuous tone at full power.

Modern audio amplifiers are bridged then require a power supply voltage almost half to produce the same power as the old single-ended amplifier. Look at a TDA7377 amplifier IC.
Wait a minute. ST Micro buys the TDA7377 from China and the datasheet has NO DETAILS AND NO GRAPHS!!

Texas Instruments make many modern audio amplifier ICs. Most are bridged and some use class-D for only a small amount of battery-wasting heating.

You and I have many "dangerous" products that use a Lithium-ion battery with no problems. Laptop, phone etc.
 

Irving

Joined Jan 30, 2016
763
Li-ion are 'safe' as long as you follow the guidelines - Samsung Note7 being a case in point if you don't.

LiFe are 100% safe even if you abuse them mercilessly!
 

Audioguru again

Joined Oct 21, 2019
1,562
LiFe04 batteries have a low capacity. They can still cause a fire or explosion if abused. One article says, "There is a whole lot of energy stored in a charged battery and in case of an unplanned discharge the results can get very interesting very quickly!"
 

Irving

Joined Jan 30, 2016
763
Sorry, but you're well out of date. LiFePO4 are virtually indestructable - I know because I've tried. There are many articles on the web that are ill informed or confused about which chemistry they refer to. If you puncture a modern LiFePO4 cell nothing happens. We even drove a tank over one! Of course if you short it out the shorting wire gets hot and burns, but the cell itself remains inert - whereas a Li-Ion gets hot and burns itself out internally til it explodes, throwing burning electrolyte everwhere. A LiFePO4 cell will gas at high temperatures but won't ignite til about 450degC compared to Li-Ion at <170C and even then its self-extinguishing.

As to capacity... yes LiFePO4 are lower capacity than Li-Ion, maybe 80%, but that's not the important measure. Compared to any other battery technology they are the best balance of safety and capacity. As I write this I'm in my powerchair, sitting on 5Kw of LiFePO4, that's 24v @ 210Ah, of 100% usable energy in less space than 2 x 60Ah mobility batteries which struggle at best to give more than 50-60% capacity, say 850W when new. I wouldn't be sitting here (unable to run away) if I didn't know they were safe. I'm one of a group worldwide that have actively been building and testing LiFePO4 packs for mobility and over nearly 10y or more there hasn't been one recorded incident of battery failure... plenty of human failure which can be an issue when you have a pack with a short circuit capability well in excess of 1200A. New LiFePO4 cells coming in 2021 will increase usable capacity to 300Ah - thats 5x what the best lead-acid battery can deliver and getting closer to Li-Ion capacity with better discharge and charge performance.

My current pack has a life of around 4000 charge cycles, that's easily a 15y+ lifespan to 80% capacity for about £1100, compared to a set of lead-acid gel batteries every 12-14 months at £400 a set (power wheelchairs used properly murder batteries!) It gives me a range of about 60 miles, day in/day out compared to 12 or less on lead-acid (when new, <6miles after about 9 months). I don't have range anxiety any more! The chair has more range than I do! And I don't have to worry about charging every night; anyway they charge at 40A next to my bed, empty to 97%+ full in just over 5h, 100% in around 8h. I do have a cell voltage/SOC/temperature monitor (my own design, charging/monitoring controlled from phone by BT) but they barely get warm - the 10AWG silicone-sheathed charging cable gets warmer! I'm more at risk having the phone on charge on the bed.

A friend of mine is head of a research team developing the next thing in batteries. He reckons 500Ah in the same space is certainly possible within 2 to 3 year.
 

Thread Starter

Mattumber92

Joined Jun 26, 2020
9
Okay well the amplifier I'm building is an a/b amp that has 2 channels 100w each. It's rated at 24v 2a. I'm going to be hooking up 2 14" 75w speakers and 2 20w tweeters. I of each per channel Hoping to have atleast 6 hours of play time. I'm thinking lithium would be a good choice. The battery cell I'm planning is going to be4 18650 in parallel and 4 cells in series to increase Ah, with a voltage regulator to make it put out 24v. What are everyones thoughts on that?
 

sparky 1

Joined Nov 3, 2018
246
The 13.6V small garden/mower tractor battery on sale might give you more amp hours. Compatible with car charging.
A 12 to 24V converter and capacitor bank might deliver what 24V is capable of.
The battery box cables, connectors and carrying handle makes the project mobile ready.

available at harbor freight discount auto parts
 
Last edited:

Irving

Joined Jan 30, 2016
763
4S = 14.4v @ 100%soc down to 12v @10%soc
4P = 12Ah typical with 80A peak discharge

Capacity 12v * 12Ah = 144Wh

Use Boost or step-up converter to 24v (not voltage regulator) @ 85% efficiency gives 144 * .85 = 122Wh

@24v 122Wh = 5Ah or 2.5 hour runtime @ 2A continuous drain, less for peaky current draw like music

But 24v@2A = 48W, not 100W. The peak output is 100W * 2 channels = 200W peak @ 24v = 8A so your boost converter needs to be capable of 8A peak output to avoid distortion on loud passages, or you'll need some big hold-up capacitors on the power leads to the amp.

So you have less than 1/2 the capacity you need and a boost converter is a poor alternative to having a properly sized 24v battery to start with.

BTW, if you were planning on using 6000mAh or similar capacity cells be very wary, there's a reason the main manufacturers Panasonic Samsung Sony etc only offer 3500mA Max... Fakes abound.
 
Last edited:

Audioguru again

Joined Oct 21, 2019
1,562
How can an amplifier produce 200W when powered from only 24V?

Since the amplifier is 2 x 100W class-AB then the power to the speakers is 200W and it heats with about 90W then the battery must produce 290W. Then the battery current is 290W/24V= 12.1A!

You want a 24V battery then the amplifier with no part number will produce at least 20Vp-p into each speaker. 20Vp-p is 7.07V RMS which is a power of only 11W into 4 ohms, but your speakers have no impedance.
If the amplifier with no part number is bridged then its output power is 39W into 4 ohms.

See the problem? Not enough battery voltage for the amplifier to produce 100W per channel.
If the amplifier is bridged and can drive 4 ohm speakers then a 32V battery can be used to make 100W per channel.

18650 lithium battery cells are 3.8V each when run for a while. Then four in series make only 15.2V which is way too low.
 

Thread Starter

Mattumber92

Joined Jun 26, 2020
9
Also 200w x2 is the max it can produce. I'm not expecting it to be continuous. I'll probably need 2 to produce what I want.
 
Top