Looking for Li-Ion advice

[Jack.Straw]

Hello. I realize that this is an electronic vehicle and tech forum, but i'm hoping to get some sound battery advice and I figure you guys probably know your stuff. I like to build portable amplifiers for media players/phones. Up until now i've been using NiMH batteries in low power 9v amps. The battery life is incredible, but the sound is just ok. I'm looking to move up to a 12v class-D amp board and a 12v Li-Ion pack. I've been doing lots of reading on the subject, but the product searching has been intimidating. Of course i've found lots of ebay packs that claim big mAH, but a lot of forum scouring has revealed that these are mostly very low quality packs that don't actually live up to their claims, and are also potentially dangerous. I'm wondering if anyone here has any recommendations for a battery & charger solution. I've built my own NiMH charging circuits in the past, but the Li-Ion tech is more intimidating and potentially dangerous, so i would prefer a pre-fab solution if possible. Here is what my project requires:

1) The battery will be enclosed inside of the amplifier, and therefor can not be removed for charging. Should the battery fail, i can get inside to replace it, but regularly removing it for charging is not an option.

2) The charger can be internal or external... i'm not picky here. I can easily put a power input on the unit and jump that to the internal battery.

3) I'm looking for as much battery life as i can get (of course!)

4) My budget is low. I cannot entertain top of the line, high-dollar solutions for this.

5) I would prefer an easy solution, but would certainly entertain using things like replacement laptop batteries, and even creating my own battery pack if a protection circuit / charger can be recommended.

Thanks for any advice you can offer!

 

[Sunder]

I don't know how sensitive your equipment is to voltage variations, but odds are, if you use Li-Ion, you'll need voltage regulation. For example, Panasonic NCRs are full at 4.2v, and dead at 2.5v. So for a 4 cell pack, your equipment needs to tolerate 16.8v to 10v.

If you use LiFePo4 cells, they're heavier, but fully charged is 3.6v, and there's not much left after 3.0v. So your equipment would only need to tolerate 14.4v to 12v - much more like a 10 cell NiMh.

Here is one example:

http://m.ebay.com/itm/201358956387

As it says, with a built in battery management system, just supply 14v, and it takes care of the rest.

 

[e-beach]

I run my Roland Cube Street and Roland Mobil Cube on a 3s2p generic 18650 cells I got from a new 12 cell computer battery I got from ebay. I pulled the cells out of the computer battery pack and printed battery holders on my 3D printer. I built the pack with charge and balance wires for a 3s pack. I charge and balance them with a generic iMax B6 charger. From testing through an inexpensive RC watt meter I know that my batteries will last at least 20 hours of usage, I never worry about over discharging them because I charge the after every use. For me that is usually less then 6 hours of usage in between charges. I do not use a volt meter or a bms on these packs, but for a layer of protection both a bms and a volt meter would be useful in a amp with an internal battery.

At full charge my pack is 12.6v. I would only let the pack discharge to 9v for a required recharge, although if it dropped to 7.5v the cells would probably be ok. At 12.6v I am over-volting both amps from their power supply specs, but they seem to tolerate it.

Questions:
What are the highest and lowest voltages your amp can tolerate? The manufacture might not help you out on that one but it is worth a try contacting them.

Do you know their amp draw?

 

[Jack.Straw]

Sunder, thanks for the reply! My application is very tolerant of voltage variance, accepting anywhere from 8v to 24v. I had not heard of a Lithium "smart battery" with the charging and protection built in. That would be rather ideal if i can find something like that with smaller dimensions (and as a result, likely lower mAH i suppose). Thanks for the suggestion!

E-each, thanks also. The amp can tolerate 8-24 volts. As far as draw, i'm not really sure. I expect that depends party on the speaker's ohm rating and the volume? The amp chip says 1x 60w, but i expect that is with a 4 ohm speaker. I'll likely use an 8 ohm speaker. I'm afraid i'm not knowledgeable about such things to make any sort of calculation. I will ask and see what kind of answer i get. Oh, and i guess i should mention that the bluetooth module will run on the same battery, but it requires very little power. I've got an email in with the seller to find out the unit's voltage requirements. I expect 5v is what it actually uses, but it likely has a larger tolerance range than that. If not, i can easily build a simple voltage regulator for it.

 

[e-beach]

I would suggest making your amp setup the way you want it to be, and the use an existing battery or power supply wired through a rc watt meter to learn the exact watt draw needed for your internal battery. Then get the lithium batteries and bms and charger to build a pack, or to purchase a pre-made pack that fits your requirments.

Knowing your amp draw is critical for the use lithium cells as they can heat up when used improperly.

 

[Sunder]

Smart battery is just marketing fluff for a battery with a BMS pre-installed. Depending on their intended use, this is either common, or not heard of. E.g. A battery for an eBike, Almost always has a BMS installed. For use in a RC car or drone? Almost never has a BMS.

As ebeach said, best to measure the load before getting a pack. NiMH can deliver a lot of current, especially for "bursty" applications (e.g. subwoofer heavy music?)

 

[dogman dan]

Here is where you get a quality 12v pack, in a chemistry that is known to be both long lasting, and typically safe. 5 ah,, 10 ah, whatever you need.

http://www.pingbattery.com/custom-lifepo4-lithium-batteries/

A 5 ah cell ping uses can easily put out 8 amps continuous, and 15 amps burst. Go as big as you can of course, for both lifespan and ability to make the speaker powerful. Lifespan should be 3-4 years of heavy use, if you keep it down to 1.5c rates of discharge.

The other thing that comes to mind right away, is an external battery,, using 18v 4 ah drill batteries. You'd plug a drill pack into the box, likely using the socket from a drill or other cheaper device. Then just charge with the same charger you'd use for a drill. These packs are about $100 on full retail.

Definitely,, you need to run your device on a 12v power supply first, so you can put a watt meter on it and get real world specs for the draw in watts. Otherwise you are just taking wild guesses as to how to spec the battery. Once you have the data, you will know what size you need to keep the c rate lower, and to increase the run time.

One last thing,, do you mail this stuff? If you do, mailing lithium batteries is a bitch legally. Have them supply their own drill battery?