Best value Lithium ion/polymer cells/batteries

Question

I've built a sound reactive, android controlled, wearable RGB LED matrix/suit which I'm currently powering with a rather heavy 12V 3Ah SLA battery. I'm looking to switch to lithium or ni-mh packs and leaning toward lipo for weigh/size/easy of charging/low self discharge etc etc. The suit's current draw is 500mA - 1000mA average and about 3000mA peak @12v depending on mode, and I'd like it to last a night out parting ( ^-:D-^ ) so atleast ~5 hours, I also think it would be best to have the battery rated 14v or above and run a switchmode buck regulator to 12v to provide full brightness over the runtime of the battery and also reduce the direct current draw from the battery and thus ESR losses (although I think these would be low with decent lipos but maybe if I get really cheap cells I'll get better capacity from drawing less current? ) thus I think I'll need about 5000mAh capacity @ ~14v

Now the crux of the matter what are the best value cells to go for in terms of cost per energy density (or cost per capacity/weight)? I don't mind buying packs or building ones from single cells. At the moment I can get 10x 18650 lipo cells off ebay for ~$17 shipped and a 5S2P arrangement should give me ~5000mAh @ ~18.5V which should buck down nicely even if the cells don't have a great ESR? opinions welcome :)

Answer 1

Among mainstream battery technologies, LiPo has the best energy density, so you will probably want to go this way. In your case, since you will use a regulator anyway, every configuration possible (including single cell combined with a step-up boost regulator). Once you have chosen the total energy capacity you need for your application and the cell configuration, there are two parameters to consider:

1. Quality: for the best specific energy, you will need to get high quality cells, which are going to cost more.
2. Discharge rating: cells are usually rated for a given maximum discharge current, usually expressed relatively to its capacity (for example, a 10C rating for a 1000mAh cell means that the max current is 10*1000mA = 10A). While higher C rating may mean a lower ESR, there is a trade-off and increasing C rating significantly lowers the specific energy. As an example, a 40C 2000mAh cell from a high quality chinese manufacturer I was looking at recently had a specific energy of about 140 Wh/kg while the 15C version had a specific energy of 170 Wh/kg. This difference can largely offset any loss due to slightly increased ESR.

Whatever configuration you will end up using, your require that the battery lasts at least 5h. Assuming little loss in DC-DC regulation, this means that the average current would be at most 0.2C and the peak current (from your numbers) about 0.6-1C. I would thus look for 5C-rated good-quality cells, which should get you 180Wh/kg or more.

Answer 2

I know that it has been a while, but if our still looking at batteries for your suit..... I agree with above, Lipo is the chemistry you would want to give you the run time and lower weight.

A few considerations: 1) Those e-bay 5000mAh Lipo batteries are not going to work. I know from experience. I have bought some and tested them - averaged at around 550 mAh - 1/10 of what they advertised!

2) Lipo can catch fire/blow up etc if you really don't know what you are doing.

3) LiPo have nominal voltage of 3.7 so 4 in series would give you 14.4 volts, but fully charged they are 4.2 volts giving you 16.8. If your lights require exactly 12 volts, you will need that buck.

4) LiPo should NOT be overdischarged - your batteries or battery pack or LED driver will need something to tell it to turn off when the voltage drops to 3.0 volts per battery. (you could go a little lower, but 3 volts is safe and healthier and keeps you at the 12 volts you need)

5) If you build a battery pack out of LiPo, and in this case it would likely be a 4S2P, need to be able to charge the pack properly. Overtime voltage of each of the cells in series can start to vary and need to be "balanced" so that they all are charged to the same 4.2. This requires you to either charge each battery separately(which might be hard to do if you have soldered them together), or buy a balance charger and wire balance wires to your pack.

6) Probably the most economical place to buy quality Lithium ion batteries is laptop battery packs. Used or even batter, brand new laptop replacement battery packs are reasonably priced and will give you quality batteries at a rating of 2200 to 2600 mAh.

7) After you get this suit working with Lithium batteries, you must post a picture of yourself out partying! This battery pack will be a lot lighter than the SLA.