Lots of these Li-Po battery packs on eBay, going for around £12 from various vendors:
Nice hand-sized battery pack, with a coax plug and socket on flying leads, and a handy little power switch and LED. Comes with a somewhat underspecced charger (350mA – so, around 20 hours to charge).
The battery specs seem too good to be true, though. 6800mAh at 12V? Nearly 7Ah at a fraction of the size of an old fashioned gel battery. Hmm.
To test the battery capacity, I hooked it up to a little LCD telly via a Turnigy power analyser.
The TV takes about 0.3A, so it ought to run for about 20 hours. Ha! To start the test, I charged the battery fully with the supplied charger:
12.77V is a little high, but it’s straight off the charger; it’ll relax back to around 12.6V.
About 6 hours later, it was time to switch it off before the battery flattened itself permanently (10.8V is as low as you should go):
So. Not really 6.8Ah at all, more like a third of that. I think I know what they’ve done, though (assuming they aren’t just out and out dishonest): to create a nominally 12 volt battery, they’ve strung 3 cells together in series – but then they’ve mistakenly added the Ah capacities of the cells together. Connect 3 cells in series, you add the voltages, but Ah stays the same as a single cell; connect them in parallel instead, and you add the Ah up but the voltage stay the same as a single cell. They’ve mixed it up.
The headline, therefore: these battery packs only have a third of their marked capacity. Caveat emptor etc.
The upside (!) is that the little charger that comes with it takes half the time to charge it.
UPDATE: A kind commenter, Unferium, notes below that Li-Po cells can be safely discharged to about 3V per cell rather than the 3.6V I used, which does change the results slightly. The voltage drops off very fast from 10.8V down, so you only get an extra 0.25Ah out of the pack. I think the headline stands
Anyway, let’s see what’s inside. Note: Lithium batteries can explode or burst into flames if mishandled, or if they’re faulty. Do not take one apart unless you know what you’re doing, or you’re an idiot. Thankfully, I’m fully qualified in at least one of those categories.
First, off comes the outer blue shrink-wrap:
… revealing a stiff cardboard “case”. It comes apart easily to reveal:
The actual battery of cells is just the silver chunk; the dark strip on the left is just dense packing foam. Shame they couldn’t just make the whole pack a bit smaller instead – I can’t see what benefit that padding does given that it’s only on one side of the cells. Even if something bad happens to the battery and the cells start expanding, they’ll blow up like a pillow, not out sideways. Ho hum.
Let’s pull the cells out:
Yup, three Li-Po cells in series. Thankfully they’ve each got their own protection circuit (which disconnects the cells before they get overcharged, or so discharged they’re unsafe to charge again):
The 3 cells are extremely securely glued together, so don’t try pulling them apart. If the foil envelope around a cell is punctured, they give off a strangely fruity smell and need to be disposed of safely in a neighbour’s bin. (Not the nice neighbours, the ones on the other side)
Despite their misleading label, they’re still useful battery packs, particularly for Arduino / microcontroller use. 2.2Ah at 12V is still plenty of juice for some projects. Here’s a little wireless monitor I rustled up to monitor our solar panels:
It’s just an LCD, an Arduino and an nRF24L01+ radio module, all cable-tied to the battery:
Lasts for a couple of days between charges, and it’s surprisingly robust. Easy to recharge with the charger that came with the battery (albeit a bit slow).
They’re not as good as they say they are, but they’re still handy.