Hi All,
I have an LED project that needs to be powered by battery and offer at least 4 hours battery life when all LED's are lit (it has 18 1/2W LED's).
Please be patient with me, lithium powered systems are new to me, and after working in the aviation industry I am well aware of the safety precautions and what happens when cells go bad!
Cost is an issue as I want to make several of these, but happy to spend a little more if it makes it reliable and SAFE!
I have tried using 10x NiMh AA's, and in all fairness this didn't work too badly. These were 2500mah rated batteries, with discharge graphs (done by people on other forums) suggesting they aren't far off the mark. The problem with these is that as the voltage drops, so does the led intensity. They are also quite bulky in reality.
My next idea, was to use Lithium cells. So I bought a ready made pack that came with a charger as well. It is made up of a 2s3p 18650 configuration and is rated at a rather conservative 1600mah per battery thus giving me 8.4V, 4800mah with a nominal voltage of 7.4V. I wired it up and to my disappointment, only got 2 hours 37 mins out of it. Needless to say I think I've been hit by the ebay false battery claims! I've attached a discharge graph, and at 4.3V the battery protection kicks in and stops the pack functioning until it receives charge.
So now I'm looking for a better solution. I've decided on two possibilities so far, but this is where I need help. The Turnigy branded packs (at HobbyKing UK) have had good reviews and in some cases even exceed their rated current. These have no protection though so would need that and a charging system. I found a 2s/3s constant current Lithium charger on ebay, which is fine but as this only handles charging, I need a protection circuit too - can the likes of these:
eBay auction: #http://www.ebay.co.uk/itm/1X-2S-Li-ion-Lithium-Battery-18650-Charger-Protection-Board-Module-3A-7-4V-8-4V-/191887828279?hash=item2cad67e537:g:hMMAAOSwnNBXUPcV be wired without the balance connection just to offer undervoltage protection, or is there a better solution?
My next idea was to use a bank of 8 individual 18650's in holders and wire them up in a 2s4p configuration with the same 2s protection board above, but then I'm basically in the same boat, and individual batteries were coming out quite a bit more expensive than the Turnigy packs.
The main problem I have is that I need to be able to power the device AND charge at the same time. My thoughts for this were to use a dpdt relay and simply switch the output terminals off from the battery when plugged in.
A bit more info on the project is that for the Lithium battery option, I've got a DC-DC Boost Converter based on the XL6009E1. I'm not keen on it's quality as the output cap gets quite hot really considering I'm only drawing about an amp when it's rated for 4A. I also only got about 87% efficiency from it (calculated by doing input power consumption vs output power consumption). I have therefore opted for the MT3608 based unit. This runs at 1.2Mhz instead of 400Khz, doesn't need as higher rated parts and should therefore be more reliable and cooler equaling greater efficiency. I'm still waiting for this to arrive. In all fairness, the battery module I have currently could be OK and the efficiency of the boost converter could be the problem, because as the voltage drops, efficiency goes way down.
Any advice is very welcome. Many thanks,
James