(I used to design and build electronics – HiFi, automation, digital systems up to and including CPU-based controllers – so I have a good understanding of the basics. I just stopped being involved in the field about 15 years ago, so my knowledge of what's currently available in terms of components is quite foggy.)
The device is a controller for a scientific instrument. Removing batteries for charging every once in a while will be cumbersome. Plus, I want seamless transition between batteries and AC (well, not AC per se, but external DC at a higher voltage).
The battery needs to be 12 V or a bit higher. In terms of how much energy it needs to store, something like a small laptop battery should be enough.
So, basically, I need to integrate the battery charging circuit with the device. The battery will not be (easily) removable. The device will provide a port for an external DC supply for charging. Switching between internal battery and external DC must be seamless. It must handle automatically all the chores related to battery charging (regulate the current, stop charging when fully charged, stop the device when fully discharged and there's no external DC, etc). Indicating the current charge level would be nice.
Any suggestions?
I'm pretty sure I could use an actual laptop battery for this purpose, I just don't know how to connect it.
Could I use 4 x LiPo or 4 x 18650 elements in series? I'd have to use some kind of battery charger/controller, I'm pretty sure such a thing already exists, I just don't know where to start looking for it. I know these elements are finicky.
Any help would be appreciated, thanks.
Best Answer
A laptop battery is liable to be a good choice if the Li-ion characteristics suit you. These may have 2 or 3 or 4 cells in series. Some provide access to all cell connection points, some don't. Those that don't may have an internal controller to maintain cell balance.
If this is a one off or low volume application you may want to look at using whatever the related laptop uses. If this is for large volume use then Digikey and others sell a range of suitable ICs. An alternative is to use fewer cells and a boost converter. There are many LiPo single cell batteries available for tablets/phones/pdas/ ... . There are numerous 2 cell batteries available for cameras. These are usually dearer per capacity unless you buy aftermarket batteries. An advantage of a camera battery is that there are usually low cost aftermarket chargers available which target a particular camera battery type and which do a good enough [tm] job of battery charging. The price of such chargers is often low enough that building in a commercial charger into a product may be $ attractive.
If you want 12V minimum then you will need 4 Li-ion cells - about 12V minimum (you choose) and just under 17V fully charged.
A possible alternative are sealed lead acid cells. Cheaper per capacity but lower mass and volume energy densities and lower cycle life in deep discharge use.
LiFePO4 (Lithium Ferro phosphate) has lower voltage per cell than LiIon and lower energy density but potentially much greater cycle life. Long term LiFePO4 offers best cost pe cycle but initial price is high.
NiMH - not recommended.
Added:
Florin commented:
This IC will provide up to 80 mA AT 18V (100 mA+ AT 12v) or 280 mA at 5V out from a single LiIon or LiFePO4 cell. In stock at Digikey for ~ $2/1.
TI / NatSemi LM4510 Synchronous Step-Up DC/DC Converter with True Shutdown Isolation
Efficiency is "OK" across a reasonable load range.
And circuit offers a bearable level of complexity.
An evaluationkit is available - whose PCB gives good pointers to proper layout:
A single Li-ion or LiFePO4 cell will power this well. The latter has lower energy density but has the advantage of longer cycle and better table manners generally. A Li-ion 18650 cell (as used in most laptop battery packs) will give about 7 - 9 Wh when new or say 6 Wh after boost conversion. A LiFePO4 18650 cell will give about 50% of the energy content of a std Li-ion cell. Say 3.5 to 4 Watt hours.