Let's take Kodak Playfull HD camera, inside it has such battery with 3 wires:
video
Fully charged it works for about 2 hours, and I need more, so I want to add external source. I think I can connect batteries in parallel, it means just connecting to these wires (2 of them, and third works for charging probably). Is it right?
Add a battery to some camera device
batteriescamera
Related Solutions
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:
Since at this stage I don't need a lot of energy stored in the battery, the single Lithium cell + booster circuit idea sounds really appealing, especially since I can easily find a housekeeping module that does the charging while also feeding DC to my device. Of course, now I have to find a suitable booster; ideally something based on an IC with few components around it. Amperage requirements are pretty meager. I need to build two, one for 5V and another for 12V.
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.
The optimal solution would be to use rechargeable battery cells that have adequate capacity to run the fish pump for the duration required. Assuming that you've already got the batteries and chargers and are trying to make this work.
You haven't told us, but I'm also assuming from your post that the battery chargers you have are designed to charge 4 cells in series, rather than individually. I'm also assuming that this is a backup DC-powered pump and that you have an AC pump that aerates the fishbowl when the power is on.
You need a line-powered DPDT relay with contacts that are rated for the larger of the charger current or discharge current. Also note that you may want to design some form of low voltage cutoff, as your pump will fully deplete your cells, likely causing cell reversal.
Apologies for the chicken scratch schematic:
When the AC power is on, the relay is energized and the batteries are connected to the chargers as independent series (1P4S x2) strings. When the power fails, both packs are connected to the pump in parallel (2P4S), increasing your runtime. Note that there are a few aspects of this approach that will lead to shortened battery life, but hopefully its enough to get you up and running.
Best Answer
Probably not without risk.
It MAY just work well enough, but more likely it wont.
More information needed, but, the battery is probably a LiIon (Lithium Ion) and the 3 wires are probably either
+ve, -ve, temperature sensor OR
+ve, -ve, dark and nasty special circuit.
If the 3rd wires is a temperature sensor it will usually be s NTC thermistor. Placing two in parallel makes it appear that the battery is too hot.
Disconnecting the feed from one leaves it unprotected.
If the 3rd wire is connected to a special circuit, connecting two in parallel may make nothing happen or just about anything happen. Not usually recommended.
In either case, parallel connecting two LiIon batteries can cause, in some cases, enough flame and smoke inside your camera to make it look like a small firework device. This is usually not what you want to happen. .