Charging a battery with a solar pannel while its in use?

I want to keep a project of mine running for days without having to stop. Ill be out in the fields so i can't use an AC to DC adaptor. I was thinking can i use two batteries in parallel(same voltage double capacity) and charge them during the day while the project is still running?

I'll be using two http://global.ebay.com/Li-ion-Battery-SB-L480-SBL480-for-Samsung-74V-6000mAh/130417784146/item batterie

I wil have a diode to stop my batteries from discharging during the night

The project will be my camera(nikon D80) doing time-lapse for days without stopping.

Can i use the two batteries in parallel and charging them while they are being used?

If yes can i do it without destroying my camera?

What should i do and how?

Yes, you can parallel the batteries, and use them while they are charging.

You don't mention what you are using to charge them. You need a charge controller, as Li-ion batteries can't handle ANY overcharge without damage. The charge controller needs to be configured for this battery type and size.

You may be better off with Lead-Acid batteries, which you can overcharge forever, as long as you keep the charge rate below the trickle charge level.

"destroying my camera"? As long as the voltage is correct for the camera, you should be OK.

edit, missed the "solar panel" reference in the heading. But problems still remain, see above. I looked up the camera, but they don't list battery voltage or current, so those are unknown.

Lithium ion batteries have special charge requirements, particularly the voltage must never exceed certain limits, so you would need an appropriate (specialised for lithium ion) charge controller between the solar panel and the battery. These are probably hard to find. This applies in particular because your battery is not a 12V battery, which is what most solar panels and solar chargers are designed for. Panels usually have 36 cells designed for a 12V lead acid battery, and that is about 21V no load and is intended to deliver about 14V to the battery when the panel is hot and under full load charging the battery.

There are lead acid batteries of similar capacity (6Ah) or you could get a single one of higher capacity that is around 12Ah. These are more suited to the purpose, in that the suitable chargers are readily available. The voltage is 7.4V for the lithium type, so I assume this is the voltage for your camera. A cursory glance at the user manual doesn't enlighten much, though it does mention 6 cell batteries like Ni-MH which are 7.2V nominal, but higher up to 9V when fully charged. You could use a 12V battery with an 8V regulator between it and the camera it seems. This can be quite simple, there are 3 terminal regulators like the 7808 that handle up to 1.5A with a suitable heat-sink. I think you might need to ask Nikon how to connect the battery, particularly for voltage and current needs of the supply regulator. Presumably it replaces the AC power unit, so its specs would help (output voltage and current). FInd one in a shop and check its ratings, maybe.

The next question is how much battery capacity do you need. How did you arrive at 12Ah? This is how long the battery delivers a particular average current for. In theory you need enough to last for a day or two considering it is being recharged, but there may be cloudy days to deal with.

The size of the solar panel:
To know the minimum battery size and the panel size you need to know how many ampere hours a day are being used by the camera. The panel then has to replace this with 1.4 times as many Ah. If you use 4Ah per day then you recharge with 5.6Ah. If there are 3 equivalent full sun hours a day, then the panel has to deliver 5.6Ah/3h = 2A. You would need at least a 2A panel for this scenario. Panels need to be well ventilated (not on the grass or a table).

The link below shows a suitable charger, but it is not suitable for a solar panel as it stands. Best to charge a 12V battery from the panel, then charge your lithium ion battery from this charger, despite inefficiencies.

I recommend using a 12V battery and a suitable regulator to suit the camera (8V seems likely). The 7808 is fixed at 8V and up to 1A or a little more. See second link. There are also adjustable regulators (LM317) for other voltages and higher current versions if the camera needs more than 1A peak. You may find a ready made car type supply to suit the camera from a 12V car battery.

It is not unreasonable to charge a battery while it is being used, but the camera needs to accept these voltages, so the panel output needs to be controlled to suit the battery, which in turn is suitable for the camera.