Of course you can charge the batteries on other ways. Have a look at http://www.sounddevices.com/download/guides/xl-charge_en.pdf
Cheeper chargers you can maybe find in onlineshops, have a look with google. Also there are chargers you can plug-in in the car, I don't know the name in english, the thing you could light your cigarette, you know.
But be carfull, not all of the cheep chargers brings the correct/enough power to load the battery in usefull time. For my 788 I use external SWIT-Batteries. I use http://www.proav.co.uk/Swit-Shop/NP-Style-Batteries/sc10151.aspx the NP ones but probably older than this. With a SWIT-Adapter and sound devices power-plug you can connect it directly to the recorders power supply.
With that, I can record with 6 Channels for about 4 houres. If the external battery is empty, you can change it without digging the internal recorder's battery out. If you have a second SWIT-Battery, you can connect it and load the empty one or use the internal.
I'm quite happy with the SWIT's, you have to be careful if it get's cold. Instead of lithium batteries, they discharge if they get too cold. But I used them at my recording-trip in Tibet this year (have a look at my site: https://www.soundeffects.ch/news/new-sound-recordings-from-china-and-tibet.php ) and had not too much problems. Put the battery in your jacket to keep it warm if the recorder is off and take it in your sleeping bag like a teddybear. They like it and keep charged.