Power requirements

The gumstix require 3.4V min, 5V max. Without wifi running, a 400Mhz gumstix draws < 250mA. The Ambicon WL1100C-CF 802.11b card dissipates < 370mA for Tx, < 250mA for Rx, < 100mA for standby.

This is nicely compatible with Li-Ion cells which charge up to 4.2V and then discharge down to 3V with a nominal voltage of 3.6V. A standard cylindrical Li-Ion cell such as those used in building laptop battery packs has a capacity of 1400mAh to 2400mAh. The LG 18650 series of Li-Ion cells are 18mm in diameter by 65mm in length.

When considering the choices for battery power it's useful to know the dimensions of the various gumstix cards:

• cfstix: 28mm x 80mm
• gumstix 400 XM: 20mm x 80mm
• audiostix: 34mm x 80mm

Battery hookup

The power connector on the gumstix expansion boards has a 0.65mm center pin (positive voltage goes here). Connector is a CUI Inc. (cui.com) PJ1-022-SMT, digikey part number CP1-022PJCT-ND ($.65 each).

The corresponding plug is CUI #PP-012, digikey part number CP-012-ND ($.90 each).

A good choice for a battery might be a two-cell module with a built-in battery protection circuit (prevents against overcharging and shorts):

• picture
• dimensions: 34mm x 15mm x 63.5mm
• capacity: 4800mAh (= 19 hours @ 250mA)

We could just stick the appropriate plug on the end of the battery pack leads and plug it into the gumstix cards. For recharging, the pack could be unplugged and connected to a recharger equipped with the same power connector as on the gumstix boards.