After lack of potential voltage on this battery (modem sierra 875U/881U) then "the red light" on the lower indicator LED is glowing, and the connection will interrupt, will close. Normally when the potential voltage strong enough for the modem the green light that will glow, and the connection will occur if the POWER LED indicator and DATA LED indicator (upper LED) both is glowing.
REPLACEMENT FOR THE BATTERY
The best replacement for this battery is the original one bought on Sierra's Battery Shops, but when you just like me, not found the best one,.... one of probabilities to make modification with other battery. With the certain voltage as high as sierra battery's voltage, that : 3,7 V with 950mAh, you can buy battery with the same voltage and ampere (electric current), I bought BL-5C battery with 3,7V and 1000mAh, it doesn't matter when ampere (electric current) bigger/higher than default (950mAh), because the machine (modem) will use electric current as much as it needed (aka:950mAh). But when we talk about voltage, you can not use the higher voltage for every machine (include modem, TV, radio, etc).
Prepare the tools that we needed, wires (use two different color, red and black, each length about one inch), soldering /iron soldering, tin soldering, 20-40K Ohm resistor, and duck tape.
The BL-5C battery contain 3 poles of pins, Positive Pole (left), Negative Pole (middle) and Negative pole (right), For making the BL-5C battery charging you must join the Negative pole (middle) and Negative pole (right) with 20K Ohm resistor. See the diagrams below:
You join the Positive Pole on BL-5C and the Positive Pole on the Sierra modem, join them with soldering, and so the outer negative pole on the BL-5C joined with outer (right) negative pole on the modem. You can see the diagram below. For placing the replacement battery is according to your creativity.